arXiv subjects

Michael J. Hudson

Publications and source records attributed to Michael J. Hudson.

At least 109 records · Page 6Linked to original sources

arXiv2009

Ages and metallicities for quiescent galaxies in the Shapley Supercluster: Driving parameters of the stellar populations

We use high signal-to-noise spectroscopy for a sample of 232 quiescent galaxies in the Shapley Supercluster, to investigate how their stellar populations depend on velocity dispersion, luminosity and stellar mass. The sample spans a large range in velocity dispersion (sigma from 30-300 km/s) and in luminosity (M_R from -18.7 to -23.2). Estimates of age, total metallicity (Z/H) and alpha-element abundance ratio (a/Fe) were derived from absorption-line analysis, using single-burst models. Age, Z/H and a/Fe are all correlated positively with velocity dispersion, but we also find significant residual trends with luminosity: at given sigma, the brighter galaxies are younger, less alpha-enriched, and have higher Z/H. At face value, these results might suggest that the stellar populations depend on stellar mass as well as on velocity dispersion. However, we show that the observed trends can be reproduced by models in which the stellar populations depend systematically only on sigma, and are independent of stellar mass Mstel. For age, the observed luminosity correlation arises because young galaxies are brighter, at fixed Mstel. For metallicity, the observed luminosity dependence arises because metal-rich galaxies, at fixed mass, tend also to be younger, and hence brighter. We find a good match to the observed luminosity correlations with Age ~ sigma^0.40, Z/H ~ sigma^0.35, a/Fe ~ sigma^0.20, where the slopes are close to those found when fitting traditional scaling relations. We conclude that the star formation and enrichment histories of galaxies are determined primarily by the depth of their gravitational potential wells. The observed residual correlations with luminosity do not imply a corresponding dependence on stellar mass.

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arXiv2009

The Star Formation Histories of Red-Sequence Galaxies, Mass-to-Light Ratios and the Fundamental Plane

This paper addresses the challenge of understanding the typical star formation histories of red sequence galaxies, using linestrength indices and mass-to-light ratios as complementary constraints on their stellar age distribution. We construct simple parametric models of the star formation history that bracket a range of scenarios, and fit these models to the linestrength indices of low-redshift cluster red-sequence galaxies. For giant galaxies, we confirm the downsizing trend. We find, however, that this trend flattens or reverses at sigma < 70 km/s. We then compare predicted stellar mass-to-light ratios with dynamical mass-to-light ratios derived from the Fundamental Plane (FP), or by the SAURON group. For galaxies with sigma ~ 70 km/s, models with a "frosting" of young stars and models with exponential star formation histories have stellar mass-to-light ratios that are larger than observed dynamical mass-to-light ratios by factors of 1.7 and 1.4, respectively, and so are rejected. The SSP model is consistent with the FP, and requires a modest amount of dark matter (20-30%) to account for the difference between stellar and dynamical mass-to-light ratios. A model in which star formation was "quenched" at intermediate ages is also consistent with the observations. We find that the contribution of stellar populations to the "tilt" of the FP is highly dependent on the assumed star-formation history: for the SSP model, the tilt of the FP is driven primarily by stellar-population effects. For a quenched model, two-thirds of the tilt is due to stellar populations and only one third is due to dark matter or non-homology.

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arXiv2009

Abundance ratios in red-sequence galaxies over a wide mass range: The "X-planes" for magnesium, calcium, carbon and nitrogen

We analyse the abundance ratios of the light elements Mg, Ca, C and N, relative to Fe, for 147 red-sequence galaxies in the Coma cluster and the Shapley Supercluster. The sample covers a six-magnitude range in luminosity, from giant ellipticals to dwarfs at M^*+4. We exploit the wide mass range to investigate systematic trends in the abundance ratios Mg/Fe, Ca/Fe, C/Fe and N/Fe. We find that each of these ratios can be well modelled using two-parameter relations of the form [X/Fe] = a0 + a1 log sigma + a2 [Fe/H], where sigma is the velocity dispersion. Analysing these "X-planes" reveals new structure in the abundance patterns, beyond the traditional one-parameter (e.g. Mg/Fe-sigma) correlations. The X-planes for the alpha elements, Mg and Ca, indicate a positive correlation with velocity dispersion, and simultaneously an anti-correlation with Fe/H (i.e. a1>0 and a2<0). Taking both effects into account dramatically reduces the scatter, compared to the traditional X/Fe-sigma relations. For C and N, a similar correlation with velocity dispersion is recovered, but there is no additional dependence on Fe/H (i.e. a1>0 and a2~0). The explicit dependence of X/Fe on two parameters is evidence that at least two physical processes are at work in setting the abundance patterns. The Fe/H dependence of Mg/Fe and Ca/Fe, at fixed sigma, may result from different durations of star formation, from galaxy to galaxy. The absence of corresponding Fe/H dependence for C and N is consistent with these elements being generated in lower-mass stars. The increase with sigma, at fixed Fe/H, is similar for elements Mg, C and N, and slightly shallower for Ca. This pattern of trends cannot be explained solely by a systematic variation of star-formation time-scale with sigma.

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arXiv2009

The Radio Luminosity Function and Galaxy Evolution in the Coma Cluster

We investigate the radio luminosity function (RLF) and radio source population for two fields within the Coma cluster of galaxies. Our VLA data reach down to log(L) = 20.23 W/Hz for Coma, and we associate 249 sources with optical counterparts from the SDSS. Comprehensive optical spectroscopy identifies 38 of these as members of the Coma cluster, evenly split between AGN and star-forming galaxies (SFG). The radio-detected SFG are the dominant population only for ~21 < log(L) < ~22 W/Hz, an interesting result given that star formation dominates field RLFs for log(L) < ~23. The majority of the radio-detected SFGs have characteristics of starbursts, including high specific star formation rates and optical spectra with strong emission lines. In conjunction with prior studies on post-starburst galaxies within the Coma cluster, this is consistent with a picture in which late-type galaxies entering Coma undergo a starburst prior to a rapid cessation of star formation. Optically bright elliptical galaxies (Mr <= -20.5) make the largest contribution to the radio luminosity function at both the high (log(L) >~ 22.48 W/Hz) and low (log(L) <~ 21 W/Hz) ends. Through a stacking analysis of these optically-bright ellipticals we find that they continue to harbor radio sources down to log(L) = 19.48. However, contrary to published results for the Virgo cluster we find no evidence for the existence of a population of optically faint (Mr ~ -14) dwarf ellipticals hosting strong radio AGN.

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arXiv2008

Consistently Large Cosmic Flows on Scales of 100 Mpc/h: a Challenge for the Standard LCDM Cosmology

Peculiar velocity surveys have non-uniform spatial distributions of tracers, so that the bulk flow estimated from them does not correspond to that of a simple volume such as a sphere. Thus bulk flow estimates are generally not strictly comparable between surveys, even those whose effective depths are similar. In addition, the sparseness of typical surveys can lead to aliasing of small scale power into what is meant to be a probe of the largest scales. Here we introduce a new method of calculating bulk flow moments where velocities are weighted to give an optimal estimate of the bulk flow of an idealized survey, with the variance of the difference between the estimate and the actual flow being minimized. These "minimum variance" estimates can be designed to estimate the bulk flow on a particular scale with minimal sensitivity to small scale power, and are comparable between surveys. We compile all major peculiar velocity surveys and apply this new method to them. We find that most surveys we studied are highly consistent with each other. Taken together the data suggest that the bulk flow within a Gaussian window of radius 50 Mpc/h is 407 km/s toward l=287 and b=8. The large-scale bulk motion is consistent with predictions from the local density field. This indicates that there are significant density fluctuations on very large scales. A flow of this amplitude on such a large scale is not expected in the WMAP5-normalized LCDM cosmology, for which the predicted one-dimensional r.m.s. velocity is ~110 km/s. The large amplitude of the observed bulk flow favors the upper values of the WMAP5 error-ellipse, but even the point at the top of the WMAP5 95% confidence ellipse predicts a bulk flow which is too low compared to that observed at >98% confidence level.

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arXiv2008

A spectroscopic survey of dwarf galaxies in the Coma Cluster: Stellar populations, environment and downsizing

We investigate the stellar populations in a sample of 89 faint (M*+2 to M*+4) red galaxies in the Coma cluster, using high S/N spectroscopy from the MMT. Our sample is drawn from two 1-degree fields, one centred on the cluster core and the other located a degree to the south west of the cluster centre. For a comparison sample we use published high-S/N data for red-sequence galaxies in the Shapley Supercluster. We use state-of-the-art stellar population models to infer the SSP-equivalent age and metallicity (Fe/H) for each galaxy, as well as the abundances of the light elements Mg, Ca, C and N. The ages of the Coma dwarfs span a wide range from <2 Gyr to >10 Gyr, with a strong environmental dependence. The oldest galaxies are found only in the core, while most of the galaxies in the outer south-west field have ages ~3 Gyr. The galaxies have a metallicity range -1.0 < [Fe/H] < 0.0, and follow the same age-metallicity-mass plane as high-mass galaxies, but with increased intrinsic scatter. The Mg/Fe abundance ratios are on average slightly super-solar, and span a range -0.1 < [Mg/Fe] < +0.4. The highest Mg enhancements are found only in the cluster core, while solar ratios predominate in the outskirts. Depending on the assumed star-formation history (quenched versus burst-dominated), the number of dwarf galaxies on the red sequence in the Coma core has doubled since z~0.4-0.7. These estimates bracket the red-sequence growth timescales found by direct studies of distant clusters. In the south-west field, the red sequence was established only at z~0.1-0.2. Our observations confirm previous indications of very recently quenched star formation in this part of the cluster. Our results support the picture in which much of the cluster passive dwarf population was generated by environment-driven transformation of infalling late-type galaxies.

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arXiv2008

Cosmic Flows on 100 Mpc/h scales

To study galactic motions on the largest available scales, we require bulk flow moments whose window functions have as narrow a peak as possible and having as small an amplitude as possible outside the peak. Typically the moments found using the maximum likelihood estimate weights do not meet these criteria. We present a new method for calculating weights for moments that essentially allow us to "design" the moment's window function, subject, of course, to the distribution and uncertainties of the available data.

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arXiv2008

A large population of recently-quenched red-sequence dwarf galaxies in the outskirts of the Coma Cluster

We analyse the stellar populations of 75 red-sequence dwarf galaxies in the Coma cluster, based on high signal-to-noise spectroscopy from the 6.5m MMT. The sample covers a luminosity range 3-4 magnitudes below M*, in the cluster core and in a field centred 1 deg to the south-west. We find a strong dependence of the absorption line strengths with location in the cluster. Galaxies further from the cluster centre have stronger Balmer lines than inner-field galaxies of the same luminosity. The magnesium lines are weaker at large radius, while the iron lines are not correlated with radius. Converting the line strengths into estimates of stellar age, metallicity and abundance ratios, we find the gradients are driven by variations in age (>6 sigma significance) and in the iron abundance Fe/H (~2.7 sigma significance). The light element (Mg, C, N, Ca) abundances are almost independent of radius. At radius of 0.4-1.3 degree (~0.3-1.0x the virial radius), dwarf galaxies have ages ~3.8 Gyr on average, compared to ~6 Gyr near the cluster centre. The outer dwarfs are also ~50% more iron-enriched, at given luminosity. Our results confirm earlier indications that the ages of red-sequence galaxies depend on location within clusters, and in Coma in particular. The exceptionally strong trends found here suggest that dwarf galaxies are especially susceptible to environmental "quenching", and/or that the south-west part of Coma is a particularly clear example of recent quenching in an infalling subcluster.

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arXiv2008

The HST/ACS Coma Cluster Survey: I - Survey Objectives and Design

We describe the HST ACS Coma cluster Treasury survey, a deep two-passband imaging survey of one of the nearest rich clusters of galaxies, the Coma cluster (Abell 1656). The survey was designed to cover an area of 740 square arcmin in regions of different density of both galaxies and intergalactic medium within the cluster. The ACS failure of January 27th 2007 leaves the survey 28% complete, with 21 ACS pointings (230 square arcmin) complete, and partial data for a further 4 pointings (44 square arcmin). Predicted survey depth for 10 sigma detections for optimal photometry of point sources is g' = 27.6 in the F475W filter, and IC=26.8 mag in F814 (AB magnitudes). Initial simulations with artificially injected point sources show 90% recovered at magnitude limits of g' = 27.55 and IC = 26.65. For extended sources, the predicted 10 sigma limits for a 1 square arcsecond region are g' = 25.8 mag/sq. arcsec and IC = 25.0 mag/sq. arcsec. We highlight several motivating science goals of the survey, including study of the faint end of the cluster galaxy luminosity function, structural parameters of dwarf galaxies, stellar populations and their effect on colors and color gradients, evolution of morphological components in a dense environment, the nature of ultra compact dwarf galaxies, and globular cluster populations of cluster galaxies of a range of luminosities and types. This survey will also provide a local rich cluster benchmark for various well known global scaling relations and explore new relations pertaining to the nuclear properties of galaxies.

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arXiv2008

NUV-IR colours of red sequence galaxies in local clusters

We present GALEX near-UV (NUV) and 2MASS J band photometry for red sequence galaxies in local clusters. We define quiescent samples according to a strict emission threshold, removing galaxies with very recent star formation. We analyse the NUV-J colour-magnitude relation (CMR) and find that the intrinsic scatter is an order of magnitude larger than for the analogous optical CMR (~0.35 rather than 0.05 mag), in agreement with previous studies. Comparing the NUV-J colours with spectroscopically-derived stellar population parameters, we find a strong (> 5.5sigma) correlation with metallicity, only a marginal trend with age, and no correlation with the alpha/Fe ratio. We explore the origin of the large scatter and conclude that neither aperture effects nor the UV upturn phenomenon contribute significantly. We show that the scatter could be attributed to simple `frosting' by either a young or a low metallicity subpopulation.

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arXiv2007

Ages and metallicities of faint red galaxies in the Shapley Supercluster

We present results on the stellar populations of 232 quiescent galaxies in the Shapley Supercluster, based on spectroscopy from the AAOmega spectrograph at the AAT. The key characteristic of this survey is its coverage of many low-luminosity objects (sigma ~ 50 km/s), with high signal-to-noise (~45 per Angstrom). Balmer-line age estimates are recovered with ~25% precision even for the faintest sample members. We summarize the observations and absorption line data, and present correlations of derived ages and metallicities with mass and luminosity. We highlight the strong correlation between age and alpha-element abundance ratio, and the anti-correlation of age and metallicity at fixed mass, which is shown to extend into the low-luminosity regime.

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arXiv2007

A deep AAOmega survey of low-luminosity galaxies in the Shapley Supercluster: Stellar population trends

We present new optical spectroscopy for 342 R<18 galaxies in the Shapley Supercluster obtained with the AAOmega facility at the Anglo-Australian Telescope. We describe the observations and measurements of central velocity dispersion, emission line equivalent widths and absorption line indices. The distinguishing characteristic of the survey is its coverage of a very wide baseline in velocity dispersion (30-300 km/s), while achieving high signal-to-noise ratio throughout (median 60 per Ang). Significant emission at H-alpha was detected in ~20 per cent of red-sequence Shapley members. Using line-ratio diagnostics, we find that the emission is LINER-like at high luminosity, but driven by star-formation in low-luminosity galaxies. We use Lick indices to characterise the absorption spectra. We define a subset of galaxies with very low emission contamination, and fit the index-sigma relations for this subset. Comparing the index-sigma slopes against predictions from single-burst stellar population models, we infer the scaling relations of age, total metallicity, [Z/H], and alpha-element abundance ratio, [a/Fe]. To reproduce the observed index-sigma slopes, all three parameters must increase with increasing velocity dispersion. Specifically, we recover: Age \propto sigma^0.52+/-0.10, Z/H \propto sigma^0.34+/-0.07, and a/Fe \propto sigma^0.23+/-0.06 (error reflects systematic effects), derived over a decade baseline in velocity dispersion. The recovered age-sigma relation is shown to be consistent with the observed evolution in the giant-to-dwarf galaxy ratio in clusters at redshifts z=0.4-0.8. A companion paper will analyse the distribution of age, [Z/H] and [a/Fe] for individual galaxies. (Abridged.)

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arXiv2007

The Masses and Shapes of Dark Matter Halos from Galaxy-Galaxy Lensing in the CFHTLS

We present the first galaxy-galaxy weak lensing results using early data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). These results are based on ~22 sq. deg. of i' data. From this data, we estimate the average velocity dispersion for an L* galaxy at a redshift of 0.3 to be 137 +- 11 km/s, with a virial mass, M_{200}, of 1.1 +- 0.2 \times 10^{12} h^{-1} Msun and a rest frame R-band mass-to-light ratio of 173 +- 34 h Msun/Lsun. We also investigate various possible sources of systematic error in detail. Additionally, we separate our lens sample into two sub-samples, divided by apparent magnitude, thus average redshift. From this early data we do not detect significant evolution in galaxy dark matter halo mass-to-light ratios from a redshift of 0.45 to 0.27. Finally, we test for non-spherical galaxy dark matter halos. Our results favor a dark matter halo with an ellipticity of ~0.3 at the 2-sigma level when averaged over all galaxies. If the sample of foreground lens galaxies is selected to favor ellipticals, the mean halo ellipticity and significance of this result increase.

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arXiv2007

Line Emission in the Brightest Cluster Galaxies of the NOAO Fundamental Plane and Sloan Digital Sky Surveys

We examine the optical emission line properties of Brightest Cluster Galaxies (BCGs) selected from two large, homogeneous datasets. The first is the X-ray selected National Optical Astronomy Observatory Fundamental Plane Survey (NFPS), and the second is the C4 catalogue of optically selected clusters built from the Sloan Digital Sky Survey Data Release ~3 (SDSS DR3). Our goal is to better understand the optical line emission in BCGs with respect to properties of the galaxy and the host cluster. Throughout the analysis we compare the line emission of the BCGs to that of a control sample made of the other bright galaxies near the cluster centre. Overall, both the NFPS and SDSS show a modest fraction of BCGs with emission lines (~15%). No trend in the fraction of emitting BCGs as a function of galaxy mass or cluster velocity dispersion is found. However we find that, for those BCGs found in cooling flow clusters, 71^{+9}_{-14}% have optical emission. Furthermore, if we consider only BCGs within 50kpc of the X-ray centre of a cooling flow cluster, the emission-line fraction rises further to 100^{+0}_{-15}%. Excluding the cooling flow clusters, only ~10% of BCGs are line emitting, comparable to the control sample of galaxies. We show that the physical origin of the emission line activity varies: in some cases it has LINER-like line ratios, whereas in others it is a composite of star-formation and LINER-like activity. We conclude that the presence of emission lines in BCGs is directly related to the cooling of X-ray gas at the cluster centre.

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arXiv2007

The Peculiar Velocities of Local Type Ia Supernovae and their Impact on Cosmology

We quantify the effect of supernova Type Ia peculiar velocities on the derivation of cosmological parameters. The published distant and local Ia SNe used for the Supernova Legacy Survey first-year cosmology report form the sample for this study. While previous work has assumed that the local SNe are at rest in the CMB frame (the No Flow assumption), we test this assumption by applying peculiar velocity corrections to the local SNe using three different flow models. The models are based on the IRAS PSCz galaxy redshift survey, have varying beta = Omega_m^0.6/b, and reproduce the Local Group motion in the CMB frame. These datasets are then fit for w, Omega_m, and Omega_Lambda using flatness or LambdaCDM and a BAO prior. The chi^2 statistic is used to examine the effect of the velocity corrections on the quality of the fits. The most favored model is the beta=0.5 model, which produces a fit significantly better than the No Flow assumption, consistent with previous peculiar velocity studies. By comparing the No Flow assumption with the favored models we derive the largest potential systematic error in w caused by ignoring peculiar velocities to be Delta w = +0.04. For Omega_Lambda, the potential error is Delta Omega_Lambda = -0.04 and for Omega_m, the potential error is Delta Omega_m < +0.01. The favored flow model (beta=0.5) produces the following cosmological parameters: w = -1.08 (+0.09,-0.08), Omega_m = 0.27 (+0.02,-0.02) assuming a flat cosmology, and Omega_Lambda = 0.80 (+0.08,-0.07) and Omega_m = 0.27 (+0.02,-0.02) for a w = -1 (LambdaCDM) cosmology.

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arXiv2006

The NOAO Fundamental Plane Survey - III. Variations in the stellar populations of red-sequence galaxies from the cluster core to the virial radius

We analyse absorption line-strength indices for ~3000 red-sequence galaxies in 94 nearby clusters, to investigate systematic variations of their stellar content with location in the host cluster. The data are drawn from the NOAO Fundamental Plane Survey. Our adopted method is a generalization of that introduced by Nelan et al. to determine the global age-mass and metallicity-mass relations from the same survey. We find strong evidence for a change in galaxy properties, at fixed mass, over a range from the cluster centre to the virial radius, R_200. For example, red-sequence galaxies further out in the clusters have weaker Mgb5177 (at ~8 sigma significance) and stronger Hbeta and Hgamma absorption (~3 sigma, ~4 sigma) than galaxies of the same velocity dispersion in the cluster cores. The Fe5270 and Fe5335 indices show only very weak trends with radius. Using a total of twelve indices, the pattern of cluster-centric gradients is considered in light of their different dependences on stellar age and chemical composition. The measured gradients for all twelve indices can be reproduced by a model in which red-sequence galaxies at ~1 R_200 have on average younger ages (by 15+/-4%) and lower alpha-element abundance ratios (by 10+/-2%), than galaxies of the same velocity dispersion but located near the cluster centre. For the total metallicity, Z/H, no significant gradient is found (2+/-3% larger at R_200 than in the core). The cluster-centric age and alpha/Fe gradients are in the sense expected if galaxies in the cluster core were accreted at an earlier epoch than those at larger radii, and if this earlier accretion contributed to an earlier cessation of star formation. The size of the observed age trend is comparable to predictions from semi-analytic models of hierarchical galaxy formation.

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arXiv2005

Cosmological Parameters from the Comparison of the 2MASS Gravity Field with Peculiar Velocity Surveys

We compare the peculiar velocity field within 65 $h^{-1}$ Mpc predicted from 2MASS photometry and public redshift data to three independent peculiar velocity surveys based on type Ia supernovae, surface brightness fluctuations in ellipticals, and Tully-Fisher distances to spirals. The three peculiar velocity samples are each in good agreement with the predicted velocities and produce consistent results for $β_{K}=Ω\sbr{m}^{0.6}/b_{K}$. Taken together the best fit $β_{K} = 0.49 \pm 0.04$. We explore the effects of morphology on the determination of $β$ by splitting the 2MASS sample into E+S0 and S+Irr density fields and find both samples are equally good tracers of the underlying dark matter distribution, but that early-types are more clustered by a relative factor $b\sbr{E}/b\sbr{S} \sim 1.6$. The density fluctuations of 2MASS galaxies in $8 h^{-1}$ Mpc spheres in the local volume is found to be $σ\sbr{8,K} = 0.9$. From this result and our value of $β_{K}$, we find $σ_8 (Ω\sbr{m}/0.3)^{0.6} = 0.91\pm0.12$. This is in excellent agreement with results from the IRAS redshift surveys, as well as other cosmological probes. Combining the 2MASS and IRAS peculiar velocity results yields $σ_8 (Ω\sbr{m}^/0.3)^{0.6} = 0.85\pm0.05$.

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arXiv2005

Mass-to-Light Ratios of Galaxy Groups from Weak Lensing

We present the findings of our weak lensing study of a sample of 116 CNOC2 galaxy groups. The lensing signal is used to estimate the mass-to-light ratio of these galaxy groups. The best fit isothermal sphere model to our lensing data has an Einstein radius of 0.88"+/-0.12", which corresponds to a shear-weighted velocity dispersion of 245+/-18 km/s. The mean mass-to-light ratio within 1 h^-1 Mpc is 185+/-28 h times solar in the B-band and is independent of radius from the group center. The signal-to-noise ratio of the shear measurement is sufficient to split the sample into subsets of "poor" and "rich" galaxy groups. The poor galaxy groups were found to have an average velocity dispersion of 193+/-38 km/s and a mass-to-light ratio of 134+/-26 h times solar in the B-band, while the rich galaxy groups have a velocity dispersion of 270+/-39 km/s and a mass-to-light ratio of 278+/-42 h times solar in the B-band, similar to the mass-to-light ratio of clusters. This steep increase in the mass-to-light ratio as a function of mass, suggests that the mass scale of ~10^13 solar masses is where the transition between the actively star-forming field environment and the passively-evolving cluster environment occurs. This is the first such detection from weak lensing.

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