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.     

Diffuse thermal emission from very hot gas in starburst galaxies: Spatial results

New BeppoSAX observations of the nearby prototypical starburst galaxies NGC 253 and M82 are presented. A companion paper (Cappi et al. 1998) shows that the hard (2–10 keV) spectrum of both galaxies, extracted from the source central regions, is best described by a thermal emission model with kT ∼ 6–9 keV and abundances ∼ 0.1-0.3 solar. The spatial analysis yields clear evidence that this emission is extended in NGC 253, and possibly also in M82. This quite clearly rules out a LLAGN as the main responsible for their hard X-ray emission. Significant contribution from point-sources (i.e. X-ray binaries (XRBs) and Supernovae Remnants (SNRs)) cannot be excluded; neither can we at present reliably estimate the level of Compton emission. However, we argue that such contributions shouldn't affect our main conclusion, i.e., that the BeppoSAX results show, altogether, compelling evidence for the existence of a very hot, metal-poor interstellar plasma in both galaxies.     

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.     

LMXB luminosity function and connection to globular cluster in early type galaxies

We derive bias-corrected X-ray luminosity functions (XLFs) of LMXBs detected in 14 E and S0 galaxies observed with Chandra. After correcting for incompleteness, the individual XLFs are statistically consistent with a single power-law. A break at or near L_X,Eddington , as previously reported, is not required in any individual case. The combined XLF with a reduced error, however, suggests a possible break at L_X = 5 × 10³⁸ erg s⁻¹, which may be consistent with the Eddington luminosity of neutron stars with the largest possible mass (3 M), or of He-enriched neutron star binaries. We confirm that the total X-ray luminosity of LMXBs is correlated with the the near-IR luminosities, but the scatter exceeds that expected from measurement errors. The scatter in L_X(LMXB)/L_K appears to be correlated with the specific frequency of globular clusters, as reported earlier.

We cross-correlate X-ray binaries with globular clusters determined by ground-based optical and HST observations in 6 giant elliptical galaxies. With the largest sample reported so far (300 GC LMXBs with a 5:2 ratio between red and blue GCs), we compare their X-ray properties, such as X-ray hardness, XLF and L_X/L_B and find no statistically significance difference between different groups of LMXBs. Regardless of their association with GCs, both GC and field LMXBs appear to follow the radial profile of the optical halo light, rather than that of more extended GCs. This suggests that while metallicity is a primary factor in the formation of LMXBs in GCs, there may be a secondary factor (e.g., encounter rate) playing a non-negligible role.     

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.     

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.     

Galactic outflows and the pollution of the galactic environment by supernovae

We here explore the effects of the SN explosions into the halo of star-forming galaxies like the Milky Way. Successive randomly distributed and clustered SNe explosions cause the formation of hot superbubbles that drive either fountains or galactic winds above the galactic disk, depending on the amount and concentration of energy that is injected by the SNe. In a galactic fountain, the ejected gas is re-captured by the gravitational potential and falls back onto the disk. From 3D non-equilibrium radiative cooling hydrodynamical simulations of these fountains, we find that they may reach altitudes up to about 5 kpc in the halo and thus allow for the formation of the so called intermediate-velocity-clouds (IVCs) which are often observed in the halos of disk galaxies. The high-velocity-clouds that are also observed but at higher altitudes (of up to 12 kpc) require another mechanism to explain their production. We argue that they could be formed either by the capture of gas from the intergalactic medium and/or by the action of magnetic fields that are carried to the halo with the gas in the fountains. Due to angular momentum losses to the halo, we find that the fountain material falls back to smaller radii and is not largely spread over the galactic disk. Instead, the SNe ejecta fall nearby the region where the fountain was produced, a result which is consistent with recent chemical models of the galaxy. The fall back material leads to the formation of new generations of molecular clouds and to supersonic turbulence feedback in the disk.     

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.     

Obscured AGN and the X-ray background

There is growing evidence that the hard X-ray background (XRB) can be explained by a large population of obscured AGN. I review some of the results of recent deep X-ray surveys, and in particular I discuss the nature of the X-ray luminous emission-line galaxies which have emerged at the faintest X-ray fluxes. If obscured AGN do explain the XRB, a direct implication is that the majority of the energy produced by accretion in the universe is absorbed and not emitted directly. Deep submillimetre surveys with SCUBA have recently attracted a lot of attention, with the potential to allow us an unobscured view dust-enshrouded starformation at high redshift. It has generally been assumed that these sources are purely high redshift starforming galaxies, but if models for the XRB are correct then a significant fraction (∼20%) could contain a luminous AGN.     

EUV emission from normal galaxies

Using data from the Wide Field Camera EUV all-sky survey, we have established upper limits to the EUV flux from a sample of 30 bright, nearby, non-active spiral galaxies. These galaxies were chosen to be those most likely to be detected in the EUV on the basis of (i) low interstellar absorption within our own galaxy, (ii) brightness in other wavebands, (iii) high star formation activity, and (iv) proximity. The derived EUV upper limits are restrictive, and establish for the first time that the EUV flux escaping from galaxies does not constitute a major component of their bolometric luminosity, and in particular that it cannot be the sink for the energy injected into the interstellar medium by supernova explosions, as had been suggested following the failure to detect this power in the X-ray band.     

ROSAT PSPC spectra of six Seyfert 1 galaxies

We present six ROSAT PSPC observations of Seyfert 1 galaxies chosen to have low Galactic line-of-sight absorption (N_H 10²⁰ cm⁻²). As expected, it is found that all of these sources possess significantly steeper spectra below 1 keV, than that observed at higher X-ray energies. In addition we find evidence for soft X-ray spectral features, which are best parameterized as line emission at 0.63 keV in NGC7469 and 0.75 keV in ESO198-G24. We examine these results in the light of the accuracy of the PSPC spectral calibration.     

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.     

Extragalactic and galactic U.V. observations owing to the balloon borne scap telescope at the wavelength 2000 Å

The use of the 2 000 Å transmission window of the atmosphere permitted to observe for the first time from a balloon gondola the nearest galaxies ; 30 arc/sec. resolution images were obtained for M 31, spiral arms and nucleus, M 33, M 101, with their external spiral structure, M 82 and NGC 3 077. Nearly three hundred more distant galaxies have been detected up to the magnitude V = 13. Several HII regions of the Galaxy have been observed, chiefly NGC 7 000 and the Cyghus Loop. Numerous blue stars of the halo have been identified.     

Testing the ADAF paradigm for supermassive black holes in elliptical galaxies

The growing evidence for supermassive black holes in the centres of nearby galaxies has brought into sharper focus the question of why elliptical galaxies, rich in hot gas, do not possess quasar-like luminosities. Recent studies suggest that the presence of advection-dominated accretion flows (ADAFs) with their associated low radiative efficiency, might provide a promising explanation for the observed quiescence of these systems. Here, we present new high-frequency radio observations of the three giant, low-luminosity elliptical galaxies NGC 4649, NGC 4472 and NGC 4636 obtained using the Very Large Array (VLA) and the sub-millimetre common-user bolometer array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The new radio limits disagree severely with the canonical ADAF predictions which significantly overestimate the observed flux. If the accretion in these objects occurs in an advection-dominated mode then our radio limits imply that the emission from their central regions must be suppressed. We present the possibility that the magnetic field in the flow is extremely low or that synchrotron emission is free-free absorbed by cold material in the accretion flow. We also discuss whether slow non-radiating accretion flows may drive winds/outflows to remove energy, angular momentum and mass so that the central densities, pressures and emissivities are much smaller than in a standard ADAF (Di Matteo et al. 1998).     

Are Boltzmann plots of hydrogen Balmer lines a tool for identifying a subclass of S1 AGN?

It is becoming clear that we can define two different types of nearby AGN belonging to the Seyfert 1 class (S1), on the basis of the match of the intensities of their Broad Balmer Lines (BBL) with the Boltzmann Plots (BP). These two types of S1 galaxies, that we call BP-S1 and NoBP-S1, are characterized, in first approximation, by Broad Line Regions (BLR) with different structural and physical properties. In this communication, we show that these features can be well pointed out by a multi-wavelength analysis of the continuum and of the broad recombination Hydrogen lines, that we carry out on a sample of objects detected at optical and X-ray frequencies. The investigation is addressed to verify whether BP-S1 are the ideal candidates for the study of the kinematical and structural properties of the BLR, in order to derive reliable estimates of the mass of their central engine and to constrain the properties of their nuclear continuum spectrum.     

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.     

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.     

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.     

VLBA imaging of parsec-scale gas in Seyfert galaxies

Seyfert galaxies contain small-scale radio jets that indicate the presence of compact active galactic nuclei of the same type found in more powerful quasars and radio galaxies. Since most Seyferts are relatively nearby, their nuclear environments can be probed at much higher resolution than in those more powerful sources. In addition, the relative weakness of the active nucleus makes the effects of the circumnuclear gas more important. VLBA images of a number of Seyfert galaxies have been produced, often at multiple frequencies, in order to reveal the presence and nature of the gas in the inner 1–100 parsecs of the galaxies. Absorption effects at gigahertz frequencies, due to both synchrotron self-absorption and free-free absorption, are quite common; the free-free absorption indicates the presence of large quantities of ionized gas in the inner few parsecs of the galaxies. Changes in radio position angles on parsec scales also occur in several galaxies, implying that Seyfert nuclei may have multiple symmetry axes.     

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.