Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/people/Ray.Norris/papers/n261.pdf Äàòà èçìåíåíèÿ: Thu Apr 21 07:19:10 2011 Äàòà èíäåêñèðîâàíèÿ: Fri Feb 28 08:07:50 2014 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 63

Mon. Not. R. Astron. Soc. 000, 1­?? (2010)

Printed 31 March 2011

A (MN L TEX style file v2.2)

CO observations of high-z radio galaxies MRC 2104-242 and MRC 0943-242: sp ectral-line p erformance of the Compact Array Broadband Backend

arXiv:1103.5806v1 [astro-ph.CO] 30 Mar 2011

B. H. C. Emonts1, R. P. Norris1, I. Feain1, G. Miley2, E. M. Sadler3, M. Villar-Mart´n4, M. Y. Mao5,6,1, T. A. Oosterloo7,8, R. D. Ekers1 i 1 J. B. Stevens , M. H. Wieringa1, K. E. K. Coppin9 and C. N. Tadhunter
1

10

CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, Epping NSW, 1710, Australia 2 Leiden Observatory, University of Leiden, P.O. Box 9513, 2300 RA Leiden, Netherlands 3 School of Physics, University of Sydney, NSW 2006, Australia 4 Instituto de Astrof´sica de Andaluc´a (CSIC), Aptdo. 3004, Granada, Spain i i 5 School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart, 7001, Australia 6 Australian Astronomical Observatory, PO Box 296, Epping, NSW, 1710, Australia 7 Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, the Netherlands 8 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, the Netherlands 9 Institute for Computational Cosmology, Durham University, South Road,Durham, DH1 3LE, UK 10 Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK

ABSTRACT

We present the first 7-millimetre observations of two high-redshift, Ly-bright radio galaxies (MRC 2104-242 and MRC 0943-242) performed with the 2 â 2 GHz instantaneous bandwidth of the Compact Array Broadband Backend (CABB) at the Australia Telescope Compact Array (ATCA). The aim was to search for 12 CO(1-0) emission in these systems and test the millimetre capabilities of CABB for performing spectral line observations at high redshifts. We show that the stable band and enhanced velocity coverage of CABB, combined with hybrid array configurations, provide the ATCA with excellent 7-mm capabilities that allow reliable searches for the ground transition of CO at high redshifts. In this paper we explicitly discuss the calibration procedures used to reach our results. We set a firm upper limit to the mass of molecular gas in MRC 2104-242 (z = 2.5) of MH2 < 2 â 1010 (x /0.8) M . For MRC 0943-242 (z = 2.9) we derive an upper limit of MH2 < 6 â 1010 (x /0.8) M . We also find a tentative 3 CO detection in the outer part of the giant Ly halo that surrounds MRC 0943242. The 30-33 GHz radio continuum of MRC 2104-242 and MRC 0943-242 is reliably detected. Both radio sources show a spectral index of -1.5 between 1.4 and 30 GHz, with no evidence for spectral curvature within this range of frequencies. Key words: galaxies: high-redshift ­ galaxies: active ­ galaxies: ISM ­ galaxies: individual: MRC 2104-242 ­ galaxies: individual: MRC 0943-242 ­ techniques: interferometric

1

INTRODUCTION

Cold gas is a primary comp onent in galaxy formation processes such as star formation and disk growth. However, despite detailed studies of cold gas in the nearby Universe, it is still difficult to trace similar quantities of cold gas b eyond our Galactic backyard. Recently, Tacconi et al. (2010) and

E-mail:b jorn.emonts@csiro.au Bolton Fellow


Daddi et al. (2010) observed that star-forming galaxies at high redshifts are likely to contain a much larger fraction of their total mass in the form of molecular gas compared with nearby massive spiral galaxies. Recent simulations supp ort this idea that the molecular gas content of galaxies increases when going to higher redshifts (Obreschkow & Rawlings 2009a,b; Obreschkow et al. 2009a,b). These results demonstrate that extensive studies of cold molecular gas in the early Universe are b ecoming feasible with existing radio telescop es.

c 2010 RAS


2

B. H. C. Emonts et al.
thermally excited1 (Greve et al. 2003; Hainline et al. 2006; Dannerbauer et al. 2009; Riechers et al. 2010) or distributed in extended reservoirs (Daddi et al. 2010; Carilli et al. 2010; Ivison et al. 2010, 2011). Cold CO gas distributed across the host galaxy may thus b e much easier to detect in the lower CO transitions than generally assumed from studies of the higher transitions. Moreover, with uncertainties in excitation prop erties of the gas, observations of the rotational ground-transition of the CO molecule [12 CO(1-0) ­ referred to as CO in the remainder of this pap er] provide the most accurate mass estimate of the overall molecular gas content in these systems. Since April 2009, the Australia Telescop e Compact Array (ATCA) has a new broad-band backend system (the Compact Array Broadband Backend or CABB). CABB offers an instantaneous bandwidth of 4 GHz, split over 2 â 2 GHz observing bands, b oth with all Stokes p olarisation parameters and 2048 channels (i.e. sp ectral resolution of 1 MHz); see Ferris & Wilson (2002); Wilson et al. (2011). ATCA/CABB has millimetre observing capabilities at 3mm (83.9 - 104.8 GHz), 7mm (30.0 - 50.0 GHz) and 15mm (16.0 - 25.0 GHz). This, in combination with hybrid array configurations with baselines as short as 31m, makes the upgraded ATCA an excellent facility to detect and spatially resolve molecular gas in high-z radio galaxies by targeting the lower rotational CO transitions (see Sect. 2 for more details). A remarkable example of this is the recent detection of CO(2-1) in the distant (z = 4.8) sub-millimetre galaxy LESS J033229.4-275619 by Coppin et al. (2010). To test the sp ectral-line p erformance of CABB over the 2 â 2 GHz bandwidth, we used the 7mm band to search for CO(1-0) in two high-z radio galaxies from the Molonglo Reference Catalogue (McCarthy et al. 1990), namely MRC 2104-242 (z = 2.5) and MRC 0943-242 (z = 2.9). These two sources are part of a larger sample of high-z radio galaxies that we aim to target with CABB in order to p erform a systematic search for CO(1-0) in these systems. MRC 2104-242 and MRC 0943-242 b oth have a redshift that corresp onds to a critical ep och in galaxy formation (z 2.5 - 3), at which there is a dramatic increase in sub-mm flux (Archibald et al. 2001; Smail et al. 2002; Chapman et al. 2005) and the space-density of (radio-loud) quasars reaches a maximum (e.g. Pei 1995; Shaver et al. 1996; Richards et al. 2006). HST observations by Pentericci et al. (2001) show that MRC 2104-242 and MRC 0943-242 b oth have an optical continuum that is clumpy and elongated in the direction of the radio source (Pentericci et al. 2000b; Carilli et al. 1997). Villar-Mart´n et al. (2003) show that they b oth contain a i giant Ly-halo ( 100 kp c in diameter). For MRC 2104242 the Ly gas is distributed roughly along the radio axis in what app ears to b e a rotating structure with a diameter 120 kp c (Villar-Mart´n et al. 2006). MRC 0943-242 i shows a quiescent Ly-halo that extends well b eyond the radio structure (Villar-Mart´n et al. 2003). MRC 0943-242 i also shows a deep Ly absorption (Rottgering et al. 1995; Jarvis et al. 2003), indicating that large amounts of neu-

Powerful radio galaxies enable comprehensive studies of the cold ISM throughout the Universe. Their strong radio sources provide a background continuum against which we can search for foreground neutral and molecular gas in absorption (e.g. Uson et al. 1991; Vermeulen et al. 2003; Kanekar et al. 2007; Carilli et al. 2007), while their host galaxies are generally in a very sp ecific stage of galaxy evolution. Detailed studies at low and intermediate redshifts reveal that p owerful radio galaxies are frequently associated with gas-rich galaxy mergers (e.g. Heckman et al. 1986; Baum et al. 1992), often contain young stellar p opulations (Tadhunter et al. 2005; Holt et al. 2007; Labiano et al. 2008), and many display strong jet-ISM interactions (Tadhunter 1991; Villar-Mart´n et al. i 1999; Clark et al. 1998; Emonts et al. 2005; Morganti et al. 2005a,b; Holt et al. 2008). At high redshifts (z > 2), luminous radio galaxies (L500MHz > 1027 W Hz-1 ) are among the most massive galaxies in the early Universe (see Miley & De Breuck 2008, for a review). They are typically surrounded by proto-clusters, which are thought to b e the ancestors of rich local clusters (e.g. Pentericci et al. 2000a; Venemans et al. 2007). The high-z radio galaxies and surrounding proto-cluster gas and galaxies often interact with one another (e.g. Nesvadba et al. 2008; Ivison et al. 2008) and are therefore lab oratories for studying the formation and evolution of galaxies and clusters as well as investigating the relationship b etween early star formation and AGN activity. Since Brown & Vanden Bout (1991) first observed CO gas (the strongest tracer for molecular hydrogen) at a redshift b eyond z = 2, intensive searches for CO in highz radio galaxies during the early 1990s were unsuccessful (Evans et al. 1996; van Ojik et al. 1997). Since then, studies of individual radio galaxies at z 2 - 5 with synthesis radio telescop es have found CO emission (tracing molecular gas masses of a few â1010 - 1011 M ) in a numb er of these systems (e.g. Scoville et al. 1997; Papadop oulos et al. 2000; De Breuck et al. 2003a,b, 2005; Klamer et al. 2005; Nesvadba et al. 2009, see also Solomon & Vanden Bout (2005); Omont (2007); Miley & De Breuck (2008) for reviews). In some cases CO is observed to b e resolved on scales of several tens of kp c (e.g. Papadop oulos et al. 2000). This indicates that large amounts of cold molecular gas may b e relatively common in high-z radio galaxies. However, the ma jor observational limitations for starting comprehensive studies of CO in high-z radio galaxies have b een the very limited velocity coverage of existing mm-sp ectrometers (often not much wider than the velocity range of the CO gas and/or the accuracy of the redshift) plus the fact that most observatories can only target the higher order rotational transitions of 12 CO. Although the higher order CO lines are likely to have a higher flux density than the lower ones in the nuclear starburst/AGN regions, where gas is dense and thermally excited, Papadop oulos et al. (2000, 2001) suggest that the opp osite may b e true for large reservoirs of less dense and sub-thermally excited gas that is more widely distributed. In fact, various studies of the low-order CO transitions in different typ es of high-z galaxies reveal molecular gas that is sub-

1

Harris et al. (2010) and Danielson et al. (2010) point out that a multi-component inter-stellar medium, rather than sub-thermal excitation, may better reflect the physical properties of the molecular gas in high-z systems. c 2010 RAS, MNRAS 000, 1­??


CO observations of two high-z radio galaxies with CABB
Table 1. Observations Source MRC 2104-242 Array H75 H168 Obs. date 09JUL21 09JUL23 09SEP18 09SEP19 09SEP20 09JUL11 09JUL12 09JUL14 09MAY10 09MAY11 09MAY12 ti (h) 99 15 00 14 19 10 20 88 29 68 18

3

nt

central

(GHz)

MRC 0943-242

H75

H168 Figure 1. Observing frequency of the first six rotational transitions of CO that can be targeted with the CABB millimetre system (3, 7 and 15mm) plotted against the redshift of the COemitters.

6. 4. 3. 3. 2. 3. 3. 2. 3. 2. 3.

33. 33. 33. 33. 33. 30. 30. 30. 30. 30. 30.

000 020 000 000 000 001 010 001 001 015 001

Notes ­ tint is the effective on-source integration time (i.e. not including overheads).

tral gas are present in this system. From fitting the sp ectral energy distributions of the host galaxies with Spitzer, Seymour et al. (2007) derive a total stellar mass of a few â1011 M for b oth systems. In Sect. 2 we present our CO observations and explain in more detail the enhanced capabilities of the ATCA for studying molecular gas at high redshifts. Section 3 shows the result regarding b oth the p erformance of CABB for doing these high-z CO studies as well as the scientific outcome of our observations of MRC 2104-242 and MRC 0943-242. In Sect. 4 and 5 we discuss the scientific results and conclude that the upgraded ATCA is a world-class facility for sp ectral line observations of the cold molecular gas at high-z .

2

OBSERVATIONS

During the p eriod May - Septemb er 2009, MRC 2104-242 and MRC 0943-242 were observed with ATCA/CABB. Details of the observations are given in Table 1. Figure 1 shows the observing windows for the various transitions of extra-galactic CO currently available with CABB in the 3, 7 and 15 millimetre bands. For MRC 2104242 and MRC 0943-242 we targeted the ground-transition CO(1-0) with the CABB 7mm system. The redshift of MRC 2104-242 (z = 2.491) corresp onds to an observing frequency of 33.0 GHz, which is also one of the optimum CABB frequencies for centring the 7mm band.2 The redshift of MRC 0943-242 (z = 2.9185)3 corresp onds to an observing frequency of 29.4 GHz, which is outside the nominal 7mm CABB band (30.0 - 50.0 GHz). Nevertheless, when centring the band at 30.001 GHz, data is obtained down to 29 GHz. Observations of MRC 0943-242 therefore served as a good test of how well the CABB system b ehaves at the very edge of the 7mm band. The total observing time for each source ­ including overhead and calibration ­ was roughly 40 hours (see Table 1 and Sect. 2.1). The observations were spread over the two
2

most compact hybrid array configurations (H75 and H168) in order to minimise the effect of atmospheric phase fluctuations (which worsen with increase in baseline length; Klamer 2006). Both array configurations include five antennas that are spread across b oth an east-west as well as a north-south spur. This ensures a reasonably good uv-coverage (see Fig.2) even from observing runs as short as 6-8h, during which a source is targeted only ab ove an elevation of 30 (in order to avoid high airmasses, which dramatically increase the system temp erature; see Klamer 2006). The system temp eratures ranged from 70 - 132K for the observations of MRC 2104-242 and from 130 - 180K for MRC 0943-242.4 At the high central frequencies of our observations (30-33 GHz), the coarsest sp ectral-line mode of CABB (2 â 2 GHz bands with 1 MHz sp ectral resolution) provides a velocity resolution of 9.5-11 km s-1 across an effective velocity coverage of at least 17,000 km s-1 p er 2 GHz band. Both 2 GHz bands were centred around the same observing frequency, but b ecause they are not mutually indep endent, only one 2 GHz band was used in the final data analysis. Observations were done as much as p ossible during the night and in good weather conditions (to avoid decorrelation due to atmospheric phase instabilities) and only ab ove 30 elevation (to avoid high system temp eratures due to large airmasses). Because our observations also served as a test for the p erformance of CABB for sp ectral-line observations, in the following Section we will explain the details of several crucial calibration steps. For the data reduction and analysis we used MIRIAD and KARMA.

2.1

Calibration, overheads and data reduction

See the online ATCA Users Guide for details: http://www . narrabri.atnf.csiro.au/observing/users guide/html/atug.html 3 This redshift corresp onds to the velo city of the most prominent H I absorption in the Ly profile of MRC 0943-242 (Jarvis et al. 2003); see Sect. 2.1.3 for more details. c 2010 RAS, MNRAS 000, 1­??

Our general observing strategy was as follows: a strong calibrator was observed at least three times during each run in order to check the reliability of the bandpass calibration. A secondary (phase/gain) calibrator was observed roughly every 10 minutes. Flux calibration was done at least once during each run. Pointing solutions of the antennas were checked and up dated every hour, or every time the telescop e

4 More details on theoretical estimates of T sys values at 7mm can be found in the online ATCA Users Guide.


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B. H. C. Emonts et al.

Figure 2. The uv-coverage of the combined observing runs of both MRC 2104-242 and MRC 0943-242 (Table 1), using the hybrid H75 and H168 arrays.

slewed more than 20 on the sky. Taking into consideration the conservative nature of this calibration strategy, the overheads due to calibration and slewing were ab out 50%.5

2.1.1

Phase/gain calibration

For phase calibration we p erformed a 2 minute scan on a calibrator close to our target source roughly every 10 minutes, although target scans were decreased to 5 minutes in p oor weather conditions and increased to 15 minutes when atmospheric phase stability was excellent. For MRC 2104-242 we used PKS B2008-159, PKS B2128-123 or PKS 2149-306 as phase calibrator. For MRC 0943-242 we used PKS 0919-260. Phase calibration was done in a standard way.

of F33 GHz 1.9 and 1.8 Jy resp ectively) were suitable for bandpass calibration. This allowed us to obtain a bandpass solution roughly every 10-15 minutes. We used a new feature in the MIRIAD task mfcal to interp olate b etween consecutive bandpass solutions in order to comp ensate for p ossible frequency dep endent gain variations that slowly fluctuate in time.

2.1.3

Flux calibration

2.1.2

Bandpass calibration

In order to test the quality of the bandpass calibration at 7mm across the full 2 GHz band, we observed a strong calibrator (PKS B0537-441, PKS B1253-055, PKS B1334-127, PKS B1921-293 or PKS B2223-052) at least three times during each run (unless the run was cut short due to weather). We noticed that weather and atmospheric conditions at the ATCA site can introduce frequency dep endent temp oral gain fluctuations across the wide CABB band, which can have a significant effect on the quality of the bandpass calibration at 7mm. It is therefore essential to obtain at least one good scan on the bandpass calibrator during good atmospheric conditions. For MRC 0943-242 we chose the b est quality bandpass calibrator scan for calibrating our data. In case more than one bandpass calibrator scan was deemed suitable, we applied the bandpass solutions to that part of the data observed closest in time to the resp ective calibrator. For MRC 2104-242 the strong phase calibrators PKS B2008-159 and PKS B2128-123 (with observed fluxes

5

We estimate that in order to reach the potential maximum efficiency with less conservative calibration, overheads should be considered to be at least 30%.

For MRC 2104-242, flux calibration was done by observing Uranus at the time that it was at roughly the same elevation as the phase calibrator and target source during each run. The presence of a weak radio continuum from the lob edominated high-z radio galaxies in our 7mm data (which are not exp ected to significantly change their flux densities over time-scales of a few months) allowed us to compare the relative flux calibration b etween the various runs, which remained constant within 15%. Our absolute flux calibration used the available MIRIAD-model for Uranus. This model did not take into account changes in the planet's orientation, which introduce time-variations of up to 10% in its brightness temp erature (see Kramer et al. 2008; Weiland et al. 2010), p otentially leading to a significant error in absolute flux calibration. During one of the runs we also observed PKS B1934-638, which confirmed our Uranus-based absolute flux calibration to an accuracy of 18%. We therefore estimate the overall (relative + absolute) uncertainty in the flux calibration of MRC 2104-242 to b e within 30%. For MRC 0943-242, Uranus was not visible during our observing runs. For flux calibration we therefore observed the ultra-compact H II region G309 [G309.9206+00.4790; Urquhart et al. (2007), with our p ointing centred at RA(J2000)=13:50:42.35, dec(J2000)=-61:35:09.78] when it was at roughly the same elevation as the phase calibrator. We calibrated the flux of G309 against Uranus, which we observed roughly half a day later for each run. The flux of G309 was stable over our six observing ep ochs and the relative flux calibration b etween the six different runs was within 13%. From our data we derive a value of S30 GHz = 1.31 ± 0.07 Jy
c 2010 RAS, MNRAS 000, 1­??


CO observations of two high-z radio galaxies with CABB
for the shortest baselines at which the source is unresolved. Recently, Murphy et al. (2010) derived a flux density of S32 GHz = 1.1 ± 0.11 Jy for G309, also using Uranus as flux calibrator. In order to verify the accuracy of our absolute flux calibration, we observed PKS B1934-638 during three of our observing ep ochs. When using PKS B1934-638 as flux calibrator instead of Uranus, the absolute fluxes derived from our data are on average 15 % lower. This uncertainty in absolute flux calibration is consistent with the difference b etween our flux estimate for G309 (which we used to calibrate our data) and that made by Murphy et al. (2010). This may again reflects variations in the brightness of Uranus that were not accounted for by the existing models (see previous paragraph). The sp ectral index of G309 changes at most a few p ercent across the 2 GHz band at 30 GHz, in agreement with Murphy et al. (2010). In all, we therefore estimate that for MRC 0943-242 the overall (relative + absolute) uncertainty in our flux calibration is within 30%. After flagging and bandpass, gain and flux calibration, we subtracted the continuum from the line data in the uvdomain by applying a linear fit to the channels across the full 2 GHz band (for MRC 0943-242 we excluded from this fit the channels in which we found a tentative CO signal, see Sect. 3.3, although this has no significant effect when fitting the full 2 GHz band). Subsequently, a robust +1 weighted (Briggs 1995) continuum map and line data set were created by Fourier transforming the uv-data and, in case of the continuum map, cleaning the signal. We then translated the velocity axis to match the optical, barycentric rest-frame velocity at the redshift of MRC 2104-242 and MRC 0943242. The redshift of MRC 2104-242 (z = 2.491) has b een confirmed by Overzier et al. (2001) and Villar-Mart´n et al. i (2003) through observations of the Ly and various metal emission-lines. For MRC 0943-242, we chose to centre our observations at the redshift of the prominent H I absorption in the Ly profile (z = 2.9185; Jarvis et al. 2003), which likely represents the bulk of the cold neutral gas in this system (see also De Breuck et al. 2003b). Table 2 shows details of the final data products that we obtained from our observations (some of these are describ ed further in Sect. 3).
Table 2. Data MRC 2104-242 Effective int. time (h) Target frequency (GHz) Redshift Beam size (arcsecâarcsec) Beam PA [PA ( )] v (km s-1 ) cont (µJy bm-1 ) Scont (mJy) line (mJy bm-1 ch-1 ) L O (K km s-1 pc2 ) C 19.5 33.02 2.491 11.7 â 7.6 97.2 9.6 29 4.0 0.45 < 2.6 â 1010

5

MRC 0943-242 18.3 29.417 2.9185 11.5 â 9.0 87.5 11.0 33 3.3 0.90 < 7.3 â 1010

Notes ­ Effective int. time is the total effective on-source integration time of all runs in both configurations combined. Target frequency (GHz) is the observing frequency of the expected CO(1-0) line at the redshift of our sources (see text for details). v (km s-1 ) is the velocity resolution per 1 MHz channel. cont is the rms noise level of the continuum image after tint . Scont is the integrated continuum flux of the radio source at the target frequency. line is the rms noise level of the full-resolution line data per 1 MHz channel after tint . L O gives C the upper limit on the CO luminosity (see text for details).

3 3.1

RESULTS CABB performance

Figure 3 shows the 33 GHz radio continuum map of MRC 2104-242 and a sp ectrum at the location of the centre of the host galaxy. The continuum image has an rms noise level of 29 µJy b eam-1 (after tint = 19.5h; see Table 2), demonstrating the effectiveness of ATCA/CABB for deep millimetre continuum studies. The 2 GHz sp ectrum has a large velocity coverage of 17, 000 km s-1 with an rms noise in each 1 MHz channel (v = 9.6 km s-1 ) of = 0.45 mJy bm-1 , with no significant systematic bandpass effects. Figure 4 shows the 30 GHz radio continuum map of MRC 0943-242 (with an rms noise of 33 µJy b eam-1 ) and an off-nuclear sp ectral line profile. In this case, at the edge of the 7mm band, half the observing band lies outside the nominal CABB range (Sect. 2), where there are instrumental low-level structures in the noise or in the bandpass at
c 2010 RAS, MNRAS 000, 1­??

ab out the 1 level of the full (1 MHz) resolution data (Fig. 4b). In addition, the noise starts to vary b eyond the nominal CABB range (Fig. 4c). After an effective on-source integration time of 18.3h, we derive a noise level at 29.4 GHz of = 0.9 mJy bm-1 p er 1 MHz channel (v = 11 km s-1 ), i.e. twice the noise level at the optimum observing frequency of 33 GHz (see ab ove). However, as can b e seen in the Hanning-smoothed data of Fig. 4c, the noise level p eaks at our target frequency of 29.4 GHz and is significantly lower throughout most part of the band, even b elow the nominal edge of 30 GHz. We therefore conclude that up to 0.8 GHz b elow the nominal 7mm band, CABB is still suitable for sp ectral-line work. Coppin et al. (2010) detected CO(2-1) in a z = 4.8 submillimetre galaxy, which was observed with CABB at 40.0 GHz (i.e. towards the other end of the 7mm band compared to our 33/30 GHz observations). They find noise levels of 0.44 mJy bm-1 p er 1 MHz channel and a bandpass stable enough to detect their CO signal at ab out the 5 level when binning across >10 channels. The data quality at 40 GHz thus app ears comparable to that at 33 GHz as presented in this pap er, giving a good indication for the excellent p erformance of CABB across the entire ATCA 7mm band. 3.2 MRC 2104-242

MRC 2104-242 (Fig. 3a) is resolved at 33 GHz with a total flux of 3.5 mJy. The continuum structure consists of two comp onents on either side of the optical host galaxy, in agreement with 4.7 and 8.2 GHz VLA continuum observations that identified it as a double lob ed radio source (Pentericci et al. 2000b). The bright northern lob e has a p eak flux density of S33GHz = 2.7 mJy bm-1 , while the fainter southern lob e has S33GHz = 0.39 mJy bm-1 . Even at 33 GHz the radio continuum structure is dominated by the radio lob es and no core comp onent (at the location of the optical nucleus) is seen in our data. We set a conservative


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B. H. C. Emonts et al.

upp er limit to the 33 GHz core flux density of Score-33GHz < 0.4 mJy bm-1 . Figure 5 shows that the integrated flux of MRC 2104-242 has a steep sp ectral index b etween 1.4 GHz and 33.0 GHz, with = -1.56 (where F ). There is no evidence for sp ectral curvature within this range of frequencies. This is in agreement with sp ectral index observations of high-z ultra-steep sp ectrum radio sources by Klamer et al. (2006). No CO is detected in MRC 2104-242, either at the location of the host galaxy or at the p osition of the radio source. We derive a firm upp er limit on the CO emission-line luminosity in MRC 2104-242 by assuming a p otential 3 signal smoothed across 500 km s-1 , using S
CO

V = 3 v

500 km s-1 Jy · km s-1 , v

(1)

with the noise level p er 1 MHz channel in one b eam (in Jy) and v the width of one 1 MHz channel (in km s-1 ). The CO luminosity (upp er limit) can then b e calculated following Solomon & Vanden Bout (2005, and references therein): L O = 3.25 â 107 ( C SCO V DL 2 rest )( )( ) Jy km/s Mp c GHz
-2

(1 + z )

-1

, (2)

with L O expressed in K km/s p c2 and with DL = 20018 C Mp c the luminosity distance of MRC 2104-242 (following Wright 2006)6 . For MRC 2104-242, SCO V < 0.094 Jy km s-1 , hence L O < 2.6 â 1010 K km s-1 p c2 . C

3.3

MRC 0943-242
Figure 5. Spectral index of MRC 2104-242 (top) and MRC 0943242 (bottom). Shown is the total integrated flux. Data at 1.4, 4.7 and 8.2 GHz are taken from Carilli et al. (1997) and Pentericci et al. (2000b), with errors corresponding to 2% of the flux at these wavelengths (as estimated by Carilli et al. 1997).

The radio source MRC 0943-242 (Fig. 4a) has a flux density of 3.3 mJy bm-1 and is unresolved in our data. Higher resolution continuum observations at 4.7 and 8.2 GHz by Carilli et al. (1997) show that the radio source consists of two lob es that are separated by 4 arcsec. When comparing the flux of our 30 GHz data with the integrated flux at 1.5, 4.7 and 8.2 GHz (Carilli et al. 1997), Figure 5 shows that MRC 0943-242 has a steep sp ectral index b etween 1.5 GHz and 30 GHz with = -1.44. Similar to the case of MRC 2104-242, there is no evidence for sp ectral curvature within this range of frequencies. No CO is detected at the central (nuclear) location of MRC 0943-242. When estimating an upp er limit on L O C in MRC 0943-242 (p otential 3 detection smoothed across 500 km s-1 ), we derive L O < 7.3 â 1010 K km s-1 p c2 (for C DL = 24242 Mp c, which corresp onds to a angular-size scale of 7.65 kp c/arsec for MRC 0943-242; Wright 2006).

3.3.1

Tentative off-nuclear CO detection

As can b e seen in Fig. 4b,d,e, we find a tentative, off-nuclear 3 CO(1-0) detection in the Hanning smoothed data of MRC 0943-242 (with the noise level at the frequency that corresp onds to the tentative detection, see the arrow in Fig. 4c). The tentative CO signal spreads over an area ab out the

size of one synthesised b eam roughly 60 kp c NE of the centre of the host galaxy. It p eaks at v -100 km s-1 with a flux density of 1.8 mJy bm-1 (with a tentative second p eak present at the 1 mJy bm-1 level around v -500 km s-1 ). The estimated luminosity of the tentative doublep eaked CO signal is L O 8 â 1010 K km s-1 p c2 (EquaC tion 2). Both the H75 and H168 array data show indications for this tentative CO signal (Fig. 4e). However, b ecause of the low-level (1 ) structure in the noise/bandpass b eyond the nominal 7mm observing band (see Sect. 3.1), our results did not improve by further smoothing/binning the data in velocity. Our tentative 3 detection thus needs to b e verified with additional observations b efore conclusions can b e drawn. 3.4 The environments of high-z radio galaxies

6

See http://www.astro.ucla.edu/wright/CosmoCalc.html for Ned Wright's online cosmology calculator that we used to deriving luminosity and angular-size distances. Throughout this paper we use H0 = 71 kms-1 Mpc-1 , M = 0.3 and = 0.7.

The large instantaneous velocity coverage of CABB (see Sect. 2) also makes it p ossible to search for CO emitters in the field of our high-z radio galaxies. The full width half maximum (FWHM) of the primary b eam is 87 / 95 arcsec at 33 / 30 GHz, corresp onding to ab out 0.69 / 0.73 Mp c at the redshift of MRC 2104-242 / MRC 0943-242. High-z radio galaxies are generally located in
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CO observations of two high-z radio galaxies with CABB

7

Figure 3. a). 33 GHz radio continuum map of MRC 2104-242 (contour levels: 0.1, 0.3, 1.0, 1.7, 2.4 mJy bm-1 ). The cross indicates the location of the radio host galaxy. b). spectral line profile against the centre of the radio host galaxy. Shown is the full 1 MHz velocity resolution of CABB across the 2 GHz band. The x-axis shows the velocity in the rest-frame of the radio host galaxy (see Sect 2.1.3). The zoom-in shows a portion of the CABB data with the approximate bandwidth coverage of the old pre-CABB ATCA system (2 â 128 MHz). c). rms noise per 1 MHz channel in the region of the radio source. d). Same as figure b, but data binned to 4 MHz channels (i.e. similar to the pre-CABB system; the zoom-in therefore gives a good representation of the data that could be obtained with the old 2 â 128 MHz pre-CABB backend). e). rms noise per channel of 4 MHz in the region of the radio source.

Figure 4. a). 30 GHz radio continuum map of MRC 0943-242 (contour levels: 0.4, 1.0, 2.0, 3.0 mJy bm-1 ). The cross indicates the location of the radio host galaxy. b). Off-nuclear spectral line profile of redshifted CO. The spectrum is taken at the location marked by the arrow in figure a) and Hanning smoothed to a velocity resolution of 2 MHz across the 2 GHz CABB band. The x-axis shows the optical barycentric velocity in the rest-frame of the radio host galaxy (see Sect 2.1.3). c). rms noise per channel in the Hanning smoothed data of figure b) across the CABB band, derived across the central region. The arrow marks the rms noise level at the velocity of the tentative CO detection (right plot). d). Zoom-in of figure b, showing the tentative off-nuclear CO detection. The arrow indicates the redshift of the deep Ly absorption of H I gas (Jarvis et al. 2003) at which we centred our zero-velocity. The range of velocities of the emission-line gas in the giant Ly halo (Villar-Mart´n et al. 2003) is also indicated in the plot. e). separate data-sets of the H75 and i H168 array observations, both showing the tentative CO signal (for illustration purposes, the x-axis of the H75-array data is scaled-down by 3 mJy bm-1 in this plot).

proto-cluster environments (e.g. Pentericci et al. 2000a; Venemans et al. 2007). MRC 0943-242 is known to b e located in a proto-cluster with many nearby companions detected in Ly and with known redshifts (Venemans et al. 2007). There are 12 known Ly companions within the
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primary b eam and observing band of our observations (van Breukelen et al. 2005; Venemans et al. 2007, see also Fig. 6). None of these galaxies shows a clear CO detection ab ove a 3 limit, after correcting for primary b eam attenuation.


8

B. H. C. Emonts et al.
well as other nearby spiral galaxies (Dickman et al. 1986; Solomon & Barrett 1991). Based on our 3 upp er limits on L O and assumC ing x = 0.8, we estimate that MH2 < 2 â 1010 M for MRC 2104-242 and MH2 < 6 â 1010 M for MRC 0943-242. The tentative off-nuclear CO detection in MRC 0943-242 has an estimated molecular gas mass of MH2 = 6 â 1010 M . 4.2 Molecular gas properties of high-z radio galaxies

Figure 6. Continuum image of MRC 0943-242. Shown is an area the size of the FWHM of the ATCA's primary beam at 30 GHz. Flux densities are corrected for the primary beam response pattern, which causes the effective noise level to rise significantly towards the edge of the image. Contours: -0.4, -1.0, -2.0 (grey); 0.4, 1.0 2.0, 3.0 (black/white) mJy bm-1 . The red circles indicate the 12 Ly-bright galaxies in the field of MRC 0943-242 (and with redshifts within the CABB observing band) detected by van Breukelen et al. (2005) and Venemans et al. (2007).

Ly observations of the environment of MRC 2104-242 are lacking and hence the cluster prop erties are unknown. No CO was detected within the primary b eam ab ove 3 after correcting for primary b eam attennuation.

4 4.1

DISCUSSION H2 masses

CO is an excellent tracer of molecular hydrogen, b ecause the rotational transitions of CO are excited primarily by collisions with H2 . A standard conversion factor x = MH2 /L O C [M (K km s-1 p c2 )-1 ] is generally used to calculate the mass of the cold molecular gas (where MH2 includes a fraction of the molecular gas that is in the form of helium ­ see for example Solomon & Vanden Bout 2005, for a review). For ultra-luminous infra-red galaxies (ULIRGs), Downes & Solomon (1998) derived a conversion factor of x 0.8 M (K km s-1 p c2 )-1 . This is in agreement with other observations of ULIRGs (Solomon et al. 1997; Evans et al. 2002) as well as high-z sub-mm and starforming galaxies (Tacconi et al. 2008; Stark et al. 2008), which imply that x 0.8 - 1.6 M (K km s-1 p c2 )-1 . We adopt a value of x = 0.8 M (K km s-1 p c2 )-1 also for the two high-z radio galaxies that we study in this pap er. We note, however, that there is a significant uncertainty in this conversion factor, since values as high as x 5 have b een derived for molecular clouds in the Milky Way (Scoville et al. 1987; Strong et al. 1988, see also Dickman (1978), Bloemen et al. (1986), Solomon et al. (1987)) as

The upp er H2 mass limits that we derive for MRC 2104242 and MRC 0943-242 are comparable to H2 masses derived from CO detections in high-z radio galaxies (e.g. Scoville et al. 1997; Papadop oulos et al. 2000; De Breuck et al. 2003a,b, 2005; Klamer et al. 2005; Nesvadba et al. 2009, see also Solomon & Vanden Bout (2005); Miley & De Breuck (2008) for reviews). However, as discussed in Sect. 1, most of these observations have targeted the higher rotational CO transitions, which could underestimate the total molecular gas content in these systems. CO(1-0) detections have b een claimed for two high-z radio galaxies, namely 4C 60.07 (z = 3.8 Greve et al. 2004) and TNJ 0924-2201 (z = 5.2 Klamer et al. 2005), b oth with MH2 = 1 â 1011 M . Our derived upp er limit on molecular gas mass in MRC 2104-242 (z = 2.491) is a factor 5 lower than this. Sub millimetre galaxies (SMGs) are likely merging systems with a short-lived burst of extreme star formation and are b elieved to b e the progenitors of local massive ellipticals (e.g. Greve et al. 2005; Tacconi et al. 2008). In this sense, high-z radio galaxies and SMGs could b e the same typ e of ob jects that differ only in their level of AGN activity (e.g. Reuland et al. 2007), although Ivison et al. (2008) argue that the violent AGN activity may occur predominantly during the early evolutionary stages of these systems. Greve et al. (2005) derived a median cold gas mass of MH2 = 3.0 â 1010 M among 12 SMGs detected in CO (see also Neri et al. 2003). This is of the same order as the upp er limits that we derive for the mass of cold gas in MRC 2104242 and MRC 0943-242. Our derived upp er limits on the molecular gas mass of MRC 2104-242 and MRC 0943-242 are lower than the H2 mass estimates for a non-negligible fraction of normal massive star forming galaxies at z 1 - 2, derived from CO(3-2) observations by Tacconi et al. (2010, even when accounting for the much larger CO-to-H2 conversion factor that they used). A similar result is seen by comparing the upp er limits on CO in samples of high-z radio galaxies (Evans et al. 1996; van Ojik et al. 1997) with the results of Tacconi et al. (2010). Confirmation by observations of larger samples in the same CO transitions might indicate imp ortant differences in molecular gas fraction, excitation prop erties or chemical enrichment processes b etween high-z radio galaxies and distant massive star forming galaxies. The H2 mass limit of MRC 2104-242 is only a factor 3 higher than the H2 content of the most CO-bright radio galaxies in the low redshift Universe, as studied from CO(10) observatons of a large sample of IR-bright radio galaxies by Evans et al. (2005, corrected for the difference in the used x -value and cosmological parameters). The vast ma jority of the low-z radio galaxies in the sample of Evans et al. (2005),
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CO observations of two high-z radio galaxies with CABB
however, contain significantly less molecular gas. This was recently confirmed by Ocana Flaquer et al. (2010) with a ~ large sample of low-z radio galaxies not selected on IRprop erties, for which they derive a median H2 mass of only MH2 = 2.2 â 108 M . We note that many high-z CO detections to date are case-studies of galaxies that were pre-selected based on their prop erties at other wavelengths, such as a large sub-mm dust content or high infra-red (IR) luminosity. Both at low- and high-z there app ears to b e a relation b etween the far-IR (FIR) and CO luminosity in different typ es of galaxies (see Evans et al. 2005; Greve et al. 2005, and references therein). Such a relation would indicate that (radio) galaxies with a FIR luminosity in the range of ULIRGs (LFIR > 1012 L ) contain a CO luminosity similar to the upp er limit that we derive for MRC 2104-242 (L O few â 1010 K km s-1 p c2 ). C From Spitzer observations of MRC 0943-242 at 24, 70 and 160µm (Seymour et al. 2007), we estimate an upp er limit on the total IR luminosity of LTIR < 2 â 1013 L when using the approximation by Dale & Helou (2002). Following the IRCO relation found by Evans et al. (2005) and Greve et al. (2005), this IR limit corresp onds to an average CO luminosity roughly a factor 2 lower than the L O upp er limit C that we derive for MRC 0943-242. The lack of detectable amounts of CO gas in MRC 0943-242 is therefore not unusual based on its IR prop erties, but it shows that unbiased CO(1-0) observations of high-z radio galaxies are b ecoming feasible. Systematic searches for various CO transitions in unbiased samples of high-z (radio) galaxies are necessary to ob jectively investigate the overall content of cold molecular gas in the Early Universe. Our results show that systematic and reliable searches for the ground-transition of CO in high-z (radio) galaxies are b ecoming feasible with existing broadband facilities that can target the 20-50 GHz regime, such as the ATCA and EVLA. 4.2.1 CO in the vicinity of MRC 0943-242?

9

galaxy (Klamer et al. 2004; Ivison et al. 2008). In particular 4C 60.07 shows an apparent deficit of molecular gas in the radio host galaxy, while CO app ears to b e present in a merging companion and associated tidal debris (Ivison et al. 2008). If confirmed, a more detailed comparison b etween the CO(1-0) prop erties of these systems deserves further attention. The p osition angle of the radio source in MRC 0943-242 (which has a total linear size of ab out 4 and is therefore unresolved in our observations) is PA = -74 (Carilli et al. 1997). This is roughly within 45 of the location of the tentative CO detection from the central region of the radio host galaxy. If confirmed, this may resemble alignments that Klamer et al. (2004) argue exist among other high-z radio galaxies. 4.3 Radio continuum

Both radio sources are clearly detected in our sensitive ( 30 µJy) 7mm continuum observations. Their sp ectral indices are relatively steep from 30 GHz down to 1.4 GHz, with no evidence for sp ectral curvature within this large range of frequencies. This indicates that there is no turn-over due to synchrotron losses or inverse Compton cooling up to 115 GHz in the restframe of these radio sources. This is consistent with continuum observations of a large sample of high-z ultra-steep sp ectrum radio galaxies by Klamer et al. (2006), who also find relatively steep p ower law sp ectral energy distributions (SEDs) with no evidence for sp ectral steep ening up to several tens of GHz in the rest frame. A detailed analysis of this phenomenon is crucial for understanding the electron acceleration mechanism or environmental prop erties of high-z radio sources, but is b eyond the scop e of this pap er.

5 In this Section we briefly discuss the p ossible nature of the tentative CO detection in the vicinity of MRC 0943-242, which needs to b e confirmed b efore a more detailed analysis is deemed suitable. The tentative CO detection ( 60 kp c NE of the host galaxy) may b e associated with a nearby companion galaxy, although no companion has b een detected in Ly at that location (van Breukelen et al. 2005; Venemans et al. 2007), so any such galaxy would have to b e Ly-faint. Alternatively, the tentative CO detection may represent cold gas in the outer part of the quiescent Ly halo (Villar-Mart´n et al. i 2003). Binette et al. (2000) show that C IV absorption is associated with the deep Ly absorption in MRC 0943-242 and derive that this reservoir of absorbing gas is also located in the outer halo (i.e. ouside the radio cocoon). If confirmed, the cold gas prop erties of MRC 0943-242 resemble those found in the high-z radio galaxies TXS 0828+193 (z = 2.6 Nesvadba et al. 2009) and B3 J2330+3927 (z = 3.1 De Breuck et al. 2003a). The only two known high-z radio galaxies in which CO(1-0) has b een detected (4C 60.07 and TNJ 0924-2201; see Sect. 4.2) also show indications that the CO gas may not b e aligned with the central location of the host
c 2010 RAS, MNRAS 000, 1­??

CONCLUSIONS

We presented the first 7mm observations of two high-z radio galaxies (MRC 2104-242 and MRC 0943-242) with the 2 â 2 GHz Compact Array Broadband Backend. Our results demonstrate the feasibility of using ATCA/CABB for sp ectral-line work at high redshift. We also presented 7mm continuum images of the two high-z radio galaxies, with a typical rms noise level of 30 µJy b eam-1 . The enhanced sp ectral-line and continuum capabilities of ATCA/CABB in the millimetre regime complement those of other large existing and up coming observatories, such as PdbI, EVLA and ALMA. From our CO(1-0) data we derive upp er limits on the H2 mass of MH2 < 2 â 1010 M for MRC 2104-242 and MH2 < 6 â 1010 M for MRC 0943-242 (x = 0.8). These upp er limits are of the same order as H2 mass estimates derived from CO detections of other high-z radio galaxies and SMGs, but lower than the mass of molecular gas detected in a non-negligible fraction of normal star forming galaxies at z 1 - 2. For MRC 0943-242 we also find a tentative CO(1-0) detection at ab out 60 kp c distance from the central region of the host galaxy, but this needs to b e confirmed with additional observations. The sp ectral index of b oth MRC 2104-242 and MRC 0943-242 is relatively steep with -1.5 b etween


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1.4 and 30 GHz. There is no evidence for sp ectral curvature up to 115 GHz in the rest frame of these radio sources.

ACKNOWLEDGMENTS We are tremendously grateful to Warwick Wilson, Dick Ferris and their team and to the engineers and system scientists in Narrabri for making CABB such a great success. We also thank the anonymous referee for good suggestions that significantly improved this pap er. The Australia Telescop e is funded by the Commonwealth of Australia for op eration as a National Facility managed by CSIRO.

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