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Publ. Astron. Soc. Aust., 2000, 53, 56­71
.

The Optical/Near-IR Colours of Red Quasars
Paul J. Francis,1,2 Matthew T. Whiting3 and Rachel L. Webster3
1

Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 0200, Australia pfrancis@mso.anu.edu.au 2 Joint appointment with the Department of Physics and Theoretical Physics, Faculty of Science, Australian National University, Canberra, ACT 0200, Australia 3 School of Physics, University of Melbourne, Parkville, Vic. 3052, Australia mwhiting, rwebster@physics.unimelb.edu.au Received 1999 July 5, accepted 2000 January 7

Abstract: We present quasi-simultaneous multi-colour optical/near-IR photometry for 157 radio selected quasars, forming an unbiassed sub-sample of the Parkes Flat-Spectrum Sample. Data are also presented for 12 optically selected QSOs, drawn from the Large Bright QSO Survey. The spectral energy distributions of the radio- and optically-selected sources are quite different. The optically selected QSOs are all very similar: they have blue spectral energy distributions curving downwards at shorter wavelengths. Roughly 90% of the radio-selected quasars have roughly power-law spectral energy distributions, with slopes ranging from F 0 to F -2 . The remaining 10% have spectral energy distributions showing sharp peaks: these are radio galaxies and highly reddened quasars. Four radio sources were not detected down to magnitude limits of H 19 · 6. These are probably high redshift (z > 3) galaxies or quasars. We show that the colours of our red quasars lie close to the stellar locus in the optical: they will be hard to identify in surveys such as the Sloan Digital Sky Survey. If near-IR photometry is added, however, the red power-law sources can be clearly separated from the stellar locus: IR surveys such as 2MASS should be capable of finding these sources on the basis of their excess flux in the K -band. Keywords: quasars: general--methods: observational

1 Introduction It was long believed that quasars are blue. The optical/near-IR colours of optically selected QSOs are indeed uniformly very blue (e.g. Neugebauer et al. 1987; Francis 1996). It was therefore a surprise when substantial numbers of extremely red quasars were identified in radio-selected samples (e.g. Rieke, Lebofsky & Wisniewski 1982; Ledden & O'Dell 1983; Webster et al. 1995; Stickel, Rieke & Kuhr 1996). ¨ The biggest sample of these ob jects is that of Webster et al., who were studying a sample of radio-loud quasars with flat radio spectra: the Parkes HalfJansky Flat-Spectrum survey, a complete sample of 323 sources with fluxes at 2 · 7 GHz (S2 · 7 ) of greater than 0 · 5 Jy, and radio spectral indices (S ) with > -0 · 5 as measured between 2 · 7 and 5 · 0 GHz (Drinkwater et al. 1997). While some of these Parkes sources had BJ - Kn colours as blue as any optically selected QSOs, most had redder BJ - Kn colours, and some were amongst the reddest ob jects on the sky. Why should the Parkes sources be so red? A variety of theories were proposed:
Astronomical Society of Australia 2000

· The BJ magnitudes of the Parkes sample were measured many years before the Kn magnitudes. Quasars with flat radio spectra are known to be highly variable: this could thus introduce a scatter into the BJ - Kn colours, though it is hard to see why it should introduce a systematic reddening. · Elliptical galaxies with redshifts z > 0 · 1 have very red BJ - Kn colours, due to the redshifted 400 nm break. If the host galaxies make a significant contribution to the integrated light from the Parkes sources, this could produce the red colours. Masci, Webster & Francis (1998), however, used spectra to show that this effect was only significant for 10% of the sample. · The BJ magnitudes were derived from COSMOS scans of UK Schmidt plates, and are sub ject to substantial systematic errors, which could introduce scatter into the BJ - Kn colours (O'Brian, Webster & Francis 2000, in preparation), though this too should not introduce a systematic reddening.
10.1071/AS00001 1323-3580/00/010056$05.00


Optical/Near­IR Colours of Red Quasars

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· Parkes quasars could have the same intrinsic colours as optically selected QSOs, but be reddened by dust somewhere along the line of sight (Webster et al. 1995). · Flat-radio-spectrum quasars are thought to have relativistic jets: if the synchrotron emission from these jets has a very red spectrum and extended into the near-IR, it could account for the red colours (Serjeant & Rawlings 1996). In this paper, we test the results of Webster et al. (1995) by obtaining much better photometry of a large sub-set of the Parkes sources. To minimise the effects of variability, all our photometry for a given source was obtained within a period of at most six days. All the data were obtained from photometrically calibrated images and, rather than relying on only two bands (BJ and Kn ), we obtained photometry in every band from B to Kn . In principle, multi-colour photometry should enable us to discriminate between the dust and synchrotron models. If quasars have intrinsically blue power-law continua (e.g. F -0 · 3 , Francis 1996), reddened by a foreground dust screen with an extinction E (B - V ) between the B and V bands (in magnitudes) and an optical depth inversely proportional to wavelength, then the observed continuum slope will be F e-
2E (B -V )/ -1 · 7

will have a characteristic `u' shape, dominated by the underlying blue flux at short wavelengths but by the new synchrotron component at longer wavelengths (Figure 2).

Figure 2--Continuum shapes of quasars with an additional red emission component. To show some of the possibilities, two different arbitrary functional forms have been chosen for the red component: a power-law (left ) and an exponential (right ). The strength of this red component increases upwards. Note the characteristic `u' shape. The plausibility of synchrotron models is discussed in Whiting, Webster & Francis (2000). Table 1. Night code Observing log Telescope/Instrument 1m 1m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 1m 1m 1m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 1m 1m 2 · 3m 2 · 3m 2 · 3m 2 · 3m 2 · 3m



,

(1)
A B C D E F I J K L M N O P Q R S T U V W X Y Z

Date April April April April April April July July July July July July July July July July July Sept Sept Sept Sept Sept Sept Sept 12, 1997 13, 1997 14, 1997 15, 1997 16, 1997 17, 1997 12, 1997 13, 1997 14, 1997 13, 1997 14, 1997 15, 1997 16, 1997 17, 1997 18, 1997 19, 1997 20, 1997 7, 1997 8, 1997 9, 1997 10, 1997 11, 1997 12, 1997 13, 1997

where is the wavelength in µm. This is plotted in Figure 1. Note the very characteristic `n' shape, as the dust absorption increases exponentially into the blue.

Imager Imager Caspir Caspir

Imager Imager Caspir Caspir Caspir Caspir Caspir Caspir

Imager Imager Caspir Caspir Caspir

Figure 1--Continuum shapes of dust affected quasars. The extinction E (B - V ) increases downwards: values are 0, 0 · 1, 0 · 2, 0 · 3 and 0 · 4. Note the characteristic `n' shape.

If, alternatively, the redness is caused by the addition of some red synchrotron emission component to the underlying blue continuum, continuum shapes

If radio-quiet red quasars exist, they cannot be selected by conventional optical surveys. We show that by combining optical and near-IR data, it should be possible to select any radio-quiet sources with the colours of most of our radio-selected red quasars.


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Table 2.
Name PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS 0036­216 0038­020 0048­071 0048­097 0048­427 0056­001 0131­001 0153­410 0202­172 0213­026 0216+011 0220­349 0221+067 0226­038 0229­398 0232­042 0237+040 0238­084 0240­060 0240­217 0256+075 0301­243 0316­444 0537­441 0829+046 0912+029 0922+005 1016­311 1020­103 1021­006 1032­199 1034­293 1036­154 1038+064 1042+071 1045­188 1048­313 1055­243 1055+018 1101­325 1107­187 1110­217 1133­172 1136­135 1156­094 1244­255 1256­229 1313­333 1330+022 1353­341 1404­342 1411+094 1430­155 1430­178 1435­218 1437­153 1438­347 1450­338 1454­060 1456+044 1504­166 1508­055 1509+022 1510­089 1511­100 1511­210 1514­241 1518+045 1519­273 1532+016 1535+004 1542+042 1546+027 1548+056 1550­269 1555­140 1555+001 1556­245 1601­222 1602­001 1614+051 1615+029 Redshift z ... 1 · 178 1 · 974 ... 1 · 749 0 · 717 0 · 879 0 · 226 1 · 740 1 · 178 1 · 610 1 · 490 0 · 510 2 · 066 1 · 646 1 · 437 0 · 978 0 · 005 1 · 800 0 · 314 0 · 895 ... 0 · 076 0 · 893 ... 0 · 427 1 · 717 0 · 794 0 · 197 2 · 549 2 · 198 0 · 312 0 · 525 1 · 264 0 · 698 0 · 595 1 · 429 1 · 086 0 · 888 0 · 355 0 · 497 ... 1 · 024 0 · 557 ... 0 · 638 1 · 365 1 · 210 0 · 216 0 · 223 1 · 122 0 · 162 1 · 573 2 · 326 1 · 187 0 · 254 1 · 159 0 · 368 1 · 249 0 · 394 0 · 876 1 · 185 0 · 219 0 · 362 1 · 513 1 · 179 0 · 049 0 · 052 0 · 510 1 · 435 3 · 497 2 · 184 0 · 415 1 · 422 2 · 145 0 · 097 1 · 770 2 · 818 ... 1 · 624 3 · 217 1 · 341 Morphology galaxy p oint faint p oint p oint p oint faint galaxy p oint faint faint faint p oint p oint p oint p oint p oint galaxya p oint galaxyb p oint p oint galaxy p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint faint faint faint p oint faint p oint p oint p oint galaxy galaxyc p oint galaxyd faint p oint p oint p oint p oint faint p oint p oint p oint p oint galaxye p oint p oint p oint galaxyf galaxyg p oint p oint faint p oint p oint p oint p oint galaxyh faint p oint faint p oint p oint p oint W U W W W U M V V V V V V V V V W W W W W W W A A C ... B A B B B C A B B B B B A D C D A C A A A A A K K L J J J J D I ... I I I A B L B K K K M J A K B J M I ... J K A B 600 360 300 180 300 300 1200 300 180 600 300 300 300 180 600 180 300 60 180 180 960 180 180 300 300 600 ... 300 300 300 300 300 600 300 600 600 600 300 300 300 2400 2400 1800 300 600 300 300 300 600 1200 300 300 1800 300 300 300 300 2400 300 ... 600 300 300 300 300 900 300 300 300 300 600 300 300 900 300 300 600 360 ... 300 900 300

Observation dates and exposure times
V R I J H Letters indicate observation dates, numb ers are exp osure times (seconds) W U W W W U M V V V V V V V V V W W W W W W W A A C ... B A B B B C A B B B B B A D C D A C A A A A A K K L J J J J D I ... I I I A B L B K K K ... J A K B J M I ... J K AD 300 360 300 180 300 300 600 180 180 600 300 300 300 180 600 180 180 60 180 180 600 180 180 300 300 300 ... 300 300 300 300 300 300 300 300 300 300 300 300 300 1800 2400 1800 300 600 300 300 300 300 300 300 300 1800 300 300 300 300 1200 300 ... 300 300 300 300 300 600 60 300 300 300 ... 300 300 300 300 300 600 360 ... 300 600 2100 W U W W W U M V V V V V V V V V W W W W W W W A A C D B A B B B C A B B B B B A D C D A C A A A A A K K L J J J J D I C I I ... A B L B K K K M J A K B J M I L J K AD 300 360 300 180 300 300 600 180 180 600 300 300 300 180 600 180 180 60 180 180 600 180 180 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 1200 3600 1200 300 600 300 300 300 300 300 300 300 1200 300 300 300 300 600 300 900 300 300 ... 300 300 600 60 300 300 300 2400 300 300 300 300 300 900 300 2400 300 600 900 W U W W W ... M V V V V V V V V V W W W W W W W A A C D B A B B B C A B B B B B A D C D A C A A A A A K K L J J J J D I C I I ... A B L B K K K M J A K B J M I L J K AD 300 360 300 180 300 ... 600 180 180 600 300 300 300 180 600 180 180 60 180 180 600 180 180 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 1200 1800 1200 300 600 300 300 300 300 300 300 300 1200 300 300 300 300 600 300 900 300 300 ... 300 300 600 60 300 300 300 2400 300 300 300 300 300 300 300 2400 300 900 900 X X X X X X N X X X X X Z Z Z Z Z Z Z ... ... ... ... F F E ... F F F F F F ... ... F ... ... ... F E E E F F F F F F F O O O P P P P E P E P P P E F Q P P P P ... Q F Q F Q Q Q ... O O E 240 240 1200 240 240 240 1200 240 240 240 480 1200 240 240 1200 240 240 300 240 ... ... ... ... 240 240 1200 ... 240 240 240 240 240 240 ... ... 240 ... ... ... 240 1200 1200 240 240 240 240 240 240 240 240 240 240 1200 240 240 240 240 240 240 1200 240 240 240 240 240 240 240 240 240 240 ... 240 240 240 240 240 240 240 ... 240 240 1200 X X X X X X N X X X X X Z Z X Z Z Z Z Z ... ... ... E E E ... F F ... F F E ... ... F ... ... ... E E E E F F F F E F F O O O P P P P E P E P P P E F Q P P P P R Q E Q F Q Q Q Q O O E 240 240 1200 240 240 240 1200 240 240 240 1200 1200 240 240 1200 240 240 300 240 240 ... ... ... 240 240 1200 ... 240 240 ... 240 240 240 ... ... 240 ... ... ... 240 1200 1200 240 240 240 240 240 240 240 240 240 240 2400 240 240 240 240 240 240 1200 240 240 240 240 240 240 240 240 240 240 3600 240 240 240 240 240 240 240 3600 240 240 1200 X X X X X X N X X X X X Z Z X Z Z Z Z Z ... ... ... F F E ... F F ... F F F ... ... F ... ... ... F E E E F F F F F F F O O O P P P P E P F P P P E F Q P P P P ... Q F Q F Q Q Q Q P O E Kn 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 240 1200 1200 1200 1200 300 240 300 0 0 0 240 240 2240 0 240 240 0 1200 240 240 0 0 2240 0 0 0 240 1200 1200 240 1200 1200 240 240 240 240 240 240 1200 1200 240 240 240 240 240 240 1200 240 240 240 240 240 240 240 240 240 240 0 240 240 240 1200 240 240 240 3600 1200 1200 1200


Optical/Near­IR Colours of Red Quasars

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Table 2.
Name PKS 1616+063 PKS 1635­035 PKS 1648+015 PKS 1649­062 PKS 1654­020 PKS 1655+077 PKS 1656+053 PKS 1705+018 PKS 1706+006 PKS 1725+044 PKS 1732+094 PKS 1933­400 PKS 1953­325 PKS 1954­388 PKS 1958­179 PKS 2000­330 PKS 2002­185 PKS 2004­447 PKS 2008­159 PKS 2021­330 PKS 2022­077 PKS 2037­253 PKS 2044­168 PKS 2047+098 PKS 2053­044 PKS 2056­369 PKS 2058­135 PKS 2058­297 PKS 2059+034 PKS 2106­413 PKS 2120+099 PKS 2121+053 PKS 2126­158 PKS 2127­096 PKS 2128­123 PKS 2131­021 PKS 2134+004 PKS 2135­248 PKS 2140­048 PKS 2143­156 PKS 2144+092 PKS 2145­176 PKS 2145+067 PKS 2149­307 PKS 2149+056 PKS 2149+069 PKS 2155­152 PKS 2200­238 PKS 2203­188 PKS 2206­237 PKS 2208­137 PKS 2210­257 PKS 2212­299 PKS 2215+020 PKS 2216­038 PKS 2223­052 PKS 2227­088 PKS 2227­399 PKS 2229­172 PKS 2233­148 PKS 2239+096 PKS 2240­260 PKS 2243­123 PKS 2245­328 PKS 2245+029 PKS 2252­090 PKS 2312­319 PKS 2313­438 PKS 2314­409 PKS 2329­415 PKS 2337­334 PKS 2344­192 PKS 2345­167 PKS 2351­154 PKS 2354­117 L 2110­4509 L 2111­4335 L 2111­4506 L 2113­4305 L 2113­4345 L 2113­4538 L 2114­4335 Redshift z 2 · 088 2 · 856 ... ... 2 · 000 0 · 621 0 · 887 2 · 576 0 · 449 0 · 296 ... 0 · 965 1 · 242 0 · 626 0 · 652 3 · 783 0 · 859 0 · 240 1 · 178 1 · 471 1 · 388 1 · 574 1 · 937 ... 1 · 177 ... 0 · 029 1 · 492 1 · 012 1 · 055 0 · 932 1 · 941 3 · 266 0 · 780 0 · 499 1 · 285 1 · 937 0 · 821 0 · 344 0 · 698 1 · 113 2 · 130 0 · 999 2 · 345 0 · 740 1 · 364 0 · 672 2 · 120 0 · 619 0 · 086 0 · 392 1 · 833 2 · 703 3 · 572 0 · 901 1 · 404 1 · 561 0 · 323 1 · 780 0 · 609 1 · 707 0 · 774 0 · 630 2 · 268 ... ... 1 · 323 1 · 847 2 · 448 0 · 671 ... ... 0 · 576 2 · 675 0 · 960 0 · 555 1 · 708 1 · 376 1 · 249 2 · 053 0 · 946 1 · 318 Morphology p oint p oint faint faint faint p oint p oint p oint faint p oint faint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint faint p oint faint galaxyi p oint p oint p oint p oint p oint p oint faint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint faint p oint p oint p oint p oint galaxyj p oint p oint p oint p oint p oint p oint p oint p oint faint p oint p oint p oint p oint p oint p oint faint p oint p oint p oint p oint faint faint p oint p oint p oint p oint p oint p oint p oint p oint p oint p oint J D C ... M V I I V A M A I I I I B A I J J J J ... J ... J J J J T J L T K K K K K K T T V T M V M T T V V V V V L V V V V V W W W W W W J J J J L LM K K K U U U U U U W B 300 600 600 ... 1800 600 300 300 1200 600 1800 300 300 300 300 300 600 300 300 300 600 300 300 ... 300 ... 120 300 300 300 300 300 600 600 300 900 300 300 300 300 300 300 120 300 1800 120 1200 300 300 120 120 120 120 600 600 120 120 120 600 600 300 180 180 180 1200 1800 300 300 300 300 600 1800 300 300 300 300 300 300 300 300 300 180

(Continued )
Kn O F E F Q Y P Q X E P F N N N P E E N NQ N O N ... O OQ Q P Q Q X Q P X O Q Q Q Q Q X X X X Q X Q Y Y Y Y Y Y Y P Z Z Z Z Z Z Z Z Z X X O Q Q Q O PQ Q Q Q Y Y Y Y Y ... ... 1200 1200 1200 2240 1200 240 240 1200 240 1200 1200 240 240 240 240 1200 240 240 240 720 240 240 240 0 240 2400 240 240 240 240 240 240 2240 240 1240 240 240 240 240 240 240 240 240 240 240 240 960 1200 1200 240 1200 1320 1200 240 1200 1200 240 240 1200 1200 240 1260 240 240 240 240 1200 1200 1200 1200 660 2400 1200 240 1200 240 240 240 240 240 0 0

V R I J H Letters indicate observation dates, numb ers are exp osure times (seconds) J D C ... M V I I V A L A I I I I B A I I J J J ... J M J J J J T J L T K K K K K K T T V T M V M T T V V V V V L V V V V V W W W W W W J J J J L ... K K K U U U U U U W 300 600 600 ... 1800 600 300 300 600 300 1200 300 300 300 300 300 300 300 300 300 600 300 300 ... 300 2400 30 300 300 300 300 300 1860 300 300 300 300 300 300 300 300 300 120 300 1800 120 1200 300 300 120 120 120 120 600 600 120 120 120 600 300 300 180 180 180 600 1800 300 300 300 300 600 ... 300 300 300 300 300 300 300 300 300 180 J D C C M V I I V A L A I I I I B A I I J J J VW J L J J J J T J L T K K K K K K T T V T M V M T T V V V V V L V V V V V W W W W W W J J J J L L K K K U U U U U U W 300 600 600 3000 1800 300 300 300 300 300 1200 300 300 300 300 300 300 300 300 300 600 300 300 1800 300 1800 30 300 300 300 300 300 1860 300 300 300 300 300 300 300 300 300 120 300 1200 120 1800 300 300 120 120 120 120 300 1860 120 120 120 300 300 300 180 180 180 600 600 300 300 300 300 600 600 300 300 300 300 300 300 300 300 300 180 J D C C M V I I V A L A I I I I B A I I J J J W J L J J J J T J L T K K K K K K T T U T M V M T T V V V V V L V V V V V W W W W W W J J J J L L K K K U U U U U U W 300 600 600 2400 1800 300 300 300 300 300 1200 300 300 300 300 300 300 300 300 300 600 300 300 4200 300 1800 30 300 300 300 300 300 1800 300 300 300 300 300 300 300 300 300 300 300 600 120 1800 300 300 120 120 120 120 300 1800 120 120 120 300 300 300 180 180 180 600 600 300 300 300 300 1200 600 300 300 300 300 300 300 300 300 300 180 O F E F Q Y P Q X E P F N N N P F F N NQ N O N ... O O Q P Q Q X Q P X O Q Q Q Q Q X X X X Q X Q Y Y Y Y Y Y Y P Z Z Z Z Z Z Z Z Z X X O Q Q Q O PQ Q Q Q Y Y Y Y Y ... ... 240 2240 1200 2240 1200 240 240 240 240 1200 1200 240 240 240 240 1200 240 240 240 480 240 240 240 ... 240 1200 240 480 240 240 240 240 1200 240 1200 240 240 240 240 240 240 240 240 240 240 240 1200 240 240 240 240 240 240 240 600 240 240 240 480 240 240 240 240 240 300 240 240 240 240 240 960 2400 240 240 240 240 240 240 240 240 ... ... O F E F Q Y P Q X E P F N N N P E F N NQ N O N X O O Q P Q Q X Q P X O Q Q Q Q Q X X X X Q X Q Y Y Y Y Y Y Y P Z Z Z Z Z Z Z Z Z X X O Q Q Q O PQ Q Q Q Y Y Y Y Y ... ... 240 2240 1200 2240 1200 240 240 240 240 1200 1200 240 240 240 240 1200 240 240 240 480 240 240 240 3600 240 1200 240 480 240 240 240 240 1200 240 1200 240 240 240 240 240 240 240 240 240 240 240 1200 240 240 240 240 240 240 360 1200 240 240 240 240 240 240 240 240 240 240 240 240 240 240 240 600 2400 240 240 240 240 240 240 240 240 ... ...


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Table 2.
Name L L L L L 2114­4346 2114­4501 2116­4439 2118­4702 2119­4415 Redshift z 2 0 1 1 0 · · · · · 041 597 480 332 728 Morphology p p p p p oint oint oint oint oint W W W W W B 180 180 180 180 180

(Continued )
Kn ... ... ... Z Z 0 0 0 240 240

V R I J H Letters indicate observation dates, numb ers are exp osure times (seconds) W W W W W 180 180 180 180 180 W W W W W 180 180 180 180 180 W W W W W 180 180 180 180 180 ... ... ... ... Z ... ... ... ... 240 ... ... ... ... Z ... ... ... ... 240

a b c d e Use circular photometric ap erture of radius 45 Ap erture radius 10 Ap erture radius 8 Ap erture radius 15 Ap erture f g h i j Ap erture radius 20 Ap erture radius 35 Ap erture radius 5 Ap erture radius 10 Ap erture radius 14 . radius 8

Table 3. Filter B V R I J H K

Assumed fluxes of a zero magnitude star Mean wavelength (µm) 0 0 0 0 1 1 2 · · · · · · · 440 550 700 880 239 649 132 Flux of zero magnitude star (F , W m-2 nm-1 ) 6 3 2 1 3 1 4 · · · · · · · 32â 64â 18â 13â 11â 15â 10â 10- 10- 10- 10- 10- 10- 10-
11 11 11 11 12 12 13

This paper describes the observations, presents the data, includes some simple phenomenological analyses of the results, and discusses the colour selection of red quasars in the optical and near-IR. We defer the detailed modelling of the data to the companion paper by Whiting, Webster & Francis (2000). 2 Observations We obtained quasi-simultaneous B, V , R, I , J, H and Kn photometry of a subset of the Parkes sample. Observations were taken during 26 nights in 1997 (Table 1) at Siding Spring Observatory. Optical images were obtained with either the 1 m telescope, or with the imager on the 2 · 3 m telescope. Near-IR images were obtained with the CASPIR 256â256 InSb array camera (McGregor et al. 1994) on the 2 · 3 m telescope. A total of 157 Parkes sources were observed in some or all of the bands, as well as a small control sample of 12 optically selected QSOs randomly selected from the Large Bright QSO survey (LBQS, Morris et al. 1991); an optical QSO survey well matched in size and redshift distribution to the Parkes sample. To minimise the effects of variability, all the observations of an individual source were made within, at most, a six-day period (Table 2). Flat spectrum quasars typically vary by 10% or less on these timescales, though very occasionally greater variations are seen, typically in BL Lac ob jects (e.g. Wagner et al. 1990; Heidt & Wagner 1996). Only data taken in photometric conditions were used: seeing was typically 1­2 . Bright ob jects were typically observed for approximately five minutes in each band. Fainter ob jects were observed for up to two hours in our most sensitive bands (R, I and H ). If they were seen in these bands, we observed them in progressively bluer bands as time allowed. Four sources

were not detected in any band: PKS 1535+004, PKS 1601­222, PKS 1649­062 and PKS 2047+098. About five standard stars, spanning a range of colours, were observed each night: in the optical, the Graham E regions (Graham 1982) were used, while in the near-IR, photometric calibration was obtained using the IRIS standard stars, which have magnitudes on the Carter SAAO system (Carter & Meadows 1995). Within individual nights, the scatter in photometric zero points (without using colour corrections) was 3% rms, so all the standards in a given band were simply averaged to give the final calibration. All 98 Parkes sources lying in the R.A. ranges 00:36­00:57, 01:53­02:40 and 14:50­22:52 (B1950) were observed in both the optical and the IR: these should thus form an unbiassed, complete subsample of the whole Parkes Half-Jansky sample. The remaining 59 sources were selected for observation mainly on the basis of prevailing weather conditions, and so should also form a reasonably unbiassed subsample. No selection was made against radio galaxies: sources with resolved optical or near-IR images (as classified by the COSMOS plate measuring machine from UK Schmidt plates, and checked by visual inspection of our images) are listed in Table 2. Where appropriate, they are excluded from the following analysis. Optical images were bias- and overscan-subtracted, and then flat fielded using twilight sky flats. For the fainter sources, multiply dithered 300- or 600-second exposures were taken: these were combined using inverse variance weighting. The infrared exposures were made up of multiple dithered 60 s images, each made up of two averaged 30 s exposures in J , six averaged 10 s exposures in H and twelve averaged 5 s exposures in Kn . These were biasand dark-subtracted, and then corrected for the nonlinearity of the CASPIR detector using a simple quadratic correction term (derived from plots of median counts against exposure time obtained from dome flats). Known bad pixels were replaced by the interpolated flux from neighbouring pixels. Flat fields were obtained by taking exposures of the dome with lamps on and off, and subtracting one from the other: this removes the contribution from telescope emission, and substantially improves the photometric accuracy attainable. Individual images were sky subtracted, using a median of the 10


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Figure 3--Distribution of B - Kn colours for the Parkes sample (top ), and the optically selected LBQS sample (bottom ). Sources with spatially extended images (radio galaxies) have been excluded, as have sources with redshift z > 3 (as the Ly forest depressed the B -band flux). Only Parkes sources within the complete sub-sample have been used. The LBQS data from this paper have been supplemented by data from Francis (1996).

images taken nearest in time. The dithered images were then aligned and combined, using the median to remove residual errors. The radio sources were identified from the radio positions by using astrometry from nearby stars, bootstrapped from positions in the COSMOS/UKST and APM/POSS sky catalogues, maintained on-line at the Anglo-Australian Observatory. Magnitudes were then measured using circular apertures, with the sky level determined from the median flux in an annulus around the sky aperture. For unresolved sources, the photometric apertures were set by the seeing: typical aperture radii were 5 . For resolved sources (mostly low redshift radio galaxies) larger circular apertures were used, centred on the galactic nucleus. These larger aperture radii are listed in the footnotes to Table 2. Standard stars were measured with similar aperture sizes. Quoted errors are the sum (in quadrature) of random errors and an assumed 5% error in the photometric zero points. Random errors were determined by measuring the rms (root-meansquared) pixel-to-pixel variation in sky regions, and scaling to the aperture size used. This will be accurate for fainter (sky or read-noise limited) sources, but will underestimate random errors for the brightest few sources. The photometric zero point errors were estimated from the scatter in zero points between different standard star measurements in an individual night: typical rms scatters are 3%,

so we adopted a conservative value of 5% as our zero point error. For modelling and plotting purposes, we converted the magnitudes into fluxes. We assumed fluxes for zero magnitude ob jects as listed in Table 3. In the optical, our filter sets approximate the Johnson & Cousins system, and were calibrated using the Graham standards (also approximating Johnson & Cousins). The zero magnitude star fluxes for this system were taken from Bessell, Castelli & Plez (1998). In the infrared, our observations used the CASPIR filter set calibrated by the IRIS standards. Zero magnitude fluxes were calculated by P. McGregor, assuming that Vega is well represented in the near-IR by a black body of temperature 11200 K, and normalisation F (555 nm) = 3 · 44 â 10-12 W cm-2 µm-1 (Bersanelli, Bouchet & Falomo 1991). These normalisations agree closely with those quoted for UKIRT near-IR standards (MacKenty et al. 1997). Our observations were made with the Kn filter, but were calibrated using the quoted K magnitudes of the IRIS standards without applying a colour correction term, and should thus be normalised to a K -band zero point. 3 Results and Discussion 3.1 The Colour Distribution The results are listed in Table 4. Quoted errors are 1 ; upper limits are 3 .


62

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Table 4.
Name PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS PKS 0036­216 0038­020 0048­071 0048­097 0048­427 0056­001 0131­001 0153­410 0202­172 0213­026 0216+011 0220­349 0221+067 0226­038 0229­398 0232­042 0237+040 0238­084 0240­060 0240­217 0256+075 0301­243 0316­444 0537­441 0829+046 0912+029 0922+005 1016­311 1020­103 1021­006 1032­199 1034­293 1036­154 1038+064 1042+071 1045­188 1048­313 1055­243 1055+018 1101­325 1107­187 1110­217 1133­172 1136­135 1156­094 1244­255 1256­229 1313­333 1330+022 1353­341 1404­342 1411+094 1430­155 1430­178 1435­218 1437­153 1438­347 1450­338 1454­060 1456+044 1504­166 1508­055 1509+022 1510­089 1511­100 1511­210 1514­241 1518+045 1519­273 1532+016 1535+004 1542+042 1546+027 1548+056 1550­269 1555­140 1555+001 1556­245 1601­222 1602­001 1614+051 1615+029 1616+063 21 18 20 16 18 17 23 19 17 21 20 21 19 17 19 15 18 12 19 19 21 16 16 17 16 18 · · · · · · · · · · · · · · · · · · · · · · · · · · B 20±0 · 20 74±0 · 07 74±0 · 18 12±0 · 05 62±0 · 06 73±0 · 05 34±0 · 70 81±0 · 10 46±0 · 05 33±0 · 20 33±0 · 13 73±0 · 38 85±0 · 09 56±0 · 05 74±0 · 07 73±0 · 05 33±0 · 05 03±0 · 05 07±0 · 07 20±0 · 09 32±0 · 18 51±0 · 05 07±0 · 05 93±0 · 05 28±0 · 05 75±0 · 05 ... 18 · 32±0 · 06 17 · 10±0 · 05 18 · 38±0 · 06 18 · 92±0 · 06 18 · 83±0 · 06 20 · 54±0 · 11 17 · 10±0 · 05 20 · 38±0 · 11 18 · 55±0 · 05 20 · 19±0 · 10 19 · 13±0 · 07 18 · 17±0 · 05 15 · 54±0 · 05 22 · 44±0 · 35 24 · 41±1 · 08 22 · 32±0 · 35 16 · 65±0 · 05 21 · 95±0 · 33 17 · 42±0 · 05 18 · 43±0 · 06 18 · 17±0 · 06 19 · 02±0 · 06 19 · 20±0 · 06 17 · 37±0 · 05 18 · 64±0 · 09 >22 · 70 19 · 14±0 · 07 19 · 20±0 · 07 19 · 94±0 · 11 17 · 80±0 · 05 22 · 52±0 · 37 18 · 36±0 · 06 ... 20 · 28±0 · 12 17 · 37±0 · 05 19 · 75±0 · 12 17 · 26±0 · 05 18 · 18±0 · 05 22 · 01±0 · 25 15 · 18±0 · 05 16 · 11±0 · 05 18 · 21±0 · 06 19 · 12±0 · 08 >21 · 70 18 · 75±0 · 06 17 · 43±0 · 05 19 · 37±0 · 06 20 · 24±0 · 14 18 · 28±0 · 05 20 · 34±0 · 10 18 · 89±0 · 06 ... 17 · 66±0 · 05 20 · 38±0 · 11 18 · 01±0 · 05 19 · 45±0 · 08 19 18 20 15 18 17 22 18 17 20 20 21 18 17 20 16 17 11 18 17 20 16 14 17 15 18 · · · · · · · · · · · · · · · · · · · · · · · · · · V 36±0 · 07 44±0 · 06 32±0 · 13 75±0 · 05 45±0 · 06 55±0 · 05 50±0 · 50 15±0 · 06 33±0 · 05 82±0 · 14 36±0 · 13 32±0 · 27 97±0 · 06 38±0 · 05 31±0 · 10 27±0 · 05 88±0 · 05 04±0 · 05 87±0 · 07 93±0 · 06 68±0 · 13 10±0 · 05 76±0 · 05 34±0 · 05 61±0 · 05 59±0 · 06 ... 18 · 01±0 · 05 16 · 70±0 · 05 18 · 25±0 · 05 18 · 83±0 · 06 18 · 26±0 · 05 20 · 07±0 · 10 16 · 92±0 · 05 19 · 87±0 · 11 18 · 26±0 · 05 20 · 19±0 · 13 18 · 71±0 · 06 17 · 63±0 · 05 15 · 54±0 · 05 21 · 10±0 · 14 23 · 01±0 · 42 21 · 54±0 · 19 16 · 52±0 · 05 21 · 43±0 · 22 16 · 99±0 · 05 17 · 68±0 · 05 17 · 63±0 · 05 17 · 89±0 · 05 17 · 46±0 · 05 16 · 78±0 · 05 17 · 56±0 · 06 23 · 24±0 · 48 18 · 92±0 · 06 18 · 54±0 · 06 19 · 01±0 · 07 17 · 31±0 · 05 20 · 40±0 · 10 17 · 84±0 · 05 ... 19 · 75±0 · 11 16 · 94±0 · 05 18 · 27±0 · 06 17 · 12±0 · 05 18 · 00±0 · 05 21 · 45±0 · 19 14 · 50±0 · 05 14 · 84±0 · 05 17 · 74±0 · 05 18 · 92±0 · 07 ... 18 · 54±0 · 06 17 · 14±0 · 05 18 · 65±0 · 06 19 · 67±0 · 09 16 · 93±0 · 05 19 · 95±0 · 08 19 · 02±0 · 06 ... 17 · 57±0 · 05 19 · 38±0 · 07 17 · 75±0 · 05 18 · 97±0 · 06 R

Source magnitudes
I 46±0 · 05 01±0 · 08 53±0 · 15 92±0 · 05 75±0 · 06 ... 20 · 17±0 · 17 16 · 69±0 · 05 16 · 74±0 · 05 18 · 87±0 · 07 19 · 02±0 · 09 19 · 88±0 · 18 17 · 36±0 · 05 16 · 79±0 · 05 19 · 46±0 · 10 15 · 63±0 · 05 17 · 14±0 · 05 9 · 77±0 · 05 18 · 07±0 · 07 16 · 34±0 · 05 18 · 92±0 · 08 15 · 22±0 · 05 13 · 51±0 · 05 16 · 35±0 · 05 14 · 49±0 · 05 17 · 87±0 · 06 16 · 58±0 · 05 17 · 51±0 · 06 15 · 62±0 · 05 17 · 61±0 · 06 18 · 16±0 · 08 17 · 01±0 · 05 18 · 62±0 · 08 16 · 43±0 · 05 18 · 27±0 · 08 17 · 08±0 · 05 19 · 21±0 · 16 17 · 94±0 · 07 16 · 47±0 · 05 14 · 97±0 · 05 18 · 74±0 · 07 21 · 10±0 · 22 19 · 41±0 · 09 16 · 06±0 · 05 20 · 22±0 · 17 16 · 11±0 · 05 16 · 39±0 · 05 16 · 50±0 · 05 16 · 47±0 · 05 15 · 94±0 · 05 16 · 35±0 · 05 16 · 15±0 · 06 20 · 73±0 · 16 18 · 14±0 · 08 17 · 75±0 · 06 17 · 68±0 · 06 16 · 72±0 · 05 18 · 69±0 · 08 17 · 08±0 · 05 18 · 12±0 · 05 18 · 46±0 · 10 16 · 28±0 · 05 ... 16 · 17±0 · 05 17 · 07±0 · 05 20 · 30±0 · 16 13 · 31±0 · 05 13 · 45±0 · 05 16 · 55±0 · 05 17 · 98±0 · 08 >21 · 50 17 · 61±0 · 06 16 · 16±0 · 05 17 · 42±0 · 06 18 · 58±0 · 10 15 · 47±0 · 05 18 · 72±0 · 08 18 · 30±0 · 08 >22 · 40 16 · 66±0 · 05 18 · 93±0 · 10 17 · 00±0 · 05 18 · 27±0 · 08 17 18 19 14 17 · · · · · 16 17 18 13 17 16 17 15 16 17 18 18 16 16 18 15 16 8 17 · · · · · · · · · · · · · · · · · · · J 20±0 · 07 44±0 · 15 73±0 · 20 99±0 · 05 21±0 · 12 49±0 · 08 95±0 · 13 49±0 · 06 27±0 · 07 48±0 · 15 14±0 · 19 72±0 · 20 29±0 · 06 40±0 · 07 86±0 · 18 22±0 · 05 49±0 · 07 70±0 · 05 50±0 · 12 ... ... ... ... 14 · 76±0 · 05 13 · 45±0 · 05 16 · 97±0 · 06 ... 16 · 70±0 · 08 14 · 85±0 · 05 17 · 12±0 · 10 17 · 35±0 · 12 15 · 76±0 · 06 17 · 23±0 · 11 ... ... 16 · 30±0 · 07 ... ... ... 14 · 55±0 · 05 17 · 46±0 · 07 19 · 06±0 · 19 18 · 37±0 · 22 15 · 54±0 · 05 18 · 16±0 · 22 15 · 43±0 · 05 15 · 16±0 · 05 15 · 46±0 · 05 15 · 23±0 · 05 15 · 20±0 · 06 15 · 67±0 · 06 15 · 10±0 · 07 19 · 07±0 · 27 17 · 75±0 · 24 17 · 64±0 · 22 16 · 29±0 · 08 16 · 36±0 · 09 16 · 99±0 · 08 16 · 56±0 · 10 16 · 17±0 · 05 16 · 33±0 · 08 15 · 56±0 · 06 16 · 04±0 · 07 15 · 24±0 · 05 16 · 46±0 · 07 18 · 62±0 · 32 11 · 95±0 · 05 12 · 06±0 · 05 15 · 65±0 · 06 17 · 40±0 · 18 ... 17 · 38±0 · 12 14 · 97±0 · 05 16 · 30±0 · 07 17 · 63±0 · 15 14 · 25±0 · 05 17 · 73±0 · 16 17 · 59±0 · 14 ... 16 · 28±0 · 07 18 · 04±0 · 24 16 · 66±0 · 05 17 · 87±0 · 21 15 16 17 13 16 15 17 14 15 16 17 17 15 15 17 14 15 7 17 14 · · · · · · · · · · · · · · · · · · · · H 39±0 · 06 82±0 · 13 93±0 · 16 24±0 · 05 86±0 · 14 66±0 · 07 24±0 · 12 81±0 · 06 67±0 · 07 40±0 · 10 55±0 · 12 77±0 · 14 26±0 · 06 86±0 · 06 55±0 · 12 38±0 · 05 97±0 · 07 92±0 · 05 33±0 · 16 23±0 · 05 ... ... ... 13 · 85±0 · 05 12 · 52±0 · 05 16 · 01±0 · 05 ... 16 · 42±0 · 09 14 · 11±0 · 05 ... 16 · 80±0 · 11 14 · 83±0 · 05 16 · 32±0 · 07 ... ... 15 · 30±0 · 06 ... ... ... 13 · 93±0 · 05 17 · 21±0 · 07 17 · 82±0 · 11 17 · 69±0 · 19 14 · 90±0 · 05 17 · 51±0 · 20 14 · 66±0 · 05 14 · 21±0 · 05 14 · 36±0 · 05 14 · 43±0 · 05 14 · 38±0 · 05 15 · 13±0 · 06 14 · 36±0 · 06 17 · 97±0 · 13 16 · 78±0 · 17 17 · 18±0 · 23 15 · 11±0 · 06 15 · 86±0 · 09 16 · 03±0 · 07 15 · 86±0 · 09 15 · 71±0 · 05 15 · 59±0 · 07 14 · 87±0 · 06 15 · 12±0 · 06 14 · 09±0 · 05 15 · 60±0 · 06 17 · 81±0 · 25 11 · 10±0 · 05 11 · 31±0 · 05 14 · 84±0 · 06 16 · 22±0 · 11 >19 · 73 16 · 46±0 · 09 13 · 88±0 · 05 15 · 13±0 · 06 17 · 01±0 · 13 13 · 43±0 · 05 16 · 90±0 · 12 16 · 97±0 · 13 >20 · 02 15 · 41±0 · 06 17 · 83±0 · 30 16 · 04±0 · 05 17 · 54±0 · 24 14 16 17 12 16 14 16 14 15 15 17 16 14 15 17 14 15 7 16 13 · · · · · · · · · · · · · · · · · · · · Kn 46±0 · 06 16±0 · 13 54±0 · 39 51±0 · 05 19±0 · 13 92±0 · 06 78±0 · 26 14±0 · 06 35±0 · 08 17±0 · 07 38±0 · 34 82±0 · 22 47±0 · 07 13±0 · 06 51±0 · 38 28±0 · 05 15±0 · 06 68±0 · 05 35±0 · 24 63±0 · 06 ... ... ... 13 · 02±0 · 05 11 · 80±0 · 05 15 · 29±0 · 06 ... 15 · 59±0 · 14 13 · 27±0 · 05 ... 16 · 04±0 · 10 13 · 70±0 · 06 15 · 63±0 · 15 ... ... 14 · 59±0 · 05 ... ... ... 13 · 16±0 · 05 15 · 95±0 · 08 16 · 74±0 · 15 16 · 63±0 · 27 14 · 40±0 · 05 16 · 87±0 · 20 13 · 85±0 · 06 13 · 44±0 · 05 13 · 69±0 · 06 13 · 71±0 · 06 13 · 62±0 · 06 14 · 57±0 · 08 13 · 78±0 · 06 17 · 50±0 · 37 15 · 69±0 · 23 16 · 77±0 · 53 14 · 24±0 · 08 15 · 24±0 · 16 15 · 23±0 · 09 15 · 16±0 · 15 14 · 30±0 · 05 14 · 01±0 · 07 14 · 29±0 · 08 14 · 31±0 · 08 13 · 27±0 · 05 14 · 70±0 · 08 16 · 79±0 · 36 10 · 50±0 · 05 11 · 02±0 · 05 14 · 08±0 · 07 15 · 48±0 · 19 ... 15 · 82±0 · 17 13 · 02±0 · 05 14 · 30­0 · 07 16 · 07±0 · 11 13 · 08±0 · 05 16 · 24±0 · 24 16 · 58±0 · 31 >19 · 49 15 · 19±0 · 08 17 · 31±0 · 32 15 · 89±0 · 08 16 · 52±0 · 17

18 · 33±0 · 05 18 · 11±0 · 06 19 · 84±0 · 10 15 · 39±0 · 05 18 · 15±0 · 05 17 · 43±0 · 05 20 · 78±0 · 14 17 · 41±0 · 05 17 · 11±0 · 05 19 · 99±0 · 08 19 · 54±0 · 08 20 · 83±0 · 20 18 · 24±0 · 05 17 · 17±0 · 05 19 · 90±0 · 08 15 · 93±0 · 05 17 · 53±0 · 05 10 · 44±0 · 05 18 · 61±0 · 06 17 · 07±0 · 05 19 · 92±0 · 08 15 · 69±0 · 05 14 · 12±0 · 05 16 · 92±0 · 05 15 · 11±0 · 05 18 · 29±0 · 05 17 · 09±0 · 05 17 · 82±0 · 05 16 · 30±0 · 05 18 · 03±0 · 05 18 · 60±0 · 06 17 · 71±0 · 05 19 · 43±0 · 08 16 · 63±0 · 05 19 · 12±0 · 08 17 · 76±0 · 05 19 · 70±0 · 11 18 · 40±0 · 06 17 · 15±0 · 05 15 · 31±0 · 05 19 · 30±0 · 06 21 · 57±0 · 13 20 · 52±0 · 11 16 · 39±0 · 05 20 · 47±0 · 11 16 · 63±0 · 05 17 · 11±0 · 05 17 · 16±0 · 05 17 · 24±0 · 05 16 · 74±0 · 05 16 · 56±0 · 05 16 · 77±0 · 05 22 · 91±0 · 48 18 · 55±0 · 06 18 · 04±0 · 05 18 · 47±0 · 06 16 · 98±0 · 05 19 · 39±0 · 07 17 · 45±0 · 05 18 · 83±0 · 05 19 · 35±0 · 09 16 · 66±0 · 05 ... 16 · 72±0 · 05 17 · 50±0 · 05 20 · 65±0 · 11 13 · 98±0 · 05 14 · 20±0 · 05 17 · 18±0 · 05 18 · 44±0 · 06 >22 · 30 18 · 21±0 · 06 16 · 75±0 · 05 18 · 13±0 · 06 19 · 18±0 · 08 16 · 28±0 · 05 20 · 00±0 · 07 18 · 82±0 · 07 >22 · 90 17 · 10±0 · 05 19 · 31±0 · 08 17 · 31±0 · 05 18 · 76±0 · 06


Optical/Near­IR Colours of Red Quasars

63

Table 4.
Name PKS 1635­035 PKS 1648+015 PKS 1649­062 PKS 1654­020 PKS 1655+077 PKS 1656+053 PKS 1705+018 PKS 1706+006 PKS 1725+044 PKS 1732+094 PKS 1933­400 PKS 1953­325 PKS 1954­388 PKS 1958­179 PKS 2000­330 PKS 2002­185 PKS 2004­447 PKS 2008­159 PKS 2021­330 PKS 2022­077 PKS 2037­253 PKS 2044­168 PKS 2047+098 PKS 2053­044 PKS 2056­369 PKS 2058­135 PKS 2058­297 PKS 2059+034 PKS 2106­413 PKS 2120+099 PKS 2121+053 PKS 2126­158 PKS 2127­096 PKS 2128­123 PKS 2131­021 PKS 2134+004 PKS 2135­248 PKS 2140­048 PKS 2143­156 PKS 2144+092 PKS 2145­176 PKS 2145+067 PKS 2149­307 PKS 2149+056 PKS 2149+069 PKS 2155­152 PKS 2200­238 PKS 2203­188 PKS 2206­237 PKS 2208­137 PKS 2210­257 PKS 2212­299 PKS 2215+020 PKS 2216­038 PKS 2223­052 PKS 2227­088 PKS 2227­399 PKS 2229­172 PKS 2233­148 PKS 2239+096 PKS 2240­260 PKS 2243­123 PKS 2245­328 PKS 2245+029 PKS 2252­090 PKS 2312­319 PKS 2313­438 PKS 2314­409 PKS 2329­415 PKS 2337­334 PKS 2344­192 PKS 2345­167 PKS 2351­154 PKS 2354­117 L 2110­4509 L 2111­4335 L 2111­4506 L 2113­4305 L 2113­4345 L 2113­4538 L 2114­4335 L 2114­4346 L 2114­4501 B 21 · 58±0 · 32 21 · 87±0 · 31 ... 23 · 51±0 · 68 20 · 18±0 · 09 17 · 33±0 · 05 19 · 07±0 · 07 22 · 15±0 · 29 17 · 92±0 · 05 >23 · 50 18 · 46±0 · 06 19 · 60±0 · 09 17 · 99±0 · 05 19 · 48±0 · 08 18 · 90±0 · 06 19 · 39±0 · 07 19 · 52±0 · 09 17 · 50±0 · 05 19 · 50±0 · 08 19 · 10±0 · 06 19 · 69±0 · 09 17 · 70±0 · 05 ... 18 · 86±0 · 06 ... 15 · 88±0 · 05 18 · 85±0 · 06 18 · 09±0 · 05 19 · 57±0 · 09 19 · 65±0 · 10 19 · 86±0 · 10 17 · 97±0 · 05 19 · 43±0 · 07 15 · 28±0 · 05 20 · 61±0 · 13 17 · 04±0 · 05 19 · 64±0 · 10 17 · 22±0 · 05 18 · 02±0 · 05 18 · 78±0 · 07 19 · 43±0 · 09 16 · 15±0 · 05 18 · 00±0 · 05 >23 · 50 18 · 79±0 · 07 18 · 42±0 · 05 18 · 45±0 · 06 18 · 85±0 · 07 17 · 41±0 · 06 16 · 79±0 · 05 18 · 74±0 · 07 17 · 47±0 · 05 21 · 84±0 · 31 17 · 03±0 · 05 18 · 59±0 · 06 18 · 31±0 · 06 17 · 94±0 · 05 21 · 26±0 · 19 19 · 24±0 · 06 19 · 00±0 · 06 17 · 95±0 · 05 16 · 65±0 · 05 19 · 08±0 · 07 21 · 71±0 · 22 22 · 03±0 · 24 18 · 10±0 · 05 19 · 89±0 · 11 18 · 33±0 · 06 19 · 40±0 · 08 22 · 93±0 · 60 23 · 52±0 · 68 17 · 23±0 · 05 18 · 97±0 · 07 18 · 81±0 · 07 18 · 89±0 · 07 17 · 29±0 · 05 17 · 54±0 · 05 17 · 87±0 · 05 18 · 56±0 · 06 17 · 67±0 · 05 18 · 22±0 · 06 18 · 56±0 · 06 18 · 67±0 · 06 V 21 · 00±0 · 20 21 · 14±0 · 18 ... 23 · 90±0 · 87 19 · 88±0 · 07 16 · 86±0 · 05 18 · 66±0 · 06 20 · 59±0 · 12 17 · 46±0 · 05 21 · 15±0 · 11 18 · 03±0 · 05 18 · 91±0 · 06 17 · 55±0 · 05 18 · 89±0 · 06 17 · 48±0 · 05 18 · 82±0 · 06 18 · 74±0 · 06 17 · 07±0 · 05 19 · 06±0 · 07 18 · 55±0 · 05 19 · 56±0 · 09 17 · 55±0 · 05 ... 18 · 47±0 · 06 >23 · 50 14 · 76±0 · 05 18 · 68±0 · 06 17 · 65±0 · 05 18 · 85±0 · 06 19 · 24±0 · 08 19 · 22±0 · 07 16 · 92±0 · 05 18 · 65±0 · 06 15 · 13±0 · 05 20 · 16±0 · 15 16 · 80±0 · 05 19 · 23±0 · 08 17 · 18±0 · 05 17 · 69±0 · 05 18 · 36±0 · 06 19 · 11±0 · 08 15 · 80±0 · 05 17 · 75±0 · 05 22 · 05±0 · 23 18 · 48±0 · 06 17 · 90±0 · 05 18 · 36±0 · 06 18 · 52±0 · 06 16 · 25±0 · 05 16 · 70±0 · 05 18 · 76±0 · 07 17 · 36±0 · 05 20 · 42±0 · 10 16 · 49±0 · 05 18 · 33±0 · 06 18 · 15±0 · 06 17 · 77±0 · 05 21 · 14±0 · 18 18 · 61±0 · 06 18 · 68±0 · 06 17 · 37±0 · 05 16 · 50±0 · 05 18 · 82±0 · 07 21 · 00±0 · 17 21 · 51±0 · 16 17 · 86±0 · 05 19 · 53±0 · 08 18 · 08±0 · 05 19 · 17±0 · 07 21 · 89±0 · 27 ... 16 · 79±0 · 05 18 · 71±0 · 06 18 · 03±0 · 05 18 · 64±0 · 07 16 · 78±0 · 05 17 · 37±0 · 05 17 · 62±0 · 05 18 · 22±0 · 06 17 · 49±0 · 05 17 · 90±0 · 05 18 · 42±0 · 06 18 · 54±0 · 06 R

(Continued )
I 20 · 73±0 · 33 19 · 50±0 · 10 >23 · 30 23 · 02±0 · 88 18 · 42±0 · 07 16 · 10±0 · 05 17 · 86±0 · 07 18 · 95±0 · 09 16 · 37±0 · 05 19 · 33±0 · 07 17 · 43±0 · 06 18 · 00±0 · 07 16 · 89±0 · 05 17 · 54±0 · 06 16 · 71±0 · 05 18 · 05±0 · 07 17 · 35±0 · 06 16 · 33±0 · 05 18 · 31±0 · 08 18 · 50±0 · 08 19 · 24±0 · 16 17 · 05±0 · 05 >22 · 19 17 · 99±0 · 07 22 · 22±0 · 44 13 · 45±0 · 05 18 · 09±0 · 07 17 · 20±0 · 06 17 · 76±0 · 06 18 · 36±0 · 10 17 · 93±0 · 07 16 · 23±0 · 05 17 · 39±0 · 06 14 · 70±0 · 05 19 · 11±0 · 17 16 · 11±0 · 05 18 · 41±0 · 10 16 · 44±0 · 05 17 · 09±0 · 06 17 · 68±0 · 07 18 · 23±0 · 09 15 · 29±0 · 05 17 · 20±0 · 06 19 · 54±0 · 10 17 · 74±0 · 06 16 · 88±0 · 05 17 · 64±0 · 07 17 · 46±0 · 06 15 · 04±0 · 05 16 · 10±0 · 05 17 · 95±0 · 07 16 · 81±0 · 05 20 · 00±0 · 20 15 · 80±0 · 05 17 · 43±0 · 06 17 · 39±0 · 06 16 · 69±0 · 05 19 · 83±0 · 17 17 · 27±0 · 05 17 · 91±0 · 06 16 · 36±0 · 05 16 · 04±0 · 05 18 · 31±0 · 08 19 · 48±0 · 11 19 · 48±0 · 11 17 · 09±0 · 05 18 · 60±0 · 10 17 · 36±0 · 06 18 · 41±0 · 09 20 · 51±0 · 14 20 · 96±0 · 27 15 · 88±0 · 05 18 · 42±0 · 10 17 · 44±0 · 06 17 · 78±0 · 08 15 · 99±0 · 05 16 · 73±0 · 05 16 · 92±0 · 06 17 · 62±0 · 07 16 · 89±0 · 06 17 · 25±0 · 05 17 · 79±0 · 06 18 · 05±0 · 07 J 19 · 11±0 · 18 17 · 84±0 · 08 >18 · 88 >20 · 00 17 · 50±0 · 13 15 · 54±0 · 06 17 · 14±0 · 10 17 · 52±0 · 16 15 · 76±0 · 05 18 · 40±0 · 20 16 · 26±0 · 06 17 · 32±0 · 16 16 · 27±0 · 08 16 · 26±0 · 08 16 · 10±0 · 06 17 · 03±0 · 10 16 · 48±0 · 07 16 · 18±0 · 08 17 · 70±0 · 12 18 · 11±0 · 30 18 · 52±0 · 34 16 · 48±0 · 09 ... 17 · 81±0 · 20 19 · 55±0 · 39 12 · 35±0 · 05 17 · 52±0 · 15 16 · 34±0 · 07 16 · 77±0 · 08 17 · 98±0 · 23 16 · 52±0 · 07 15 · 45±0 · 05 16 · 38±0 · 07 14 · 58±0 · 05 17 · 58±0 · 14 15 · 67±0 · 06 17 · 50±0 · 13 15 · 59±0 · 05 16 · 61±0 · 08 16 · 37±0 · 07 17 · 74±0 · 19 14 · 54±0 · 05 16 · 60±0 · 08 18 · 26±0 · 24 16 · 90±0 · 10 15 · 67±0 · 05 17 · 27±0 · 11 16 · 69±0 · 08 13 · 94±0 · 05 15 · 13±0 · 05 17 · 34±0 · 12 15 · 92±0 · 06 19 · 20±0 · 50 14 · 81±0 · 05 16 · 45±0 · 07 16 · 47±0 · 07 15 · 87±0 · 06 18 · 88±0 · 27 16 · 38±0 · 06 17 · 42±0 · 12 15 · 15±0 · 05 15 · 51±0 · 05 17 · 61±0 · 13 17 · 45±0 · 14 18 · 98±0 · 49 16 · 82±0 · 09 17 · 97±0 · 19 16 · 74±0 · 08 17 · 58±0 · 14 19 · 88±0 · 54 18 · 71±0 · 13 14 · 53±0 · 05 17 · 88±0 · 18 16 · 56±0 · 07 16 · 19±0 · 06 15 · 25±0 · 05 16 · 16±0 · 06 16 · 39±0 · 07 17 · 12±0 · 10 ... ... ... ... H 18 · 84±0 · 22 16 · 85±0 · 06 >17 · 77 19 · 92±0 · 64 16 · 59±0 · 10 15 · 04±0 · 06 16 · 44±0 · 09 16 · 38±0 · 10 14 · 91±0 · 05 17 · 44±0 · 14 15 · 79±0 · 07 16 · 78±0 · 16 15 · 28±0 · 06 14 · 97±0 · 06 15 · 64±0 · 06 16 · 87±0 · 10 15 · 46±0 · 06 15 · 58±0 · 07 16 · 82±0 · 09 16 · 61±0 · 14 18 · 01±0 · 34 15 · 99±0 · 09 >19 · 92 16 · 96±0 · 15 19 · 07±0 · 39 11 · 58±0 · 05 17 · 00±0 · 15 16 · 18±0 · 08 15 · 74±0 · 06 17 · 66±0 · 26 15 · 49±0 · 06 14 · 89±0 · 05 15 · 41±0 · 06 13 · 89±0 · 05 16 · 71±0 · 11 15 · 18±0 · 06 16 · 76±0 · 11 14 · 76±0 · 05 15 · 91±0 · 07 15 · 69±0 · 07 17 · 25±0 · 19 14 · 17±0 · 05 16 · 02±0 · 08 17 · 42±0 · 18 15 · 82±0 · 07 14 · 72±0 · 05 16 · 72±0 · 11 15 · 93±0 · 07 13 · 23±0 · 05 14 · 34±0 · 05 16 · 42±0 · 09 15 · 19±0 · 06 18 · 21±0 · 29 14 · 42±0 · 05 15 · 51±0 · 06 15 · 70±0 · 06 15 · 02±0 · 05 17 · 71±0 · 21 15 · 42±0 · 06 16 · 73±0 · 10 14 · 32±0 · 05 15 · 00±0 · 05 17 · 20±0 · 14 16 · 63±0 · 12 17 · 02±0 · 16 16 · 04±0 · 08 17 · 37±0 · 18 16 · 15±0 · 08 16 · 67±0 · 10 18 · 42±0 · 32 17 · 64±0 · 09 13 · 57±0 · 05 17 · 06±0 · 14 15 · 96±0 · 07 15 · 37±0 · 06 14 · 55±0 · 05 15 · 35±0 · 06 15 · 96±0 · 07 16 · 86±0 · 12 ... ... ... ... Kn 17 · 48±0 · 32 16 · 56±0 · 13 >17 · 03 18 · 25±0 · 56 15 · 38±0 · 12 14 · 39±0 · 09 15 · 92±0 · 10 15 · 63±0 · 17 14 · 13±0 · 05 16 · 37±0 · 20 15 · 00±0 · 09 15 · 92±0 · 26 14 · 39±0 · 08 14 · 39±0 · 08 15 · 10±0 · 08 16 · 92±0 · 33 14 · 70±0 · 07 14 · 97±0 · 12 16 · 49±0 · 18 16 · 08±0 · 30 17 · 40±0 · 66 15 · 18±0 · 15 ... 16 · 13±0 · 25 18 · 21±0 · 41 11 · 39±0 · 05 16 · 22±0 · 35 15 · 63±0 · 15 14 · 81±0 · 08 16 · 52±0 · 34 14 · 71±0 · 08 14 · 30±0 · 05 14 · 61±0 · 08 13 · 22±0 · 05 15 · 85±0 · 17 14 · 73±0 · 08 15 · 89±0 · 18 13 · 95±0 · 06 15 · 24±0 · 11 14 · 82±0 · 09 16 · 41±0 · 32 13 · 48±0 · 06 15 · 24±0 · 13 17 · 17±0 · 49 15 · 76±0 · 19 13 · 92±0 · 05 15 · 82±0 · 09 15 · 25±0 · 07 12 · 87±0 · 06 13 · 40±0 · 05 15 · 65±0 · 08 14 · 59±0 · 06 19 · 34±1 · 78 13 · 96±0 · 05 14 · 69±0 · 06 15 · 06±0 · 09 14 · 11±0 · 06 16 · 77±0 · 17 14 · 57±0 · 05 16 · 01±0 · 18 13 · 52±0 · 05 14 · 50±0 · 07 16 · 08±0 · 19 15 · 49±0 · 15 16 · 48±0 · 33 15 · 63±0 · 09 16 · 72±0 · 17 15 · 59±0 · 08 15 · 67±0 · 08 16 · 39±0 · 20 17 · 25±0 · 20 12 · 76±0 · 05 16 · 37±0 · 26 15 · 06±0 · 06 15 · 13±0 · 10 14 · 33±0 · 07 15 · 32±0 · 12 15 · 21±0 · 11 16 · 21±0 · 23 ... ... ... ...

20 · 75±0 · 18 20 · 41±0 · 11 >23 · 20 23 · 05±0 · 52 19 · 36±0 · 07 16 · 64±0 · 05 18 · 37±0 · 06 19 · 95±0 · 10 16 · 97±0 · 05 19 · 74±0 · 06 17 · 72±0 · 05 18 · 52±0 · 06 17 · 28±0 · 05 18 · 34±0 · 06 16 · 92±0 · 05 18 · 55±0 · 06 18 · 18±0 · 06 16 · 76±0 · 05 18 · 70±0 · 06 18 · 44±0 · 05 19 · 21±0 · 08 17 · 18±0 · 05 >22 · 99 18 · 18±0 · 06 23 · 45±0 · 61 14 · 15±0 · 05 18 · 29±0 · 06 17 · 40±0 · 05 18 · 45±0 · 06 18 · 96±0 · 08 18 · 68±0 · 06 16 · 63±0 · 05 18 · 00±0 · 06 15 · 00±0 · 05 19 · 58±0 · 11 16 · 56±0 · 05 18 · 96±0 · 08 16 · 87±0 · 05 17 · 55±0 · 05 18 · 01±0 · 06 18 · 85±0 · 07 15 · 56±0 · 05 17 · 31±0 · 05 20 · 85±0 · 11 17 · 99±0 · 06 17 · 45±0 · 05 18 · 10±0 · 06 18 · 10±0 · 06 15 · 70±0 · 05 16 · 42±0 · 05 18 · 53±0 · 06 17 · 12±0 · 05 20 · 14±0 · 12 16 · 14±0 · 05 17 · 89±0 · 05 17 · 81±0 · 05 17 · 19±0 · 05 21 · 34±0 · 30 18 · 06±0 · 05 18 · 34±0 · 06 16 · 95±0 · 05 16 · 32±0 · 05 18 · 51±0 · 06 20 · 26±0 · 10 20 · 49±0 · 12 17 · 50±0 · 05 19 · 10±0 · 07 17 · 75±0 · 05 18 · 89±0 · 07 21 · 40±0 · 20 21 · 68±0 · 25 16 · 44±0 · 05 18 · 46±0 · 06 17 · 80±0 · 05 18 · 21±0 · 06 16 · 54±0 · 05 16 · 94±0 · 05 17 · 18±0 · 05 17 · 99±0 · 06 17 · 27±0 · 05 17 · 52±0 · 05 18 · 09±0 · 06 18 · 49±0 · 06


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Table 4.
Name L 2116­4439 L 2118­4702 L 2119­4415 B 18 · 61±0 · 06 19 · 21±0 · 08 18 · 02±0 · 05 V 18 · 20±0 · 06 18 · 98±0 · 07 17 · 72±0 · 05 R 17 · 80±0 · 05 18 · 64±0 · 06 17 · 43±0 · 05

(Continued )
I 17 · 40±0 · 06 18 · 40±0 · 08 16 · 88±0 · 05 J ... ... 16 · 07±0 · 06 H ... ... 15 · 68±0 · 06 Kn ... 16 · 67±0 · 30 15 · 03±0 · 09

Figure 4--Optical and infrared colours of the complete subset of the Parkes sample (triangles and crosses), compared with a small sample of optically selected LBQS QSOs (circles). Solid triangles denote unresolved sources: crosses are galaxies. The solid line shows where a pure power-law continuum slope would lie: it runs from F 0 on the left end, to F -2 on the right end. Error bars are not shown for the unresolved Parkes sources, but are comparable with those of the optically selected QSOs. The reddening vector is for an extinction E (B - V ) = 0 · 2, a redshift of one, and dust extinction as in equation (1). The direction of the reddening vector is independent of redshift.

Our data confirm the basic result of Webster et al. (1995): the Parkes quasars have very different B - K colours from optically selected QSOs (Figure 3). The difference is significant: a Kolmogorov­ Smirnov test shows that the the probability of getting two samples this different from the same parent population is only 9 · 1 â 10-5 . The bluest Parkes sources have colours very similar to those of optically selected QSOs, but the distribution of colours extends much further into the red. 3.2 The `Main Sequence' Are the Parkes sources uniformly red everywhere between B and Kn ? In Figure 4 we plot a measure of the optical colour (B - I ) against a measure of the near-IR colour (J - Kn ) for the complete sub-sample. Ob jects whose continuum shape approximates a featureless power-law all the way from B to Kn should lie close to the solid line in this plot.

Approximately 90% of all the Parkes sources do indeed lie close to the power-law line in Figure 4. These sources form a `main sequence' of quasar colours, stretching from blue ob jects with F 0 to red ob jects with F -2 . Examples of quasars from both ends of this `main sequence' are shown in Figure 5. Note that these quasars can lie on either side of the power-law line: i.e. they can have both `n' and `u' shaped continuum spectra. The ma jority, however, lie above the line, consistent with slightly `u' shaped spectra (redder in the near-IR than in the optical). This supports the synchrotron model for these sources. We defer discussion of this point to the detailed synchrotron modelling in the companion paper by Whiting, Webster & Francis (2000). 3.3 Optical ly Selected QSOs As Figure 4 shows, the optically selected QSOs all have very similar colours, and lie at the blue end of


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Figure 5--Spectral energy distributions of representative Parkes quasars from the blue (left six plots) and red (right six plots) ends of the `main sequence', as defined in the text. Sources on the left have J - Kn 1 · 5 and B - I 1 · 5; sources on the right have J - Kn > 1 · 8 and 3 > B - I > 1 · 8.

the `main sequence'. They lie systematically below the power-law line, however, indicating that they have `n' shaped spectra: i.e. they are redder in the optical than in the near-IR. This can be seen in their spectra energy distributions, shown in Figure 6. This spectral curvature matches the predictions of the dust model. Wills, Netzer & Wills (1985), however, suggested that it may be partially due to blended Fe II and Balmer-line emission, though Francis et al. (1991) argued that this curvature is too large to be plausibly explained by emission-line contributions. The position of the optically selected QSOs at the blue end of the `main sequence' would be expected if the cause of redness in the Parkes quasars is the addition of a red synchrotron component to an underlying blue continuum which is identical to that in radio-quiet QSOs (Whiting, Webster & Francis 2000). 3.4 Galaxies and Extremely Red Objects The spectra of the spatially extended sources in the Parkes sample are sharply peaked in the red, as would be expected from moderate redshift galaxies (Figure 7). They therefore lie far below the `main sequence' in Figure 4, the one exception being

PKS 1514­241, which is a galaxy at z = 0 · 049 with a BL Lac nucleus, which is presumably diluting the galaxy colours. Higher redshift galaxies lie further to the right on this plot, as would be expected due to the 400 nm break reducing the B -band flux. What are the other, red, highly `n' shaped ob jects lying far below the `main sequence' which are not spatially resolved? A few are high redshift QSOs, in which the B -band flux has been reduced by Ly forest absorption (Figure 8). The reddest ob jects, however, with B - I > 3 (Figure 9), do not lie at high redshifts. We have obtained spectra of four of these very red ob jects (Francis et al. 2000, in preparation). Three show hybrid spectra: they look like galaxies at short wavelengths, but at longer wavelengths a red power-law continuum component is seen, along with broad emission lines. The ratios of H to H are around 20: far above those seen in normal AGN ( 5) and evidence of substantial rdening (Figure 10). Note that these hybrid ob jects all have radio spectra indices near the steep spectrum cut-off of our sample, as do the galaxies in the sample. The reddest ob jects are thus a heterogeneous group: some are high redshift quasars, some are galaxies, and some are heavily dust-reddened quasars.


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Figure 6--Spectral energy distributions of all six optically selected QSOs with complete photometric data.

Figure 7--Spectral energy distributions of three representative galaxies from the Parkes sample.

3.5 Unidentified Objects Four Parkes sources were not detected in any band. After correction for galactic foreground absorption (Schlegel, Finkbeiner & Davis 1998), our non-detections impose 3 upper limits of H > 19 · 61 for PKS 1532+004, H > 19 · 76 and K > 19 · 29 for PKS 1601-222, H > 17 · 22 and K > 16 · 61 for PKS 1649-062 (which is sub jected

to substantial galactic reddening) and H > 19 · 82 for PKS 2047+098. If unified schemes for radio-loud AGN are correct, the host galaxies of our flat-radio-spectrum sources should be very similar to those of steep-radiospectrum radio galaxies. This enables us to place a lower-limit on the redshift of these unidentified sources: even if their AGN light is completely obscured, we should still see the host galaxy, which


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Figure 8--Spectral energy distributions of three representative Parkes quasars with redshifts z > 3, showing the dip in the B -band caused by Ly forest absorption.

Figure 9--Spectral energy distributions of the six Parkes sources with B - I > 3. The data for PKS 1706+006 have been adjusted for galactic dust extinction of E (B - V ) = 0 · 23 (Schlegel, Finkbeiner & Davis 1998), assuming a dust extinction law as described in the text.

should lie on the K -band Hubble diagram for radio galaxies (e.g. McCarthy 1992). To be undetected at our magnitude limits, therefore, all these sources must lie above redshift 1, and apart from PKS 1649-062, probably lie above redshift 3. 3.6 Anomalous Objects Three sources have colours that do not fit any of these categories (Figure 11). We discuss these in turn.

PKS 1648+015 shows a smooth optical power-law rising into the red, until at around 1 · 4 µm, the flux abruptly decreases. As all the IR data points were taken within minutes of each other in good weather conditions, we believe that this near-IR turn-over is real. We obtained a somewhat noisy optical spectrum of this source (Drinkwater et al. 1997) which shows a featureless, very red power-law, in excellent agreement with the photometry. We cannot explain this source.


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Figure 10--Optical spectra of four extremely red Parkes sources. With the exception of PKS 0131­001, the spectra show features both of galaxy light (the 400 nm break and narrow [O II] 372 · 7 nm and [O III] 495 · 9/500 · 7 lines) and of dust-reddened quasar light (a red continuum at long wavelengths, broad H 656 · 3 nm line emission, and the notable weakness of the broad H 486 · 1 nm line with respect to H).

Figure 11--Spectral energy distributions of three anomalous Parkes sources.

PKS 1732+094 is blue longwards of around 0 · 6 µm, but drops dramatically at shorter wavelengths. Our spectrum of this source (Drinkwater et al.) is too poor to be of any use. We hypothesise that this may be a very high redshift z > 4 quasar, and that the drop in the blue is due to Ly absorption.

PKS 2002­185 has optical colours typical of the bluest Parkes sources, but in the near-IR is bluer still: far bluer than any other source at these wavelengths. An optical spectrum, covering a very restricted wavelength range (Wilkes et al. 1983) shows only a single broad emission-line: on the


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Figure 12--Optical and near-IR colours of the Parkes sources (triangles) compared to photometry of 6400 high galactic latitude point sources drawn from the 2MASS survey (crosses) and sources with K 22 from the EIS Hubble Deep Field data release (circles, Benoist et al. 1999).

Figure 13--Near-IR colours of the Parkes sources (triangles) compared to photometry of 6400 high galactic latitude point sources drawn from the 2MASS survey (crosses) and sources with K 22 from the EIS Hubble Deep Field data release (circles, Benoist et al. 1999).


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Figure 14--Optical colours of the Parkes sources (triangles) compared to photometry of 3200 high galactic latitude point sources drawn from the EIS wide survey (crosses, Prandoni et al. 1999).

assumption that this is Mg II (279 · 8 nm) a redshift of 0 · 859 is determined. 4 Multicolour Selection of Red Quasars Could there be a population of radio-quiet QSOs with the same colours as our radio-loud red quasars? Webster et al. (1995) showed that it is virtually impossible to find such QSOs in any sample with a blue optical magnitude limit. In this section we ask whether red QSOs could be identified by colour selection in the red optical and near-IR. In Figure 12, we compare the optical and near-IR colours of the Parkes sources against the colours of high galactic latitude point sources drawn from the Two-Micron All Sky Survey (2MASS, K 15) and the ESO Imaging Survey (EIS, K 22). The `Main Sequence' sources, both red and blue, are clearly separated from the foreground ob jects. This separation is due to their power-law spectral energy distributions: as compared to the convex spectral energy distributions of stars and galaxies, the quasars have excess flux in B and/or K . This selection technique is similar to the `KX' technique proposed by Warren, Hewett & Foltz (2000). Unfortunately, the very red sources lying below the `main sequence' have colours within the stellar locus and will be hard to find. Can red quasars be identified purely on the basis of their near-IR colours? In Figure 13, we show that most of the Parkes quasars lie in regions of the near-IR colour­colour plot with substantial stellar contamination, but that the reddest move away

from the stellar locus, and could be detectable in the IR alone. Figure 14 shows that purely optical colour selection is not likely to be effective.

5 Conclusions The Parkes quasars can, we conclude, be crudely divided into three populations: (1) The `Main Sequence': About 90% of the Parkes sources have approximately powerlaw spectral energy distributions, with spectral indices (F ) in the range 0 > > -2. The nature of these sources is discussed by Whiting, Webster & Francis (2000). (2) Very Red Sources: These sources, which comprise 10% of the Parkes sample, are characterised by much redder continuum slopes in the optical than in the IR. They tend to have relatively steep radio spectra. Half these sources are radio galaxies, while most of the remainder are highly dustreddened quasars. The undetected sources are probably high redshift members of this class. (3) Oddballs: Roughly 2% of the Parkes sample defy this categorisation. The `main sequence' sources, both red and blue, should be easily detectable in combined near-IR and optical QSO surveys, due to their excess flux in the K and/or B bands.


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Acknowledgments We wish to thank Mike Bessell and Peter McGregor for their help with the details of the photometry, and Tori Ibbetson for her assistance with the observations. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint pro ject of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation, and of catalogues from the ESO Imaging Survey, obtained from observations with the ESO New Technology Telescope at the La Silla observatory under program-ID Nos 59.A-9005(A) and 60.A9005(A). References
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