X. Y. Xia

Chandra Observations of Markarian 273: Unveiling the Central Active Galactic Nucleus and the Extended Hot Gas Halo

We report X-ray observations of the field containing the ultraluminous IRAS galaxy Mrk 273. The data were obtained using the Advanced CCD Imaging Spectrometer S3 instrument on board Chandra. The high-resolution X-ray image for the first time reveals a compact hard X-ray nucleus in Mrk 273. Its position is coincident with the northern nucleus identified in the optical, infrared, and radio and in molecular CO maps. Its X-ray energy distribution is well described by a heavily obscured active galactic nucleus (AGN) spectrum with an absorbed power law plus a narrow Fe Kα emission line at 6.4 keV. The neutral hydrogen column density is about 4 × 1023 cm-2, implying an absorption-corrected X-ray luminosity (0.1-10 keV) for the nucleus of LX ≈ 6.5 × 1043 ergs s-1 for H0 = 50 km s-1 Mpc-1. The X-ray properties therefore firmly establish the northern nucleus of Mrk 273 as a Seyfert 2 AGN. There are also bright soft X-ray clumps and diffuse soft X-ray emissions surrounding the central hard X-ray nucleus within the 10 of the nuclear region. Its spectrum can be fitted by a MEKAL thermal model with a temperature of about 0.8 keV and high metallicity (Z ~ 1.5 Z☉) plus emission lines from α elements and ions. We find that a soft X-ray clump, about 4 (projected separation of about 4 kpc) southwest of the northern hard X-ray nucleus, is coincident with a nebula with strong [O III] λ5007 emissions. Further outside the central region, the Chandra observations reveal a very extended hot halo in Mrk 273. The X-ray halo encompasses the entire optical tidal tail and plume, with a projected diameter of about 108 × 68 kpc. The total soft X-ray luminosity (0.1-2.4 keV) of the hot halo is LX ≈ 1.9 × 1041 ergs s-1, in the range of the soft X-ray luminosity of bright elliptical galaxies. The temperature of the hot gas is about 0.62 keV with a low metallicity (Z ~ 0.1 Z☉). We discuss the nature of the AGN in Mrk 273 and the implications of our results on the origin of X-ray halos in elliptical galaxies. We also discuss the properties of Mrk 273x, a background AGN at redshift z = 0.46 in the Mrk 273 field. The AGN has an X-ray luminosity of LX ≈ 2.43 × 1044 ergs s-1 in the 0.5-10 keV band. Its X-ray properties resemble those of Seyfert 1 galaxies, while its optical properties are similar to Seyfert 2 galaxies. Such mixed classifications in the optical and X-ray may be a challenge for the unification scheme of AGNs.

The Nature of Ultraluminous X-Ray Sources in NGC 4565

We report the optical identifications of two X-ray-luminous point sources in the spiral galaxy NGC 4565 based on archive data of Chandra and the Hubble Space Telescope. The central X-ray point source, RX J1236.3+2559, is found to be the nucleus of NGC 4565 with an X-ray luminosity of LX ≈ 4.3 × 1039 ergs s-1. We show that its multiband properties are consistent with its being a low-luminosity active galactic nucleus. A faint optical counterpart with B ≈ 25.1 and I ≈ 24.0 was identified for the off-nucleus X-ray point source, RX J1236.2+2558. Its extinction-corrected B magnitude is estimated to be 24.5. The X-ray-to-optical flux ratio (fX/fB) is about 540. From the optical and X-ray properties, we argue that RX J1236.2+2558 is an ultraluminous X-ray compact source with LX ≈ 6.5 × 1039 ergs s-1. The source is probably located in a faint globular cluster at the outer edge of NGC 4565s bulge.

STATISTICAL PROPERTIES OF ULTRALUMINOUS IRAS GALAXIES FROM AN HST IMAGING SURVEY

We perform photometric measurements on a large HST snapshot imaging survey sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative nuclei from bright clumps in all the sample targets, mainly based on a quantitative criterion of I-band luminosity as well as the global and local morphological information. All the sources are then classified into three categories with multiple, double, and single nucleus/nuclei. The resultant fractions of multiple, double, and single nucleus/nuclei ULIRGs are 18%, 39%, and 43%, respectively. This supports the multiple merger scenario as a possible origin of ULIRGs, in addition to the commonly accepted pair merger model. Further statistical studies indicate that the AGN fraction increases from multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULIRGs. For the single nucleus category, there is a high-luminosity tail in the luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This indicates that active galactic nuclei tend to appear at final merging stage. For multiple/double nuclei galaxies, we also find a high fraction of very close nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This strengthens the conclusion that systems at late merging phase preferentially host ULIRGs.

Chandra observations of ultraluminous infrared galaxies: Extended hot gaseous halos in merging galaxies

We study the properties of hot gaseous halos in 10 nearby ultraluminous IRAS galaxies observed with the ACIS instrument on board Chandra. For all sample galaxies, diffuse soft X-ray emissions are found within ~10 kpc of the central region; their spectra are well fitted by a MEKAL model plus emission lines from α-elements and other ions. The temperature of the hot gas is about 0.7 keV and metallicity is about 1 Z☉. Outside the central region, extended hot gaseous halos are found for 9 out of the 10 ultraluminous infrared galaxies (ULIRGs). Most spectra of these extended halos can be fitted with a MEKAL model with a temperature of about 0.6 keV and a low metallicity (Z ~ 0.1 Z☉). We discuss the implications of our results on the origin of X-ray halos in elliptical galaxies and the feedback processes associated with starbursts.

A STATISTICAL STUDY OF THE SPECTRA OF VERY LUMINOUS IRAS GALAXIES. I. DATA

This paper presents the results of spectral observations for the largest complete sample of very luminous IRAS galaxies obtained to date. The sample consists of those 73 objects for which , and , and was extracted from the 2 Jy IRAS redshift catalog. All the spectra were obtained using the 2.16 m telescope of Beijing Astronomical Observatory during the years 1994-1996. A total of 123 galaxy spectra were obtained with spectral ranges of to and to at resolutions of and respectively. In addition to the 73 spectra for sample galaxies, we also present spectra for ten non-sample galaxies and a further 40 for the companions of sample galaxies. The data presented include nuclear spectrum and the parameters describing the emission lines, absorption lines and continua as well as DSS images and environmental parameters.

The Nature of the Diffuse Clumps and the X-Ray Companion of Markarian 273*

We present an optical spectrum for Mrk 273x, the X-ray source 13 to the northeast of Mrk 273. The new spectrum indicates that the object is at a much higher redshift (0.458) than the value previously reported by us (0.0376). All the detected emission lines show properties of Seyfert 2 galaxies. Mrk 273x has one of the highest X-ray and radio luminosities (LX ≈ 1.1 × 1044 ergs s-1, L1.37 GHz ≈ 2.0 × 1040 ergs s-1) among Seyfert 2 galaxies, yet it has a low neutral hydrogen column density, NH ≈ 4.4 × 1020 cm-2. These properties seem difficult to explain in the unified scheme of active galactic nuclei. The spectrum that we previously used to identify the redshift for Mrk 273x turns out to belong to diffuse clumps in the northeast tail/plume about 20 (~20 kpc in projected distance) from the nuclear region of Mrk 273. A new spectrum for this region was obtained; this spectrum is essentially the same as the previous one. These observations indicate that these clumps have the same redshift as Mrk 273 and are physically associated with the merger. The spectrum is dominated by strong emission from the [O III] doublet, with Seyfert 2-like line ratios, which indicate that these northeast clumps are probably excited by the shock + precursor mechanism during the merging process. This mechanism may be operating in other ultraluminous IRAS galaxies as well.

Mid-to-Far Infrared Spectral Energy Distribution of Galaxies in the Spitzer First Look Survey Field

We made model fitting to the mid-to-far infrared spectral energy distributions (SEDs) for different categories of galaxies in the main extragalactic field of the Spitzer First Look Survey with the aid of spectroscopic information from the Sloan Digital Sky Survey. We find that the mid-to-far infrared SEDs of HII galaxies, mixture type galaxies and LINERs can be well fitted by the one-parameter (α) dust model of Dale et al. plus the 13 Gyr dust-free elliptical galaxy model. The statistics of α values indicates that all these galaxies tend to be quiescent, although the HII galaxies are relatively more active than the LINERs. The mid-infrared SEDs of absorption galaxies are well fitted simply by the 13 Gyr dust-free elliptical galaxy template, and the near-to-mid infrared SEDs of QSOs can be represented by AGN NGC 5506.

Soft X-ray properties of Ultraluminous IRAS Galaxies

We report on the results of cross-correlation of a sample of 903 Ultraluminous IRAS galaxies (ULIRGs) with the ROSAT-All Sky Survey Bright Source Catalogue and the ROSAT archived pointing observations. The sample of ULIRGs has been compiled from the recently released PSCz redshift survey. In total, 35 ULIRGs are securely detected by the ROSAT All-Sky Survey and pointing observations, five of which are blazars. The statistical properties of these sources in the soft X-ray band are determined and compared with their properties in other wavebands. We find that the ratio of the soft X-ray to the far-infrared flux spans about five orders of magnitude and reaches values of about unity. This ratio is a good indicator of the main energy source of ULIRGs. Those with soft X-ray to far-infrared flux exceeding 0.01 are probably powered by accretion onto central supermassive black holes while those with ratios smaller than 0.001 are probably powered by starbursts or other heating processes, or are Compton thick sources. Some ULIRGs have energy contributions from both. This ratio is low for most ULIRGs and hyperluminous infrared galaxies, which explains their low detection rate by ROSAT and ASCA. We also find that some ULIRGs have a similar soft X-ray luminosity vs. temperature relation to that for groups of galaxies and elliptical galaxies, suggesting a common origin of these systems. Our study also reveals a tight correlation between the hardness ratio and the soft X-ray luminosity for Seyfert 1s/QSOs.

Optical Identifications of Companion Soft X-ray Sources of Mrk 231

We present optical identification results for four ROSAT PSPC soft X-ray companions of Mrk 231 based on the deep BATC 6660 A-band image and the optical spectra obtained by the 60/90-cm Schmidt telescope and the 2.16-m telescope at the Xinglong Station, NAOC. Three optical counterparts are quasars with redshifts z > 1 and the remaining X-ray source is probably a background galaxy cluster. Therefore, none of these soft X-ray companions are physically connected with the central X-ray source Mrk 231. Incorporating the previous results of Arp 220 and Mrk 273 (Xia et al. 1998, 1999), we suggest that the apparent soft X-ray associations with ULIRGs are chance coincidence in most cases.

Chandra observations of isolated early-type galaxies

In this paper, we present a statistical study of the X-ray properties of 11 isolated early-type galaxies (IEGs) observed with the Chandra ACIS-S3, and then discuss the implications of these properties.