Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea-www.harvard.edu/~kim/pap/N1316_2003.pdf Äàòà èçìåíåíèÿ: Mon Dec 6 20:20:37 2010 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:03:14 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: dust disk

The Astrophysical Journal, 586: 826­849, 2003 April 1
# 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.

CHANDRA X-RAY OBSERVATIONS OF NGC 1316 (FORNAX A) Dong-Woo Kim and G. Fabbiano
Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 Received 2002 June 25; accepted 2002 December 9

ABSTRACT We report the results of the Chandra ACIS subarcsecond resolution X-ray observation of the archetypal merger radio galaxy NGC 1316 (Fornax A). We confirm the presence of fine substructures in the hot interstellar medium (ISM). Some of these are likely to result from interaction with the radio jets, while others may be related to a complex intermingling of different phases of the ISM. We detect a low-luminosity X-ray active galactic nucleus (AGN) with LX ¼ 5 á 1039 ergs sþ1 (in 0.3­8 keV) and a þ ¼ 1:7 power-law energy spectrum. We also detect 81 point sources within the 25th magnitude isophotal ellipse of NGC 1316 (LX in the range of 2 á 1037 to 2 á 1039 ergs sþ1), with hard (kT $ 5 keV) X-ray spectra, typical of X-ray binaries, and a spatial radial distribution consistent with that of the optical (i.e., stellar) surface brightness. We derive the X-ray luminosity function (XLF) of these sources, correcting for the incompleteness at the faint end caused by the presence of the diffuse emission from the hot ISM in the central regions of NGC 1316 and by the widening of the Chandra point-spread functions at increasing distance from the aim point. With these corrections, the XLF is well reproduced by a single--unbroken--power law with a slope of þ1.3 down to our threshold luminosity of $3 á 1037 ergs sþ1. The hot ISM has temperatures in the 0.5­0.6 keV range, its surface brightness distribution is more centrally concentrated than that of the point sources, and its temperature appears to decrease at larger radii. These properties suggest that the ISM may be subject to partial winds. Taking into account the spectral complexity of the ISM, and the presence of unresolved low luminosity X-ray sources (which can be inferred from the spectra), we constrain the metal abundance of the hot ISM to be Z ¼ 0:25­ 1.3 Z (90% confidence). Subject headings: galaxies: abundances -- galaxies: individual (NGC 1316, NGC 1317) -- galaxies: ISM -- X-rays: galaxies

1. INTRODUCTION

The radio galaxy NGC 1316 (Fornax A) is a disturbed elliptical galaxy with numerous tidal tails. To explain this morphology, Schweizer (1980) suggested several low-mass, gas-rich mergers occurring over the last 2 Gyr (see also Ekers et al. 1983; Kim, Fabbiano, & Mackie 1998, hereafter KFM; Mackie & Fabbiano 1998). NGC 1316 has been extensively observed in a wide range of wavelengths (see a summary of previous observations in KFM). In the radio band, Fornax A is the third brightest object in the sky, with giant radio lobes (Wade 1961; Ekers et al. 1983), separated by $200 kpc, consisting of polarized filaments (Fomalont et al. 1989) and S-shaped nuclear radio jets (Geldzahler & Fomalont 1984). In X-rays, it was observed with Einstein (Fabbiano, Kim, & Trinchieri 1992), ROSAT PSPC (Feigelson et al. 1995), ASCA (Kaneda et al. 1995; Iyomoto et al. 1998), and ROSAT HRI (KFM). In particular, the ROSAT observations revealed $109 M of hot ISM, with an inhomogeneous distribution. The substructures of the ISM were not highly significant, given the ROSAT HRI data quality, but nevertheless suggested a multiphase ISM interacting with the radio jets (KFM). NGC 1316 is the first elliptical galaxy for which this type of jet/hot ISM interaction has been reported. Previous evidence of this phenomenon had been limited to galaxy clusters (Cygnus A, Carilli, Perley, & Harris 1994; the Perseus cluster, Bohringer et al. 1993). Chandra observations are now revealing many other examples in several other elliptical galaxies and clusters (e.g., NGC 4374, Finoguenov & Jones 2001; and Hydra A, McNamara et al. 2000). 826

In this paper we report the results of a high spatial resolution, spectrally resolved Chandra ACIS (Weisskopf et al. 2000) observations of NGC 1316. The motivation of this work is twofold. First, we attempt to confirm and study in detail the features of the hot ISM suggested by the ROSAT HRI (KFM). Second, NGC 1316 is an X-ray­faint elliptical (Fabbiano et al. 1992), where we would expect a large contribution to the X-ray emission to originate from low-mass X-ray binaries (LMXBs; e.g., Trinchieri & Fabbiano 1985; Canizares, Fabbiano, & Trinchieri 1987; Kim, Fabbiano, & Trinchieri 1992; Eskridge, Fabbiano, & Kim 1995a, 1995b; Fabbiano, Kim, & Trinchieri 1994b; Pellegrini & Fabbiano 1994). With Chandra we can detect individual LMXBs (e.g., Sarazin, Irwin, & Bregman 2000), study their properties, and quantitatively estimate their contribution to the diffuse emission. We can also significantly reduce the bias introduced by the presence of unresolved sources in past discussions of the spectral and spatial characteristics of the ISM. This paper is organized as follows. In x 2, we describe the Chandra observations, the data reduction, and the results of spatial and spectral analyses. In x 3, we derive the X-ray luminosity function (XLF) of point sources associated with NGC 1316 and estimate the uncertainties derived from the varying detection thresholds across the field. In x 4, we summarize various emission components. In x 5, we discuss the implications of our results for the nature and properties of the hot ISM and its interaction with radio jets and other phases of ISM. Finally, we summarize our conclusions in x 6. We adopt a distance D ¼ 18:6 Mpc throughout this paper, based on the Hubble Space Telescope (HST ) measurements of Cepheid variables (Madore et al. 1999)


CHANDRA X-RAY OBSERVATIONS OF NGC 1316 in NGC 1365. At the adopted distance, 100 corresponds to 90 pc.
2. CHANDRA OBSERVATIONS AND DATA ANALYSIS

827

NGC 1316 was observed for 30 ks on 2001 April 17 (ObsID 2022), with the Chandra Advanced CCD Imaging Spectrometer (ACIS; Garmire 1997). We used the backilluminated CCD S3 (CCD ID 7) because of its sensitivity at low energies. To include NGC 1317 (6<3 away from NGC 1316) in the same S3 chip, a small offset was applied to the SIM (Science Instrument Module) position. NGC 1316 was kept close to on-axis to achieve the best spatial resolution. The data were reduced with XPIPE (Kim et al. 2003), a custom pipeline specifically developed for the Chandra Multiwavelength Project (ChaMP). XPIPE takes the CXC pipeline level 2 data products and then applies additional data corrections (e.g., gain correction, removing bad pixels/ columns) and additional data screening (e.g., removing background flares), performs source detection, and determines source properties, such as flux, X-ray colors, variability, and extent. Specifically, for this observation, we have identified two bad pixels in CCD S2 (or CCD ID 6), which resulted in five spurious sources. The CXC pipeline processing was done before the new S3 gain file was released in 2001 September. We have rerun acis_process_events to correct the ACIS gain and used follow-up tools available in CIAO1 to generate CXC pipeline­like products. Background flares are often seen in back-illuminated (BI) chips (CCD S1 and S3). They are excluded when the background rate is beyond 3 from the average rate, which is measured iteratively after initial exclusion of flares. Removal of background flares reduced the effective exposure time of CCD S3 to 24.7 ks. To detect X-ray sources, we used WAVDETECT (Freeman & Vinay 2002), a wavelet detection algorithm available in CIAO. WAVDETECT is more reliable in finding individual sources (and not detecting false sources) in a crowded field containing extended emission than the traditional sliding-box CELLDETECT algorithm, although it requires a longer processing time. We set the WAVDETECT significance threshold parameter to be 10þ6, which corresponds to 1 possibly spurious source, and the scale parameter to cover seven steps between 1 and 64 pixels. This made us sensitive to sources ranging from pointlike to 3200 in size, and in particular accommodates the variation of the point-spread function (PSF) as a function of the off-axis angle. WAVDETECT tends to detect spurious sources near the detector edge. To avoid such false detections, we have utilized an exposure map (made at 1.5 keV) and applied a 10% exposure threshold. The exposure map was tailored to each CCD, based on the aspect history. To extract source properties (such as count rates, spectra, etc.), we have used the 95% encircled energy (at 1.5 keV) radius centered at the WAVDETECT centroid, with a minimum of 300 to accommodate the radial variation of PSF. Background counts were determined locally for each source from an annulus from 2 to 5 times the source radius, after excluding nearby sources. 2.1. Point Sources The detected X-ray sources are shown in Figure 1 superimposed on the X-ray (left panel ) and on the Digitized Sky
1

Survey2 optical (right panel ) images. In this figure (and throughout this paper), north is to the top and east is to the left in the image. Extended sources are found at the locations of NGC 1316 and NGC 1317 as seen in ROSAT data (KFM). In addition, the Chandra observations reveal 94 sources, 83 of them in CCD S3. Of these, 81 sources (77 in S3 and 4 in S2) are within the D25 ellipse (taken from de Vaucouleurs et al. 1991 [RC3]) of NGC 1316. The source density increases toward the center of NGC 1316, indicating that most of them are related to NGC 1316. Three sources are found within D25 of NGC 1317, with the brightest, extended one at the center of NGC 1317. The list of sources (including sources found in other CCDs) is given in Table 1. The net source counts estimated by XPIPE (i.e., by aperture photometry as described above) are usually in good agreement with those determined by WAVDETECT. Exceptions are those sources within 2000 from the center of NGC 1316, where the extended diffuse X-ray emission makes it difficult to accurately measure source and background counts. Consequently, the net counts and rates are subject to a large uncertainty, and these sources are marked in the table (they are not used below in determining XLF and in spectral fitting). Also marked in the table are those sources that have one or more sources within their source extraction radius, introducing additional uncertainty in their count rates. Three sources are found at the edge of the chip, and their counts should be considered as a lower limit. These are marked in Table 1. After correcting for effective exposure and vignetting, the X-ray flux in the 0.3­8.0 keV band is calculated with an energy conversion factor (ECF) assuming a powerlaw source spectrum with þph ¼ 1:7 and NH ¼ 3 á 1020 cmþ2; ECF ¼ 6:037 á 10þ12 ergs cmþ2 sþ1 per 1 count sþ1 for the back-illuminated (BI) chips and 9:767 á 10þ12 ergs cmþ2 sþ1 for the front-illuminated (FI) CCD chips. With the adopted distance of 18.6 Mpc, the X-ray luminosities of the point sources range from $2 á 1037 to $8 á 1039 ergs sþ1. We have identified one possible off-center ultraluminous X-ray source (ULX) with LX ¼ 2:4 á 1039 ergs sþ1 (CXOU J032251.2þ370949). Following Zezas & Fabbiano (2002), we define the ULX as a non-AGN (i.e., off-center) point source with LX > 2 á 1039 ergs sþ1. This source is marked in Figure 1 and also in Table 1. No optical counterpart is seen in the DSS image (see Fig. 1). There are three more sources with net count above 200 (or, LX > a few á 1039 ergs sþ1) within the D25 ellipse of NGC 1316 (see Table 1). Two of them correspond to the elongated feature at the center of NGC 1316 (see x 2.2). The third source, CXOU J032241.2þ371235, is 1000 SW from the center; hence its X-ray flux is highly uncertain because of the strong diffuse X-ray emission. Counting pixel values manually suggests that its count might be overestimated by as much as $50%. Five sources are found to be coincident with globular clusters identified in HST images (Shaya et al. 1996) and in ground observations (Goudfrooij et al. 2001). They are marked as such in Table 1. Also identified is one potential super­soft X-ray source (SSS), CXOU J032235.9þ371135, for which all X-ray photons are in the soft band (0.3­0.9 keV). Based on the log N log S of Chandra Deep Field sources (Brandt et al. 2001; Giacconi et al. 2001) and ChaMP sources (Kim et al. 2003), we expect fewer than seven
2

See http://cxc.harvard.edu/ciao.

See http://stdatu.stsci.edu/dss.


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Fig. 1.--X-ray sources superimposed on the X-ray (left) and on the Digitized Sky Survey optical (right) images. NGC 1316 is at the center of the DSS image, and NGC 1317 is to the north of NGC 1316. The horizontal bar in the lower left corner indicates 10 . The circle radius indicates the PSF size, while the circle color indicates WAVDETECT source significance (red,>5 ; blue, 3­5 ; green, <3 ). The source marked with a square is a possible ULX source. North is to the top and east is to the left.

serendipitous sources within D25 of NGC 1316 in S3. The radial distribution of point sources is also consistent with the radial profile of the optical light, i.e., the stellar distribution (see x 2.3), suggesting that most of them are LMXBs associated with NGC 1316. The spectral properties of these sources are also consistent with this hypothesis (x 2.4.1). 2.2. Surface Brightness Distribution of the Diffuse X-Ray Emission To identify the substructures of extended, diffuse X-ray emission, we have applied CSMOOTH, an adaptive smoothing algorithm available in CIAO. Figure 2 shows the smoothed image within the central 30 á 30 region. In this arcminute-scale image, the diffuse X-ray emission is elongated in the north-south direction, and does not follow either the major axis (P:A: ¼ 50 ) or the minor axis of NGC 1316. The ellipse in the center of Figure 2 indicates the optical figure of NGC 1316 with size equal to 1/10 of D25. Also noticeable is a secondary maximum of the surface brightness (`` blob '') at 3000 ­6000 north of the center. This feature is extended when compared to the PSF, and its X-ray emission is possibly soft (see x 2.4.3). We address the nature of these large-scale north-south elongations in x 5. In the subarcminute scale, the X-ray surface brightness is elongated along the major axis of the optical figure, but it is far from featureless. The enlarged view of the central region of NGC 1316 is shown in Figure 3. It is clearly noticeable that the X-ray surface brightness distribution forms valleys

(cavities) toward the southeast (P:A: ¼ 120 ) and northwest (P:A: ¼ 315 ). These are the directions along which the radio jets propagate (see Fig. 2 in Geldzahler & Fomalont 1984). In Figure 3, red lines indicate the directions and approximate sizes of the radio jets (see x 5.2 for further discussions of the interaction between the hot ISM and radio jets). It was one of the main purposes of this observation to confirm the reality of these X-ray valleys, which were suggested by the ROSAT HRI image (KFM). If we compare the counts within a 300 radius circle in the X-ray valley and in the surrounding region at a similar distance from the center, the difference is significant at a $10 level. The central 300 region exhibits a double-peak feature, elongated toward the northeast (see also Fig. 4). Figure 4 shows the central 30 á 30 smoothed X-ray images in different energy bands: soft (0.3­0.9 keV), medium (0.9­ 2.5 keV), and hard (2.5­8.0 keV). The diffuse emission is most prominent in the soft band, as expected from the hot gaseous X-ray emission. The northern blob is also most clearly seen in the soft band (see below for more discussion). The point sources, on the other hand, are seen more distinctly in the medium band, as expected for LMXBs with a harder spectrum. A point source at the nucleus is unambiguously identified in the hard band. The location of the nucleus determined from this hard-band image is marked as a circle at the center of Figure 3. A true-color image made by combining the three band images (red for the soft band, green for the medium band, and blue for the hard band) is shown in Figure 5.


TABLE 1 Source List Source Radius (arcsec)

Source

CCD No.

Detect No.

R.A. (J2000)

Decl. (J2000)

Net Counts

Error

Counts ksþ1

LXa (1038 ergs sþ1)

Notesb

Within the D25 Ellipse of NGC 1316 CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU J032219.5þ371338 J032219.9þ371322 J032228.9þ371443 J032229.5þ371511 J032225.6þ371145 J032227.5þ371223 J032228.9þ371242 J032229.1þ371023 J032230.4þ371025 J032232.6þ371309 J032235.2þ371127 J032235.2þ371205 J032235.3þ371248 J032235.5þ371313 J032235.6þ371248 J032235.7þ371218 J032235.9þ371135 J032235.9þ371256 J032236.4þ370842 J032236.4þ371324 J032236.6þ371054 J032236.6þ371306 J032237.2þ371212 J032237.4þ371301 J032237.5þ371428 J032237.6þ371252 J032238.7þ371221 J032238.7þ371422 J032239.1þ371148 J032239.3þ371313 J032239.4þ371252 J032239.4þ371256 J032239.6þ371243 J032239.9þ371056 J032240.1þ371310 J032240.4þ371244 J032240.5þ371227 J032240.7þ371151 J032240.8þ371224 J032241.1þ371235 J032241.3þ371117 J032241.7þ371229 J032241.7þ371343 J032241.8þ371207 J032241.8þ371225 J032241.9þ371238 J032241.9þ371305 J032242.0þ371218 J032242.0þ371246 J032242.0þ371256 J032242.1þ371123 J032242.3þ371260 J032242.5þ371222 J032242.5þ371258 J032242.7þ371217 J032242.8þ371222 J032243.2þ371206 J032243.3þ371104 J032243.7þ371215 J032243.8þ371233 J032243.9þ371203 ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... ........... 6 6 6 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 6 5 1 4 83 78 63 62 61 60 26 59 25 24 58 57 23 56 82 22 55 54 21 53 52 51 50 20 19 18 49 48 76 47 17 46 16 15 14 13 45 12 11 10 44 43 42 9 41 8 7 6 5 40 4 39 38 3 75 2 37 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 19.5 19.9 28.9 29.5 25.6 27.5 28.9 29.1 30.4 32.6 35.2 35.2 35.3 35.5 35.6 35.7 35.9 35.9 36.4 36.4 36.6 36.6 37.2 37.4 37.5 37.6 38.7 38.7 39.1 39.3 39.4 39.4 39.6 39.9 40.1 40.4 40.5 40.7 40.8 41.1 41.3 41.7 41.7 41.8 41.8 41.9 41.9 42.0 42.0 42.0 42.1 42.3 42.5 42.5 42.7 42.8 43.2 43.3 43.7 43.8 43.9 þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 13 13 14 15 11 12 12 10 10 13 11 12 12 13 12 12 11 12 08 13 10 13 12 13 14 12 12 14 11 13 12 12 12 10 13 12 12 11 12 12 11 12 13 12 12 12 13 12 12 12 11 12 12 12 12 12 12 11 12 12 12 37.6 21.8 42.7 11.3 45.2 23.3 41.6 23.1 25.1 09.4 26.8 05.3 47.5 13.2 47.6 18.1 34.9 56.1 42.0 23.7 54.5 06.0 11.6 00.9 28.2 51.5 20.9 21.5 48.4 12.8 52.5 56.0 43.0 55.9 10.2 44.5 26.7 51.4 23.9 35.4 17.4 29.0 43.2 07.4 25.1 38.1 04.9 18.5 45.5 56.1 23.3 59.5 21.5 57.5 17.1 22.2 06.3 03.8 15.0 32.7 02.7 3.44 3.22 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 5.24 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 11.32 6.34 3.87 8.69 8.75 7.87 22.48 4.92 7.26 5.72 32.78 10.90 26.80 75.54 23.96 11.10 12.04 8.67 11.89 72.40 3.77 17.24 7.53 7.76 7.18 20.72 4.66 31.72 23.71 19.95 8.68 10.75 22.06 22.02 1.48 31.87 32.08 28.12 54.97 248.57 28.52 805.64 3.95 14.45 440.46 52.86 11.35 5.46 10.00 19.77 65.08 42.73 56.18 18.42 6.69 29.86 3.03 36.39 25.81 21.89 8.33 4.72 3.97 3.41 4.29 4.44 4.29 6.09 3.80 4.30 3.98 7.16 4.99 6.56 9.94 6.28 5.12 5.12 4.60 5.48 9.77 3.81 5.57 4.88 4.60 4.14 6.20 4.61 6.91 7.02 6.40 5.50 5.72 7.12 6.29 4.64 9.22 10.57 7.82 11.86 19.24 7.26 31.17 4.47 6.61 24.47 13.06 6.04 9.88 8.96 7.13 9.61 8.44 11.76 6.50 9.06 10.43 4.91 7.48 7.84 7.92 5.14 0.40 0.22 0.15 0.32 0.36 0.32 0.93 0.22 0.32 0.23 1.37 0.46 1.09 3.06 0.97 0.47 0.51 0.35 0.59 2.94 0.15 0.70 0.31 0.33 0.29 0.89 0.19 1.29 1.00 0.85 0.35 0.44 0.92 0.91 0.06 1.32 1.34 1.18 2.30 10.39 1.18 33.70 0.16 0.60 18.40 2.20 0.47 0.23 0.41 0.82 2.69 1.77 2.34 0.76 0.27 1.23 0.12 1.55 1.07 0.90 0.34 1.62 0.89 0.61 1.29 0.90 0.81 2.31 0.55 0.81 0.57 3.43 1.16 2.72 7.66 2.42 1.18 1.27 0.86 1.47 7.35 0.39 1.74 0.77 0.83 0.73 2.22 0.48 3.23 2.50 2.13 0.88 1.10 2.30 2.28 0.15 3.31 3.34 2.94 5.75 25.97 2.94 84.21 0.40 1.51 45.98 5.51 1.18 0.57 1.03 2.04 6.72 4.43 5.84 1.89 0.68 3.09 0.31 3.87 2.66 2.26 0.84

2 2 9

5

2 2 5

1, 2 1, 2 1, 7 1, 2, 6

1, 6 1, 5 1, 5 1 2 2 1, 2 2 1, 2, 5 1, 2


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KIM & FABBIANO
TABLE 1--Continued Source Radius (arcsec) 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 4.10 8.18 3.21 5.43 3.68 4.03 5.54 6.48 9.57

Vol. 586

Source CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU J032244.2þ371053 ........... J032244.8þ371210 ........... J032245.2þ371423 ........... J032245.4þ371226 ........... J032245.4þ371306 ........... J032245.6þ371207 ........... J032245.8þ371214 ........... J032247.1þ371408 ........... J032247.5þ371240 ........... J032248.3þ371159 ........... J032249.3þ371152 ........... J032249.4þ371033 ........... J032250.9þ370813 ........... J032251.0þ371206 ........... J032251.2þ370949 ........... J032252.2þ371158 ........... J032253.6þ371211 ........... J032254.3þ371027 ........... J032259.7þ371128 ........... J0323 5.7þ371113 ............

CCD No. 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

Detect No. 36 35 73 34 72 33 1 71 70 32 31 30 79 69 29 28 27 68 66 65

R.A. (J2000) 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 23 44.2 44.8 45.2 45.4 45.4 45.6 45.8 47.1 47.5 48.3 49.3 49.4 50.9 51.0 51.2 52.2 53.6 54.3 59.7 05.7 þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ

Decl. (J2000) 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 37 10 12 14 12 13 12 12 14 12 11 11 10 08 12 09 11 12 10 11 11 53.2 09.7 22.7 25.5 06.2 06.7 13.8 07.5 39.7 58.9 52.0 33.0 13.3 05.8 49.2 57.9 11.1 26.8 27.7 13.0

Net Counts 9.48 11.36 8.78 2.00 12.63 8.11 19.58 4.25 9.46 6.68 16.62 5.23 17.98 10.68 227.86 14.45 15.28 14.82 26.05 92.49

Error 4.87 5.60 4.29 4.17 5.24 5.01 6.21 3.62 4.73 4.30 5.46 4.75 7.54 4.86 16.54 5.47 5.58 5.91 7.35 11.82

Counts ksþ1 0.41 0.46 0.52 0.08 0.51 0.33 0.80 0.23 0.38 0.27 0.68 0.21 0.78 0.43 9.46 0.59 0.62 0.65 1.09 3.96

LXa (1038 ergs sþ1) 1.03 1.16 1.30 0.20 1.29 0.83 2.00 0.57 0.95 0.68 1.71 0.51 1.95 1.08 23.65 1.47 1.56 1.62 2.72 9.90

Notesb

3

3

4

Within the D25 Ellipse of NGC 1317 CXOU J032240.7þ370518 ........... CXOU J032244.3þ370614 ........... CXOU J032246.1þ370552 ........... 7 7 7 81 74 80 3 22 40.7 3 22 44.3 3 22 46.1 þ 37 05 18.4 þ 37 06 14.0 þ 37 05 51.9 15.26 11.79 13.79 1.03 366.52 47.13 9.23 21.45 11.82 0.07 16.39 2.16 0.17 40.97 5.40 3 8

Beyond D25 of NGC 1316 and NGC 1317 CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU CXOU J032145.8þ370430 ........... J032147.1þ370723 ........... J032159.4þ370515 ........... J032137.5þ371454 ........... J0322 0.7þ371918 ............ J032211.1þ370954 ........... J032212.6þ371212 ........... J032215.4þ371656 ........... J032225.7þ370912 ........... J032230.8þ370736 ........... J032254.7þ370706 ........... 2 2 2 5 6 6 6 6 7 7 7 3 2 1 1 16 12 11 7 64 77 67 3 3 3 3 3 3 3 3 3 3 3 21 21 21 21 22 22 22 22 22 22 22 45.8 47.1 59.4 37.5 00.7 11.1 12.6 15.4 25.7 30.8 54.7 þ þ þ þ þ þ þ þ þ þ þ 37 37 37 37 37 37 37 37 37 37 37 04 07 05 14 19 09 12 16 09 07 07 29.9 23.4 15.3 53.7 18.2 53.8 11.8 55.9 12.0 36.1 06.5 40.00 31.57 28.50 34.29 22.29 8.56 5.84 8.03 5.22 7.77 11.84 113.70 87.04 180.51 384.08 197.23 30.20 26.37 78.53 136.60 49.72 74.48 19.81 16.67 18.68 32.73 17.55 7.66 6.75 10.45 13.09 9.28 11.84 4.447 3.177 6.489 17.639 8.064 1.111 0.958 3.034 6.268 2.234 3.461

a L (in 0.3­8.0 keV) is determined with D ¼ 18:6 Mpc and ECF ¼ 6:037 (or 9:767÷á 10þ15 ergs sþ1 for 1 count ksþ1, which is appropriate in BI (or FI) X CCDs for a source with photon index 1.7 and N ÏH÷ ¼ 3 á 1020 cmþ2. b Notes: (1) Sources within 2000 from the center of NGC 1316. The source count may be contaminated by the diffuse emission. (2) There is another source(s) within the source extraction radius. (3) Sources fall near the edge of the detector. The source count should be considered as a lower limit. (4) A potential ULX source. (5) Coincident with globular clusters listed in Shaya et al. 1996 and Goudfooij et al. 2001. (6) At the center of NGC 1316. (7) Closeto the center of NGC 1316. Its count may be overestimated. (8) At the center of NGC 1317. (9) A potential super soft X-ray source.

2.3. Radial Profiles of Diffuse X-Ray Emission and Point Sources We have derived radial surface brightness profiles to study the radial distribution of the hot gas and point-source components and compare them with the optical (stellar) distribution. Given the obvious lack of circular symmetry of the diffuse X-ray emission, we have extracted radial profiles from different angular sectors, after excluding detected point sources. Beta models, ôx $ ½1 ×Ïr=a÷2 þ3×0:5 , were then used to model these profiles. Toward the east-west direction at P:A: ¼ 30 ­150 and 210 ­330 , excluding the northern blob and the less prominent southern extension, the radial profile is smooth and the best-fit  is 0.57­0.59 at 90% confidence, corresponding to a radial slope of þ2.4 to þ2.5. Toward the north-south direc-

tion, however, the radial profile steepens around 2000 ­3000 and then flattens because of the elongated features. The best-fit  is 0.43­0.49 at 90% confidence, which corresponds to a radial slope of þ1.6 to þ1.9. The X-ray radial profile measured toward the east-west direction is shown in Figure 6, where observed data points are marked by open circles with error bars and the best-fit beta model is represented by the dashed line. Although the data are well fitted in most radial bins, there is a significant deviation from the beta model at the nucleus, which requires an additional point source. This nuclear source is most prominent in the hard-band X-ray image (Fig. 4). We have used a CIAO tool mkpsf to reproduce a point source at the center. The intensity of this nuclear source is consistent with the results of the X-ray spectral fitting (see x 2.4.2). The solid line in Figure 6 indicates the composite beta model+central point source


No. 2, 2003

CHANDRA X-RAY OBSERVATIONS OF NGC 1316

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Fig. 2.--Chandra X-ray image of the central 30 á 30 region of NGC 1316. The data were adaptively smoothed. The ellipse indicates the optical figure of NGC 1316, with size equal to 1/10 of D25 taken from RC3. The horizontal bar in the lower left corner is 3000 long. North is to the top and east is to the left.

profile. The central source profile is marked by the dotted line in the same figure. The estimated X-ray flux of the nuclear component is 1:2 á 10þ13 ergs sþ1 (with a 15% error) in the broad energy band (0.3­8 keV). Also plotted in the Figure 6 (green) is the radial profile of the stellar light distribution from the HST I band (Shaya et al. 1996). The I-band profile decreases radially as rþ1:16 and is much flatter than the radial distribution of the diffuse X-ray emission. The X-ray point sources detected in the Chandra observations (Fig. 6, red squares) are distributed like the stellar light, as would be expected from LMXBs in NGC 1316, instead of following the diffuse X-ray emission. This means that the contribution from LMXBs to the X-ray emission becomes more significant at the outer radii. This has an important implication when considering the effect of hidden populations of LMXBs in X-ray­faint early-type galaxies observed with lower angular resolution than Chandra. In particular, lower resolution data would give a biased distribution of the hot ISM, and wrong (in excess) mass measurements, if the contribution of LMXBs is ignored. Ignoring the LMXBs would also affect spectral results (see x 5 for further discussion).

2.4. Spectral Analysis Following the science threads,3 we have produced response files (rmf and arf ) to be used in the spectral fitting. We have also applied a new method to produce weighted response files to take into account the variation with detector location,4 but the results are almost the same. After this paper was submitted, a time-dependent degradation of the low-energy ACIS quantum efficiency was reported.5 We have regenerated the spectral response files and rerun all the spectral fitting. We found that the spectral parameter most affected is NH, which can become lower by as much as a factor of 2. The kT of the softer spectral component in multicomponent fits also becomes slightly lower. However, the main results remain the same. We report here the results from the latest analysis, performed with the best available calibration.
See http://cxc.harvard.edu/ciao/documents_threads.html. As described in the science thread available at http://cxc.harvard.edu/ ciao. 5 See http://cxc.harvard.edu/cal/Links/Acis/acis/Cal_prods/qeDeg/ index.html.
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KIM & FABBIANO

Vol. 586

Fig. 3.--Enlarged view of the central region of NGC 1316 from the Chandra X-ray data (adaptive smoothing applied). The horizontal bar in the lower left corner is 1000 long. The location of the nucleus determined in the hard-band image is marked by a green circle. A few contours are drawn to illustrate the X-ray valleys along P:A: $ 120 and $315 where the radio jets (red lines) are propagating. North is to the top and east is to the left.

The background spectrum was extracted from five circular, source-free regions with a 6000 radius in CCD S3, situated between 20 and 50 from the center of NGC 1316. Different choices of background regions within the same CCD do not change our results, because point sources (the nucleus and LMXBs) are well within very small source regions and the diffuse emission is extended only to $10 ­20 . We have fitted, within an energy range of 0.3­5.0 keV, emission models including a thermal gas model (MEKAL), bremsstrahlung, and power-law, alone and in combination. For solar abundances, we take the photospheric values given in Anders & Grevesse (1989), which is about 40% larger than the meteoric value in Fe. The spectral