arXiv subjects

Joel N. Bregman

Publications and source records attributed to Joel N. Bregman.

At least 109 records · Page 6Linked to original sources

arXiv2009

Redshift Evolution in the Iron Abundance of the Intracluster Medium

Clusters of galaxies provide a closed box within which one can determine the chemical evolution of the gaseous baryons with cosmic time. We studied this metallicity evolution in the hot X-ray emitting baryons through an analysis of XMM-Newton observations of 29 galaxy clusters in the redshift range 0.3 < z < 1.3. Taken alone, this data set does not show evidence for significant evolution. However, when we also include a comparable sample of 115 clusters observed with Chandra (Maughan et al. 2008) and a lower redshift sample of 70 clusters observed with XMM at z < 0.3 (Snowden et al. 2008), there is definitive evidence for a decrease in the metallicity. This decrease is approximately a factor of two from z = 0 to z \approx 1, over which we find a least-squares best-fit line Z(z) / Z_{\odot} = (0.46 \pm 0.05) - (0.38 \pm 0.03)z. The greatest uncertainty in the evolution comes from poorly constrained metallicities in the highest redshift bin.

astro-ph.CO↗

arXiv2009

Oxygen Metallicity Determinations from Optical Emission Lines in Early-type Galaxies

We measured the oxygen abundances of the warm (T$\sim 10^{4}K$) phase of gas in seven early-type galaxies through long-slit observations. A template spectra was constructed from galaxies void of warm gas and subtracted from the emission-line galaxies, allowing for a clean measurement of the nebular lines. The ratios of the emission lines are consistent with photoionization, which likely originates from the UV flux of post-asymototic giant branch (PAGB) stars. We employ H II region photoionization models to determine a mean oxygen metallicity of $1.01\pm0.50$ solar for the warm interstellar medium (ISM) in this sample. This warm ISM 0.5 to 1.5 solar metallicity is consistent with modern determinations of the metallicity in the hot (T$\sim 10^{6}-10^{7}K$) ISM and the upper range of this warm ISM metallicity is consistent with stellar population metallicity determinations. A solar metallicity of the warm ISM favors an internal origin for the warm ISM such as AGB mass loss within the galaxy.

astro-ph.CO↗

arXiv2009

Elemental Abundances in the X-Ray Gas of Early-Type Galaxies with XMM and Chandra Observations

The source of hot gas in elliptical galaxies is thought to be due to stellar mass loss, with contributions from supernova events and possibly from infall from a surrounding environment. This picture predicts supersolar values for the metallicity of the gas toward the inner part of the galaxy, which can be tested by measuring the gas phase abundances. We use high-quality data for 10 nearby early-type galaxy from XMM-Newton, featuring both the EPIC and the Reflection Grating Spectrometer, where the strongest emission lines are detected with little blending; some Chandra data are also used. We find excellent consistency in the elemental abundances between the different XMM instruments and good consistency with Chandra. Differences in abundances with aperture size and model complexity are examined, but large differences rarely occur. For a two-temperature thermal model plus a point source contribution, the median Fe and O abundances are 0.86 and 0.44 of the Solar value, while Si and Mg abundances are similar to that for Fe. This is similar to stellar abundances for these galaxies but supernovae were expected to enhance the gas phase abundances considerably, which is not observed.

astro-ph.HE↗

arXiv2008

High-Velocity Clouds in the Nearby Spiral Galaxy M 83

We present deep HI 21-cm and optical observations of the face-on spiral galaxy M 83 obtained as part of a project to search for high-velocity clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M 83, with 5.6x10^7 Msolar of HI contained in a disk rotating 40-50 km/s more slowly in projection than the bulk of the gas. We interpret this as a vertically extended thick disk of neutral material, containing 5.5% of the total HI within the central 8 kpc. Using an automated source detection algorithm to search for small-scale HI emission features, we find eight distinct, anomalous-velocity HI clouds with masses ranging from 7x10^5 to 1.5x10^7 Msolar and velocities differing by up to 200 km/s compared to the HI disk. Large on-disk structures are coincident with the optical spiral arms, while unresolved off-disk clouds contain no diffuse optical emission down to a limit of 27 r' mag per square arcsec. The diversity of the thick HI disk and larger clouds suggests the influence of multiple formation mechanisms, with a galactic fountain responsible for the slowly-rotating disk and on-disk discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the HI clouds are consistent with the mass exchange rate predicted by the galactic fountain model. If the HVC population in M 83 is similar to that in our own Galaxy, then the Galactic HVCs must be distributed within a radius of less than 25 kpc.

astro-ph↗

arXiv2008

Mass Loss From Evolved Stars in Elliptical Galaxies

Most of the X-ray emitting gas in early-type galaxies probably originates from red giant mass loss and here we model the interaction between this stellar mass loss and the hot ambient medium. Using two-dimensional hydrodynamic simulations, we adopt a temperature for the ambient medium of 3E6 K along with a range of ambient densities and stellar velocities. When the stellar velocity is supersonic relative to the ambient medium, a bow shock occurs, along with a shock driven into the stellar ejecta, which heats only a fraction of the gas. Behind the bow shock, a cool wake develops but the fast flow of the hot medium causes Kelvin-Helmholtz instabilities to grow and these fingers are shocked and heated (without radiative cooling). Along with the mixing of this wake material with the hot medium, most of the stellar ejecta is heated to approximately the temperature of the hot ambient medium within 2 pc of the star. With the addition of radiative cooling, some wake material remains cool (< 1E5 K), accounting for up to 25% of the stellar mass loss. Less cooled gas survives when the ambient density is lower or when the stellar velocity is higher than in our reference case. These results suggest that some cooled gas should be present in the inner part of early-type galaxies that have a hot ambient medium. These calculations may explain the observed distributed optical emission line gas as well as the presence of dust in early-type galaxies.

astro-ph↗

arXiv2008

On the lack of strong O-line excess in the Coma cluster outskirts from Suzaku

About half of the baryons in the local Universe are thought to reside in the so-called warm-hot intergalactic medium (WHIM) at temperatures of 0.1-10 million K. Thermal soft excess emission in the spectrum of some cluster outskirts that contains OVII and/or OVIII emission lines is regarded as evidence of the WHIM, although the origin of the lines is controversial due to strong Galactic and solar system foreground emission. We observed the Coma-11 field, where the most prominent thermal soft excess has ever been reported, with Suzaku XIS in order to make clear the origin of the excess. We did not confirm OVII or OVIII excess emission. The OVII and OVIII intensity in Coma-11 is more than 5 sigma below that reported before and we obtained 2 sigma upper limits of 2.8 and 2.9 photons cm^-2 s^-1 sr^-1 for OVII and OVIII, respectively. The intensities are consistent with those in another field (Coma-7) that we measured, and with other measurements in the Coma outskirts (Coma-7 and X Com fields with XMM-Newton). We did not confirm the spatial variation within Coma outskirts. The strong oxygen emission lines previously reported are likely due to solar wind charge exchange.

astro-ph↗

arXiv2008

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

Quillen et al.(2007) presented an imaging survey with the {\it 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 infrared 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-infrared 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 time scale is about 1 Gyr. The physical extent of the infrared excess is consistent with that of the optical emission line nebulae. This supports the hypothesis that the 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 (${\dot M}_X$) estimated from the X-ray emission and the star formation rate estimated from the infrared luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates suggesting that the re-heating of the ICM is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

astro-ph↗

arXiv2007

An infrared survey of brightest cluster galaxies: Paper I

We report on an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission. These galaxies are located in the cores of X-ray luminous clusters selected from the ROSAT All-Sky Survey. We find that about half of these sources have a sign of excess infrared emission; 22 objects out of 62 are detected at 70 microns, 18 have 8 to 5.8 micron flux ratios above 1.0 and 28 have 24 to 8 micron flux ratios above 1.0. Altogether 35 of 62 objects in our survey exhibit at least one of these signs of infrared excess. Four galaxies with infrared excesses have a 4.5/3.6 micron flux ratio indicating the presence of hot dust, and/or an unresolved nucleus at 8 microns. Three of these have high measured [OIII](5007A)/Hbeta flux ratios suggesting that these four, Abell 1068, Abell 2146, and Zwicky 2089, and R0821+07, host dusty active galactic nuclei (AGNs). 9 objects (including the four hosting dusty AGNs) have infrared luminosities greater than 10^11 L_sol and so can be classified as luminous infrared galaxies (LIRGs). Excluding the four systems hosting dusty AGNs, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation.

astro-ph↗

arXiv2007

Different Methods of Forming Cold Fronts in Non-Merging Clusters

Sharp edges in X-ray surface brightness with continuous gas pressure called cold fronts have been often found in relaxed galaxy clusters such as Abell 496. Models that explain cold fronts as surviving cores of head-on subcluster mergers do not work well for these clusters and competing models involving gas sloshing have been recently proposed. Here, we test some concrete predictions of these models in a combined analysis of density, temperature, metal abundances and abundance ratios in a deep Chandra exposure of Abell 496. We confirm that the chemical discontinuities found in this cluster are not consistent with a core merger remnant scenario. However, we find chemical gradients across a spiral "arm" discovered at 73 kpc north of the cluster center and coincident with the sharp edge of the main cold front in the cluster. Despite the overall SN Ia iron mass fraction dominance found within the cooling radius of this cluster, the metal enrichment along the arm, determined from silicon and iron abundances, is consistent with a lower SN Ia iron mass fraction (51% +- 14%) than that measured in the surrounding regions (85% +- 14%). The "arm" is also significantly colder than the surroundings by 0.5-1.6 keV. The arm extends from a boxy colder region surrounding the center of the cluster, where two other cold fronts are found. This cold arm is a prediction of current high resolution numerical simulations as a result of an off-center encounter with a less massive pure dark matter halo and we suggest that the cold fronts in A496 provide the first clear corroboration of such model, where the closest encounter happened ~ 0.5 Gyr ago. We also argue for a possible candidate dark matter halo responsible for the cold fronts in the outskirts of A496.

astro-ph↗

arXiv2007

X-Ray Absorption from the Milky Way Halo and the Local Group

Million degree gas is present at near-zero redshift and is due either to a gaseous Galactic Halo or a more diffuse but very massive Local Group medium. We can discriminate between these models because the column densities should depend on location in the sky, either relative to the Galaxy bulge or to the M31-Milky Way axis. To search for these signatures, we measured the OVII Kalpha absorption line strength toward 25 bright AGNs, plus LMC X-3, using XMM-Newton RGS archival data. The data are in conflict with a purely Local Group model, but support the Galactic Halo model. The strongest correlation is between the OVII equivalent widths and the ROSAT background emission measurement in the R45 band (0.4-1 keV), for which OVII emission makes the largest single contribution. This suggests that much of the OVII emission and absorption are cospatial, from which the radius of a uniform halo appears to lie the range 15-110 kpc. The present data do not constrain the type of halo gas model and an equally good fit is obtained in a model where the gas density decreases as a power-law, such as r^(-3/2). For a uniform halo with a radius of 20 kpc, the electron density would be 9E-4 cm^(-3), and the gas mass is 4E8 Msolar. The redshift of the four highest S/N OVII measurements is consistent with a Milky Way origin rather than a Local Group origin.

astro-ph↗

arXiv2007

The Search for the Missing Baryons at Low Redshift

At low redshift, only about one-tenth of the known baryons lie in galaxies or the hot gas seen in galaxy clusters and groups. Models posit that these "missing baryons" are in gaseous form in overdense filaments that connect the much denser virialized groups and clusters. About 30% are cool (<1E5 K) and are detected in Ly alpha absorption studies, but about half is predicted to lie in the 1E5-1E7 K regime. Gas is detected in the 2-5E5 K range through OVI absorption studies (7% of the baryons) and possibly near 1E5 K from broad Ly absorption (20% of the baryons). Hotter gas (0.5-2E6 K) is detected at zero redshift by OVII and OVIII K X-ray absorption, and the OVII line strengths seem to correlate with the Galactic soft X-ray background, so it is probably produced by Galactic Halo gas, rather than a Local Group medium. There are no compelling detections of the intergalactic hot gas (0.5-10E6 K) either in absorption or emission and these upper limits are consistent with theoretical models. Claimed X-ray absorption lines are not confirmed, while most of the claims of soft emission are attributable to artifacts of background subtraction and field-flattening. The missing baryons should become detectable with moderate improvements in instrumental sensitivity.

astro-ph↗

arXiv2007

The Merger in Abell 576: A Line of Sight Bullet Cluster?

Using a combination of Chandra and XMM observations, we confirmed the presence of a significant velocity gradient along the NE/E-W/SW direction in the intracluster gas of the cluster Abell 576. The results are consistent with a previous ASCA SIS analysis of this cluster. The error weighted average over ACIS-S3, EPIC MOS 1 & 2 spectrometers for the maximum velocity difference is >3.3E03 km/s at the 90% confidence level, similar to the velocity limits estimated indirectly for the "bullet" cluster (1E0657-56). The probability that the velocity gradient is generated by standard random gain fluctuations with Chandra and XMM is <0.1%. The regions of maximum velocity gradient are in CCD zones that have the lowest temporal gain variations. It is unlikely that the velocity gradient is due to Hubble distance differences between projected clusters (probability<~0.01%). We mapped the distribution of elemental abundance ratios across the cluster and detected a strong chemical discontinuity using the abundance ratio of silicon to iron, equivalent to a variation from 100% SN Ia iron mass fraction in the West-Northwest regions to 32% in the Eastern region. The "center" of the cluster is located at the chemical discontinuity boundary, which is inconsistent with the radially symmetric chemical gradient found in some regular clusters, but consistent with a cluster merging scenario. We predict that the velocity gradient as measured will produce a variation of the CMB temperature towards the East of the core of the cluster that will be detectable by current and near-future bolometers. The measured velocity gradient opens for the possibility that this cluster is passing through a near line-of-sight merger stage where the cores have recently crossed.

astro-ph↗

arXiv2007

The Search for Million Degree Gas Through The NVII Hyperfine Line

Gas in the million degree range occurs in a variety of astronomical environments, and it may be the main component of the elusive missing baryons at low redshift. The NVII ion is found in this material and it has a hyperfine spin-flip transition with a rest frequency of 53.042 GHz, which can be observed for z > 0.1, when it is shifted into a suitably transparent radio band. We used the 42-48 GHz spectrometer on the Green Bank Telescope to search for both emission and absorption from this NVII transmission. For absorption studies, 3C273, 3C 279, 3C 345, and 4C+39.25 were observed but no feature were seen above the 5 sigma level. For emission line studies, we observed Abell 1835, Abell 2390 and the star-forming galaxy PKS 1345+12, but no features were seen exceeding 5 sigma. We examine whether the strongest emission feature, in Abell 2390 (3.7 sigma), and the strongest absorption feature, toward 4C+39.25 (3.8 sigma), might be expected from theoretical models. The emission feature would require ~1E10 Msolar of 1E6 K gas, which is inconsistent with X-ray limits for the O VII Kalpha line, so it is unlikely to be real. The NVII absorption feature requires a NVII column of 6E16 cm^-2, higher than model predictions by at least an order of magnitude, which makes it inconsistent with model expectations. The individual observations were less than 1 hr in length, so for lengthy observations, we show that NVII absorption line observations can begin to be useful in in the search for hot intergalactic gas.

astro-ph↗

arXiv2006

The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions

The mean ages of early-type galaxies obtained from the analysis of optical spectra, give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr. Independent age determinations are possible by using infrared spectra (5-21 microns), which we have obtained with the Infrared Spectrograph on the Spitzer Observatory. This age indicator is based on the collective mass loss rate of stars, where mass loss from AGB stars produces a silicate emission feature at 9-12 microns. This feature decreases more rapidly than the shorter wavelength continuum as a stellar population ages, providing an age indicator. From observations of 30 nearby early-type galaxies, 29 show a spectral energy distribution dominated by stars and one has significant emission from the ISM and is excluded. The infrared age indicators for the 29 galaxies show them all to be old, with a mean age of about 10 Gyr and a standard deviation of only a few Gyr. This is consistent with the ages inferred from the values of M/L_B, but is inconsistent with the ages derived from the optical line indices, which can be much younger. All of these age indicators are luminosity-weighted and should be correlated, even if multiple-age components are considered. The inconsistency indicates that there is a significant problem with either the infrared and the M/L_B ages, which agree, or with the ages inferred from the optical absorption lines.

astro-ph↗

arXiv2006

The Size of the Cooling Region of Hot Gas in Two Elliptical Galaxies

Some early-type galaxies show OVI emission, a tracer of gas at 10^5.5 K, and a predicted product of gas cooling from the X-ray emitting temperatures. We studied the spatial extent and velocity structure of this cooling gas by obtaining spectra of the OVI doublet in NGC 4636 and NGC 5846 with the Far Ultraviolet Spectroscopic Explorer. For NGC 4636, the central LWRS pointing shows that the OVI lines are double-peaked and symmetrical about the systemic velocity of the galaxy, with a separation of 210 km/s. A LWRS observation 30" from the center failed to show additional OVI emission. For NGC 5846, three spectra were obtained with 4" x 20" apertures (MDRS) at the center and 4" to the east and west of the center. The OVI line flux seen in the previous LWRS is contained in the sum of the smaller apertures, with most of the flux in a single non-central MDRS aperture, suggesting a size for the emission < 0.5 kpc; the emission consists of a blue and red peak. For both galaxies, the OVI velocity structure is similar to that of the optical [NII] emission and is consistent with rotation. The compactness and velocity structure of the OVI emission rules out cooling flow models with broadly distributed mass drop-out but is consistent with cooling flow models where the cooling occurs primarily in the central region. The 1E4 K gas may be the end state of the OVI gas.

astro-ph↗

arXiv2006

OVI Observations of Galaxy Clusters: Evidence for Modest Cooling Flows

A prediction of the galaxy cluster cooling flow model is that as gas cools from the ambient cluster temperature, emission lines are produced in gas at subsequently decreasing temperatures. Gas passing through 10^5.5 K emits in the lines of OVI 1032,1035, and here we report a FUSE study of these lines in three cooling flow clusters, Abell 426, Abell 1795, and AWM 7. No emission was detected from AWM 7, but OVI is detected from the centers of Abell 426 and Abell 1795, and possibly to the south of the center in Abell 1795, where X-ray and optical emission line filaments lie. In Abell 426, these line luminosities imply a cooling rate of 32+/-6 Msolar/yr within the central r = 6.2 kpc region, while for Abell 1795, the central cooling rate is 26+/-7 Msolar/yr (within r = 22 kpc), and about 42+/-9 Msolar/yr including the southern pointing. Including other studies, three of six clusters have OVI emission, and they also have star formation as well as emission lines from 1E4 K gas. These observations are generally consistent with the cooling flow model but at a rate closer to 30 Msolar/yr than originally suggested values of 100-1000 Msolar/yr.

astro-ph↗

arXiv2005

Direct Measurements of Gas Bulk Flows in the ICM of the Centaurus Cluster with the Chandra Satellite

We present the analysis of the velocity structure of the intracluster gas near the core of Abell 3526 obtained with two off-center Chandra observations, specifically designed to eliminate errors due to spatial variations of the instrumental gain. We detected a significant velocity gradient along the direction NE-SW direction, roughly perpendicular to the direction of the incoming sub-group Cen45, in agreement with previous ASCA SIS measurements. The presence of gas bulk velocities is observed both with and without the inclusion of the FeK line complex in the spectral fittings. The configuration and magnitude of the velocity gradient is consistent with near transonic circulatory motion, either bulk or eddy-like. The velocity difference obtained using the best calibrated central regions of ACIS-S3 is found to be (2.4+-1.0)E03 km/s for rectangular regions 2'.4 x 3' roughly diametrically opposed around the cluster's core. There are also indications of a high velocity zone towards the Southern region with similar magnitudes. The detection of velocity gradients is significant at the >99.4% confidence and simulations show that intrachip gain fluctuations > 1800 km/s are required to explain the velocity gradient by chance. The measurements suggest that >1% of the total merger energy can still be bulk kinetic 0.4 Gyr after the merging event. This is the first direct confirmation of velocity gradients in the intracluster gas with independent instruments and indicates that strong departure from hydrostatic equilibrium is possible even for cool clusters that do not show obvious signs of merging.

astro-ph↗

arXiv2005

Galactic Globular Clusters with Luminous X-Ray Binaries

Luminous X-ray binaries (>1E34 erg/s, LMXBs) have a neutron star or black hole primary, and in globular clusters, most of these close binaries are expected to be have evolved from wider binaries through dynamical interactions with other stars. We sought to find a predictor of this formation rate that is representative of the initial properties of globular clusters rather than of the highly evolved core quantities. Models indicate the half-light quantities best reflect the initial conditions, so we examine whether the associated dynamical interaction rate, proportional to L^1.5 r^-2.5, is useful in understanding the presence of luminous LMXBs in the Galactic globular cluster system. We find that while LMXB clusters with large values of L^1.5 r^-2.5 preferentially host LMXBs, the systems must also have half-mass relaxation times below about 1E9 yr. This relaxation time effect probably occurs because several relaxation times are required to modify binary separations, a timescale that must be shorter than cluster ages. The frequency of finding an LMXB cluster is enhanced if the cluster is metal-rich and if it is close to the bulge region. The dependence upon metallicity is most likely due either to differing initial mass functions at the high mass end, or because bulge systems evolve more rapidly from tidal interactions with the bulge. This approach can be used to investigate globular cluster systems in external galaxies, where core properties are unresolved.

astro-ph↗