George K. T. Hau

Early-type galaxies at large galactocentric radii – II. Metallicity gradients and the [Z/H]–mass, [α/Fe]–mass relations

We present the results of a study of stellar population properties at large galactocentric radii of 14 low-mass early-type galaxies in the Fornax and Virgo clusters. We derive radial profiles of Age, total metallicity [Z/H], and [�/Fe] abundance ratios out to � 1 3 effective radii by using nearly all of the Lick/IDS absorption-line indices in comparison to recent single stellar population models. We extend our study to higher galaxy mass via a novel literature compilation of 37 early-type galaxies, which provides stellar population properties out to one effective radius. We find that metallicity gradients correlate with galactic mass, and the relationship shows a sharp change in slope at a dynamical mass of � 3:5×10 10 M� . The central and mean values of the stellar population parameters (measured in r 6 re=8, and at r = re, respectively) define positive mass trends. We suggest that the low metallicities, almost solar [�/Fe] ratios and the tight mass-metallicity gradient relation displayed by the low-mass galaxies are indicative of an early star-forming collapse with extended (i.e., > 1 Gyr), low efficiency star formation, and mass-dependent galactic outflows of metal-enriched gas. The flattening of metallicity gradients in high-mass galaxies, and the broad scatter of the relationship are attributed to merger events. The high metallicities and supersolar abundances shown by these galaxies imply a rapid, high efficiency star formation. The observed [Z/H]–mass and [�/Fe]–mass relationships can be interpreted as a natural outcome of an early star-forming collapse. However, we find that hierarchical galaxy formation models implementing mass-dependent star formation efficiency, varying IMF, energy feedback via AGN, and the effects due to merger-induced starbursts can also reproduce both our observed relationships.

The colour of galaxies in distant groups

We present new optical and near-infrared imaging for a sample of 98 spectroscopicallyselected galaxy groups at 0.25 < z < 0.55, most of which have velocity dispersions � < 500 km/s. We use psf-matched aperture photometry to measure accurate colours for group members and the surrounding field population. The samp le is statistically complete above a stellar mass limit of approximately M = 1 × 10 10 M⊙. The overall colour distribution is bimodal in both the field and group samples; but at fixed luminosity the fraction of group galaxies populating the red peak is larger, by � 20±7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in Hubble Space Telescope imaging, exhibit a tight red sequence, similar to that seen f or more massive clusters. Using optical and near-infrared colours, including data fr om the Spitzer Space Telescope, we show that approximately 20‐30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spi ral morphologies. This is true of both the field and group sample, and shows little dependence on nea r infrared luminosity. Thus, the fraction of bright ( 0.4 MK < 22) group members with no sign of star formation or AGN activity, as identified by their colours or [OII] emission, is 54 ± 6 per cent. Our field sample, which includes galaxies in all environments, contains 35±3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions, that dense enviro nments atz < 0.5 are associated with a premature cessation of star formation in some galaxies; in particular we find no evidence for significantly enhanced star formation in these environment s. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the timescale of this quenching equally for all group members distorts the colour distribution in a way that is inconsistent with observations.

Discovery of an optical counterpart to the hyperluminous X-ray source in ESO 243-49

The existence of black holes of masses � 10 2 -10 5 Mhas important implications for the formation and evolution of star clusters and supermassive black holes. One of the strongest candidates to date is the hyperluminous X-ray source HLX1, possibly located in the S0-a galaxy ESO243-49, but the lack of an identifiable optical counterpart had hampered its interpretation. Using the Magellan telescope, we have discovered an unresolved optical source with R = 23:80 ± 0:25 mag and V = 24:5 ± 0:3 mag within HLX1s positional error circle. This implies an average X-ray/optical flux ratio � 500. Taking the same distance as ESO243-49, we obtain an intrinsic brightness MR = 11:0±0:3 mag, comparable to that of a massive globular cluster. Alternatively, the optical source is consistent with a main-sequence M star in the Galactic halo (for example an M4.4 star at � 2:5 kpc). We also examined the properties of ESO243-49 by combining Swift/UVOT observations with stellar population modelling. We found that the overall emission is dominated by a � 5 Gyr old stellar population, but the UV emission at � 2000 u is mostly due to ongoing star-formation at a rate of � 0:03M� yr −1 . The UV emission is more intense (at least a 9� enhancement above the mean) North East of the nucleus, in the same quadrant as HLX1. With the combined optical and X-ray measurements, we put constraints on the nature of HLX1. We rule out a foreground star and a background AGN. Two alternative scenarios are still viable. HLX1 could be an accreting intermediate mass black hole in a star cluster, which may itself be the stripped nucleus of a dwarf galaxy that passed through ESO243-49, an event which might have caused the current episode of star formation. Or, it could be a neutron star in the Galactic halo, accreting from an M4-M5 donor star.

An ultra‐compact dwarf around the Sombrero galaxy (M104): the nearest massive UCD

We report the discovery of an ultra-compact dwarf (UCD) associated with the Sombrero galaxy (M104). This is the closest massive UCD known and the first spectroscopically verified massive UCD which is located in a low-density environment. The object, we name SUCD1, was identified in Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) imaging and confirmed to be associated with the Sombrero galaxy by its recession velocity obtained from Keck spectra. The light profile is well fitted by a Wilson model. We measure a half-light size of 14.7 ± 1.4 pc, an absolute magnitude of MV = −12.3 mag (MK =− 15.1 mag) and an internal velocity dispersion of 25.0 ± 5.6 km s −1 . Such values are typical of UCDs. From Lick spectral indices we measure a luminosity-weighted central age of 12.6 ± 0.9 Gyr, [Fe/H] of −0.08 ± 0.08 dex and [α/Fe] of 0.06 ± 0.07 dex. The lack of colour gradients suggests these values are representative of the entire UCD. The derived stellar and virial masses are the same, within errors, at ∼3.3 × 10 7 M� . Thus, we find no strong evidence for dark matter or the need to invoke a non-standard initial mass function. We also report arguably the first X-ray detection of a bona fide UCD, which we attribute to the presence of low-mass X-ray binaries. The X-ray luminosity of LX = 0.56 × 10 38 erg s −1 is consistent with the values observed for globular clusters (GCs) of the same metallicity. Overall, we find SUCD1 has properties similar to other known UCDs and massive GCs.

A spectroscopic measurement of galaxy formation timescales with ROLES

We present measurements of the specific star-formation rate (SSFR)-stellar mass relation for star-forming galaxies. Our deep spectroscopic samples are based on the Redshift One LDSS3 Emission line Survey, ROLES, and European Southern Observatory, ESO, public spectroscopy at z=1, and on the Sloan Digital Sky Survey (SDSS) at z=0.1. These datasets cover an equally deep mass range of 8.5 10) the shapes of the cumulative cosmic SFRDs are very similar at both z=0.1 and z=1.0, both showing 70% of the total SFRD above a mass of log(M*/Msun)>10. Mass functions are constructed for star-forming galaxies and found to evolve by only <35% between z=1 and z=0.1 over the whole mass range. The evolution is such that the mass function decreases with increasing cosmic time, confirming that galaxies are leaving the star-forming sequence/blue cloud. The observational results are extended to z~2 by adding two recent Lyman break galaxy samples, and data at these three epochs (z=0.1, 1, 2) are compared with the GALFORM semi-analytic model of galaxy formation. GALFORM predicts an overall SFR density (SFRD) as a function of stellar mass in reasonable agreement with the observations. The star formation timescales inferred from 1/SSFR also give reasonable overall agreement, with the agreement becoming worse at the lowest and highest masses. [abridged]

The Redshift One LDSS‐3 Emission line Survey (ROLES): survey method and z∼ 1 mass‐dependent star formation rate density

Motivated by suggestions of cosmic downsizing, in which the dominant contribution to the cosmic star formation rate density (SFRD) proceeds from higher to lower mass galaxies with increasing cosmic time, we describe the design and implementation of the Redshift One LDSS3 Emission line Survey (ROLES). This survey is designed to probe low-mass, z ∼ 1 galaxies directly for the first time with spectroscopy. ROLES is a K-selected (22.5 < K AB < 24.0) survey for dwarf galaxies [8.5 ≲ log(M * /M) ≲ 9.5] at 0.89 < z < 1.15 drawn from two extremely deep fields [Great Observatories Origins Deep Survey-S (GOODS-S) and MS1054-Faint Infra-Red Extragalactic Survey]. Using the [O II]λ727 emission line, we obtain redshifts and star formation rates (SFRs) for star-forming galaxies down to a limit of ∼0.3 M yr ―1 . We present the [O II] luminosity function measured in ROLES and find a faint-end slope of α faint ∼ ―1.5, similar to that measured at z ∼ 0.1 in the Sloan Digital Sky Survey. By combining ROLES with higher mass surveys (Gemini Deep Deep Survey and European Southern Observatory GOOD-S public spectroscopy) we measure the SFRD as a function of stellar mass using [O II] (with and without various empirical corrections) and using spectral energy distribution fitting to obtain the SFR from the rest-frame UV luminosity for galaxies with spectroscopic redshifts. Our best estimate of the corrected [O II] SFRD and UV SFRD both independently show that the SFRD evolves equally for galaxies of all masses between z ∼ 1 and z ∼ 0.1. The exact evolution in normalization depends on the indicator used, with the [O II]-based estimate showing a change of a factor of ≈2.6 and the UV-based estimate a factor of ≈6. We discuss possible reasons for the discrepancy in normalization between the indicators, but note that the magnitude of this uncertainty is comparable to the discrepancy between indicators seen in other z ∼ 1 works. Our result that the shape of the SFRD as a function of stellar mass (and hence the mass range of galaxies dominating the SFRD) does not evolve between z ∼ 1 and z ∼ 0.1 is robust to the choice of indicator.

Dependence of star formation activity on stellar mass and environment from the Redshift One LDSS-3 Emission line Survey

Using the sample from the Redshift One LDSS-3 Emission line Survey (ROLES), we probe the dependence of star formation rate (SFR) and specific star formation rate (sSFR) as a function of stellar mass M * and environment as defined by local galaxy density, in the Chandra Deep Field South field. Our spectroscopic sample consists of 312 galaxies with K AB 8.5, and with [O II] derived SFR > 0.3 M ⊙ yr -1 , at 0.889 ≤ z ≤ 1.149. The results have been compared directly with the Sloan Digital Sky Survey Stripe 82 sample at 0.032 ≤ z ≤ 0.05. For star-forming galaxies, we confirm that there is little correlation between SFR and density at z ∼ 0. However, for the lowest mass galaxies in our z ∼ 1 sample, those with log(M * /M ⊙ ) < 10, we find that both the median SFR and sSFR increase significantly with increasing local density. The downsizing trend for low-mass galaxies to be quenched progressively later in time appears to be more pronounced in moderately overdense environments. Overall we find that the evolution of star formation in galaxies is most strongly driven by their stellar mass, with local galaxy density playing a role that becomes increasingly important for lower mass galaxies.

Bridging the gap between low- and high-mass dwarf galaxies

While the dark matter content within the most-massive giant and smallest dwarf galaxies has been probed – spanning a range of over one million in mass – an important observational gap remains for galaxies of intermediate mass. This gap covers K-band magnitudes of approximately −16 > MK > −18 mag (for which dwarf galaxies have B−K∼ 2). On the high-mass side of the gap are dwarf elliptical (dE) galaxies that are dominated by stars in their inner regions. While the low-mass side includes dwarf spheroidal (dSph) galaxies that are dark matter dominated and ultracompact dwarf (UCD) objects that are star-dominated. Evolutionary pathways across the gap have been suggested but remain largely untested because the ‘gap’ galaxies are faint, making dynamical measurements very challenging. nWith long exposures on the Keck telescope using the Echelle Spectrograph and Imager instrument, we have succeeded in bridging this gap by measuring the dynamical mass for five dwarf galaxies with M_K ∼ −17.5 (M_B ∼ −15.5). With the exception of our brightest dwarf galaxy, they possess relatively flat velocity dispersion profiles of around 20 km s^(−1). By examining their 2D scaling relations and 3D fundamental manifold, we found that the sizes and velocity dispersions of these gap galaxies reveal continuous trends from dE to dSph galaxies. We conclude that low-luminosity dE galaxies are dominated by stars, not by dark matter, within their half light radii. This finding can be understood if internal feedback processes are operating most efficiently in gap galaxies, gravitationally heating the centrally located dark matter to larger radii, whereas external environmental processes, which can strip away stars, have a greater influence on dSph galaxies, resulting in their higher dark matter fractions. UCD objects appear to be more similar to massive compact star clusters than to small galaxies. Our dynamical study of low-mass dE galaxies provides further constraints on the processes that shape some of the smallest and most-numerous galaxies in the Universe.

The outer halo globular clusters of M31

We present Keck/HIRES spectra of three globular clusters in the outer halo of M31, at projected distances beyond ≈80 kpc from M31. The measured recession velocities for all three globular clusters confirm their association with the globular cluster system of M31. We find evidence for a declining velocity dispersion with radius for the globular cluster system. Their measured internal velocity dispersions, derived virial masses and mass-to-light ratios are consistent with those for the bulk of the M31 globular cluster system. We derive old ages and metallicities which indicate that all three belong to the metal-poor halo globular cluster subpopulation. We find indications that the radial gradient of the mean metallicity of the globular cluster system interior to 50 kpc flattens in the outer regions, however it is still more metal-poor than the corresponding field stars at the same (projected) radius.

Early-type galaxies at large galactocentric radii: I. Stellar kinematics and photometric properties

We present the results of a combined analysis of the kinematic and photometric properties at large galactocentric radii of a sample of 14 low-luminosity early-type galaxies in the Fornax and Virgo clusters. From Gemini South GMOS long-slit spectroscopic data, we measure radial profiles of the kinematic parameters v rot , σ, h 3 and h 4 out to ∼1―3 effective radii. Multiband imaging data from the Hubble Space Telescope/ACS are employed to evaluate surface brightness profiles and isophotal shape parameters of ellipticity, position angle and discyness/boxiness. The galaxies are found to host a cold and old stellar component which extend to the largest observed radii and that is the dominant source of their dynamical support. The prevalence of discy-shaped isophotes and the radial variation of their ellipticity are signatures of a gradual gas dissipation. An early star-forming collapse appears to be the main mechanism acting in the formation of these objects. Major mergers are unlikely to have occurred in these galaxies. We cannot rule out a minor merging origin for these galaxies, but a comparison of our results with model predictions of different merger categories places some constraints on the possible merger progenitors. These merger events are required to happen at high-redshift (i.e. z ≥ 1), between progenitors of different mass ratio (at least 3:1) and containing a significant amount of gas (i.e. ≥10 per cent). A further scenario is that the low-luminosity galaxies were originally late-type galaxies, whose star formation has been truncated by removal of gas and subsequently the disc has been dynamically heated by high-speed encounters in the cluster environment.