X-ray populations in galaxies

High-resolution Chandra observations have allowed the detection of populations of X-ray sources in galaxies of all morphological types. The X-ray Luminosity Functions (XLFs) of these X-ray source populations have been derived and studied to uncover the drivers for the formation and evolution of binaries in different stellar populations and environments. These XLFs also provide a tool for identifying the nature of the X-ray source population, since different XLFs characterize X-ray sources belonging to young and old stellar populations. Similarly, X-ray colors can be used for identifying different types of X-ray sources. Ultra-Luminous X-ray sources (ULXs, L_X > 10³⁹ ergs s⁻¹) are found to be associated with star-forming stellar populations. The study of the ULX population of the Antennae galaxies points to compact accreting binaries.     

Luminous star-forming galaxies in the SDSS-GALEX database

We use the combined photometric GALEX + SDSS database to look for populations of luminous blue star-forming galaxies. These were initially identified from such a sample at redshifts near 0.4, using SDSS spectra. We make use of the NUV, g, and i colour index previously defined in our previous paper, to separate stars and QSOs, to locate more of these unusual galaxies, to fainter limits. They are found in significant numbers in two different regions of the related colour-magnitude plot. Within these regions, we use the ensemble 7-colour photometry (FUV, NUV, u, g, r, i, z) to postulate the populations of blue star-forming galaxies at redshifts near 0.4 and 1.0, from a full photometric sample of over half a million.     

Ultra-luminous sources in nearby galaxies

This paper presents an update of what we have learned in the last year about the ULX phenomenon. New results are presented on radio emission from Holmberg II and a review is given on the recent X-ray data on timing and spectra. The new X-ray spectroscopic and optical imaging survey of nearby ULX with XMM allows us, for the first time, to place the average properties of these objects on a statistical basis. Direct examination of the sites of ULXs in nearby galaxies shows that 1/3 of them are not in or near star forming regions, indicating that a substantial fraction of ULX are not directly associated with young star formation. There are two ULX which have been identified with B stars as the optical counterparts on the basis of optical spectroscopy. Radio imaging of the Holmberg II ULX shows that it lies in a luminous extended radio source and that the radio emission is not beamed. A statistical study of ULX spectra in nearby galaxies shows that the ratio of ‘high state’ to ‘low state’ ULXs is 1:1 and that the high state objects, in general, are best fit with low temperature black bodies with a steep power law index. The objects with high state spectra are systematically more luminous than the objects with low state spectra consistent with the hypothesis that both are drawn from a population which shows state changes similar to those of black holes in the Milky Way. If this is true then the masses implied for the objects with the low state spectra are greater than 50M.     

ROSAT observations of nearby galaxies

First results of pointed and All Sky Survey observations of galaxies with the X-ray observatory satellite ROSAT are reported. During observations of the Magellanic Clouds and the Andromeda galaxy new super-soft X-ray sources have been detected. This new class of luminous X-ray sources may help to solve the millisecond pulsar progenitor problem. Due to the improved sensitivity and longer observation times of ROSAT new X-ray point sources have been resolved in several nearby galaxies. The diffuse emission of the LMC that was already reported by EINSTEIN has been mapped in detail. It shows a lot of fine structure and temperatures around 5 × 10⁶ K. The improved low energy response of ROSAT led to the discovery of 10⁶ K gas from the spiral galaxy M101 and the halo of the starburst galaxy NGC 253. No diffuse emission was detected from the halo of the edge-on spiral galaxy NGC 5907.     

Nuclear activity in a volume-limited sample of nearby galaxies

We report on some of the most recent results from a multiwavelength study of a volume-limited sample of galaxies. This paper will concentrate on the detection of variability in NGC 4395, the least luminous known Seyfert galaxy, and the high resolution X-ray imaging of the sample.     

Molecular gas in the central regions of nearby active galaxies

We are carrying out a program of 0.6″–0.8″ resolution ¹²CO J=2→1 observations with unprecedented high sensitivity in a sample of nearby AGN with the IRAM interferometer. Here we give some general results of the program and highlight results from the Seyfert 1 galaxy NGC 7469. This source shows tightly wound molecular spiral arms, a bar-like gas distribution running from the arms to the central regions, and a barely resolved, rapidly rotating central component.     

Compact X-ray sources in nearby galaxy nuclei

We have found compact, near-nuclear X-ray sources in 21 (54%) of a complete sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT HRI data. ROSAT X-ray luminosities (0.2 – 2.4 keV) of these compact X-ray sources are ∼10³⁷ – 10⁴⁰ erg s⁻¹. The mean displacement between the location of the compact X-ray source and the optical photometric center of the galaxy is ∼390 pc. ASCA spectra of six of the 21 galaxies show the presence of a hard component with relatively steep (Γ ≈ 2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the data from the spiral galaxies well, suggesting that the X-ray objects in these galaxies may be similar to a black hole candidate (BHC) in its soft (high) state. ASCA data from the elliptical galaxies indicate that hot (kT ≈ 0.7 keV) gas dominates the emission. The fact that the spectral slope of the spiral galaxy sources is steeper than in normal type 1 active galactic nuclei (AGNs) and that relatively low absorbing columns (N_H ≈ 10²¹ cm⁻²) were found to the power-law component indicates that these objects are somehow geometrically and/or physically different from AGNs in normal active galaxies. The X-ray sources in the spiral galaxies may be BHCs, low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs or AGNs) to be ∼10²–10³ M_⊙. The X-ray sources in the elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.     

The X-ray structure of nearby galaxy clusters

The combination of the large effective area and the very low internal background of the ROSAT Position Sensitive Proportional Counter provides an extremely sensitive instrument for the study of diffuse X-ray sources. In this paper we review new results on the X-ray structure of nearby clusters as measured with ROSAT. Substructure is a common feature in these objects. Such structure provides evidence that clusters have formed relatively recently through mergers of relatively large subunits. This behavior is predicted by hierarchical formation theories in a dense universe.     

NMA survey of CO and HCN emission from nearby active galaxies

The Nobeyama Millimeter Array has been used to map CO(1-0) and HCN(1-0) emission in nearby Seyfert galaxies. A wide variety of molecular gas distributions are found, and there appears to be no “typical” gas distribution either in type-1 Seyferts or type-2s. All the gas distributions and kinematics in the observed Seyferts can be understood as a response to a non-axisymmetric potential in the central regions, suggesting that a small scale (a few 100 pc — a few kpc) distortion of the underlying potential is necessary for Seyfert activity, although it is not a sufficient condition. Circumnuclear star formation in the host of the observed Seyferts can occur via gravitational instabilities of the molecular gas, as in the case of star forming regions in non-Seyfert galaxies. Our results may support the idea that the host galaxies of Seyferts (both type-1s and 2s) and non-Seyferts are the same in terms of the fuel and trigger of star formation. Near the center of Seyfert nuclei (r < a few 100 pc), we find that the molecular gas tends to be gravitationally stable. We find that the R_HCN/CO value ranges over an order of magnitude, from 0.086 to 0.6 among Seyfert galaxies. It seems that the presence of kpc scale jet/outflows is related to the extremely high R_HCN/CO values.     

Medium resolution near-infrared spectra of the host galaxies of nearby quasars

We present medium resolution near-infrared host galaxy spectra of low redshift quasars, PG 0844+349$0844+349$ (z = 0.064), PG 1226+023$1226+023$ (z = 0.158), and PG 1426+015$1426+015$ (z = 0.086). The observations were done by using the Infrared Camera and Spectrograph (IRCS) at the Subaru 8.2 m telescope. The full width at half maximum of the point spread function was about 0.3 arcsec by operations of an adaptive optics system, which can effectively resolve the quasar spectra from the host galaxy spectra. We spent up to several hours per target and developed data reduction methods to reduce the systematic noises of the telluric emissions and absorptions. From the obtained spectra, we identified absorption features of Mg I (1.503 μm), Si I (1.589 μm) and CO (6-3) (1.619 μm), and measured the velocity dispersions of PG 0844+349$0844+349$ to be 132 ± 110 km s⁻¹ and PG 1426+015$1426+015$ to be 264 ± 215 km s⁻¹. By using an _MBH–σ$M_{\text{BH}}''–''\sigma$ relation of elliptical galaxies, we derived the black hole (BH) mass of PG 0844+349$0844+349$, log(_MBH/_M⊙)=7.7±5.5$\log (M_{\text{BH}}/M_{\odot })=7.7\pm 5.5$ and PG 1426+015,log(_MBH/_M⊙)=9.0±7.5$1426+015\text{,}\log (M_{\text{BH}}/M_{\odot })=9.0\pm 7.5$. These values are consistent with the BH mass values from broad emission lines with an assumption of a virial factor of 5.5.     

Superclustering of galaxies and X-ray sources

An attempt is made to compare optical properties of Abell clusters with X-ray sources.     

The X-ray properties of normal galaxies

An extensive program to study nearby normal galaxies was carried out by various observers using the imaging instruments on the $\text{Einstein}$ Observatory; more than 50 such galaxies were detected with 0.5 – 3.0 keV luminosities ranging from 2 × 10³⁸ ergs s^?1 to 3 × 10⁴¹ergs s^?1. The X-ray luminosity of normal galaxies is ~2 × 10^?4 of the optical luminosity and shows no strong correlation with morphological type. For the nearest galaxies, (the Large and Small Magellanic Clouds, M31 and M33,) studies, performed with the Observatory, were comparable to the $\text{Uhuru}$ survey of the Galaxy. Approximately 30 new SNR were recognized in the Magellanic Clouds as a result. Over 90 sources were detected in M31 of which at least 20 are identified with globular cluster. The numbers of luminous (>10³⁷ ergs s^?1) sources detected in the nearest galaxies per unit mass are similar to that found in our own galaxy. Individual X-ray sources in the arms of nearby spirals can be very luminous; seven with luminosities in excess of 10³⁹ergs s^?1 have been discovered. The nuclei of some, but not all, normal galaxies are luminous X-ray sources; X-ray activity is not presently predictable from the radio or optical properties of the nucleus.     

Einstein X-ray observations of clusters of galaxies

The X-ray imaging capability of the Einstein Observatory has provided new observational material in many branches of astrophysics. In this contribution we will review the implications of the X-ray observations for the classification of clusters, the formation of SO galaxies, and the interaction of the galaxies with the intracluster gas.     

The X-ray evolution of galaxies: implications for future X-ray observatories

The X-ray evolution of the luminosity of normal galaxies is primarily driven by the evolution of their X-ray binary populations. The imprints left by a cosmological evolution of the star formation rate (SFR) will cause the average X-ray luminosity of galaxies to appear higher in the redshift range 1–3. As reported by White and Ghosh [ApJ, 504 (1998) L31] the profile of X-ray luminosity with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. In order to observe the high redshift (z>3) universe in the X-ray band, it is necessary to avoid confusion from foreground field galaxies. We report on the predictions of these models of the X-ray flux expected from galaxies and the implications for the telescope parameters of future deep universe X-ray observatories.     

How well do available data describe the distribution of galaxies in the nearby universe?

To study the distribution of galaxies in the Universe data on their positions magnitudes and redshifts are needed. A review of all large samples of galaxy counts, galaxy catalogues and redshift surveys as well as catalogues and redshifts of clusters is given. It is shown that the sky has been unevenly studied, the strongest asimmetry being between the Northern and Southern hemispheres. Particular attention is paid to the Zwicky near clusters. It is shown that only 14% have a well determined redshift and at least 28% are not single clusters but superposition of two or more groups.From an analysis of the available literature it appears that 1) there is more data about redshifts and positions of galaxies than are normally used. 2) The available data are far from uniform and complete.It is argued that a lot more new observations are needed before one can confidently draw conclusions about the structure of the Universe.     

Extreme starburst or central activity in X-ray luminous IRAS galaxies

ROSAT All Sky Survey observations of IRAS galaxies have revealed up to now a number of 10 optically non-Seyfert galaxies with X-ray (0.1–2.4) luminosities up to a few 10⁴³erg · s⁻¹ (Boller et al. 1992). The sources are brighter than previous detection limits of a few 10⁴¹erg · s⁻¹ as found by Stocke et al. (1991) or Green, Anderson and Ward (1992) for Einstein sources. The optical classification is based on follow-up observations which indicate clearly the non-Seyfert (LINER and HII region-like galaxies) nature. Our investigations reveal that galaxies classified as non-Seyferts on the basis of optical spectroscopy may reach exceptionally high X-ray luminosities which are similar to that of Seyfert galaxies. On the basis of the present observational material we suppose a hidden low luminosity AGN in the centre of these objects as the source of energy production. The objects are of interest when evaluating starburst versus central activity.     

X-ray study of accretion flow in narrow-line Seyfert 1 galaxies

We present the results of a systematic study of narrow-line Seyfert 1 galaxies (NLS1s) observed with XMM-Newton. The 2–12 keV X-ray spectra of NLS1s are well represented by a single power law with a photon index Γ ∼ 2. When this hard power law continuum is extrapolated into the low energy band, we found that all objects in our sample show prominent soft excess emission. This excess emission is well parameterized by the thermal emission expected from an optically thick accretion disk, and we found the following three peculiar features: (1) The derived disk temperatures are significantly higher than the expectation from a standard Shakura-Sunyaev accretion disk, if we assume a central mass of a black hole to be 10^6–8M_⊙. (2) The temperatures are distributed within narrow range (ΔkT ∼ 0.08 keV) with an average temperature of 0.18 keV in spite of the range of four orders of magnitude in luminosity (10^41–45 erg s⁻¹). (3) We found a peculiar temperature–luminosity relation, where the luminosity seems to be almost saturated in spite of the significant change in temperature, during the observations of the most luminous NLS1 PKS 0558-504. These results strongly suggest that the standard accretion disk picture is no longer appropriate in the nuclei of NLS1s. We discuss a possible origin for the soft excess component, and suggest that a slim disk may be able to explain the observational results, if the photon trapping effect is properly taken into account.     

High resolution X-ray spectroscopy and chemical evolution in elliptical galaxies

We discuss the elemental composition in the hot ISM of elliptical galaxies derived from new and recent X-ray spectral analysis in the context of new phenomenological models of their chemical evolution. Star formation histories, the IMF, the astrophysics of supernovae, and the nature of galactic winds impact the metal content and relative abundances in the hot ISM. We evaluate how X-ray spectroscopy may be utilized to deconstruct how elliptical galaxies, and the stellar populations that compose them, form and evolve, with an emphasis on present and future high resolution spectroscopic analysis.