arXiv subjects

John S. Mulchaey

Publications and source records attributed to John S. Mulchaey.

At least 91 records · Page 5Linked to original sources

arXiv2006

The stellar mass content of distant galaxy groups

We have obtained near-infrared imaging of 58 galaxy groups, in the redshift range 0.1<z<0.6, from the William Herschel Telescope and from the Spitzer IRAC data archive. The groups are selected from the CNOC2 redshift survey, with additional spectroscopy from the Baade telescope (Magellan). Our group samples are statistically complete to K(Vega}=17.7 (INGRID) and [3.6](AB)=19.9 (IRAC). From these data we construct near-infrared luminosity functions, for groups in bins of velocity dispersion, up to 800 km/s, and redshift. The total amount of near-infrared luminosity per group is compared with the dynamical mass, estimated from the velocity dispersion, to compute the mass-to-light ratio, M/Lk. We find that the M/Lk values in these groups are in good agreement with those of their statistical descendants at z=0, with no evidence for evolution beyond that expected for a passively evolving population. There is a trend of M/Lk with group mass, which increases from M/Lk~10 for groups with sigma<250 km/s to M/Lk~100 for sigma=425-800 km/s. This trend is weaker, but still present, if we estimate the total mass from weak lensing measurements. In terms of stellar mass, stars make up ~2 per cent of the mass in the smallest groups, and ~1 per cent in the most massive groups. We also use the near-infrared data to consider the correlations between stellar populations and stellar masses, for group and field galaxies at 0.1<z<0.6. We find that fewer group galaxies show strong [OII] emission compared with field galaxies of the same stellar mass and at the same redshift. We conclude that most of the stellar mass in these groups was already in place by z~0.4, with little environment-driven evolution to the present day.

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arXiv2006

First results of the XI Groups Project: Studying an unbiased sample of galaxy groups

X-ray observations of hot, intergalactic gas in galaxy groups provide a useful means of characterizing the global properties of groups. However, X-ray studies of large group samples have typically involved very shallow X-ray exposures or have been based on rather heterogeneous samples. Here we present the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for the first time, a redshift-selected, statistically unbiased sample of galaxy groups using deep X-ray data. Combining this with radio observations of cold gas and optical imaging and spectroscopy of the galaxy population, the project aims to advance the understanding of how the properties and dynamics of group galaxies relate to global group properties. Here, X-ray and optical data of the first four galaxy groups observed as part of the project are presented. In two of the groups we detect diffuse emission with a luminosity of L_X ~ 10^41 erg/s, among the lowest found for any X-ray detected group thus far, with a comparable upper limit for the other two. Compared to typical X-ray selected groups of similar velocity dispersion, these four systems are all surprisingly X-ray faint. We discuss possible explanations for the lack of significant X-ray emission in the groups, concluding that these systems are most likely collapsing for the first time. Our results strongly suggest that, unlike our current optically selected sample, previous X-ray selected group samples represented a biased picture of the group population. This underlines the necessity of a study of this kind, if one is to reach an unbiased census of the properties of galaxy groups and the distribution of baryons in the Universe.

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arXiv2006

X-ray Selected Intermediate-Redshift Groups of Galaxies

We present spectroscopic confirmation of nine moderate redshift galaxy groups and poor clusters selected from the ROSAT Deep Cluster Survey. The groups span the redshift range z=0.23-0.59 and have between 4 and 20 confirmed members. The velocity dispersions of these groups range from 125 to 650 km/s. Similar to X-ray groups at low redshift, these systems contain a significant number of early-type galaxies. Therefore, the trend for X-ray luminous groups to have high early-type fractions is already in place by at least z=0.5. In 4 of the 9 groups, the X-ray emission is clearly peaked on the most luminous early-type galaxy in the group. However, in several cases the central galaxy is composed of multiple luminous nuclei, suggesting that the brightest group galaxy may still be undergoing major mergers. In at least 3 of the groups in our sample, a dominant early-type galaxy is not found at the center of the group potential. This suggests that many of our groups are not dynamically evolved despite their high X-ray luminosities. While similar systems have been identified at low redshift, the X-ray luminosities of the intermediate redshift examples are one to three orders of magnitude higher than those of their low redshift counterparts. We suggest that this may be evidence for group downsizing: while massive groups are still in the process of collapsing and virializing at intermediate redshifts, only low-mass groups are in the process of forming at the present day.

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arXiv2006

Are Red Tidal Features Unequivocal Signatures of Major Dry Mergers?

We use a cosmological numerical simulation to study the tidal features produced by a minor merger with an elliptical galaxy. We find that the simulated tidal features are quantitatively similar to the red tidal features, i.e., dry tidal features, recently found in deep images of elliptical galaxies at intermediate redshifts. The minor merger in our simulation does not trigger star formation due to active galactic nuclei heating. Therefore, both the tidal features and the host galaxy are red, i.e. a dry minor merger. The stellar mass of the infalling satellite galaxy is about 10^10 Msun, and the tidal debris reach the surface brightness of mu_R~27 mag arcsec^-2. Thus, we conclude that tidal debris from minor mergers can explain the observed dry tidal features in ellipticals at intermediate redshifts, although other mechanisms (such as major dry mergers) may also be important.

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arXiv2006

Gas stripping in galaxy groups - the case of the starburst spiral NGC 2276

Ram pressure stripping of galactic gas is generally assumed to be inefficient in galaxy groups due to the relatively low density of the intragroup medium and the small velocity dispersions of groups. To test this assumption, we obtained Chandra X-ray data of the starbursting spiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for a strong galaxy interaction with hot intragroup gas. The data reveal a shock-like feature along the western edge of the galaxy and a low-surface-brightness tail extending to the east, similar to the morphology seen in other wavebands. Spatially resolved spectroscopy shows that the data are consistent with intragroup gas being pressurized at the leading western edge of NGC 2276 due to the galaxy moving supersonically through the intragroup medium at a velocity ~850 km/s. Detailed modelling of the gravitational potential of NGC 2276 shows that the resulting ram-pressure could significantly affect the morphology of the outer gas disc but is probably insufficient to strip large amounts of cold gas from the disc. We estimate the mass loss rates due to turbulent viscous stripping and starburst outflows being swept back by ram pressure, showing that both mechanisms could plausibly explain the presence of the X-ray tail. Comparison to existing HI measurements shows that most of the gas escaping the galaxy is in a hot phase. With a total mass loss rate of roughly 5 M_Sun/yr, the galaxy could be losing its entire present HI supply within a Gyr. This demonstrates that the removal of galactic gas through interactions with a hot intragroup medium can occur rapidly enough to transform the morphology of galaxies in groups. Implications of this for galaxy evolution in groups and clusters are briefly discussed.

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arXiv2006

Spectroscopic Confirmation of a Large AGN Population in Clusters of Galaxies

We have completed a spectroscopic survey of X-ray point sources in eight low-redshift clusters of galaxies (0.05 10^{41}) ~ 5%. We stress that additional, lower-luminosity AGN are expected to be present in the M_R < -20 mag cluster members. Our data unambiguously demonstrate that cluster galaxies host AGN more frequently than previously expected. Only four of these galaxies have obvious visible-wavelength AGN signatures, even though their X-ray luminosities are too high for their X-ray emission to be due to populations of low-mass X-ray binaries or hot, gaseous halos. We attribute the significant difference in visible and X-ray AGN identification to dilution of low-luminosity AGN spectral signatures by host galaxy starlight and/or obscuration of accretion onto the central, supermassive black hole.

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arXiv2006

Probing the IGM/Galaxy Connection Toward PKS0405-123 III: The Galaxy Survey and Correlations with OVI Absorbers

We present a galaxy survey of the field surrounding PKS0405-123 performed with the WFCCD spectrometer at Las Campanas Observatory. The survey is comprised of two datasets: (1) a greater than 95% complete survey to R = 20 of the field centered on PKS0405-123 with 10' radius (L~0.1 L_* and radius of 1Mpc at z=0.1); and (2) a set of four discontiguous (i.e. non-overlapping), flanking fields covering ~1 square degree area with completeness ~90% to R=19.5mag. With these datasets, one can examine the local and large-scale galactic environment of the absorption systems identified toward PKS0405-123. In this paper, we focus on the OVI systems analyzed in Paper I. The results suggest that this gas arises in a diverse set of galactic environments including the halos of individual galaxies, galaxy groups, filamentary-like structures, and also regions devoid of luminous galaxies. In this small sample, there are no obvious trends between galactic environment and the physical properties of the gas. Furthermore, we find similar results for a set of absorption systems with comparable N(HI) but no detectable metal-lines. The observations indicate that metals are distributed throughout a wide range of environments in the local universe. Future papers in this series will address the distribution of galactic environments associated with metal-line systems and the Lya forest based on data for over 10 additional fields. All of the data presented in this paper is made public at a dedicated web site.

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arXiv2005

Spatially-Resolved Narrow Line Region Kinematics in Active Galactic Nuclei

We have analyzed HST spectroscopy of 24 nearby AGNs to investigate spatially-resolved gas kinematics in the Narrow Line Region (NLR). These observations effectively isolate the nuclear line profiles on less than 100 pc scales and are used to investigate the origin of the substantial scatter between the widths of strong NLR lines and the stellar velocity dispersion sigma_* of the host galaxy, a quantity which relates with substantially less scatter to the mass of the central, supermassive black hole, and more generally characterize variations in the NLR velocity field with radius. We find that line widths measured with STIS at a range of spatial scales systematically underestimate both sigma_* and the line width measured from ground-based observations, although they do have comparably large scatter to the relation between ground-based NLR line width and sigma_*. There are no obvious trends in the residuals when compared with a range of host galaxy and nuclear properties. The widths and asymmetries of [OIII] 5007 and [SII] 6716, 6731 as a function of radius exhibit a wide range of behavior. Some of the most common phenomena are substantial width increases from the STIS to the large-scale, ground-based aperture and almost no change in line profile between the unresolved nuclear spectrum and ground-based measurements. We identify asymmetries in a surprisingly large fraction of low-ionization [SII] line profiles and several examples of substantial red asymmetries in both [OIII] and [SII]. These results underscore the complexity of the circumnuclear material that constitutes the NLR and suggest that the scatter in the NLR width and sigma_* correlation can not be substantially reduced with a simple set of empirical relations.

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arXiv2005

Probing the IGM-Galaxy Connection Toward PKS0405-123 II : A Cross-Correlation Study of Lya Absorbers and Galaxies at z<0.5

We present a pilot study of the clustering properties of Lya absorbers with respect to known galaxies based on 112 Lya absorbers and 482 galaxies identified at z<0.5 along the sightline toward PKS0405-123. The principal goal is to determine the origin of Lya absorbers based on their cross-correlation amplitude with known galaxies and investigate a possible N(HI) dependence of the cross-correlation function. The main results of our study are as follows. (1) The cross-correlation function xi_{ga} measured using only emission-line dominated galaxies and Lya absorbers of log N(HI) >= 14 shows a comparable strength to the galaxy auto-correlation function xi_{gg} on co-moving, projection distance scales < 1 h^{-1} Mpc, while there remains a lack of cross-correlation signal when using only absorption-line dominated galaxies. This signifies a morphology-dependent xi_{ga} and indicates that strong absorbers of log N(HI) >= 14 and emission-line galaxies reside in the same halo population. (2) A maximum-likelihood analysis shows that Lya absorbers of log N(HI) < 13.6 are consistent with being more randomly distributed with respect to known galaxies. Finally, (3) we find based on this single sightline that the amplitude of xi_{ga} does not depend sensitively on N(HI) for strong absorbers of log N(HI) >= 13.6.

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arXiv2003

The First Detailed X-ray Observations of High-Redshift, Optically-Selected Clusters: XMM-Newton Results for Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90

We present the first detailed X-ray observations of optically-selected clusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of 0.5 Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similarly to other optically-selected clusters at redshifts of z > 0.4, both clusters are modest X-ray emitters with bolometric luminosities of only Lx = 1.4 - 2.0 x 10^(44) erg/s. We measure gas temperatures of T = 2.88 (+0.71/-0.49) keV for Cl 1324+3011 and 2.51 (+1.05/-0.69) keV for Cl 1604+4304. The X-ray properties of both clusters are consistent with the high-redshift Lx-T relation measured from X-ray-selected samples at z > 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for their measured velocity dispersions (sigma). The clusters are cooler by a factor of 2 - 9 compared to the local sigma-T relation. We briefly discuss the possible explanations for these results.

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arXiv2003

An X-ray Atlas of Groups of Galaxies

A search was conducted for a hot intragroup medium in 109 low-redshift galaxy groups observed with the ROSAT PSPC. Evidence for diffuse, extended X-ray emission is found in at least 61 groups. Approximately one-third of these detections have not been previously reported in the literature. Most of the groups are detected out to less than half of the virial radius with ROSAT. Although some spiral-rich groups do contain an intragroup medium, diffuse emission is restricted to groups that contain at least one early-type galaxy.

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arXiv2002

Circumnuclear Dust in Nearby Active and Inactive Galaxies. I. Data

The detailed morphology of the interstellar medium (ISM) in the central kiloparsec of galaxies is controlled by pressure and gravitation. The combination of these forces shapes both circumnuclear star formation and the growth of the central, supermassive black hole. We present visible and near-infrared Hubble Space Telescope images and color maps of 123 nearby galaxies that show the distribution of the cold ISM, as traced by dust, with excellent spatial resolution. These observations reveal that nuclear dust spirals are found in the majority of active and inactive galaxies and they possess a wide range in coherence, symmetry, and pitch angle. We have used this large sample to develop a classification system for circumnuclear dust structures. In spite of the heterogeneous nature of the complete sample, we only find symmetric, two-arm nuclear dust spirals in galaxies with large scale bars and these dust lanes clearly connect to dust lanes along the leading edges of the large scale bars. Not all dust lanes along large scale bars form two arm spirals, however, and several instead end in nuclear rings. We find that tightly wound, or low pitch angle, nuclear dust spirals are more common in unbarred galaxies than barred galaxies. Finally, the extended narrow line region in several of the active galaxies is well-resolved. The connection between the ionized gas and circumnuclear dust lanes in four of these galaxies provides additional evidence that a significant fraction of their extended narrow line region is ambient gas photoionized in situ by the active nucleus. In a future paper, we will use our classification system for circumnuclear dust to identify differences between active and inactive galaxies, as well as barred and unbarred galaxies, in well-matched subsamples of these data.

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arXiv2002

Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei

(Abridged) We present a detailed study of the relation between circumnuclear dust morphology, host galaxy properties, and nuclear activity in nearby galaxies. We use our sample of 123 nearby galaxies with visible--near-infrared colormaps from the Hubble Space Telescope to create well-matched, "paired" samples of 28 active and 28 inactive galaxies, as well as 19 barred and 19 unbarred galaxies, that have the same host galaxy properties. Comparison of the barred and unbarred galaxies shows that grand design nuclear dust spirals are only found in galaxies with a large-scale bar. Tightly wound nuclear dust spirals, in contrast, show a strong tendency to avoid galaxies with large-scale bars. Comparison of the AGN and inactive samples shows that nuclear dust spirals, which may trace shocks and angular momentum dissipation in the ISM, occur with comparable frequency in both active and inactive galaxies. The only difference between the active and inactive galaxies is that several inactive galaxies appear to completely lack dust structure in their circumnuclear region, while none of the AGN lack this structure. The comparable frequency of nuclear spirals in active and inactive galaxies, combined with previous work that finds no significant differences in the frequency of bars or interactions between well-matched active and inactive galaxies, suggests that no universal fueling mechanism for low-luminosity AGN operates at spatial scales greater than ~100 pc radius from the galactic nuclei. The similarities instead suggest that the lifetime of nuclear activity is less than the characteristic inflow time from these spatial scales. An order-of-magnitude estimate of this inflow time is the dynamical timescale. This sets an upper limit of several million years to the lifetime of an individual episode of nuclear activity.

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arXiv2002

An Unexpectedly High AGN Fraction in Red Cluster Galaxies

As part of a program to study the evolution of active galactic nuclei (AGN) in clusters of galaxies, we present our results for Abell 2104. A deep Chandra observation of this massive, z = 0.154 cluster reveals a significant X-ray point source excess over the expectations of blank fields, including eight X-ray counterparts with R<20 mag. Our spectroscopy shows that all six X-ray sources associated with red counterparts are cluster members and their X-ray properties are consistent with all of them being AGN. Only one of the six has the emission lines characteristic of optically selected AGN; the remaining five would not have been classified as AGN based on their optical spectra. This suggests the existence of a large population of obscured, or at least optically unremarkable, AGN in clusters of galaxies. These six sources correspond to a lower limit of ~5% of the AGN fraction in cluster galaxies with R<20 mag (rest-frame M_V = -19.5 mag) and is comparable to the blue galaxy fraction in the cluster. Such an obscured AGN population in clusters of galaxies has many implications for cluster galaxy evolution, the hidden growth of their central, supermassive black holes, estimates of the star formation rate at infrared and radio wavelengths, and the observed variance in the hard X-ray background.

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arXiv2001

The L_x-sigma Relation For Poor Groups of Galaxies

We use a sample of 20 poor groups of galaxies to study the low L_x tail of the L_x-sigma relationship. We have obtained redshifts for fainter members and deep X-ray imaging of these groups. We find that the L_x-sigma relationship derived for the rich clusters in the sample of Mushotzky & Scharf (1997) is consistent with the data for our 20 poor groups. However, it is not possible to reach strong conclusions about differences in the L_x-sigma relationship for groups and clusters in general at the present time because of significant differences between various cluster samples.

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arXiv2000

The Optical and Near-Infrared Morphologies of Isolated Early Type Galaxies

In order to study early type galaxies in their simplest environments, we have constructed a well-defined sample of 30 isolated galaxies. The sample contains all RC3 early-type galaxies with no other cataloged galaxy with known redshift lying within a projected radius of 1 (h_100)^{-1} Mpc and =/- 1000 km/s (where we use the recessional velocities in the RC3). We have obtained optical and near-infrared images of 23 of the galaxies and of a comparison sample of 13 early-type galaxies in X-ray detected poor groups of galaxies. We have applied the techniques of unsharp masking, galaxy model division, and color maps to search for morphological features that might provide clues to the evolution of these galaxies. Evidence for dust features is found in approximately 75% of both the isolated and group galaxies (17 of 22 and 9 of 12, respectively). However, shells or tidal features are much more prevalent in our isolated sample than in our group sample (9 of 22 or 41% versus 1 of 12 or 8%, respectively). The isolation and colors of these shell galaxies make it unlikely that tidal interactions or asymmetric star formation are the causes of such features. One model that is not ruled out is that mergers produce the shells. If shells and dust are both merger signatures, the absence of shells in group ellipticals implies that shells: 1) form more easily, 2) are younger, and/or 3) are longer-lived in isolated environments.

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arXiv2000

The Galaxy Populations of X-Ray Detected, Poor Groups

(Abridged) We determine the quantitative morphology and star formation properties of galaxies in six nearby X-ray detected, poor groups using multi-object spectroscopy and wide-field R imaging. We measure structural parameters for each galaxy by fitting a PSF-convolved, two component model to their surface brightness profiles. To compare directly the samples, we fade, smooth, and rebin each galaxy image so that we effectively observe each galaxy at the same redshift (9000 km/s) and physical resolution (0.87h^(-1) kpc). We compare results for the groups to a sample of field galaxies. We find that: 1) Galaxies spanning a wide range in morphological type and luminosity are well-fit by a de Vaucouleurs bulge with exponential disk profile. 2) Morphologically classifying these nearby group galaxies by their bulge fraction (B/T) is fairly robust on average, even when their redshift has increased by up to a factor of four and the effective resolution of the images is degraded by up to a factor of five. 3) The fraction of bulge-dominated systems in these groups is higher than in the field (~50% vs. ~20%). 4) The fraction of bulge-dominated systems in groups decreases with increasing radius, similar to the morphology-radius (~density) relation observed in galaxy clusters. 5) Current star formation in group galaxies is correlated with significant morphological asymmetry for disk-dominated systems (B/T<0.4). 6) The group galaxies that are most disk-dominated (B/T<0.2) are less star forming and asymmetric on average than their counterparts in the field.

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arXiv2000

X-ray Properties of Groups of Galaxies

ROSAT observations indicate that approximately half of all nearby groups of galaxies contain spatially extended X-ray emission. The radial extent of the X-ray emission is typically 50-500 kpc or approximately 10-50% of the virial radius of the group. Diffuse X-ray emission is generally restricted to groups that contain at least one early-type galaxy. X-ray spectroscopy suggests the emission mechanism is most likely a combination of thermal bremsstrahlung and line emission. This interpretation requires that the entire volume of groups be filled with a hot, low-density gas known as the intragroup medium. Higher temperature groups tend to follow the correlations found for rich clusters between X-ray luminosity, temperature, and velocity dispersion. However, groups with temperatures below approximately 1 keV appear to fall off the cluster L-T relationship (and possibly the L-sigma and sigma-T cluster relationships, although evidence for these latter departures is at the present time not very strong.)

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