The most massive galaxies in clusters are already fully grown at z∼0.5z \sim 0.5

By constructing scaling relations for galaxies in the massive cluster MACSJ0717.5 at $z=0.545$ and comparing with those of Coma, we model the luminosity evolution of the stellar populations and the structural evolution of the galaxies. We calculate magnitudes, surface brightnesses and effective radii using HST/ACS images and velocity dispersions using Gemini/GMOS spectra, and present a catalogue of our measurements for 17 galaxies. We also generate photometric catalogues for $\sim 3000$ galaxies from the HST imaging. With these, we construct the colour-magnitude relation, the fundamental plane, the mass-to-light versus mass relation, the mass-size relation and the mass-velocity dispersion relation for both clusters. We present a new, coherent way of modelling these scaling relations simultaneously using a simple physical model in order to infer the evolution in luminosity, size and velocity dispersion as a function of redshift, and show that the data can be fully accounted for with this model. We find that (a) the evolution in size and velocity dispersion undergone by these galaxies between $z \sim 0.5$ and $z \sim 0$ is mild, with $R_e(z) \sim (1+z)^{-0.40\pm0.32}$ and $\sigma(z) \sim (1+z)^{0.09 \pm 0.27}$, and (b) the stellar populations are old, $\sim 10$ Gyr, with a $\sim 3$ Gyr dispersion in age, and are consistent with evolving purely passively since $z \sim 0.5$ with $\Delta \log M/L_B = -0.55_{-0.07}^{+0.15} z$. The implication is that these galaxies formed their stars early and subsequently grew dissipationlessly so as to have their mass already in place by $z \sim 0.5$, and suggests a dominant role for dry mergers, which may have accelerated the growth in these high-density cluster environments.Comment: 20 pages; accepted for publication in MNRA

Measuring the low mass end of the Mbh - sigma relation

We show that high quality laser guide star (LGS) adaptive optics (AO) observations of nearby early-type galaxies are possible when the tip-tilt correction is done by guiding on nuclei while the focus compensation due to the changing distance to the sodium layer is made 'open loop'. We achieve corrections such that 40% of flux comes from R<0.2 arcsec. To measure a black hole mass (Mbh) one needs integral field observations of both high spatial resolution and large field of view. With these data it is possible to determine the lower limit to Mbh even if the spatial resolution of the observations are up to a few times larger than the sphere of influence of the black hole.Comment: 4 pages, 2 figures, LaTeX. To appear in "Hunting for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista and C.C. Popescu, AIP Conf. Ser., in pres

Rejuvenation of spiral bulges

We seek to understand whether the stellar populations of galactic bulges show fingerprints of secular evolution triggered by the presence of the disc. For this purpose we re-analyse the sample of Proctor and Sansom, deriving stellar population ages and element abundances from absorption line indices. We obtain very consistent constraints on ages from the three Balmer indices Hbeta, Hgamma, and Hdelta, in good agreement with those of Proctor and Sansom based on a completely different method. Like other studies in the literature, we find that bulges have relatively low luminosity weighted ages, the lowest age derived for the smallest bulges being 1.3 Gyr. Hence bulges are not generally old but actually rejuvenated systems. We discuss evidence that this might be true also for the bulge of the Milky Way. We show that the smallest bulges, being the youngest with the lowest alpha/Fe ratios, must have experienced star formation events involving 10-30 per cent of their total mass in the past 1-2 Gyr. No significant correlations of the stellar population parameters with Hubble Type are found instead. We show that the relationships with sigma coincide perfectly with those of early-type galaxies. Hence, bulges are typically seen younger, metal-poorer and less alpha/Fe enhanced than early-type galaxies, only because of their smaller masses. At a given velocity dispersion, bulges and elliptical galaxies are indistinguishable as far as their stellar populations are concerned. These results favour an inside-out formation scenario and indicate that the discs in spiral galaxies of Hubble types Sbc and earlier cannot have a significant influence on the evolution of the stellar populations in the bulge component. The phenomenon of pseudobulge formation must be restricted to spirals of types later than Sbc.Comment: 12 pages, 6 figures, accepted by MNRA

The Ages and Metallicities of Early Type Galaxies in the Fornax Cluster

We have measured central line strengths for a complete sample of early type galaxies in the Fornax cluster, comprising 11 elliptical and 11 lenticular galaxies, more luminous than M_B=-17. In contrast to the elliptical galaxies in the sample studied by Gonzalez (and recently revisited by Trager) we find that the Fornax ellipticals follow the locus of galaxies of fixed age in Worthey's models and have metallicities varying from roughly solar to three times solar. The lenticular galaxies however exhibit a substantial spread to younger luminosity weighted ages indicating a more extended star formation history. We present measurements of the more sensitive indices: C4668 and Hgamma_A; these confirm and re-enforce the conclusions that the elliptical galaxies are coeval and that only the lenticular galaxies show symptoms of late star-formation. The inferred difference in the age distribution between lenticular and elliptical galaxies is a robust conclusion as the models generate consistent relative ages using different age and metallicity indicators even though the absolute ages remain uncertain. The young luminosity weighted ages of the S0s in the Fornax cluster are consistent with the recent discovery that the fraction of S0 galaxies in intermediate redshift clusters is a factor of 2-3 lower than found locally and suggests that a fraction of the cluster spiral galaxy population has evolved to quiescence in the 5 Gyr interval from z=0.5 to the present. Two of the faintest lenticular galaxies in our sample have blue continua and strong Balmer-line absorption suggesting starbursts \la2 Gyrs ago. These may be the low redshift analogues of the starburst or post-starburst galaxies seen in clusters at z=0.3, similar to the Hdelta strong galaxies in the Coma cluster.Comment: 6 pages and 3 postscript figures. Accepted for publication in MNRAS (letter

On the Origin of the Colour-Magnitude Relation in the Virgo Cluster

We explore the origin of the colour-magnitude relation (CMR) of early type galaxies in the Virgo cluster using spectra of very high S/N ratio for six elliptical galaxies selected along the CMR. The data are analysed using a new evolutionary stellar population synthesis model to generate galaxy spectra at the resolution given by their velocity dispersions. In particular we use a new age indicator that is virtually free of the effects of metallicity. We find that the luminosity weighted mean ages of Virgo ellipticals are greater than ~8 Gyr, and show no clear trend with galaxy luminosity. We also find a positive correlation of metallicity with luminosity, colour and velocity dispersion. We conclude that the CMR is driven primarily by a luminosity-metallicity correlation. However, not all elements increase equally with the total metallicity and we speculate that the CMR may be driven by both a total metallicity increase and by a systematic departure from solar abundance ratios of some elements along the CMR. A full understanding of the role played by the total metallicity, abundance ratios and age in generating the CMR requires the analysis of spectra of very high quality, such as those reported here, for a larger number of galaxies in Virgo and other clusters.Comment: To appear in The Astrophysical Journal Letters, 2001 April 20 (551, number 2). 5 pages and 4 figure

Are Recent Peculiar Velocity Surveys Consistent?

We compare the bulk flow of the SMAC sample to the predictions of popular cosmological models and to other recent large-scale peculiar velocity surveys. Both analyses account for aliasing of small-scale power due to the sparse and non-uniform sampling of the surveys. We conclude that the SMAC bulk flow is in marginal conflict with flat COBE-normalized Lambda-CDM models which fit the cluster abundance constraint. However, power spectra which are steeper shortward of the peak are consistent with all of the above constraints. When recent large-scale peculiar velocity surveys are compared, we conclude that all measured bulk flows (with the possible exception of that of Lauer & Postman) are consistent with each other given the errors, provided the latter allow for `cosmic covariance'. A rough estimate of the mean bulk flow of all surveys (except Lauer & Postman) is ~400 km/s towards l=270, b=0.Comment: 8 pages, 3 figures. To appear in Proceedings of the Cosmic Flows Workshop, Victoria, B. C., Canada, July 1999, eds. S. Courteau, M. Strauss, and J. Willic