A study of a complete soft X-ray selected sample of AGN

We present preliminary results from analyses of hard X-ray and optical observations of a soft X-ray selected sample. We created a small but complete sample with 20 of the softest and brightest objects with low Galactic absorption from the ROSAT bright soft X-ray selected radio-quiet AGN sample. This sample consists of 10 narrow-line Seyfert 1 galaxies and 10 broad-line Seyfert galaxies. We analyze ASCA data in the 0.6–10 keV band and optical spectra from ground-based telescopes. We investigate the photon indices in the hard X-ray band, soft excesses in the ASCA band, and optical emission line properties. The photon indices in the 2–10 keV band are nominal for both narrow-line Seyfert 1 galaxies and broad-line Seyfert 1 galaxies in each class compared with other heterogeneous samples. All of the narrow-line Seyfert 1 galaxies show soft excesses, but this component seems to be less significant for broad-line Seyfert 1 galaxies. There seems to be a trend of steeper X-ray spectra to be accompanied by narrower Hβ for narrow-line Seyfert 1 galaxies, but this is not extended to the larger velocity width regime of broad-line Seyfert 1 galaxies, and no clear trend is seen among them.     

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.     

Ginga observations of Seyfert galaxies

We observed twenty-eight Seyfert 2 galaxies with the Japanese X-ray satellite, Ginga, and found Seyfert 2 galaxies, in general, have the X-ray spectral characteristics of obscured Seyfert 1 nuclei. This result agrees with the predictions from Unified Seyfert model proposed by Antonucci and Miller /1/. However, among the observed Seyfert 2 galaxies, there are a few galaxies with no evidence of an obscuration, contrary to the general predictions of the unified model. We note that type 2 AGN will contribute to the Cosmic Diffuse X-ray Background, if the unified Seyfert model can be extended to the far distant AGN such as quasars.     

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.     

Galaxies at the detection limits of deep X-ray surveys

The great sensitivities of the Chandra X-ray Observatory and XMM-Newton have allowed us to begin to explore the X-ray emission from galaxies at moderate to high redshift. By using the stacking method, we show that we can detect the ensemble emission from normal elliptical, spiral and irregular galaxies out to redshifts approaching unity. The average X-ray luminosity of these galaxy types can then be compared with the results of models of the evolution in the numbers of low-mass and high-mass X-ray binaries and can possibly be used to constrain models of star formation.     

The dark matter halo structure of “fossil” groups candidates

We investigated properties of four isolated giant elliptical galaxies with extended X-ray halo using ASCA data. The derived size of X-ray halo, X-ray luminosity, and gravitational mass of the dark halo are unusually large those of X-ray halo of a single galaxy, but are typical for X-ray halos of groups and poor clusters of galaxies. The measured temperatures and abundances of the X-ray halo gas in these galaxies are also similar to those of the groups and poor clusters. Based on these results we identified these galaxies as “isolated X-ray overluminous elliptical galaxy” (IOLEG). The radial profiles of dark halo in these objects were derived from X-ray data. It is found that some are similar to those of compact groups while others are the same as those of normal ellipticals. The dark halos of lOLEGs are thus indistinguishable from those of groups (and poor clusters), which appears to be consistent with a widely believed idea that lOLEGs are a product of dynamical evolution of a compact group. However, mass-to-light ratios of IOLEGs (M₂₀₀/L_B  100–1000) are far greater than those of Hickson compact groups M₂₀₀/L_B  40–60). Since it is hard to consider that total optical luminosity of a compact group decreases by an order of magnitude in the course of dynamical evolution, such difference in the observed mass-to-light ratio between IOLEGs and Hickson compact groups strongly suggests that most IOLEGs have not evolved from compact groups which are observed at present.     

X-rays from the first massive black holes

X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field.     

X-ray source populations in nearby star-forming galaxies

We present results on the X-ray source populations of nearby star-forming galaxies based on Chandra observations. First we discuss the monitoring campaigns on the Antennae and M82 galaxies. In both cases we find that the majority of the X-ray sources exhibit intensity and/or spectral variability. However, despite of this variability, we do not find any statistically significant variations of their X-ray luminosity functions (XLFs). We also find that the majority of the X-ray sources are associated with star-forming regions, although we do not always identify optical counterparts to the X-ray sources. Especially in the case of M82 we find that the most luminous sources are clustered in the central region of the galaxy. Finally, we present the first results from a study of a sample of nearby star-forming galaxies which form a starburst age sequence: although their XLFs to first order are represented by power-laws with consistent slopes, there is an indication for small variations, suggesting a change of their X-ray binary populations.     

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.     

Observations of extragalactic objects with telescope ART-P onboard Granat

During 1990–1991 more than two dozens of extragalactic objects were observed by X-ray instruments aboard the Granat satellite. Among them there were AGNs and Seyfert galaxies (NGC4151, NGC5548, MCG-30-15, IC4329 etc.), quasars (3C273, 3C390.3 etc.), radio galaxies (Cen A, M87 etc.) and clusters of galaxies (Virgo, Perseus, Coma etc.). We will discuss below the preliminary results obtained with the ART-P telescope during the observations of five sources: NGC4151, 3C273, Cen A and the galaxies M87 and NGC4388 on the Virgo cluster.     

Ultraviolet images of galaxies from the Optical Monitor on XMM-Newton

A large number of galaxies, both normal and active, have been observed in ultraviolet light by the Optical Monitor on XMM-Newton. These are some of the deepest wide-field ultraviolet images of these galaxies yet obtained, and in many cases the first collected in this waveband. We present images of five active galaxies, and discuss the potential uses of the ultraviolet surface brightness distribution and morphology, in association with X-ray data, for Active Galactic Nuclei, star formation and galaxy evolution studies.     

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.     

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.     

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.     

The dust content of clusters of galaxies

We have statistically investigated the infrared luminosity of clusters of galaxies in comparison with the known tracers of the cluster mass like the X-ray luminosity and the cluster richness (e.g. the number of member galaxies). Our results show that there is a clear positive correlation of the infrared luminosity with the cluster mass. Quantitatively speaking, the infrared luminosity is on average 20 times higher than the X-ray luminosity. Moreover, the infrared luminosity increases with the redshift. This probably shows that a major part of this infrared luminosity is due to star formation in the member galaxies. Another possible contribution would be the thermal emission from dust particles in the diffuse intracluster medium. However our method does not allow us to infer conclusions about this second hypothesis. Depending on their size and abundance, such particles would contribute to the infrared luminosity of galaxy cluster and have an impact on the cooling function of the baryons and thus on the formation of the large scale structures. This is an important cosmological question which still remains open.     

An X-ray survey of a complete sample of 3CR radio galaxies

Imaging X-ray observations of normal spiral galaxies show extended and complex x-ray emission, easily explainable with a complex of unresolved X-ray sources. A variety of nuclear sources, including starburst nuclei and miniature active nuclei are seen. The total (0.5–3.0 keV) luminosities are in the range of Lx 10³⁸ - 10⁴⁰ erg s⁻¹. The X-ray luminosity is linearly correlated with the optical luminosity. It is also correlated with the radio continuum luminosity at 21cm, but following a power law relationship with an exponent α = 0.6. This latter relationship might have implications on the Population I X-ray binary formation models and/or on the origin of the radio continuum emission in spiral galaxies     

A multi-wavelength study of the merger candidate Abell S0721 and the activities of its member galaxies

We present a multi-wavelength study of the merging history and its influence on galaxy activities of a newly detected merger candidate – Abell S0721. This cluster was only known as a poor cluster in the Shapley Supercluster, but when several survey data of different wavelengths are combined, evidence of strong merging is revealed. We have analyzed the optical and X-ray structures of the cluster, as well as galaxy dynamics and properties of the radio galaxies in the cluster. The dynamics analysis shows the velocity distribution of this system is significantly deviated from a 1-Gaussian model, indicating the existence of 2–3 dynamically different sub-systems; in addition, both optical and X-ray structures of the cluster show an elongated and multi-clump structure, two X-ray clumps imaged by ROSAT All Sky Survey are well correlated with two galaxy density peaks. This cluster is possibly a merger of several groups; our analysis shows that the projection angle for two clumps in the main structure is possibly near the projection plane, and the substructure is seen at ∼30–75° from the projection plane. The merging process could also be the origin to boost the activities of its member galaxies. Finally, as compared with other merger candidates in the Shapley Supercluster, the radio galaxies identified in Abell S0721 are all optically luminous, and the fraction of luminous radio galaxies is only lower than one merger candidate (which is proposed to be at the very beginning of a merger event), this suggests Abell S0721 is also at the beginning of a merger event.     

ROSAT PSPC observations of two starburst galaxies

We present results from ROSAT observations of NGC 1808 and NGC 2903. Exposures of 10 ksec each with the PSPC detector show X-ray sources at the central positions of both galaxies which are classified as nuclear starburst galaxies. Both targets, NGC 1808 and NGC 2903 appear slightly extended in X-ray maps in the energy band 0.1–2.4 keV. The X-ray spectrum of NGC 1808 shows almost complete absorption below 0.5 keV, indicating an extremely high hydrogen column density towards that source (N_H ≈ 8 × 10²¹cm⁻² resulting from model fits on the PSPC spectrum). In case of NGC 2903, the number of counts in the ROSAT band is significantly lower than expected from a previous EINSTEIN investigation of the source.     

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.     

Revealing the hot intergalactic medium with ROSAT

The X-ray background intensity around galaxies and rich clusters of galaxies is investigated in three energy bands using the ROSAT All-Sky Survey maps. It is found that an amplitude of the XRB enhancements surrounding the Abell clusters and high density areas in the Lick galaxy counts depends on photon energy. Excess flux generated in the surrounding of the galaxy concentrations is consistent with the thermal emission by hot gas postulated by hydrodynamic simulations.