Old and young bulges in late-type disk galaxies

ABRIDGED: We use HSTACS and NICMOS imaging to study the structure and colors of a sample of nine late-type spirals. We find: (1) A correlation between bulge and disks scale-lengths, and a correlation between the colors of the bulges and those of the inner disks. Our data show a trend for bulges to be more metal-enriched than their surrounding disks, but otherwise no simple age-metallicity connection between these systems; (2) A large range in bulge stellar population properties, and, in particular, in stellar ages. Specifically, in about a half of the late-type bulges in our sample the bulk of the stellar mass was produced recently. Thus, in a substantial fraction of the z=0 disk-dominated bulged galaxies, bulge formation occurs after the formation/accretion of the disk; (3) In about a half of the late-type bulges in our sample, however, the bulk of the stellar mass was produced at early epochs; (4) Even these "old" late-type bulges host a significant fraction of stellar mass in a young(er) c component; (5) A correlation for bulges between stellar age and stellar mass, in the sense that more massive late-type bulges are older than less massive late-type bulges. Since the overall galaxy luminosity (mass) also correlates with the bulge luminosity (mass), it appears that the galaxy mass regulates not only what fraction of itself ends up in the bulge component, but also "when" bulge formation takes place. We show that dynamical friction of massive clumps in gas-rich disks is a plausible disk-driven mode for the formation of "old" late-type bulges. If disk evolutionary processes are responsible for the formation of the entire family of late-type bulges, CDM simulations need to produce a similar number of initially bulgeless disks in addition to the disk galaxies that are observed to be bulgeless at z=0.Comment: ApJ in press; paper with high resolution figures available at http://www.exp-astro.phys.ethz.ch/carollo/carollo1_2006.pdf; B, I, and H surface brightness profiles published in electronic tabular for

The Hubble Sequence in Groups: The Birth of the Early-Type Galaxies

The physical mechanisms and timescales that determine the morphological signatures and the quenching of star formation of typical (~L*) elliptical galaxies are not well understood. To address this issue, we have simulated the formation of a group of galaxies with sufficient resolution to track the evolution of gas and stars inside about a dozen galaxy group members over cosmic history. Galaxy groups, which harbor many elliptical galaxies in the universe, are a particularly promising environment to investigate morphological transformation and star formation quenching, due to their high galaxy density, their relatively low velocity dispersion, and the presence of a hot intragroup medium. Our simulation reproduces galaxies with different Hubble morphologies and, consequently, enables us to study when and where the morphological transformation of galaxies takes place. The simulation does not include feedback from active galactic nuclei showing that it is not an essential ingredient for producing quiescent, red elliptical galaxies in galaxy groups. Ellipticals form, as suspected, through galaxy mergers. In contrast with what has often been speculated, however, these mergers occur at z>1, before the merging progenitors enter the virial radius of the group and before the group is fully assembled. The simulation also shows that quenching of star formation in the still star-forming elliptical galaxies lags behind their morphological transformation, but, once started, is taking less than a billion years to complete. As long envisaged the star formation quenching happens as the galaxies approach and enter the finally assembled group, due to quenching of gas accretion and (to a lesser degree) stripping. A similar sort is followed by unmerged, disk galaxies, which, as they join the group, are turned into the red-and-dead disks that abound in these environments.Comment: 12 pages, 12 Figures, 1 Table, accepted for publication in AP

Geometric phases and criticality in spin systems

A general formalism of the relation between geometric phases produced by circularly evolving interacting spin systems and their criticality behavior is presented. This opens up the way for the use of geometric phases as a tool to study regions of criticality without having to undergo a quantum phase transition. As a concrete example a spin-1/2 chain with XY interactions is presented and the corresponding geometric phases are analyzed. The generalization of these results to the case of an arbitrary spin system provides an explanation for the existence of such a relation.Comment: 12 pages, 4 figure

On the relation between sSFR and metallicity

In this paper we present an exact general analytic expression $Z(sSFR)=y/\Lambda(sSFR)+I(sSFR)$ linking the gas metallicity Z to the specific star formation rate (sSFR), that validates and extends the approximate relation put forward by Lilly et al. (2013, L13), where $y$ is the yield per stellar generation, $\Lambda(sSFR)$ is the instantaneous ratio between inflow and star formation rate expressed as a function of the sSFR, and $I$ is the integral of the past enrichment history, respectively. We then demonstrate that the instantaneous metallicity of a self-regulating system, such that its sSFR decreases with decreasing redshift, can be well approximated by the first term on the right-hand side in the above formula, which provides an upper bound to the metallicity. The metallicity is well approximated also by the L13 ideal regulator case, which provides a lower bound to the actual metallicity. We compare these approximate analytic formulae to numerical results and infer a discrepancy <0.1 dex in a range of metallicities and almost three orders of magnitude in the sSFR. We explore the consequences of the L13 model on the mass-weighted metallicity in the stellar component of the galaxies. We find that the stellar average metallicity lags 0.1-0.2 dex behind the gas-phase metallicity relation, in agreement with the data. (abridged)Comment: 14 pages, 6 figures, MNRAS accepte

Large stellar disks in small elliptical galaxies

We present the rotation velocities V and velocity dispersions sigma along the principal axes of seven elliptical galaxies less luminous than M_B= -19.5. These kinematics extend beyond the half-light radii for all systems in this photometrically selected sample. At large radii the kinematics not only confirm that rotation and "diskiness" are important in faint ellipticals, as was previously known, but also demonstrate that in most sample galaxies the stars at large galactocentric distances have (V/sigma)_max of about 2, similar to the disks in bona-fide S0 galaxies. Comparing this high degree of ordered stellar motion in all sample galaxies with numerical simulations of dissipationless mergers argues against mergers with mass ratios <=3:1 as an important mechanism in the final shaping of low-luminosity ellipticals, and favors instead the dissipative formation of a disk.Comment: 11 pages LaTex with 4 Postscript figure

Evolution of field early-type galaxies: The view from GOODS/CDFS

(Abridged) We explore the evolution of field early-type galaxies on a sample extracted from GOODS/CDFS. The galaxies are selected by means of a non-parametric analysis followed by visual inspection. We exclude those galaxies which are not consistent with an evolution into the Kormendy relation. The final set comprises 249 galaxies with a median redshift z=0.7. The distribution of number counts versus apparent magnitude suggests a substantial decrease of the comoving number density with redshift. The majority of the galaxies feature passively evolving old stellar populations. One third of those in the upper half of the redshift distribution have blue colors, in contrast to only 10% in the lower redshift subsample. An adaptive binning of the color maps is performed to explore the internal color distribution. We find that most blue galaxies in our sample feature blue cores whereas most of the red early-types are passively evolving stellar populations with red cores. The color gradients and scatter do not evolve with redshift and are compatible with the observations at z=0 assuming a radial dependence of the metallicity within each galaxy. This work emphasizes the need for a careful sample selection, as we found that most of those galaxies which were visually classified as early types -- but then rejected based on the Kormendy relation -- feature blue colors characteristic of recent star formation.Comment: 14 pages, 16 figures. Uses emulateapj. Accepted for publication in ApJ. Some figures in low resolutio

Oxygen Gas Abundances at 0.4<z<1.5: Implications for the Chemical Evolution History of Galaxies

We report VLT-ISAAC and Keck-NIRSPEC near-infrared spectroscopy for a sample of 30 0.47<z<0.92 CFRS galaxies and five [OII]-selected, M_B,AB<-21.5, z~1.4 galaxies. We have measured Halpha and [NII] line fluxes for the CFRS galaxies which have [OII], Hbeta and [OIII] line fluxes available from optical spectroscopy. For the z~1.4 objects we measured Hbeta and [OIII] emission line fluxes from J-band spectra, and Halpha line fluxes plus upper limits for [NII] fluxes from H-band spectra. We derive the extinction and oxygen abundances for the sample using a method based on a set of ionisation parameter and oxygen abundance diagnostics, simultaneously fitting the [OII], Hbeta, [OIII], Halpha and [NII] line fluxes. Our most salient conclusions are: a) the source of gas ionisation in the 30 CFRS and in all z~1.4 galaxies is not due to AGN activity; b) about one third of the 0.47<z<0.92 CFRS galaxies in our sample have substantially lower metallicities than local galaxies with similar luminosities and star formation rates; c) comparison with a chemical evolution model indicates that these low metallicity galaxies are unlikely to be the progenitors of metal-poor dwarf galaxies at z~0, but more likely the progenitors of massive spirals; d) the z~1.4 galaxies are characterized by the high [OIII]/[OII] line ratios, low extinction and low metallicity that are typical of lower luminosity CADIS galaxies at 0.4<z<0.7, and of more luminous Lyman Break Galaxies at z~3.1, but not seen in CFRS galaxies at 0.4<z<1.0; e) the properties of the z~1.4 galaxies suggest that the period of rapid chemical evolution takes place progressively in lower mass systems as the universe ages, and thus provides further support for a downsizing picture of galaxy formation, at least from z~1.4 to today.Comment: Proceedings contribution for "The Fabulous Destiny of Galaxies; Bridging Past and Present", Marseille, 200