Joel N. Bregman

An Infrared Survey of Brightest Cluster Galaxies. II. Why are Some Brightest Cluster Galaxies Forming Stars

Quillen et al. presented an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray-luminous clusters. They found that at least half of these sources have signs of excess IR emission. Here we discuss the nature of the IR emission and its implications for cool core clusters. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding the four systems dominated by an AGN, the excess mid-IR emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star-forming galaxies. The gas depletion timescale is about 1 Gyr. The physical extent of the IR excess is consistent with that of the optical emission-line nebulae. This supports the hypothesis that star formation occurs in molecular gas associated with the emission-line nebulae and with evidence that the emission-line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates () estimated from the X-ray emission and the star formation rates estimated from the IR luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates, suggesting that the reheating of the intracluster medium is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

CHANDRA X-RAY OBSERVATIONS OF THE X-RAY FAINT ELLIPTICAL GALAXY NGC 4697

A Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which ~80 are low-mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. Of the unresolved emission, it is likely that about half is from fainter unresolved LMXBs, while the other half (~23% of the total count rate) is from interstellar gas. The X-ray-emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. The physical state of the hot gas is uncertain; the X-ray luminosity and extended surface brightness are inconsistent with a global supersonic wind, a partial wind, or a global cooling inflow. The gas may be undergoing subsonic inflation, rotationally induced outflow, or ram pressure stripping. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with ROSAT, is due to interstellar gas. The cumulative LMXB spectrum is well fitted by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. NGC 4697 has a central X-ray source with a luminosity of LX = 8 ? 1038 ergs s-1, which may be due to an active galactic nucleus and/or one or more LMXBs. At most, the massive black hole (BH) at the center of this galaxy is radiating at a very small fraction (?4 ? 10-8) of its Eddington luminosity. Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. We suggest that all of the LMXBs may have been formed in globulars. The X-ray to optical luminosity ratio for the LMXBs in NGC 4697 is LX(LMXB, 0.3-10 keV)/LB = 8.1 ? 1029 ergs s-1 L, which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of 2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from ~5 ? 1037 to ~2.5 ? 1039 ergs s-1. The luminosity function of the LMXBs has a knee at 3.2 ? 1038 ergs s-1, which is approximately the Eddington luminosity of a 1.4 M? neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates BH and NS binaries. This characteristic luminosity could be used as a distance estimator. If they are Eddington limited, the brightest LMXBs contain fairly massive accreting BHs. The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.

X-Ray Spectral Properties of Low-Mass X-Ray Binaries in Nearby Galaxies

We have investigated the X-ray spectral properties of a collection of low-mass X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies using proprietary and archival data from the Chandra X-Ray Observatory. We find that the spectrum of the sum of the sources in a given galaxy is remarkably similar from galaxy to galaxy when only sources with X-ray luminosities less than 1039 ergs s-1 (0.3-10 keV) are considered. Fitting these lower luminosity sources in all galaxies simultaneously with a power-law model led to a best-fit power-law exponent of Γ = 1.56 ± 0.02 (90% confidence), and using a thermal bremsstrahlung model yielded kTbrem = 7.3 ± 0.3 keV. This is the tightest constraint to date on the spectral properties of LMXBs in external galaxies. The spectral properties of the LMXBs do not vary with galactic radius out to three effective radii. There is also no apparent difference between the spectral properties of LMXBs that reside within globular clusters and those that do not. We demonstrate how the uniformity of the spectral properties of LMXBs can lead to more accurate determinations of the temperature and metallicity of the hot gas in galaxies that have comparable amounts of X-ray emission from hot gas and LMXBs. Although few in number in any given galaxy, sources with luminosities of (1-2) × 1039 ergs s-1 are present in 10 of the galaxies. The spectra of these luminous sources are softer than the spectra of the rest of the sources and are consistent with the spectra of Galactic black hole X-ray binary candidates when they are in their very high state. The spatial distribution of these sources is much flatter than the optical light distribution, suggesting that a significant portion of them must reside within globular clusters. The simplest explanation of these sources is that they are ~10-15 M☉ black holes accreting near their Eddington limit. The spectra of these sources are very different from those of ultraluminous X-ray sources (ULXs) that have been found within spiral galaxies, suggesting that the two populations of X-ray-luminous objects have different formation mechanisms. The number of sources with apparent luminosities above 2 × 1039 ergs s-1 when determined using the distance of the galaxy is equal to the number of expected background active galactic nuclei and thus appears not to be associated with the galaxy, indicating that very luminous sources are absent or very rare in early-type galaxies. The lack of ULXs within elliptical galaxies strengthens the argument that ULXs are associated with recent star formation.

Creation of cosmic structure in the complex galaxy cluster merger Abell 2744

We present a detailed strong lensing, weak lensing and X-ray analysis of Abell 2744 (z = 0:308), one of the most actively merging galaxy clusters known. It appears to have unleashed ‘dark’, ‘ghost’, ‘bullet’ and ‘stripped’ substructures, each 10 14 M . The phenomenology is complex and will present a challenge for numerical simulations to reproduce. With new, multiband HST imaging, we identify 34 strongly-lensed images of 11 galaxies around the massive Southern ‘core’. Combining this with weak lensing data from HST, VLT and Subaru, we produce the most detailed mass map of this cluster to date. We also perform an independent analysis of archival Chandra X-ray imaging. Our analyses support a recent claim that the Southern core and Northwestern substructure are post-merger and exhibit morphology similar to the Bullet Cluster viewed from an angle. From the separation between X-ray emitting gas and lensing mass in the Southern core, we derive a new and independent constraint on the self-interaction cross section of dark matter particles =m

Optical and radio variability in blazars

An analysis is presented of the optical (0.44 micron) and radio flux density variations (4.8, 8.0, and 14.5 GHz) of the five best observed variable AGN, of which three are BL Lac objects and two are optically violent variable (OVV) quasars. For each object the nature of the variability at individual wavelengths is determined by using a structure function analysis and the autocorrelation function as calculated from the discrete correlation technique. Both techniques yield similar results, showing that the optical variations are similar for all objects and are a combination of flicker and shot noise. The radio variability is distinct, being described by shot noise (less variation at short times), with the two OVVs exhibiting the most extreme behavior. The time lag between the variations at the three radio frequencies and the optical flux is studied with the discrete cross-correlation function and the mean variance methods. It is concluded that the optical and radio emitting regions are physically related, exist on distinct time scales, and excite their synchrotron emitting plasmas differently.

Do Hot Halos Around Galaxies Contain the Missing Baryons

Galaxies are missing most of their baryons, and many models predict these baryons lie in a hot halo around galaxies. We establish observationally motivated constraints on the mass and radii of these halos using a variety of independent arguments. First, the observed dispersion measure of pulsars in the Large Magellanic Cloud allows us to constrain the hot halo around the Milky Way: if it obeys the standard Navarro, Frenk, and White (NFW) profile, it must contain less than 4%-5% of the missing baryons from the Galaxy. This is similar to other upper limits on the Galactic hot halo, such as the soft X-ray background and the pressure around high-velocity clouds. Second, we note that the X-ray surface brightness of hot halos with NFW profiles around large isolated galaxies is high enough that such emission should be observed, unless their halos contain less than 10%-25% of their missing baryons. Third, we place constraints on the column density of hot halos using nondetections of O VII absorption along active galactic nucleus (AGN) sightlines: in general they must contain less than 70% of the missing baryons or extend to no more than 40 kpc. Flattening the density profile of galactic hot halos weakens the surface brightness constraint so that a typical L * galaxy may hold half its missing baryons in its halo, but the O VII constraint remains unchanged, and around the Milky Way a flattened profile may only hold 6%-13% of the missing baryons from the Galaxy ((2-4) × 1010 M ☉). We also show that AGN and supernovae at low to moderate redshift—the theoretical sources of winds responsible for driving out the missing baryons—do not produce the expected correlations with the baryonic Tully-Fisher relationship and, therefore, are insufficient to explain the missing baryons from galaxies. We conclude that most of missing baryons from galaxies do not lie in hot halos around the galaxies, and that the missing baryons never fell into the potential wells of protogalaxies in the first place. They may have been expelled from the galaxies as part of the process of galaxy formation.

Interstellar matter in early-type galaxies. I. The catalog

A catalog is given of the currently available measurements of interstellar matter in the 467 early-type galaxies listed in the second edition of the Revised Shapley-Ames Catalog of Bright Galaxies. The morphological type range is E, SO, and Sa. The ISM tracers are emission in the following bands: IRAS 100 micron, X-ray, radio, neutral hydrogen, and carbon monoxide. Nearly two-thirds of the Es and SOs have been detected in one or more of these tracers. Additional observed quantities that are tabulated include: magnitude, colors, radial velocity, central velocity dispersion, maximum of the rotation curve, angular size, 60 micron flux, and supernovae. Qualitative statements as to the presence of dust or emission lines, when available in the literature, are given. Quantities derivative from the observed values are also listed and include masses of H I, CO, X-ray gas, and dust as well as an estimate of the total mass and mass-to-luminosity ratio of the individual galaxies. 204 refs.

Detection of a Hot Gaseous Halo around the Giant Spiral Galaxy NGC 1961

Hot gaseous halos are predicted around all large galaxies and are critically important for our understanding of galaxy formation, but they have never been detected at distances beyond a few kpc around a spiral galaxy. We used the ACIS-I instrument on board Chandra to search for diffuse X-ray emission around an ideal candidate galaxy: the isolated giant spiral NGC 1961. We observed four quadrants around the galaxy for 30 ks each, carefully subtracting background and point-source emission, and found diffuse emission that appears to extend to 40–50 kpc. We fit β-models to the emission and estimate a hot halo mass within 50 kpc of 5 × 10 9 M� . When this profile is extrapolated to 500 kpc (the approximate virial radius), the implied hot halo mass is 1–3 × 10 11 M� . These mass estimates assume a gas metallicity of Z = 0.5 Z� . This galaxy’s hot halo is a large reservoir of gas, but falls significantly below observational upper limits set by pervious searches, and suggests that NGC 1961 is missing 75% of its baryons relative to the cosmic mean, which would tentatively place it below an extrapolation of the baryon

Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1

There are two proposed explanations for ultraluminous X-ray sources (ULXs) with luminosities in excess of 1039 erg s−1. They could be intermediate-mass black holes (more than 100–1,000 solar masses, ) radiating at sub-maximal (sub-Eddington) rates, as in Galactic black-hole X-ray binaries but with larger, cooler accretion disks. Alternatively, they could be stellar-mass black holes radiating at Eddington or super-Eddington rates. On its discovery, M 101 ULX-1 had a luminosity of 3 × 1039 erg s−1 and a supersoft thermal disk spectrum with an exceptionally low temperature—uncomplicated by photons energized by a corona of hot electrons—more consistent with the expected appearance of an accreting intermediate-mass black hole. Here we report optical spectroscopic monitoring of M 101 ULX-1. We confirm the previous suggestion that the system contains a Wolf-Rayet star, and reveal that the orbital period is 8.2 days. The black hole has a minimum mass of 5, and more probably a mass of 20−30, but we argue that it is very unlikely to be an intermediate-mass black hole. Therefore, its exceptionally soft spectra at high Eddington ratios violate the expectations for accretion onto stellar-mass black holes. Accretion must occur from captured stellar wind, which has hitherto been thought to be so inefficient that it could not power an ultraluminous source.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. IX. Ultraviolet observations of Fairall 9

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Ly? + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Ly? + N V, for which we find that the centroid of the continuum-emission-line cross-correlation function is ?cent ? 14-20 days. We measure a lag ?cent 4 days for He II ?1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Ly? + N V and He II ?1640 at a high level of significance. We are unable to determine a reliable lag for C IV ?1550, but we note that the profiles of the variable parts of Ly? and C IV ?1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.