arXiv subjects

Alvio Renzini

Publications and source records attributed to Alvio Renzini.

At least 91 records · Page 5Linked to original sources

arXiv2016

In and out star formation in z~1.5 quiescent galaxies from rest-frame UV spectroscopy and the far-infrared

We present a sample of 34 spectroscopically confirmed BzK-selected ~1e11 Msun quiescent galaxies (pBzK) in the COSMOS field. The targets were initially observed with VIMOS on the VLT to facilitate the calibration of the photometric redshifts of massive galaxies at z >~ 1.5. Here we describe the reduction and analysis of the data, and the spectrophotometric properties of these pBzK galaxies. In particular, using a spatially resolved median 2D spectrum, we find that the fraction of stellar populations with ages <1 Gyr is at least 3 times higher in the outer regions of the pBzK galaxies than in their cores. This results in a mild age gradient of ~<0.4 Gyr over ~6 kpc and suggests either the occurrence of widespread rejuvenation episodes or that inside-out quenching played a role in the passivization of this galaxy population. We also report on low-level star formation rates derived from the [OII]3727A emission line, with SFR_OII = 3.7-4.5 Msun/yr. This estimate is confirmed by an independent measurement on a separate sample of similarly-selected quiescent galaxies in the COSMOS field, using stacked far-infrared data (SFR_FIR = 2-4 Msun/yr). This second, photometric sample also displays significant excess at 1.4 GHz, suggestive of the presence of radio-mode AGN activity.

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arXiv2016

Minor Mergers or Progenitor Bias? The Stellar Ages of Small and Large Quenched Early-Type Galaxies

We investigate the origin of the evolution of the population-averaged size of quenched galaxies (QGs) through a spectroscopic analysis of their stellar ages. The two most favoured scenarios for this evolution are either the size growth of individual galaxies through a sequence of dry minor merger events, or the addition of larger, newly quenched galaxies to the pre-existing population (i.e., a progenitor bias effect). We use the 20k zCOSMOS-bright spectroscopic survey to select bona fide quiescent galaxies at 0.2<z<0.8. We stack their spectra in bins of redshift, stellar mass and size to compute stellar population parameters in these bins through fits to the rest-frame optical spectra and through Lick spectral indices. We confirm a change of behaviour in the size-age relation below and above the ~10^11 MSun stellar mass scale: In our 10.5 < log M*/MSun < 11 mass bin, over the entire redshift window, the stellar populations of the largest galaxies are systematically younger than those of the smaller counterparts, pointing at progenitor bias as the main driver of the observed average size evolution at sub-10^11 MSun masses. In contrast, at higher masses, there is no clear trend in age as a function of galaxy size, supporting a substantial role of dry mergers in increasing the sizes of these most massive QGs with cosmic time. Within the errors, the [alpha/Fe] abundance ratios of QGs are (i) above-solar over the entire redshift range of our analysis, hinting at universally short timescales for the buildup of the stellar populations of QGs, and (ii) similar at all masses and sizes, suggesting similar (short) timescales for the whole QG population and strengthening the role of mergers in the buildup of the most massive QGs in the Universe.

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arXiv2016

The dominance of quenching through cosmic times

The evolution with cosmic time of the star formation rate density (SFRD) and of the "Main Sequence" star formation rate-stellar mass relations are two well established observational facts. In this paper the implications of these two relations combined are analytically explored, showing that quenching of star formation must start already at very early cosmic times and the quenched fraction then dominates ever since over the star forming one. Thus, a simple picture of the cosmic evolution of the global SFRD is derived, in terms of the interplay between star formation and its quenching.

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arXiv2016

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. VII. Implications from the Nearly Universal Nature of Horizontal Branch Discontinuities

The UV-initiative Hubble Space Telescope Treasury survey of Galactic globular clusters provides a new window into the phenomena that shape the morphological features of the horizontal branch (HB). Using this large and homogeneous catalog of UV and blue photometry, we demonstrate that the HB exhibits discontinuities that are remarkably consistent in color (effective temperature). This consistency is apparent even among some of the most massive clusters hosting multiple distinct sub-populations (such as NGC 2808, omega Cen, and NGC 6715), demonstrating that these phenomena are primarily driven by atmospheric physics that is independent of the underlying population properties. However, inconsistencies arise in the metal-rich clusters NGC 6388 and NGC 6441, where the discontinuity within the blue HB (BHB) distribution shifts ~1,000 K to 2,000 K hotter. We demonstrate that this shift is likely due to a large helium enhancement in the BHB stars of these clusters, which in turn affects the surface convection and evolution of such stars. Our survey also increases the number of Galactic globular clusters known to host blue-hook stars (also known as late hot flashers) from 6 to 23 clusters. These clusters are biased toward the bright end of the globular cluster luminosity function, confirming that blue-hook stars tend to form in the most massive clusters with significant self-enrichment.

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arXiv2015

Revisiting the role of the Thermally-Pulsating Asymptotic Giant Branch phase in high-redshift galaxies

We study the debated contribution from thermally pulsing asymptotic giant branch (TP-AGB) stars in evolutionary population synthesis models. We investigate the Spectral Energy Distributions (SEDs) of a sample of 51 spectroscopically confirmed, high-z ($1.3<z_{\rm spec}<2.7$), galaxies using three evolutionary population synthesis models with strong, mild and light TP-AGB. Our sample is the largest of spectroscopically confirmed galaxies on which such models are tested so far. Galaxies were selected as passive, but we model them using a variety of star formation histories in order not to be dependent on this pre-selection. We find that the observed SEDs are best fitted with a significant contribution of TP-AGB stars or with substantial dust attenuation. Without including reddening, TP-AGB-strong models perform better and deliver solutions consistent within $1σ$ from the best-fit ones in the vast majority of cases. Including reddening, all models perform similarly. Using independent constraints from observations in the mid- and far-IR, we show that low/negligible dust attenuation, i.e. $E(B-V)\lesssim 0.05$ , should be preferred for the SEDs of passively-selected galaxies. Given that TP-AGB-light models give systematically older ages for passive galaxies, we suggest number counts of passive galaxies at higher redshifts as a further test to discriminate among stellar population models.

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arXiv2015

Discovery of Massive, Mostly Star-formation Quenched Galaxies with Extremely Large Lyman-alpha Equivalent Widths at z ~ 3

We report a discovery of 6 massive galaxies with both extremely large Lya equivalent width and evolved stellar population at z ~ 3. These MAssive Extremely STrong Lya emitting Objects (MAESTLOs) have been discovered in our large-volume systematic survey for strong Lya emitters (LAEs) with twelve optical intermediate-band data taken with Subaru/Suprime-Cam in the COSMOS field. Based on the SED fitting analysis for these LAEs, it is found that these MAESTLOs have (1) large rest-frame equivalent width of EW_0(Lya) ~ 100--300 A, (2) M_star ~ 10^10.5--10^11.1 M_sun, and (3) relatively low specific star formation rates of SFR/M_star ~ 0.03--1 Gyr^-1. Three of the 6 MAESTLOs have extended Ly$α$ emission with a radius of several kpc although they show very compact morphology in the HST/ACS images, which correspond to the rest-frame UV continuum. Since the MAESTLOs do not show any evidence for AGNs, the observed extended Lya emission is likely to be caused by star formation process including the superwind activity. We suggest that this new class of LAEs, MAESTLOs, provides a missing link from star-forming to passively evolving galaxies at the peak era of the cosmic star-formation history.

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arXiv2015

GOODS-HERSCHEL: star formation, dust attenuation and the FIR-radio correlation on the Main Sequence of star-forming galaxies up to z~4

We use deep panchromatic datasets in the GOODS-N field, from GALEX to the deepest Herschel far-infrared and VLA radio continuum imaging, to explore, using mass-complete samples, the evolution of the star formation activity and dust attenuation of star-forming galaxies to z~4. Our main results can be summarized as follows: i) the slope of the SFR-M correlation is consistent with being constant, and equal to ~0.8 at least up to z~1.5, while its normalization keeps increasing with redshift; ii) for the first time here we are able to explore the FIR-radio correlation for a mass-selected sample of star-forming galaxies: the correlation does not evolve up to z~4; iii) we confirm that galaxy stellar mass is a robust proxy for UV dust attenuation in star-forming galaxies, with more massive galaxies being more dust attenuated, strikingly we find that this attenuation relation evolves very weakly with redshift, the amount of dust attenuation increasing by less than 0.3 magnitudes over the redshift range [0.5-4] for a fixed stellar mass, as opposed to a tenfold increase of star formation rate; iv) the correlation between dust attenuation and the UV spectral slope evolves in redshift, with the median UV spectral slope of star-forming galaxies becoming bluer with redshift. By z~3, typical UV slopes are inconsistent, given the measured dust attenuation, with the predictions of commonly used empirical laws. Finally, building on existing results, we show that gas reddening is marginally larger (by a factor of around 1.3) than stellar reddening at all redshifts probed, and also that the amount of dust attenuation at a fixed ISM metallicity increases with redshift. We speculate that our results support evolving ISM conditions of typical star-forming galaxies such that at z~1.5 Main Sequence galaxies have ISM conditions getting closer to those of local starbursts.

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arXiv2015

Star formation and quenching among the most massive galaxies at z~1.7

We have conducted a detailed object-by-object study of a mass-complete (M*>10^11 M_sun) sample of 56 galaxies at 1.4 < z < 2 in the GOODS-South field, showing that an accurate de-blending in MIPS/24um images is essential to properly assign to each galaxy its own star formation rate (SFR), whereas an automatic procedure often fails. This applies especially to galaxies with SFRs below the Main Sequence (MS) value, which may be in their quenching phase. After that, the sample splits evenly between galaxies forming stars within a factor of 4 of the MS rate (~45%), and sub-MS galaxies with SFRs ~10-1000 times smaller (~55%). We did not find a well defined class of intermediate, transient objects below the MS, suggesting that the conversion of a massive MS galaxy into a quenched remnant may take a relatively short time (<1 Gyr), though a larger sample should be analyzed in the same way to set precise limits on the quenching timescale. X-ray detected AGNs represent a ~30% fraction of the sample, and are found among both star-forming and quenched galaxies. The morphological analysis revealed that ~50% of our massive objects are bulge-dominated, and almost all MS galaxies with a relevant bulge component host an AGN. We also found sub-MS SFRs in many bulge-dominated systems, providing support to the notion that bulge growth, AGN activity and quenching of star formation are closely related to each other.

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arXiv2015

An Objective Definition for the Main Sequence of Star-Forming Galaxies

The Main Sequence (MS) of star-forming galaxies plays a fundamental role in driving galaxy evolution and in our efforts to understand it. However, different studies find significant differences in the normalization, slope and shape of the MS. These discrepancies arise mainly from the different selection criteria adopted to isolate star-forming galaxies, that may include or exclude galaxies with specific star formation rate (SFR) substantially below the MS value. To obviate this limitation of all current criteria, we propose an objective definition of the MS that does not rely at all on a pre-selection of star-forming galaxies. Constructing the 3D SFR-Mass-Number plot, the MS is then defined as the ridge line of the star-forming peak, as illustrated with various figures. The advantages of such definition are manifold. If generally adopted it will facilitate the inter-comparison of results from different groups using the same star formation rate (SFR) and stellar mass diagnostics, or to highlight the relative systematics of different diagnostics. All this could help understanding MS galaxies as systems in a quasi-steady state equilibrium and would also provide a more objective criterion for identifying quenching galaxies.

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arXiv2014

Why Galaxies Care about AGB Stars: Setting the Stage

In this introduction to the Third Congress of Vienna on AGB stars I first try to highlight why it is so hard to cope with the AGB evolutionary phase. This phase is indeed dominated by three main physical processes concerning bulk motions of matter inside/around stars, namely envelope convection, mixing and mass loss. They are inextricably interlaced with each other in a circular sequence of reactions and counter-reactions which has so far undermined our attempts at calibrating such processes one independent of the other. The second part of this introduction is focused on Globular Clusters, illustrating how they came to be a {\it new frontier} for the AGB evolution and a new opportunity to understand it.

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arXiv2014

The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era

We present new constraints on the star formation histories of six ultra-faint dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I. Our analysis employs a combination of high-precision photometry obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, medium-resolution spectroscopy obtained with the DEep Imaging Multi-Object Spectrograph on the W.M. Keck Observatory, and updated Victoria-Regina isochrones tailored to the abundance patterns appropriate for these galaxies. The data for five of these Milky Way satellites are best fit by a star formation history where at least 75% of the stars formed by z~10 (13.3 Gyr ago). All of the galaxies are consistent with 80% of the stars forming by z~6 (12.8 Gyr ago) and 100% of the stars forming by z~3 (11.6 Gyr ago). The similarly ancient populations of these galaxies support the hypothesis that star formation in the smallest dark matter sub-halos was suppressed by a global outside influence, such as the reionization of the universe.

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arXiv2014

A Consistent Study of Metallicity Evolution at 0.8 < z < 2.6

We present the correlations between stellar mass, star formation rate (SFR) and [NII]/Ha flux ratio as indicator of gas-phase metallicity for a sample of 222 galaxies at 0.8 < z < 2.6 and log(M*/Msun)=9.0-11.5 from the LUCI, SINS/zC-SINF and KMOS3D surveys. This sample provides a unique analysis of the mass-metallicity relation (MZR) over an extended redshift range using consistent data analysis techniques and strong-line metallicity indicator. We find a constant slope at the low-mass end of the relation and can fully describe its redshift evolution through the evolution of the characteristic turnover mass where the relation begins to flatten at the asymptotic metallicity. At fixed mass and redshift, our data do not show a correlation between the [NII]/Ha ratio and SFR, which disagrees with the 0.2-0.3dex offset in [NII]/Ha predicted by the "fundamental relation" between stellar mass, SFR and metallicity discussed in recent literature. However, the overall evolution towards lower [NII]/Ha at earlier times does broadly agree with these predictions.

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arXiv2014

Mass and Environment as Drivers of Galaxy Evolution III: The constancy of the faint-end slope and the merging of galaxies

We explore using our continuity approach the underlying connections between the evolution of the faint-end slope of the stellar mass function of star-forming galaxies, the logarithmic slope of the sSFR-mass relation and the merging of galaxies. We derive analytically the consequences of the observed constancy of the faint-end slope since redshifts of at least z ~ 2. If the logarithmic slope of the sSFR-mass relation is negative, then the faint-end slope should quickly diverge due to the differential mass increase of galaxies on the star-forming main sequence, and this will also quickly destroy the Schechter form of the mass function. This problem can be solved by removing low mass galaxies by merging them into more massive galaxies. We quantify this process by introducing the specific merger mass rate (sMMR) as the specific rate of mass added to a given galaxy through mergers. For a modest negative value of the logarithmic slope of the sSFR-mass relation of beta ~ -0.1, an average sMMR ~ 0.1sSFR across the population is required to keep the faint-end slope constant with epoch, as observed. This in turn implies a merger rate of ~ 0.2sSFR for major mergers, which is consistent with the available observational estimates. More negative values of beta require higher sMMR and higher merger rates, and the steepening of the mass function becomes impossible to control for beta < ~ -0.6, for an observed value of the faint-end slope of ~ -1.4. The close link that is required between the in situ sSFR and the sMMR probably arises because both are closely linked to the buildup of dark matter haloes. These new findings further develop the formalism for the evolving galaxy population that we introduced earlier, and show how striking symmetries in the galaxy population can emerge as the result of deep links between the physical processes involved.

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arXiv2013

The Formation History of the Ultra-Faint Dwarf Galaxies

We present early results from a Hubble Space Telescope survey of the ultra-faint dwarf galaxies. These Milky Way satellites were discovered in the Sloan Digital Sky Survey, and appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. The primary goal of the survey is to measure the star-formation histories of these galaxies and discern any synchronization due to the reionization of the universe. We find that the six galaxies of our survey have very similar star-formation histories, and that each is dominated by stars older than 12 Gyr.

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arXiv2013

WFC3 grism confirmation of the distant cluster Cl J1449+0856 at z=2.00: Quiescent and star-forming galaxy populations

We present deep Hubble Space Telescope Wide Field Camera 3 slitless spectroscopic observations of the distant cluster Cl J1449+0856. These cover a single pointing with 18 orbits of G141 spectroscopy and F140W imaging, allowing us to derive secure redshifts down to m_140~25.5 AB and 3sigma line fluxes of 5*10^(-18) erg/s/cm^2. In particular, we were able to spectroscopically confirm 12 early-type galaxies in the field up to z~3, 6 of which in the cluster core, which represents the first direct spectroscopic confirmation of passive galaxies in a z=2 cluster environment. With 140 redshifts in a ~6 arcmin^2 field, we can trace the spatial and redshift galaxy distribution in the cluster core and background field. We find two strong peaks at z=2.00 and z=2.07, where only one was seen in our previously published ground-based data. Thanks to the spectroscopic confirmation of the cluster ETGs, we can now re-evaluate the redshift of Cl J1449+0856 at z=2.00, rather than z=2.07, with the background overdensity being revealed to be sparse and "sheet"-like. This presents an interesting case of chance alignment of two close yet unrelated structures, each one preferentially selected by different observing strategies. With 6 quiescent or early-type spectroscopic members and 20 star-forming ones, Cl J1449+0856 is now reliably confirmed to be at z=2.00. The identified members can now allow for a detailed study of galaxy properties in the densest environment at z=2.

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arXiv2013

Calibrating stellar population models with Magellanic Cloud star clusters

Stellar population models are commonly calculated using star clusters as calibrators for those evolutionary stages that depend on free parameters. However, discrepancies exist among different models, even if similar sets of calibration clusters are used. With the aim of understanding these discrepancies, and of improving the calibration procedure, we consider a set of 43 Magellanic Cloud (MC) clusters taking age and photometric information from the literature. We carefully assign ages to each cluster based on up-to-date determinations ensuring that these are as homogeneous as possible. To cope with statistical fluctuations, we stack the clusters in five age bins deriving for each of them integrated luminosities and colors. We find that clusters become abruptly red in optical and optical-IR colors as they age from ~0.6 to 1 Gyr, which we interpret as due to the development of a well-populated thermally pulsing asymptotic giant branch (TP-AGB). We argue that other studies missed this detection because of coarser age binnings. Maraston (2005; M05) and Girardi et al. (2010) models predict the presence of a populated TP-AGB at ~0.6 Gyr, with a correspondingly very red integrated color, at variance with the data; Bruzual & Charlot (2003) and Conroy et al. (2009) models run within the error bars at all ages. The discrepancy between the synthetic colors of M05 models and the average colors of MC clusters results from the now obsolete age scale adopted. Finally, our finding that the TP-AGB phase appears to develop between ~0.6-1 Gyr is dependent on the adopted age scale for the clusters and may have important implications for stellar evolution.

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arXiv2013

Hubble Space Telescope Imaging of the Binary Nucleus of the Planetary Nebula EGB 6

EGB 6 is an ancient, low-surface-brightness planetary nebula. The central star, also cataloged as PG 0950+139, is a very hot DAOZ white dwarf (WD) with an apparent M dwarf companion, unresolved from the ground but detected initially through excesses in the JHK bands. Its kinematics indicates membership in the Galactic disk population. Inside of EGB 6 is an extremely dense emission knot -- completely unexpected since significant mass loss from the WD should have ceased ~10^5 y ago. The electron density of the compact nebula is very high (2.2x10^6 cm^-3), as indicated by collisional de-excitation of forbidden emission lines. Hubble Space Telescope imaging and grism spectroscopy are reported here. These resolve the WD and apparent dM companion -- at a separation of 0."166, or a projected 96(+204,-45) AU at the estimated distance of 576(+1224, -271) pc (using the V magnitude). Much to our surprise, we found that the compact emission nebula is superposed on the dM companion, far from the photoionizing radiation of the WD. Moreover, a a striking mid-infrared excess has recently been reported in Spitzer/IRAC and MIPS bands, best fit with two dust shells. The derived ratio L_IR/L_WD = 2.7 x 10^-4 is the largest yet found for any WD or planetary nucleus. The compact nebula has maintained its high density for over three decades. We discuss two possible explanations for the origin and confinement of the compact nebula, neither of which is completely satisfactory. This leaves the genesis and confinement of the compact nebula an astrophysical puzzle, yet similar examples appear in the literature.

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arXiv2013

The SINS/zC-SINF Survey of z~2 Galaxy Kinematics: The Nature of Dispersion Dominated Galaxies

We analyze the spectra, spatial distributions and kinematics of Ha, [NII] and [SII] emission in a sample of 42, z~2.2 UV/optically selected star forming galaxies (SFGs) from the SINS & zC-SINF surveys, 35 of which were observed in the adaptive optics mode of SINFONI. This is supplemented by kinematic data from 48 z~1-2.5 galaxies from the literature. We find that the kinematic classification of the high-z SFGs as `dispersion dominated' or `rotation dominated' correlates most strongly with their intrinsic sizes. Smaller galaxies are more likely `dispersion-dominated' for two main reasons: 1) The rotation velocity scales linearly with galaxy size but intrinsic velocity dispersion does not depend on size, and as such, their ratio is systematically lower for smaller galaxies, and 2) Beam smearing strongly decreases large-scale velocity gradients and increases observed dispersion much more for galaxies with sizes at or below the resolution. Dispersion dominated SFGs may thus have intrinsic properties similar to `rotation dominated' SFGs, but are primarily more compact, lower mass, less metal enriched and may have higher gas fractions, plausibly because they represent an earlier evolutionary state.

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