arXiv subjects

Russell J. Smith

Publications and source records attributed to Russell J. Smith.

At least 37 records · Page 2Linked to original sources

arXiv2017

The Initial Mass Function in the Nearest Strong Lenses from SNELLS: Assessing the Consistency of Lensing, Dynamical, and Spectroscopic Constraints

We present new observations of the three nearest early-type galaxy (ETG) strong lenses discovered in the SINFONI Nearby Elliptical Lens Locator Survey (SNELLS). Based on their lensing masses, these ETGs were inferred to have a stellar initial mass function (IMF) consistent with that of the Milky Way, not the bottom-heavy IMF that has been reported as typical for high-$σ$ ETGs based on lensing, dynamical, and stellar population synthesis techniques. We use these unique systems to test the consistency of IMF estimates derived from different methods. We first estimate the stellar $M_*/L$ using lensing and stellar dynamics. We then fit high-quality optical spectra of the lenses using an updated version of the stellar population synthesis models developed by Conroy & van Dokkum. When examined individually, we find good agreement among these methods for one galaxy. The other two galaxies show 2-3$σ$ tension with lensing estimates, depending on the dark matter contribution, when considering IMFs that extend to 0.08 Msol. Allowing a variable low-mass cutoff or a nonparametric form of the IMF reduces the tension among the IMF estimates to $<2σ$. There is moderate evidence for a reduced number of low-mass stars in the SNELLS spectra, but no such evidence in a composite spectrum of matched-$σ$ ETGs drawn from the SDSS. Such variation in the form of the IMF at low stellar masses (m <~ 0.03 Msol), if present, could reconcile lensing/dynamical and spectroscopic IMF estimates for the SNELLS lenses and account for their lighter $M_*/L$ relative to the mean matched-$σ$ ETG. We provide the spectra used in this study to facilitate future comparisons.

astro-ph.GA↗

arXiv2017

An extended cold gas absorber in a central cluster galaxy

We present the serendipitous discovery of an extended cold gas structure projected close to the brightest cluster galaxy (BCG) of the z=0.045 cluster Abell 3716, from archival integral field spectroscopy. The gas is revealed through narrow Na D line absorption, seen against the stellar light of the BCG, which can be traced for $\sim$25 kpc, with a width of 2-4 kpc. The gas is offset to higher velocity than the BCG (by $\sim$100 km/s), showing that it is infalling rather than outflowing; the intrinsic linewidth is $\sim$80 km/s (FWHM). Very weak H$α$ line emission is detected from the structure, and a weak dust absorption feature is suggested from optical imaging, but no stellar counterpart has been identified. We discuss some possible interpretations for the absorber: as a projected low-surface-brightness galaxy, as a stream of gas that was stripped from an infalling cluster galaxy, or as a "retired" cool-core nebula filament.

astro-ph.GA↗

arXiv2017

Stellar dynamics in the strong-lensing central galaxy of Abell 1201: A low stellar mass-to-light ratio, a large central compact mass, and a standard dark matter halo

We analyse the stellar kinematics of the z=0.169 brightest cluster galaxy (BCG) in Abell 1201, using integral field observations with VLT/MUSE. This galaxy has a gravitationally-lensed arc located at unusually small radius ($\sim$5 kpc), allowing us to constrain the mass distribution using lensing and stellar dynamical information over the same radial range. We measure a velocity dispersion profile which is nearly flat at $σ$ $\approx$ 285 km/s in the inner $\sim$5 kpc, and then rises steadily to $σ$ $\approx$ 360 km/s at $\sim$30 kpc. We analyse the kinematics using axisymmetric Jeans models, finding that the data require both a significant dark matter halo (to fit the rising outer profile) and a compact central component, with mass $M_{\rm cen}$ $\approx$ 2.5$\times$10$^{10}$ $M_\odot$ (to fit the flat σ in the inner regions). The latter component could represent a super-massive black hole, in which case it would be among the largest known to date. Alternatively $M_{\rm cen}$ could describe excess mass associated with a gradient in the stellar mass-to-light ratio. Imposing a standard NFW dark matter density profile, we recover a stellar mass-to-light ratio $Υ$ which is consistent with a Milky-Way-like initial mass function (IMF). By anchoring the models using the lensing mass constraint, we break the degeneracy between $Υ$ and the inner slope $γ$ of the dark matter profile, finding $γ$=1.0$\pm$0.1, consistent with the NFW form. We show that our results are quite sensitive to the treatment of the central mass in the models. Neglecting $M_{\rm cen}$ biases the results towards both a heavier-than-Salpeter IMF and a shallower-than-NFW dark matter slope ($γ$ $\approx$ 0.5).

astro-ph.GA↗

arXiv2017

KINETyS: Constraining spatial variations of the stellar initial mass function in early-type galaxies

The heavyweight stellar initial mass function (IMF) observed in the cores of massive early-type galaxies (ETGs) has been linked to formation of their cores in an initial swiftly-quenched rapid starburst. However, the outskirts of ETGs are thought to be assembled via the slow accumulation of smaller systems in which the star formation is less extreme; this suggests the form of the IMF should exhibit a radial trend in ETGs. Here we report radial stellar population gradients out to the half-light radii of a sample of eight nearby ETGs. Spatially resolved spectroscopy at 0.8-1.35μm from the VLT's KMOS instrument was used to measure radial trends in the strengths of a variety of IMF-sensitive absorption features (including some which are previously unexplored). We find weak or no radial variation in some of these which, given a radial IMF trend, ought to vary measurably, e.g. for the Wing-Ford band we measure a gradient of +0.06$\pm$0.04 per decade in radius. Using stellar population models to fit stacked and individual spectra, we infer that the measured radial changes in absorption feature strengths are primarily accounted for by abundance gradients which are fairly consistent across our sample (e.g. we derive an average [Na/H] gradient of -0.53$\pm$0.07). The inferred contribution of dwarf stars to the total light typically corresponds to a bottom heavy IMF, but we find no evidence for radial IMF variations in the majority of our sample galaxies.

astro-ph.GA↗

arXiv2017

A counter-image to the gravitational arc in Abell 1201: Evidence for IMF variations, or a $10^{10}$M$_{\odot}$ black hole?

Abell 1201 is a massive galaxy cluster at z=0.169 with a brightest cluster galaxy (BCG) that acts as a gravitational lens to a background source at z=0.451. The lensing configuration is unusual, with a single bright arc formed at small radius $\sim$2 arcsec), where stars and dark matter are both expected to contribute substantially to the total lensing mass. Here, we present deep spectroscopic observations of the Abell 1201 BCG with MUSE, which reveal emission lines from a faint counter-image, opposite to the main arc, at a radius of 0.6 arcsec. We explore models in which the lensing mass is described by a combination of stellar mass and a standard dark-matter halo. The counter-image is not predicted in such models, unless the dark-matter component is negligible, which would imply an extremely heavy stellar initial mass function (IMF) in this galaxy. We consider two modifications to the model which can produce the observed configuration without resorting to extreme IMFs. Imposing a radial gradient in the stellar mass-to-light ratio, $Υ$, can generate a counter-image close to the observed position if $Υ$ increases by $\gtrsim$60 per cent within the inner $\sim$1 arcsec (e.g. variation from a Milky-Way-like to a Salpeter-like IMF). Alternatively, the counter-image can be produced by introducing a central super-massive black hole. The required mass is $M_{\rm BH}$ = (1.3$\pm$0.6)$\times$10$^{10}$ M$_\odot$, which is comparable to the largest black holes known to date, several of which are also hosted by BCGs. We comment on future observations which promise to distinguish between these alternatives.

astro-ph.GA↗

arXiv2016

A strong-lensing elliptical galaxy in the MaNGA survey

I report discovery of a new galaxy-scale gravitational lens system, identified using public data from the MaNGA survey, as part of a systematic search for lensed background line-emitters. The lens is SDSS J170124.01+372258.0, a giant elliptical galaxy with velocity dispersion $σ=256$ km/s, at a redshift of $z_l=0.122$. After modelling and subtracting the target galaxy light, the integral-field data-cube reveals [OII], [OIII] and H$β$ emission lines corresponding to a source at $z_s=0.791$, forming an identifiable ring around the galaxy center. The Einstein radius is $R_{Ein} \approx 2.3$ arcsec, projecting to ~5 kpc at the distance of the lens. The total projected lensing mass is $(3.6\pm0.6) \times 10^{11} M_\odot$, and the total J-band mass-to-light ratio is $3.0\pm0.7$ solar units. Plausible estimates of the likely dark matter content could reconcile this with a Milky-Way-like initial mass function (for which M/L~1.5 is expected), but heavier IMFs are by no means excluded with the present data. An alternative interpretation of the system, with a more complex source plane, is also discussed. The discovery of this system bodes well for future lens searches based on MaNGA and other integral-field spectroscopic surveys.

astro-ph.GA↗

arXiv2015

The IMF-sensitive 1.14-micron Na I doublet in early-type galaxies

We present J-band spectroscopy of passive galaxies focusing on the Na I doublet at 1.14 μm. Like the Na I 0.82 μm doublet, this feature is strong in low-mass stars and hence may provide a useful probe of the initial mass function (IMF). From high signal-to-noise composite spectra, we find that Na I 1.14 μm increases steeply with increasing velocity dispersion, σ, and for the most massive galaxies (σ> 300 km/s) is much stronger than predicted from synthetic spectra with Milky-Way-like IMFs and solar abundances. Reproducing Na I 1.14 μm at high σ likely requires either a very high [Na/H], or a bottom-heavy IMF, or a combination of both. Using the Na D line to break the degeneracy between IMF and abundance, we infer [Na/H] $\approx$ +0.5 and a steep IMF (single-slope-equivalent x $\approx$ 3.2, where x = 2.35 for Salpeter), for the high-σgalaxies. At lower mass (σ = 50-100 km/s), the line strengths are compatible with MW-like IMFs and near-solar [Na/H]. We highlight two galaxies in our sample where strong gravitational lensing masses favour MW-like IMFs. Like the high-σ sample on average, these galaxies have strong Na I 1.14 μm; taken in isolation their sodium indices imply bottom-heavy IMFs which are hard to reconcile with the lensing masses. An alternative full-spectrum-fitting approach, applied to the high-σsample, recovers an IMF less heavy than Salpeter, but under-predicts the Na I 1.14 μm line at the 5σ level. We conclude that current models struggle to reproduce this feature in the most massive galaxies without breaking other constraints, and caution against over-reliance on the sodium lines in spectroscopic IMF studies.

astro-ph.GA↗

arXiv2015

The SINFONI Nearby Elliptical Lens Locator Survey: Discovery of two new low-redshift strong lenses and implications for the initial mass function in giant early-type galaxies

We present results from a blind survey to identify strong gravitational lenses among the population of low-redshift early-type galaxies. The SINFONI Nearby Elliptical Lens Locator Survey (SNELLS) uses integral-field infrared spectroscopy to search for lensed emission line sources behind massive lens candidates at $z$<0.055. From 27 galaxies observed, we have recovered one previously-known lens (ESO325-G004) at $z$=0.034, and discovered two new systems, at $z$=0.031 and $z$=0.052. All three lens galaxies have high velocity dispersions (σ>300 km/s) and α-element abundances ([Mg/Fe]>0.3). From the lensing configurations we derive total J-band mass-to-light ratios of 1.8$\pm$0.1, 2.1$\pm$0.1 and 1.9$\pm$0.2 within the $\sim$2 kpc Einstein radius. Correcting for estimated dark-matter contributions, and comparing to stellar population models with a Milky Way (Kroupa) initial mass function (IMF), we determine the "mass excess factor", α. Assuming the lens galaxies have "old" stellar populations (10$\pm$1 Gyr), the average IMF mass factor is $\langleα\rangle$=1.10$\pm$0.08$\pm$0.10, where the first error is random and the second is systematic. If we instead fit the stellar populations from 6dF optical survey spectra, all three galaxies are consistent with being old, but the age errors are 3-4 Gyr, due to limited signal-to-noise. The IMF constraints are therefore looser in this case, with $\langleα\rangle$ = $1.23^{+0.16}_{-0.13}\pm{0.10}$. Our results are thus consistent with a Kroupa IMF (α=1.00) on average, and strongly reject very heavy IMFs with α>2. A Salpeter IMF (α=1.55) is inconsistent at the 3.5$σ$ level if the galaxies are old, but cannot be excluded using age constraints derived from the currently-available optical spectra.

astro-ph.GA↗

arXiv2014

Variations in the initial mass function in early-type galaxies: A critical comparison between dynamical and spectroscopic results

I present a comparison between published dynamical (ATLAS3D) and spectroscopic (Conroy & van Dokkum) constraints on the stellar initial mass function (IMF) in early-type galaxies, using the 34 galaxies in common between the two works. Both studies infer an average IMF mass factor $α$ (the stellar mass relative to a Kroupa-IMF population of similar age and metallicity) greater than unity, i.e. both methods favour an IMF which is heavier than that of the Milky Way, on average over the sample. However, on a galaxy-by-galaxy basis, there is no correlation between $α$ inferred from the two approaches. I investigate how the two estimates of $α$ are correlated systematically with the galaxy velocity dispersion, $σ$, and with the Mg/Fe abundance ratio. The spectroscopic method, based on the strengths of metal absorption lines, yields a correlation only with metal abundance ratios: at fixed Mg/Fe, there is no residual correlation with $σ$. The dynamical method, applied to exactly the same galaxy sample, yields the opposite result: the IMF variation correlates only with dynamics, with no residual correlation with Mg/Fe after controlling for $σ$. Hence although both methods indicate a heavy IMF on average in ellipticals, they lead to incompatible results for the systematic trends, when applied to the same set of galaxies. The sense of the disagreement could suggest that one (or both) of the methods has not accounted fully for the main confounding factors, i.e. element abundance ratios or dark matter contributions. Alternatively, the poor agreement might indicate additional variation in the detailed shape of the IMF, beyond what can currently be inferred from the spectroscopic features.

astro-ph.GA↗

arXiv2014

Dissecting the Red Sequence: The Bulge and Disc Colours of Early-Type Galaxies in the Coma Cluster

We explore the internal structure of red sequence galaxies in the Coma cluster across a wide range of luminosities ($-17>M_g>-22$) and cluster-centric radii ($0<r_{\rm{cluster}}<1.3 r_{200}$). We present the 2D bulge-disc decomposition of galaxies in deep Canada-France-Hawaii Telescope $u,g,i$ imaging using GALFIT. Rigorous filtering is applied to identify an analysis sample of 200 galaxies which are well described by an `archetypal' S0 structure (central bulge + outer disc). We consider internal bulge and/or disc colour gradients by allowing component sizes to vary between bands. Gradients are required for $30\%$ of analysis sample galaxies. Bulge half-light radii are found to be uncorrelated with galaxy luminosity ($R_e \sim 1$ kpc, $n\sim2$) for all but the brightest galaxies ($M_g<-20.5$). The S0 discs are brighter (at fixed size, or smaller at fixed luminosity) than those of star-forming spirals. A similar colour-magnitude relation is found for both bulges and discs. The global red sequence for S0s in Coma hence results from a combination of both component trends. We measure an average bulge $-$ disc colour difference of $0.09\pm0.01$ mag in $g-i$, and $0.16\pm0.01$ mag in $u-g$. Using simple stellar population models, bulges are either $\sim2$-$3\times$ older, or $\sim2\times$ more metal-rich than discs. The trend towards bluer global S0 colours observed further from Coma's core is driven by a significant correlation in disc colour with cluster-centric radius. An equivalent trend is detected in bulge colours at a marginal significance level. Our results therefore favour environment-mediated mechanisms of disc fading as the dominant factor in S0 formation.

astro-ph.GA↗

arXiv2014

Barred S0 Galaxies in the Coma Cluster

This study uses r-band images from the Eighth Data Release of the Sloan Digital Sky Survey (SDSS DR8) to study bars in lenticular (S0) galaxies in one of the nearest rich cluster environments, the Coma cluster. We develop techniques for bar detection, and assess their success when applied to SDSS image data. To detect and characterise bars we perform 2D bulge+disk+bar light decompositions of galaxy images with GALFIT. Using a sample of artificial galaxy images we determine the faintest magnitude at which bars can be successfully measured at the depth and resolution of SDSS. We perform detailed decompositions of 83 S0 galaxies in Coma, 64 from a central sample, and 19 from a cluster outskirts sample. For the central sample, the S0 bar fraction is 72^{+5}_{-6}%. This value is significantly higher than that obtained using an ellipse fitting method for bar detection, 48^{+6}_{-6}%. At a fixed luminosity, barred S0s are redder in (g-r) colour than unbarred S0s by 0.02 mag. The frequency and strength of bars increase towards fainter luminosities. Neither central metallicity nor stellar age distributions differ significantly between barred and unbarred S0s. There is an increase in the bar fraction towards the cluster core, but this is at a low significance level. Bars have at most a weak correlation with cluster-centric radius.

astro-ph.GA↗

arXiv2013

Quenching star formation in cluster galaxies

In order to understand the processes that quench star formation within rich clusters, we construct a library of subhalo orbits drawn from $Λ$CDM cosmological N-body simulations of four rich clusters. The orbits are combined with models of star formation followed by quenching in the cluster environment. These are compared with observed bulge and disc colours and stellar absorption linestrength indices of satellite galaxies. Models in which the bulge stellar populations depend only on the galaxy subhalo mass while the disc quenching depends on the cluster environment are acceptable fits to the data. An exponential disc quenching timescale of 3 - 3.5 Gyr is preferred. Models with short ($\lesssim 1$ Gyr) quenching timescales yield cluster-centric gradients in disc colours and Balmer line indices that are too steep compared to observations. We also examine models in which there is quenching in lower mass groups prior to cluster infall ("pre-processing"), finding that such models are a better fit to the data than models without pre-processing and require similar quenching times. The data slightly prefer models where quenching occurs only for galaxies falling within about 0.5 $r_{200}$. Finally, we have examined models with short quenching timescales of 1 Gyr, but a long delay time of 3 Gyr prior to quenching. All models with short quenching timescales, even such "delayed-then-rapid" quenching models, produce excessively red galaxies near the cluster core and are strongly disfavoured by the data. These results imply that the environments of rich clusters must impact star formation rates of infalling galaxies on relatively long timescales -- several times longer than a typical halo spends within the virial radius of a cluster. This scenario favours gentler quenching mechanisms such as slow "strangulation" over more rapid ram-pressure stripping.

astro-ph.CO↗

arXiv2013

S0 galaxies in the Coma cluster: Environmental dependence of the S0 offset from the Tully-Fisher relation

We present deep GMOS long-slit spectroscopy of 15 Coma cluster S0 galaxies, and extract kinematic properties along the major axis to several times the disc scale-length. Supplementing our dataset with previously published data, we create a combined sample of 29 Coma S0s, as well as a comparison sample of 38 Coma spirals. Using photometry from SDSS and 2MASS, we construct the Tully-Fisher relation (TFR; luminosity versus maximum rotational velocity) for S0 galaxies. At fixed rotational velocity, the Coma S0 galaxies are on average fainter than Coma spirals by 1.10$\pm$0.18, 0.86$\pm$0.19 and 0.83$\pm$0.19 mag in the g, i and Ks bands respectively. The typical S0 offsets remain unchanged when calculated relative to large field-galaxy spiral samples. The observed offsets are consistent with a simple star formation model in which S0s are identical to spirals until abrupt quenching occurs at some intermediate redshift. The offsets form a continuous distribution tracing the time since the cessation of star formation, and exhibit a strong correlation (>6σ) with residuals from the optical colour-magnitude relation. Typically, S0s which are fainter than average for their rotational velocity are also redder than average for their luminosity. The S0 TFR offset is also correlated with both the projected cluster-centric radius and the Σ (projected) local density parameter. Since current local environment is correlated with time of accretion into the cluster, our results support a scenario in which transformation of spirals to S0s is triggered by cluster infall.

astro-ph.CO↗

arXiv2013

A giant elliptical galaxy with a lightweight initial mass function

We present new VLT observations of the closest-known strong-lensing galaxy, the sigma=330 km/s giant elliptical ESO325-G004. The low redshift of the lens (z_lens=0.035) results in arcs being formed at a small fraction of the effective radius, (R_Ein ~ R_Eff/4). At such small radii, stars dominate the lensing mass, so that lensing provides a direct probe of the stellar mass-to-light ratio, with only small corrections needed for dark matter. The redshift of the galaxy lensed by ESO325-G004 was unknown until now, so the lensing mass was not securely determined. Using X-SHOOTER, we measure a secure source redshift of z_src=2.141. Combined with the lensing configuration, this yields a total mass inside the Einstein radius of (1.50+/-0.06) 10^11 M_sun. We estimate the range of possible contribution of dark matter to the lensing mass, using statistics from cosmological N-body simulations. Subtracting this component yields a stellar M/L ratio for the lens of 3.14^+0.24_-0.42 (M/L)_sun (in F814W band). Using VIMOS, we have also obtained very high-S/N spectroscopy for the lens galaxy. The VIMOS data confirm that ESO325-G004 has a very old stellar population. For a Kroupa IMF, the stellar population fit yields a predicted stellar M/L ratio of 3.01+/-0.25 (M/L)_sun. Hence the mass attributable to stars with this IMF is consistent with the lensing estimate. By contrast, a Salpeter or heavier IMF is disfavoured at the 99.8% confidence level. A "heavyweight" IMF, with a mass twice as large as the Kroupa case, is firmly excluded. Such an IMF has been proposed for more distant elliptical lenses, and also to explain strong dwarf-star spectral features, in particular the NaI 8200-Ang doublet. A FORS2 far-red spectrum shows that this feature is as strong in ESO325-G004 as it is in other high-sigma ellipticals, suggesting tension between dwarf-star indicators and lensing-mass constraints for this galaxy.

astro-ph.CO↗

arXiv2012

The stellar initial mass function in red-sequence galaxies: 1-micron spectroscopy of Coma Cluster galaxies with Subaru/FMOS

To investigate possible variations in the stellar initial mass function (IMF) in red-sequence galaxies, we have obtained infrared spectroscopy with Subaru/FMOS for a sample of 92 red-sequence galaxies in the Coma cluster. Velocity dispersions, ages and element abundances for these galaxies have been previously determined from optical data. By stacking the FMOS spectra in the rest frame, removing sky-subtraction residuals and other artefacts fixed in the observed frame, we derive composite spectra in the 9600-10500 Angstrom range for galaxies grouped according to their velocity dispersion or Mg/Fe ratio. We measure the Wing-Ford band of FeH and a new index centred on a CaI line at 10345 Angstrom; these features are strong in cool dwarf stars, and hence reflect the form of the IMF at low mass (<0.5M_sun) The CaI line, unlike the Wing-Ford band and other `classical' IMF indicators (NaI doublet, CaII triplet), is unaffected by the abundance of sodium. We compare the measured indices against predictions from spectral synthesis models matched to the element abundances estimated from the optical data. Binning galaxies by velocity dispersion, we find that both the Wing-Ford and CaI index measurements are best reproduced by models with the Salpeter IMF. There is no clear evidence for an increase in dwarf-star content with velocity dispersion. Binning the observed galaxies instead by Mg/Fe ratio, the behaviour of both indices implies a trend of IMF from Chabrier-like, at abundance ratios close to solar, to Salpeter or heavier for highly alpha-enhanced populations. At face value, this suggests that the IMF depends on the mode of star formation, with intense rapid star-bursts generating a larger population of low-mass stars.

astro-ph.CO↗

arXiv2012

What drives the ultra-violet colours of passive galaxies?

We present and analyse optical and ultra-violet colours for passive and optically-red Coma cluster galaxies for which we have spectroscopic age and element abundance estimates. Our sample of 150 objects covers a wide range in mass, from giant ellipticals to the bright end of the dwarf-galaxy regime. We focus on the colours FUV-i, NUV-i, FUV-NUV, u*-g and g-i. We find that all of these colours are correlated with both luminosity and velocity dispersion at the >5 sigma level, with FUV-i and FUV-NUV becoming bluer with increasing `mass' while the other colours become redder. We perform an empirical analysis to assess what fraction of the variation in each colour can be accounted for by variations in the average stellar populations, as traced by the optical spectra. For u*-g and g-i, most of the observed scatter (~80% after allowing for measurement errors and for systematic errors in u*-g) is attributable to stellar population variations, with colours becoming redder with increasing age and metallicity (Mg/H). The FUV-i colour becomes bluer with increasing age and with increasing Mg/H, favouring a `metal-rich single-star' origin for the UV upturn. However, correlations with the optically-dominant stellar populations account for only about half of the large observed scatter. We propose that the excess scatter in FUV-i may be due to a varying proportion of ancient stars in galaxies with younger average ages. The NUV-i colour is sensitive to age and Mg/H, but exhibits excess scatter that can be attributed to `leakage' of the FUV upturn. Applying a correction based on the FUV-i colour we account for ~80% of the variance in NUV-i, as for the optical colours. The FUV-NUV colour shows strong correlations with age and Mg/H, and little residual scatter. Interpreting this colour is complicated however, since it mixes the effects from the main-sequence turn-off with those from the hot evolved stars.

astro-ph.CO↗

arXiv2011

Environmental quenching and hierarchical cluster assembly: Evidence from spectroscopic ages of red-sequence galaxies in Coma

We explore the variation in stellar population ages for Coma cluster galaxies as a function of projected cluster-centric distance, using a sample of 362 red-sequence galaxies with high signal-to-noise spectroscopy. The sample spans a wide range in luminosity (0.02-4 L*) and extends from the cluster core to near the virial radius. We find a clear distinction in the observed trends of the giant and dwarf galaxies. The ages of red-sequence giants are primarily determined by galaxy mass, with only weak modulation by environment, in the sense that galaxies at larger cluster-centric distance are slightly younger. For red-sequence dwarfs (with mass <10^10 Msun), the roles of mass and environment as predictors of age are reversed: there is little dependence on mass, but strong trends with projected cluster-centric radius are observed. The average age of dwarfs at the 2.5 Mpc limit of our sample is approximately half that of dwarfs near the cluster centre. The gradient in dwarf galaxy ages is a global cluster-centric trend, and is not driven by the ongoing merger of the NGC 4839 group to the south west of Coma. We interpret these results using environmental histories extracted from the Millennium Simulation for members of massive clusters. Hierarchical cluster assembly naturally leads to trends in the accretion times of galaxies as a function of projected cluster-centric radius. On average, simulated galaxies now located in cluster cores joined halos above any given mass threshold earlier than those now located in the outskirts of clusters. We test environmental quenching models, in which star formation is halted in galaxies when they enter halos of a given mass, or become satellite galaxies. The models broadly reproduce the gradients observed in Coma, but for dwarf galaxies the efficiency of environmental quenching must be very high to match the strong trends observed.

astro-ph.CO↗

arXiv2011

The spatial distribution and origin of the FUV excess in early-type galaxies

We present surface photometry of a sample of 52 galaxies from the GALEX and 2MASS data archives, these include 32 normal elliptical galaxies, 10 ellipticals with weak Liner or other nuclear activity, and 10 star forming ellipticals or early-type spirals. We examine the spatial distribution of the Far Ultra-Violet excess in these galaxies, and its correlation with dynamical and stellar population properties of the galaxies. From aperture photometry we find that all galaxies except for recent major remnants and galaxies with ongoing star formation show a positive gradient in the (FUV-NUV) colour determined from the GALEX images. The logarithmic gradient does not correlate with any stellar population parameter, but it does correlate with the central velocity dispersion. The strength of the excess on the other hand, correlates with both [alpha/Fe] and [Z/H], but more strongly with the former. We derive models of the underlying stellar population from the 2MASS H-band images, and the residual of the image from this model reveals a map of the centrally concentrated FUV excess. We examine a possible hypothesis for generating the FUV excess and the radial gradient in its strength, involving a helium abundance gradient set up early in the formation process of the galaxies. If this hypothesis is correct, the persistence of the gradients to the present day places a strong limit on the importance of dry mergers in the formation of ellipticals.

astro-ph.CO↗