arXiv subjects

John S. Mulchaey

Publications and source records attributed to John S. Mulchaey.

At least 73 records · Page 4Linked to original sources

arXiv2010

Hot Gas Halos in Early-Type Field Galaxies

We use Chandra and XMM-Newton to study the hot gas content in a sample of field early-type galaxies. We find that the L_X-L_K relationship is steeper for field galaxies than for comparable galaxies in groups and clusters. The low hot gas content of field galaxies with L_K < L_star suggests that internal processes such as supernovae driven winds or AGN feedback expel hot gas from low mass galaxies. Such mechanisms may be less effective in groups and clusters where the presence of an intragroup or intracluster medium can confine outflowing material. In addition, galaxies in groups and clusters may be able to accrete gas from the ambient medium. While there is a population of L_K < L_star galaxies in groups and clusters that retain hot gas halos, some galaxies in these rich environments, including brighter galaxies, are largely devoid of hot gas. In these cases, the hot gas halos have likely been removed via ram pressure stripping. This suggests a very complex interplay between the intragroup/intracluster medium and hot gas halos of galaxies in rich environments with the ambient medium helping to confine or even enhance the halos in some cases and acting to remove gas in others. In contrast, the hot gas content of more isolated galaxies is largely a function of the mass of the galaxy, with more massive galaxies able to maintain their halos, while in lower mass systems the hot gas escapes in outflowing winds.

astro-ph.CO↗

arXiv2010

The First Mid-IR View of the Star-forming Properties of Nearby Galaxy Groups

We present the first mid-IR study of galaxy groups in the nearby Universe based on Spitzer MIPS observations of a sample of nine redshift-selected groups from the XMM-IMACS (XI) project, at z=0.06. We find that on average the star-forming (SF) galaxy fraction in the groups is about 30% lower than the value in the field and 30% higher than in clusters. The SF fractions do not show any systematic dependence on group velocity dispersion, total stellar mass, or the presence of an X-ray emitting intragroup medium, but a weak anti-correlation is seen between SF fraction and projected galaxy density. However, even in the densest regions, the SF fraction in groups is still higher than that in cluster outskirts, suggesting that preprocessing of galaxies in group environments is not sufficient to explain the much lower SF fraction in clusters. The typical specific star formation rates (SFR/M*) of SF galaxies in groups are similar to those in the field across a wide range of stellar mass (M*>10^9.6 msun), favoring a quickly acting mechanism that suppresses star formation to explain the overall smaller fraction of SF galaxies in groups. If galaxy-galaxy interactions are responsible, then the extremely low starburst galaxy fraction (<1%) implies a short timescale (~0.1 Gyr) for any merger-induced starburst stage. Comparison to two rich clusters shows that clusters contain a population of massive SF galaxies with very low SFR (14% of all the galaxies with M*>10^10 Msun), possibly as a consequence of ram pressure stripping being less efficient in removing gas from more massive galaxies.

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arXiv2010

A Systematic Search for X-ray Cavities in the Hot Gas of Galaxy Groups

We have performed a systematic search for X-ray cavities in the hot gas of 51 galaxy groups with Chandra archival data. The cavities are identified based on two methods: subtracting an elliptical beta model fitted to the X-ray surface brightness, and performing unsharp masking. 13 groups in the sample 25% are identified as clearly containing cavities, with another 13 systems showing tentative evidence for such structures. We find tight correlations between the radial and tangential radii of the cavities, and between their size and projected distance from the group center, in quantitative agreement with the case for more massive clusters. This suggests that similar physical processes are responsible for cavity evolution and disruption in systems covering a large range in total mass. We see no clear association between the detection of cavities and the current 1.4 GHz radio luminosity of the central brightest group galaxy, but there is a clear tendency for systems with a cool core to be more likely to harbor detectable cavities. To test the efficiency of the adopted cavity detection procedures, we employ a set of mock images designed to mimic typical Chandra data of our sample, and find that the model-fitting approach is generally more reliable than unsharp masking for recovering cavity properties. Finally, we find that the detectability of cavities is strongly influenced by a few factors, particularly the signal-to-noise ratio of the data, and that the real fraction of X-ray groups with prominent cavities could be substantially larger than the 25--50% suggested by our analysis.

astro-ph.CO↗

arXiv2010

SDSS J1254+0846: A Binary Quasar Caught in the Act of Merging

We present the first luminous, spatially resolved binary quasar that clearly inhabits an ongoing galaxy merger. SDSS J125455.09+084653.9 and SDSS J125454.87+084652.1 (SDSS J1254+0846 hereafter) are two luminous z=0.44 radio quiet quasars, with a radial velocity difference of just 215 km/s, separated on the sky by 21 kpc in a disturbed host galaxy merger showing obvious tidal tails. The pair was targeted as part of a complete sample of binary quasar candidates with small transverse separations drawn from SDSS DR6 photometry. We present follow-up optical imaging which shows broad, symmetrical tidal arm features spanning some 75 kpc at the quasars' redshift. Numerical modeling suggests that the system consists of two massive disk galaxies prograde to their mutual orbit, caught during the first passage of an active merger. This demonstrates rapid black hole growth during the early stages of a merger between galaxies with pre-existing bulges. Neither of the two luminous nuclei show significant instrinsic absorption by gas or dust in our optical or X-ray observations, illustrating that not all merging quasars will be in an obscured, ultraluminous phase. We find that the Eddington ratio for the fainter component B is rather normal, while for the A component L/LEdd is quite (>3sigma) high compared to quasars of similar luminosity and redshift, possibly evidence for strong merger-triggered accretion. More such mergers should be identifiable at higher redshifts using binary quasars as tracers.

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arXiv2009

Active Galactic Nuclei in Groups and Clusters of Galaxies: Detection and Host Morphology

The incidence and properties of Active Galactic Nuclei (AGN) in the field, groups, and clusters can provide new information about how these objects are triggered and fueled, similar to how these environments have been employed to study galaxy evolution. We have obtained new XMM-Newton observations of seven X-ray selected groups and poor clusters with 0.02 < z < 0.06 for comparison with previous samples that mostly included rich clusters and optically-selected groups. Our final sample has ten groups and six clusters in this low-redshift range (split at a velocity dispersion of $σ= 500$ km/s). We find that the X-ray selected AGN fraction increases from $f_A(L_X>10^{41}; M_R 2.5}(L_X>10^{41}; M_R<M_R^*+1) = 0.048^{+0.028}_{-0.019}$ compared to $0.119^{+0.064}_{-0.044}$ for the groups (92% significance), a result consistent with the hypothesis that the change in AGN fraction is directly connected to environment.

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arXiv2009

Probing the IGM-Galaxy Connection at z < 0.5 I: A Galaxy Survey in QSO Fields and a Galaxy-Absorber Cross-Correlation Study

(Abridged) We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE0226-4110, PKS0405-123, and PG1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Lya absorbers and 13 OVI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude M_R-5log h <~ -16 and at projected physical distances rho <~ 4 h^{-1} Mpc from the QSOs. HST/WFPC2 images of the fields around PKS0405-123 and PG1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lya and OVI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are (1) both strong Lya absorbers of log N(HI) >= 14 and OVI absorbers exhibit a comparable clustering amplitude as emission-line dominated galaxies and a factor of ~ 6 weaker amplitude than absorption-line dominated galaxies on co-moving projected distance scales of r_p < 3 h^{-1} Mpc; (2) weak Lya absorbers of log N(HI)<13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line dominated galaxies at r_p <= 250 h^{-1} kpc has a corresponding OVI absorber to a sensitive upper limit of W(1031) < 0.03 A, while the covering fraction of OVI absorbing gas around emission-line dominated galaxies is found to be kappa ~ 64%; and (4) high-resolution images of five OVI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge.

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arXiv2009

The Evolution of Active Galactic Nuclei in Clusters of Galaxies to Redshift 1.3

We have measured the luminous AGN population in a large sample of clusters of galaxies and find evidence for a substantial increase in the cluster AGN population from z~0.05 to z~1.3. The present sample now includes 32 clusters of galaxies, including 15 clusters above z=0.4, which corresponds to a three-fold increase compared to our previous work at high redshift. At z<0.4 we have obtained new observations of AGN candidates in six additional clusters and found no new luminous AGN in cluster members. Our total sample of 17 low-redshift clusters contains only two luminous AGN, while at high redshifts there are 18 such AGN, or an average of more than one per cluster. We have characterized the evolution of luminous X-ray AGN as the fraction of galaxies with M_R < M_R^*(z)+1 that host AGN with rest-frame, hard X-ray [2--10 keV] luminosities L_X,H >= 10^43 \ergs. The AGN fraction increases from f_A = 0.134^{+0.18}_{-0.087} percent at a median z=0.19 to f_A = 1.00^{+0.29}_{-0.23} percent at a median z=0.72. Our best estimate of the evolution is a factor of eight increase to z=1 and the statistical significance of the increase is 3.8sigma. This dramatic evolution is qualitatively similar to the evolution of the star-forming galaxy population in clusters known as the Butcher-Oemler effect. We discuss the implications of this result for the coevolution of black holes and galaxies in clusters, the evolution of AGN feedback, searches for clusters with the Sunyaev-Zel'dovich effect, and the possible detection of environment-dependent downsizing.

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arXiv2009

The Galactic Environment of the NeVIII Absorber toward HE0226-4110

We report the discovery of a small galaxy system in the vicinity of the NeVIII absorber at z=0.20701 toward HE0226-4110. The galaxy system consists of two 0.25 L* disk galaxies and a 0.05 L* galaxy all within Δv < 300 km/s and rho< 200 h^{-1} physical kpc of the absorber. We consider various scenarios for the origin of the NeVIII absorption, including photo-ionized gas from a AGN, a starburst driven wind, a hot intragroup medium, hot gas in a galaxy halo, and a conductive front produced by cool clouds moving at high speed through a hot medium. We argue that the conductive front scenario is most likely responsible for producing the NeVIII feature, because it is consistent with the observed galactic environment around the absorber and because it naturally explains the multi-phase nature of the gas and the kinematic signatures of the absorption profiles. Although our preferred scenario suggests that NeVIII may not be directly probing the warm-hot intergalactic medium, it does imply the existence of an extended hot confining medium around a disk galaxy that may contain a significant reservoir of baryons in the form of hot gas.

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arXiv2008

Wide-Field Chandra X-Ray Observations of AGN in Abell 85 & Abell 754

To better understand the mechanism or mechanisms that lead to AGN activity today, we measure the X-ray AGN fraction in a new sample of nearby clusters and examine how it varies with galaxy properties, projected cluster-centric radius, and cluster velocity dispersion. We present new wide-field Chandra X-ray Observatory observations of Abell 85, Abell 754 and the background cluster Abell 89B out to their virial radii. Out of seventeen X-ray sources associated with galaxies in these clusters, we classify seven as X-ray AGN with L_{X,B} > 10^{41} erg/s. Only two of these would be classified as AGN based on their optical spectra. We combine these observations with archival data to create a sample of X-ray AGN from six z < 0.08 clusters and find that 3.4+1.1/-0.8% of M_R < -20 galaxies host X-ray AGN with L_{X,B} > 10^{41} erg/s. We find that more X-ray AGN are detected in more luminous galaxies and attribute this to larger spheriods in more luminous galaxies and increased sensitivity to lower Eddington-rate accretion from black holes in those spheroids. At a given X-ray luminosity limit, more massive black holes can be accreting less efficiently, yet still be detected. If interactions between galaxies are the principal drivers of AGN activity, then the AGN fraction should be higher in lower velocity dispersion clusters and the outskirts of clusters. However, the tendency of the most massive and early-type galaxies to lie in the centers of the richest clusters could dilute such trends. While we find no variation in the AGN fraction with projected cluster-centric radius, we do find that the AGN fraction increases significantly from 2.6+1.0/-0.8% in rich clusters to 10.0+6.2/-4.3% in those with lower velocity dispersions.

astro-ph↗

arXiv2008

Evolution in the Disks and Bulges of Group Galaxies since z=0.4

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness--fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Redshift survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disk--dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z=0.4 groups have ~ 5.5 +/- 2 % fewer disk--dominated galaxies than the field, while by z=0.1 this difference has increased to ~ 19 +/- 6 %. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disk population. At both redshifts, the disks of group galaxies have similar scaling relations and show similar median asymmetries as the disks of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge--dominated galaxies is 6 +/- 3 % higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z=0.4 and z=0 using the semi-analytic galaxy catalogues of Bower et al (2006). This model accurately reproduces the B/T distributions of the group and field at z=0.1. However, the model does not reproduce our finding that the deficit of disks in groups has increased significantly since z=0.4.

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arXiv2008

The Hot Gas Halos of Galaxies in Groups

We use Chandra observations of 13 nearby groups of galaxies to investigate the hot gas content of their member galaxies. We find that a large fraction of near-IR bright, early-type galaxies in groups have extended X-ray emission, indicating that they retain significant hot gas halos even in these dense environments. In particular, we detect hot gas halos in ~80% of L_K > L_star galaxies. We do not find a significant difference in the L_K-L_X relation for detected group and cluster early-type galaxies. However, we detect X-ray emission from a significantly higher fraction of galaxies brighter than L_star in groups compared to clusters, indicating that a larger fraction of galaxies in clusters experience significant stripping of their hot gas. In addition, group and cluster galaxies appear to be X-ray faint compared to field galaxies, though a Chandra based field sample is needed to confirm this result. The near-IR bright late-types galaxies in clusters and groups appear to follow the L_K-L_X relation for early-type galaxies, while near-IR fainter late-type galaxies are significantly more X-ray luminous than this relation likely due to star formation. Finally, we find individual examples of ongoing gas stripping of group galaxies. One galaxy shows a 40-50 kpc X-ray tail, and two merging galaxy systems show tidal bridges/tails of X-ray emission. Therefore, stripping of hot galactic gas through both ram pressure and tidal forces does occur in groups and clusters, but the frequency or efficiency of such events must be moderate enough to allow hot gas halos in a large fraction of bright galaxies to survive even in group and cluster cores.

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arXiv2007

Characterizing the Low-Redshift Intergalactic Medium towards PKS1302-102

We present a detailed analysis of the intergalactic metal-line absorption systems in the archival HST/STIS and FUSE ultraviolet spectra of the low-redshift quasar PKS1302-102 (z_QSO = 0.2784). We supplement the archive data with CLOUDY ionization models and a survey of galaxies in the quasar field. There are 15 strong Lya absorbers with column densities logN_HI > 14. Of these, six are associated with at least CIII 977 absorption (logN(C^++) > 13); this implies a redshift density dN_CIII/dz = 36+13/-9 (68% confidence limits) for the five detections with rest equivalent width W_r > 50 mA. Two systems show OVI 1031,1037 absorption in addition to CIII (logN(O^+5) > 14). One is a partial Lyman limit system (logN_HI = 17) with associated CIII, OVI, and SiIII 1206 absorption. There are three tentative OVI systems that do not have CIII detected. For one OVI doublet with both lines detected at 3 sigma with W_r > 50 mA, dN_OVI/dz = 7+9/-4. We also search for OVI doublets without Lya absorption but identify none. From CLOUDY modeling, these metal-line systems have metallicities spanning the range -4 < [M/H] < -0.3. The two OVI systems with associated CIII absorption cannot be single-phase, collisionally-ionized media based on the relative abundances of the metals and kinematic arguments. From the galaxy survey, we discover that the absorption systems are in a diverse set of galactic environments. Each metal-line system has at least one galaxy within 500 km/s and 600 h^-1 kpc with L > 0.1 L_*.

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arXiv2007

Strangulation in Galaxy Groups

We use a cosmological chemodynamical simulation to study how the group environment impacts the star formation properties of disk galaxies. The simulated group has a total mass of M~8x10^12 Msun and a total X-ray luminosity of L_X~10^41 erg s^-1. Our simulation suggests that ram pressure is not sufficient in this group to remove the cold disk gas from a V_rot~150 km s^-1 galaxy. However, the majority of the hot gas in the galaxy is stripped over a timescale of approximately 1 Gyr. Since the cooling of the hot gas component provides a source for new cold gas, the stripping of the hot component effectively cuts off the supply of cold gas. This in turn leads to a quenching of star formation. The galaxy maintains the disk component after the cold gas is consumed, which may lead to a galaxy similar to an S0. Our self-consistent simulation suggests that this strangulation mechanism works even in low mass groups, providing an explanation for the lower star formation rates in group galaxies relative to galaxies in the field.

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arXiv2007

The Distribution of AGN in Clusters of Galaxies

We present a study of the distribution of AGN in clusters of galaxies with a uniformly selected, spectroscopically complete sample of 35 AGN in eight clusters at z = 0.06 to 0.31. We find that the 12 AGN with L_X > 10^42 erg/s in galaxies more luminous than a rest-frame M_R < -20 mag are more centrally concentrated than typical cluster galaxies of this luminosity, although these AGN have comparable velocity and substructure distributions to other cluster members. In contrast, a larger sample of 30 cluster AGN with L_X > 10^41 erg/s do not show evidence for greater concentration than inactive members, nor evidence for a different kinematic or substructure distribution. As we do see clear differences in the spatial and kinematic distributions of the blue Butcher-Oemler and red galaxy populations, any difference in the AGN and inactive galaxy population must be less distinct than that between these two pairs of populations. Comparison of the AGN fraction selected via X-ray emission in this study to similarly-selected AGN in the field indicates that the AGN fraction is not significantly lower in clusters, contrary to AGN identified via visible-wavelength emission lines, but similar to the approximately constant radio-selected AGN fraction in clusters and the field. We also find significant evidence for variation between clusters and explore the dependence of cluster AGN fraction on redshift, velocity dispersion, cluster substructure, and Butcher-Oemler fraction. While we see weak evidence for several trends, correlations between these four parameters in our small sample precludes identification of which one(s) most strongly influence the cluster AGN fraction.

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arXiv2007

First Measurement of a Rapid Increase in the AGN Fraction in High-Redshift Clusters of Galaxies

We present the first measurement of the AGN fraction in high-redshift clusters of galaxies (z~0.6) with spectroscopy of one cluster and archival data for three additional clusters. We identify 8 AGN in all four of these clusters from the Chandra data, which are sensitive to AGN with hard X-ray (2-10keV) luminosity L_{X,H} > 10^43 erg/s in host galaxies more luminous than a rest frame M_R < -20 mag. This stands in sharp contrast to the one AGN with L_{X,H} > 10^43 erg/s we discovered in our earlier study of eight low-redshift clusters with z=0.06-0.31 (average z~0.2). Three of the four high-redshift cluster datasets are sensitive to nearly L_{X,H} > 10^42 erg/s and we identify seven AGN above this luminosity limit, compared to two in eight, low-redshift clusters. Based on membership estimates for each cluster, we determine that the AGN fraction at z~0.6 is f_A(L_X>10^42;M_R<-20) = 0.028 (+0.019/-0.012) and f_A(L_X>10^43;M_R<-20) = 0.020 (+0.012/-0.008). These values are approximately a factor of 20 greater than the AGN fractions in lower-redshift (average z~0.2) clusters of galaxies and represent a substantial increase over the factors of 1.5 and 3.3 increase, respectively, in the measured space density evolution of the hard X-ray luminosity function over this redshift range. Potential systematic errors would only increase the significance of our result. The cluster AGN fraction increases more rapidly with redshift than the field and the increase in cluster AGN indicates the presence of an AGN Butcher-Oemler Effect.

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arXiv2007

The Evolution of Galaxies in X-ray Luminous Groups

We investigate the galaxy populations in seven X-ray selected, intermediate-redshift groups (0.2<z<0.6). Overall, the galaxy populations in these systems are similar to those in clusters at the same redshift; they have large fractions of early-type galaxies (f_e~70%) and small fractions of galaxies with significant star formation (f_[OII]~30%). We do not observe a strong evolution in the galaxy populations from those seen in X-ray luminous groups at low-redshift. Both f_e and f_[OII] are correlated with radius but do not reach the field value out to ~r_500. However, we find significant variation in the galaxy populations between groups with some groups having field-like populations. Comparisons between the morphological and spectral properties of group galaxies reveal both gas-poor mergers and a population of passive spirals. Unlike low-redshift, X-ray emitting groups, in some of these groups the brightest galaxy does not lie at the center of the X-ray emission, and in several of the groups that do have a central BGG, the BGG has multiple components. These groups appear to represent a range of evolutionary stages in the formation of the BGG. Some groups have relatively large central galaxy densities, and one group contains a string of seven bright galaxies within a radius of 200 kpc that have a lower velocity dispersion than the rest of the system. None of the central galaxies, including those with multiple components, have significant [OII] emission. These observations support a scenario in which BGGs are formed relatively late through gas-poor mergers.

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arXiv2006

The Two-dimensional XMM-Newton Group Survey: z<0.012 groups

We present the results of the 2-dimensional XMM-Newton Group Survey (2dXGS), an archival study of nearby galaxy groups. In this paper we consider eleven nearby systems (z<0.012) in Mulchaey et al. (2003), which span a broad range in X-ray luminosity from 10^40 to 10^43 ergs/s. We measure the iron abundance and temperature distribution in these systems and derive pressure and entropy maps. We find statistically significant evidence for structure in the entropy and pressure of the gas component of seven groups on the 10-20% level. The XMM-Newton data for the three groups with best statistics also suggest patchy metalicity distributions within the central 20--50 kpc of the brightest group galaxy, probed with 2-10 kpc resolution. This provides insights into the processes associated with thermalization of the stellar mass loss. Analysis of the global properties of the groups reveals a subclass of X-ray faint groups, which are characterized by both higher entropy and lower pressure. We suggest that the merger history of the central elliptical is responsible for both the source and the observed thermodynamical properties of the hot gas of the X-ray faint groups.

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arXiv2006

Differences in the AGN Populations of Groups and Clusters: Clues to AGN Evolution

We combine optical and X-ray data for eight low redshift ($z\sim 0.06$) poor groups of galaxies from the {\it XI}({\it XMM/IMACS}) Groups Project to study the AGN population in the group environment. Among $\sim 140$ group members, we identify five AGN based on their optical emission lines. None of these optically-selected AGN are detected by {\it XMM-Newton}. One additional AGN is discovered in the {\it XMM-Newton} observations. This X-ray detected AGN, which has no obvious AGN emission line signatures in its optical spectrum, is a member of the only X-ray luminous group in our sample. The lack of a significant population of X-ray bright, but optically dull AGN among less dynamically evolved groups is in stark contrast to the large fraction of such objects in rich clusters of galaxies (Martini et al. 2006). We suggest this result can be explained by a physical scenario for AGN accretion evolution: AGN activity is initially triggered by galaxy merging, leading to a high accretion rate and an optically dominant phase (via thin disk accretion). As the accretion rate drops in time, the AGN gradually enters an X-ray dominant low-accretion phase (via a radiative inefficient accretion flow). In this picture, optical- and X-ray-selected AGN are the same population of supermassive black holes observed at different epochs. Within the context of this scenario, the majority of AGN in poor groups are in the high-accretion optically dominant phase, while the AGN population in rich clusters is mostly in the low-accretion X-ray dominant phase.

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