arXiv subjects

Michael McDonald

Publications and source records attributed to Michael McDonald.

102 records · Page 6Linked to original sources

arXiv2011

The Formation and Evolution of Virgo Cluster Galaxies - I. Broadband Optical & Infrared Colours

We use a combination of deep optical (gri) and near-infrared (H) photometry to study the radially-resolved colours of a broad sample of 300 Virgo cluster galaxies. For most galaxy types, we find that the median g-H colour gradient is either flat (gas-poor giants and gas-rich dwarfs) or negative (i.e., colours become bluer with increasing radius; gas-poor dwarfs, spirals, and gas-poor peculiars). Later-type galaxies typically exhibit more negative gradients than early-types. Given the lack of a correlation between the central colours and axis ratios of Virgo spiral galaxies, we argue that dust likely plays a small role, if at all, in setting those colour gradients. We search for possible correlations between galaxy colour and photometric structure or environment and find that the Virgo galaxy colours become redder with increasing concentration, luminosity and surface brightness, while no dependence with cluster-centric radius or local galaxy density is detected (over a range of ~2 Mpc and ~3-16 Mpc^-2, respectively). However, the colours of gas-rich Virgo galaxies do correlate with neutral gas deficiency, such that these galaxies become redder with higher deficiencies. Comparisons with stellar population models suggest that these colour gradients arise principally from variations in stellar metallicity within these galaxies, while age variations only make a significant contribution to the colour gradients of Virgo irregulars. A detailed stellar population analysis based on this material is presented in Roediger et al (2011b; arXiv:1011.3511).

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arXiv2011

The Formation and Evolution of Virgo Cluster Galaxies - II. Stellar Populations

We use a combination of deep optical and near-infrared light profiles for a morphologically diverse sample of Virgo cluster galaxies to study the radially-resolved stellar populations of cluster galaxies over a wide range of galaxy structure. We find that, in the median, the age gradients of Virgo galaxies are either flat (lenticulars and Sa-Sb spirals) or positive (ellipticals, Sbc+Sc spirals, gas-rich dwarfs, and irregulars), while all galaxy types have a negative median metallicity gradient. Comparison of the galaxy stellar population diagnostics (age, metallicity, and gradients thereof) against structural and environmental parameters also reveals that the ages of gas-rich systems depend mainly on their atomic gas deficiencies. Conversely, the metallicities of Virgo gas-poor galaxies depend on their concentrations, luminosities, and surface brightnesses. The stellar population gradients of all Virgo galaxies exhibit no dependence on either their structure or environment. We interpret these stellar population data for Virgo galaxies in the context of popular formation and evolution scenarios, and suggest that gas-poor giants grew hierarchically (through dissipative starbursts), gas-poor dwarfs have descended from at least two different production channels (e.g., environmental transformation and merging), while spirals formed inside-out, but with star formation in the outskirts of a significant fraction of the population having been quenched due to ram pressure stripping. (Abridged)

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arXiv2011

Star Formation Efficiency in the Cool Cores of Galaxy Clusters

We have assembled a sample of high spatial resolution far-UV (Hubble Space Telescope Advanced Camera for Surveys Solar Blind Channel) and Halpha (Maryland-Magellan Tunable Filter) imaging for 15 cool core galaxy clusters. These data provide a detailed view of the thin, extended filaments in the cores of these clusters. Based on the ratio of the far-UV to Halpha luminosity, the UV spectral energy distribution, and the far-UV and Halpha morphology, we conclude that the warm, ionized gas in the cluster cores is photoionized by massive, young stars in all but a few (Abell 1991, Abell 2052, Abell 2580) systems. We show that the extended filaments, when considered separately, appear to be star-forming in the majority of cases, while the nuclei tend to have slightly lower far-UV luminosity for a given Halpha luminosity, suggesting a harder ionization source or higher extinction. We observe a slight offset in the UV/Halpha ratio from the expected value for continuous star formation which can be modeled by assuming intrinsic extinction by modest amounts of dust (E(B-V) ~ 0.2), or a top-heavy IMF in the extended filaments. The measured star formation rates vary from ~ 0.05 Msun/yr in the nuclei of non-cooling systems, consistent with passive, red ellipticals, to ~ 5 Msun/yr in systems with complex, extended, optical filaments. Comparing the estimates of the star formation rate based on UV, Halpha and infrared luminosities to the spectroscopically-determined X-ray cooling rate suggests a star formation efficiency of 14(+18)(-8)%. This value represents the time-averaged fraction, by mass, of gas cooling out of the intracluster medium which turns into stars, and agrees well with the global fraction of baryons in stars required by simulations to reproduce the stellar mass function for galaxies. This result provides a new constraint on the efficiency of star formation in accreting systems.

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arXiv2011

The Effect of Environment on the Formation of Halpha Filaments and Cool Cores in Galaxy Groups and Clusters

We present the results of a combined X-ray and Halpha study of 10 galaxy groups and 17 galaxy clusters using the Chandra X-ray Observatory and the Maryland Magellan Tunable Filter. We find no difference in the morphology or detection frequency of Halpha filaments in groups versus clusters, over the mass range 10^13 < M_500 < 10^15 Msun. The detection frequency of Halpha emission is shown to be only weakly dependent on the total mass of the system, at the 52% confidence level. In contrast, we find that the presence of Halpha filaments is strongly correlated with both the global (89% confidence level) and core (84%) ICM entropy, as well as the X-ray cooling rate (72%). The Halpha filaments are therefore an excellent proxy for the cooling ICM. The Halpha filaments are more strongly correlated with the cooling properties of the ICM than with the radio properties of the BCG; this further supports the scenario where these filaments are directly associated with a thermally-unstable, rapidly cooling ICM, rather than radio bubbles. The ICM cooling efficiency, defined as the X-ray cooling rate per unit gas mass, is shown to correlate with the total system mass, indicating that groups are more efficient at cooling than clusters. This result implies that, in systems with cool cores, AGN feedback scales with the total mass of the system, in agreement with earlier suggestions.

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arXiv2010

Stellar Population Trends in S0 Galaxies

We present stellar population age and metallicity trends for a sample of 59 S0 galaxies based on optical SDSS and NIR J & H photometry. When combined with optical g and r passband imaging data from the SDSS archive and stellar population models, we obtain radial age and metallicity trends out to at least 5 effective radii for most of the galaxies in our sample. The sample covers a range in stellar mass and light concentration. We find an average central light-weighted age of ~ 4 Gyr and central metallicity [Z/H] ~ 0.2 dex. Almost all galaxies show a negative metallicity gradient from the center out, with an average value of Delta[Z/H]/Delta(log(r/Re)) = -0.6. An age increase, decrease, and minimal change with radius is observed for 58%, 19%, and 23%, respectively, for a mean age gradient of Delta(age)/Delta(log(r/Re)) = 2.3 Gyr dex^{-1}. For 14 out of 59 galaxies, the light-weighted age of the outer region is greater than 10 Gyr. We find that galaxies with both lower mass and lower concentration have younger light-weighted ages and lower light-weighted metallicities. This mass-metallicity relation extends into the outer regions of our S0 galaxies. Our results are consistent with the formation of S0 galaxies through the transformation of spiral galaxy disks. Determining the structural component that makes up the outer region of galaxies with old outksirts is a necessary step to understand the formation history of S0 galaxies.

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arXiv2010

On the Origin of the Extended Halpha Filaments in Cooling Flow Clusters

We present a high spatial resolution Halpha survey of 23 cooling flow clusters using the Maryland Magellan Tunable Filter (MMTF), covering 1-2 orders of magnitude in cooling rate, dM/dt, temperature and entropy. We find 8/23 (35%) of our clusters have complex, filamentary morphologies at Halpha, while an additional 7/23 (30%) have marginally extended or nuclear Halpha emission, in general agreement with previous studies of line emission in cooling flow cluster BCGs. A weak correlation between the integrated near-UV luminosity and the Halpha luminosity is also found for our complete sample, with a large amount of scatter about the expected relation for photoionization by young stars. We detect Halpha emission out to the X-ray cooling radius, but no further, in several clusters and find a strong correlation between the Halpha luminosity contained in filaments and the X-ray cooling flow rate of the cluster, suggesting that the warm ionized gas is linked to the cooling flow. Furthermore, we detect a strong enhancement in the cooling properties of the ICM coincident with the Halpha emission, compared to the surrounding ICM at the same radius. While the filaments in a few clusters may be entrained by buoyant radio bubbles, in general, the radially-infalling cooling flow model provides a better explanation for the observed trends. The correlation of the Halpha and X-ray properties suggests that conduction may be important in keeping the filaments ionized. The thinness of the filaments suggests that magnetic fields are an important part of channeling the gas and shielding it from the surrounding hot ICM.

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arXiv2010

Integrated Stellar Populations: Confronting Photometry with Spectroscopy

We investigate the ability of spectroscopic techniques to yield realistic star formation histories (SFHs) for the bulges of spiral galaxies based on a comparison with their observed broadband colors. Full spectrum fitting to optical spectra indicates that recent (within ~1 Gyr) star formation activity can contribute significantly to the V-band flux, whilst accounting for only a minor fraction of the stellar mass budget which is made up primarily of old stars. Furthermore, recent implementations of stellar population (SP) models reveal that the inclusion of a more complete treatment of the thermally pulsating asymptotic giant branch (TP-AGB) phase to SP models greatly increases the NIR flux for SPs of ages 0.2-2 Gyr. Comparing the optical--NIR colors predicted from population synthesis fitting, using models which do not include all stages of the TP-AGB phase, to the observed colors reveals that observed optical--NIR colors are too red compared to the model predictions. However, when a 1 Gyr SP from models including a full treatment the TP-AGB phase is used, the observed and predicted colors are in good agreement. This has strong implications for the interpretation of stellar populations, dust content, and SFHs derived from colors alone.

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arXiv2010

MMTF-Halpha and HST-FUV Imaging of the Filamentary Complex in Abell 1795

We have obtained deep, high spatial resolution images of the central region of Abell 1795 at Halpha and [NII] (6583A) with the Maryland Magellan Tunable Filter (MMTF), and in the far-ultraviolet (FUV) with the Advanced Camera for Surveys Solar Blind Channel on the Hubble Space Telescope (HST). The superb image quality of the MMTF data has made it possible to resolve the known SE filament into a pair of thin, intertwined filaments extending for ~50 kpc, with a width < 1 kpc. The presence of these thin, tangled strands is suggestive of a cooling wake where runaway cooling is taking place, perhaps aided by an enhanced magnetic field in this region. The HST data further resolve these strands into chains of FUV-bright stellar clusters, indicating that these filaments are indeed sites of on-going star formation, but at a rate ~2 orders of magnitude smaller than the mass-deposition rates predicted from the X-ray data. The elevated [NII]/Halpha ratio and large spatial variations of the FUV/Halpha flux ratio across the filaments indicate that O-star photoionization is not solely responsible for the ionization. The data favor collisional heating by cosmic rays either produced in-situ by magnetohydrodynamical processes or conducted in from the surrounding intracluster medium.

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arXiv2009

The Near-IR Luminosity Function and Bimodal Surface Brightness Distribution of Virgo Cluster Galaxies

We have acquired deep, H-band, imaging for a sample of 286 Virgo cluster galaxies with B <= 16 mag and extracted surface photometry from optical g,r,i,z Sloan Digital Sky Survey images of 742 Virgo Cluster Catalog galaxies, including those with H-band images. We confirm the detection of a dip in the luminosity function indicative of a discontinuity in the cluster galaxy population; the dip is more pronounced at redder wavelengths. We find, in agreement with earlier works of Tully & Verheijen and ours for Ursa Major cluster galaxies, a clear dichotomy between high and low surface brightness galaxy disks. The difference between the low and high brightness peaks of Virgo disk galaxies is ~2 H-mag arcsec^-2, significantly larger than any systematic errors. The high surface brightness disk galaxies have two distinct classes of high and low concentration bulges, while low surface brightness galaxies have only low concentration bulges. Early-type galaxies exhibit a similar structural bimodality though offset from that of the spiral galaxies towards higher surface brightnesses. Both the early- and late-type structural bimodalities are uncorrelated with colour or any other structural parameter except, possibly, circular velocity. The structural bimodality may be linked to dynamical properties of galaxies. Low angular momentum systems may collapse to form dynamically important disks with high surface brightness, while high angular momentum systems would end up as low surface brightness galaxies dominated by the dark halo at all radii. The confirmation of structural bimodality for gas-rich and gas-poor galaxies in the high-density Virgo cluster as well as the low-density UMa cluster suggests that this phenomenon is independent of environment.

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arXiv2008

Bulge-Disk Decompositions and Structural Bimodality of Ursa Major Cluster Spiral Galaxies

We present bulge and disk (B/D) decompositions of existing K'-band surface brightness profiles for 65 Ursa Major cluster spiral galaxies. This improves upon the disk-only fits of Tully et al. (1996). The 1996 disk fits were used by Tully & Verheijen (1997) for their discovery of the bimodality of structural parameters in the UMa cluster galaxies. It is shown that our new 1D B/D decompositions yield disk structural parameters that differ only slightly from the basic fits of Tully et al. and evidence for structural bimodality of UMa galaxies is maintained. Our B/D software for the decomposition of 1D surface brightness profiles of galaxies uses a non-linear minimization scheme to recover the best fitting Sersic bulge and exponential disk while accounting for the possible presence of a compact nucleus and spiral arms and for the effects of seeing and disk truncations. In agreement with Tully & Verheijen, we find that the distribution of near-infrared disk central surface brightnesses is bimodal with an F-test confidence of 80%. There is also strong evidence for a local minimum in the luminosity function at M_K' ~ -22. A connection between the brightness bimodality and a dynamical bimodality, based on new HI line widths, is identified. The B/D parameters are presented in an Appendix.

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arXiv2007

The Vcirc-sigma0 Relation of Galaxies

Courteau et al. (2007a) reported on the dependence of the ratio of a galaxy's maximum circular velocity, Vcirc, to its central velocity dispersion, sigma0, on morphology, or equivalently total light concentration. This Vcirc-sigma0 concentration relation, which involves details about the local and global galaxy physics, poses a fundamental challenge for galaxy structure models. Furthermore, not only must these models reproduce the Vcirc-sigma0 relation and its various dependences, they must simultaneously match other fundamental scaling relations such as the velocity-size-luminosity and color-luminosity relations of galaxies. We focus here on the interpretation of parameters that enter the Vcirc-sigma0 relation to enable proper data-model comparisons and follow-up studies by galaxy modelers and observers.

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arXiv2006

The Bulge-Halo Connection in Galaxies: A Physical Interpretation of the Vcirc-sigma_0 Relation

We explore the dependence of the ratio of a galaxy's circular velocity, Vcirc, to its central velocity dispersion, sigma_0, on morphology, or equivalently total light concentration. Such a dependence is expected if light traces the mass. Over the full range of galaxy types, masses and brightnesses, and assuming that the gas velocity traces the circular velocity, we find that galaxies obey the relation log(Vcirc/sigma_0)= 0.63-0.11*C28 where C28=5log(r80/r20) and the radii are measured at 80 percent and 20 percent of the total light. Massive galaxies scatter about the Vcirc = sqrt(2)*sigma_0 line for isothermal stellar systems. Disk galaxies follow the simple relation Vcirc/sigma_0=2(1-B/T), where B/T is the bulge-to-total light ratio. For pure disks, C28~2.8, B/T -> 0, and Vcirc~=2*sigma_0. Self-consistent equilibrium galaxy models from Widrow & Dubinski (2005) constrained to match the size-luminosity and velocity-luminosity relations of disk galaxies fail to match the observed Vcirc/sigma_0 distribution. Furthermore, the matching of dynamical models for Vcirc(r)/sigma(r) with observations of dwarf and elliptical galaxies suffers from limited radial coverage and relatively large error bars; for dwarf systems, however, kinematical measurements at the galaxy center and optical edge suggest Vcirc(Rmax) > 2*sigma_0 (in contrast with past assumptions that Vcirc = sqrt(2)*sigma_0 for dwarfs.) The Vcirc-sigma_0-C28 relation has direct implications for galaxy formation and dynamical models, galaxy scaling relations, the mass function of galaxies, and the links between respective formation and evolution processes for a galaxy's central massive object, bulge, and dark matter halo.

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