arXiv subjects

A. Pipino

Publications and source records attributed to A. Pipino.

At least 19 recordsLinked to original sources

arXiv2015

The Zurich Environmental Study (ZENS) of Galaxies in Groups along the Cosmic Web. V. Properties and Frequency of Merging Satellites and Centrals in Different Environments

We use the Zurich ENvironmental Study (ZENS) database to investigate the environmental dependence of the merger fraction $Γ$ and merging galaxy properties in a sample of ~1300 group galaxies with $M>10^{9.2}M_\odot$ and 0.05 10^{13.5} M_\odot$ relative to less massive systems, indicating a suppression of merger activity in large potential wells. In the fiducial case of relaxed groups only, we measure a variation $ΔΓ/Δ\log (M_{HALO}) \sim - 0.07$ dex$^{-1}$, which is almost independent of galaxy mass and merger stage. At galaxy masses $>10^{10.2} M_\odot$, most mergers are dry accretions of quenched satellites onto quenched centrals, leading to a strong increase of $Γ$ with decreasing group-centric distance at these mass scales.Both satellite and central galaxies in these high mass mergers do not differ in color and structural properties from a control sample of nonmerging galaxies of equal mass and rank. At galaxy masses $<10^{10.2} M_\odot$, where we mostly probe satellite-satellite pairs and mergers between star-forming systems, close pairs (projected distance $<10-20$ kpc) show instead $\sim2\times$ enhanced (specific) star formation rates and $\sim1.5\times$ larger sizes than similar mass, nonmerging satellites. The increase in both size and SFR leads to similar surface star-formation densities in the merging and control-sample satellite populations.

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arXiv2014

On the relation between sSFR and metallicity

In this paper we present an exact general analytic expression $Z(sSFR)=y/Λ(sSFR)+I(sSFR)$ 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, $Λ(sSFR)$ 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)

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arXiv2014

ZENS IV. Similar Morphological Changes associated with Mass- and Environment-Quenching, and the Relative importance of Bulge Growth versus the Fading of Disks

We use ZENS data at low redshift to study the dependence of the quenched satellite fraction and of the morphological mix of these quenched satellites, on three different environmental parameters: group halo mass, halo-centric distance and large-scale structure over-density. The fraction of quenched satellites is independent of halo mass and the surrounding large-scale overdensity, but increases towards the centres of the haloes, as previously found. The morphological mix is, however, constant with radial position, indicating that the well-known morphology-density relation results from the increasing fraction of quenched galaxies towards the centres of haloes. The constancy of the morphological outcome suggests that mass-quenching and satellite quenching have the same effect on the morphologies of the galaxies. The quenched satellites have larger B/T and smaller half-light radii than the star-forming satellites. These are mostly due to differences in the disks. The bulges in quenched satellites have very similar luminosities and surface brightness profiles, and any mass growth of the bulges associated with quenching cannot greatly change these quantities. The quenched disks are fainter and have smaller scale lengths than in star-forming satellites. This can be explained either by a differential fading of the disks or if disks were generally smaller in the past, both of which are expected in an inside-out growth of disks. At least at low redshifts, the structure of massive quenched satellites is produced by processes that operate before quenching takes place. A comparison with semi-analytic models argues for a reduction in the efficiency of group halos in quenching their disk satellites and for mechanisms to increase the B/T of low mass quenched satellites. [abridged]

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arXiv2013

The dust content of QSO hosts at high redshift

Infrared observations of high-z quasar (QSO) hosts indicate the presence of large masses of dust in the early universe. When combined with other observables, such as neutral gas masses and star formation rates, the dust content of z~6 QSO hosts may help constraining their star formation history. We have collected a database of 58 sources from the literature discovered by various surveys and observed in the FIR. We have interpreted the available data by means of chemical evolution models for forming proto-spheroids, investigating the role of the major parameters regulating star formation and dust production. For a few systems, given the derived small dynamical masses, the observed dust content can be explained only assuming a top-heavy initial mass function, an enhanced star formation efficiency and an increased rate of dust accretion. However, the possibility that, for some systems, the dynamical mass has been underestimated cannot be excluded. If this were the case, the dust mass can be accounted for by standard model assumptions. We provide predictions regarding the abundance of the descendants of QSO hosts; albeit rare, such systems should be present and detectable by future deep surveys such as Euclid already at z>4.

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arXiv2013

The Zurich Environmental Study of Galaxies in Groups along the Cosmic Web. III. Galaxy Photometric Measurements and the Spatially-Resolved Color Properties of Early- and Late-Type Satellites in Diverse Environments

We present photometric measurements for the galaxies - and when possible their bulges and disks - in the 0.05 10^10 Msun, these satellites have shallower color gradients in higher mass groups and in the cores of groups compared with lower mass groups and the outskirts of groups. Stellar population analyses and semi-analytic models suggest that disk-dominated satellites undergo quenching of star formation in their outer disks, on timescales ~2 Gyr, as they progressively move inside the group potential.

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arXiv2013

The Zurich Environmental Study (ZENS) of Galaxies in Groups along the Cosmic Web. I. Which Environment Affects Galaxy Evolution?

The Zurich Environmental Study (ZENS) is based on a sample of ~1500 galaxy members of 141 groups in the mass range ~10^12.5-14.5 M_sun within the narrow redshift range 0.05 10^10 M_sun, satellite galaxies in relaxed and unrelaxed groups have similar size, color and (specific) star formation rate distributions; at lower galaxy masses, satellites are marginally redder in relaxed relative to unrelaxed groups, suggesting quenching of star formation in low-mass satellites by physical processes active in relaxed halos. Finally, relaxed and unrelated groups show similar stellar mass conversion efficiencies, peaking at halo masses around 10^12.5 M_sun. In the enclosed ZENS catalogue we publish all environmental diagnostics as well as the galaxy structural and photometric measurements described in companion ZENS papers II and III.

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arXiv2013

The Zurich Environmental Study (ZENS) of Galaxies in Groups along the Cosmic Web. II. Galaxy Structural Measurements and the Concentration of Morphologically Classified Satellites in Diverse Environments

We present structural measurements for the galaxies in the 0.05 10^10 M_sun, the concentration of disk satellites increases with increasing stellar mass, separately within each morphological bin of B/T. The known increase in concentration with stellar mass for disk satellites is thus due, at least in part, to an increase in galaxy central stellar density at constant B/T. The correlation between concentration and galaxy stellar mass becomes progressively steeper for later morphological types. The concentration of disk satellites shows a barely significant dependence on delta_LSS or R/R_200. The strongest environmental effect is found with group mass for M>10^10 M_sun disk-dominated satellites, which are ~10% more concentrated in high mass groups than in lower mass groups.

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arXiv2013

Galactic and Cosmic Type Ia SN rates: is it possible to impose constraints on SNIa progenitors?

We compute the Type Ia supernova rates in typical elliptical galaxies by varying the progenitor models for Type Ia supernovae. To do that a formalism which takes into account the delay distribution function (DTD) of the explosion times and a given star formation history is adopted. Then the chemical evolution for ellipticals with baryonic initial masses $10^{10}$, $10^{11}$ and $10^{12} M_{\odot}$ is computed, and the mass of Fe produced by each galaxy is precisely estimated. We also compute the expected Fe mass ejected by ellipticals in typical galaxy clusters (e.g. Coma and Virgo), under different assumptions about Type Ia SN progenitors. As a last step, we compute the cosmic Type Ia SN rate in an unitary volume of the Universe by adopting several cosmic star formation rates and compare it with the available and recent observational data. Unfortunately, no firm conclusions can be derived only from the cosmic SNIa rate, neither on SNIa progenitors nor on the cosmic star formation rate. Finally, by analysing all our results together, and by taking into account previous chemical evolution results, we try to constrain the best Type Ia progenitor model. We conclude that the best progenitor models for Type Ia SNe are still the single degenerate model, the double degenerate wide model, and the empirical bimodal model. All these models require the existence of prompt Type Ia supernovae, exploding in the first 100 Myr since the beginning of star formation, although their fraction should not exceed 15-20% in order to fit chemical abundances in galaxies.

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arXiv2013

Colour gradients of high-redshift Early-Type Galaxies from hydrodynamical monolithic models

We analyze the evolution of colour gradients predicted by the hydrodynamical models of early type galaxies (ETGs) in Pipino et al. (2008), which reproduce fairly well the chemical abundance pattern and the metallicity gradients of local ETGs. We convert the star formation (SF) and metal content into colours by means of stellar population synthetic model and investigate the role of different physical ingredients, as the initial gas distribution and content, and eps_SF, i.e. the normalization of SF rate. From the comparison with high redshift data, a full agreement with optical rest-frame observations at z < 1 is found, for models with low eps_SF, whereas some discrepancies emerge at 1 < z < 2, despite our models reproduce quite well the data scatter at these redshifts. To reconcile the prediction of these high eps_SF systems with the shallower colour gradients observed at lower z we suggest intervention of 1-2 dry mergers. We suggest that future studies should explore the impact of wet galaxy mergings, interactions with environment, dust content and a variation of the Initial Mass Function from the galactic centers to the peripheries.

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arXiv2013

The two regimes of the cosmic sSFR evolution are due to spheroids and discs

This paper aims at explaining the two phases in the observed specific star formation rate (sSFR), namely the high (>3/Gyr) values at z>2 and the smooth decrease since z=2. In order to do this, we compare to observations the specific star formation rate evolution predicted by well calibrated models of chemical evolution for elliptical and spiral galaxies, using the additional constraints on the mean stellar ages of these galaxies (at a given mass). We can conclude that the two phases of the sSFR evolution across cosmic time are due to different populations of galaxies. At z>2 the contribution comes from spheroids: the progenitors of present-day massive ellipticals (which feature the highest sSFR) as well as halos and bulges in spirals (which contribute with average and lower-than-average sSFR). In each single galaxy the sSFR decreases rapidly and the star formation stops in <1 Gyr. However the combination of different generations of ellipticals in formation might result in an apparent lack of strong evolution of the sSFR (averaged over a population) at high redshift. The z<2 decrease is due to the slow evolution of the gas fraction in discs, modulated by the gas accretion history and regulated by the Schmidt law. The Milky Way makes no exception to this behaviour.

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arXiv2013

The dust properties of z ~ 3 MIPS-LBGs from photo-chemical models

The stacked spectral energy distribution (SED) of Multiband Imaging Photometer for Spitzer (MIPS) 24$μm$ detected Lyman break galaxies (MIPS-LBGs) is fitted by means of spectro-photometric model GRASIL with an "educated" fitting approach which benefits from results of chemical evolution models. The star formation rate(SFR)-age-metallicity degeneracies of SED modelling are broken by using SFH and chemical enrichment history suggested by chemical models, which also provide dust mass, dust abundance and chemical elements locked in dust component. We derive the total mass $M_{tot}$, stellar mass $M_{\ast}$, gas mass $M_{g}$, dust mass $M_{d}$, age and SFR of the stacked MIPS-LBG in a self-consistent way. Our estimate of $M_{\ast}= 8\times 10^{10}$ agrees with other works based on UV-optical SED fitting. We suggest that MIPS-LBGs at $z\sim3$ are young (0.3-0.6 Gyr), massive ($M_{tot} \sim 10^{11} M_{\odot}$), dusty ($M_{d} \sim 10^{8} M_{\odot}$), metal rich ($Z \sim Z_{\odot} $) progenitors of elliptical galaxies suffering a strong burst of star formation (SFR $\sim 200 M_{\odot}/yr$). Our estimate of $M_{d}=7 \times 10^{7} M_{\odot}$ of the stacked MIPS-LBG is about a factor of eight lower than the estimated value based on single temperature grey-body fitting, suggesting that self-consistent SED models are needed to estimate dust mass. By comparing with the Milky Way molecular cloud and dust properties, we suggest that denser and dustier environments and flatter dust size distribution are likely in high redshift massive star forming galaxies. These dust properties, as well as the different types of SFHs, can cause different SED shapes between high redshift star-forming ellipticals and local star-burst templates. This discrepancy of SED shapes could in turn explain the non detection at submillimeter wavelengths, of IR luminous ($L_{IR} \succeq 10^{12} L_{\odot} $) MIPS-LBGs.

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arXiv2012

Chemical evolution of the Galactic bulge: different stellar populations and possible gradients

We compute the chemical evolution of the Galactic bulge to explain the existence of two main stellar populations recently observed. After comparing model results and observational data we suggest that the old more metal poor stellar population formed very fast (on a timescale of 0.1-0.3 Gyr) by means of an intense burst of star formation and an initial mass function flatter than in the solar vicinity whereas the metal rich population formed on a longer timescale (3 Gyr). We predict differences in the mean abundances of the two populations (-0.52 dex for <[Fe/H]>) which can be interpreted as a metallicity gradients. We also predict possible gradients for Fe, O, Mg, Si, S and Ba between sub-populations inside the metal poor population itself (e.g. -0.145 dex for <[Fe/H]>). Finally, by means of a chemo-dynamical model following a dissipational collapse, we predict a gradient inside 500 pc from the Galactic center of -0.26 dex kpc^{-1} in Fe.

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arXiv2011

Abundance ratios in the hot ISM of elliptical galaxies

To constrain the recipes put forth to solve the theoretical Fe discrepancy in the hot interstellar medium of elliptical galaxies and at the same time explain the [alpha/Fe] ratios. In order to do so we use the latest theoretical nucleosynthetic yields, we incorporate the dust, we explore differing SNIa progenitor scenarios by means of a self-consistent chemical evolution model which reproduces the properties of the stellar populations in elliptical galaxies. Models with Fe-only dust and/or a lower effective SNIa rate achieve a better agreement with the observed Fe abundance. However, a suitable modification to the SNIa yield with respect to the standard W7 model is needed to fully match the abundance ratio pattern. The 2D explosion model C-DDT by Maeda et al. (2010) is a promising candidate for reproducing the [Fe/H] and the [alpha/Fe] ratios. (A&A format)

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arXiv2011

An empirical calibration of Lick indices using Milky Way Globular Clusters

To provide an empirical calibration relation in order to convert Lick indices into abundances for the integrated light of old, simple stellar populations for a large range in the observed [Fe/H] and [alpha/Fe]. This calibration supersedes the previously adopted ones because it is be based on the real abundance pattern of the stars instead of the commonly adopted metallicity scale derived from the colours. We carried out a long-slit spectroscopic study of 23 Galactic globular cluster for which detailed chemical abundances in stars have been recently measured. The line-strength indices, as coded by the Lick system and by Serven et al. (2005), were measured in low-resolution integrated spectra of the GC light. The results were compared to average abundances and abundance ratios in stars taken from the compilation by Pritzl et al. (2005) as well as to synthetic models. Fe-related indices grow linearly as a function of [Fe/H] for [Fe/H]>-2. Mg-related indices respond in a similar way to [Mg/H] variations, however Mgb turns out to be a less reliable metallicity indicator for [Z/H]<-1.5 . Despite the known Mg overabundance with respect to Fe in GC stars, it proved impossible to infer a mean [Mg/Fe] for integrated spectra that correlates with the resolved stars properties, because the sensitivity of the indices to [Mg/Fe] is smaller at lower metallicities. We present empirical calibrations for Ca, TiO, Ba and Eu indices as well as the measurements of H_alpha and NaD.

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arXiv2010

The properties of Brightest Cluster Galaxies in the SDSS DR6 adaptive matched filter cluster catalogue

We study the properties of Brightest Cluster Galaxies (BCGs) drawn from a catalogue of more than 69000 clusters in the SDSS DR6 based on the adaptive matched filter technique (AMF, Szabo et al., 2010). Our sample consists of more than 14300 galaxies in the redshift range 0.1-0.3. We test the catalog by showing that it includes well-known BCGs which lie in the SDSS footprint. We characterize the BCGs in terms of r-band luminosities and optical colours as well as their trends with redshift. In particular, we define and study the fraction of blue BCGs, namely those that are likely to be missed by either colour-based cluster surveys and catalogues. Richer clusters tend to have brighter BCGs, however less dominant than in poorer systems. 4-9% of our BCGs are at least 0.3 mag bluer in the g-r colour than the red-sequence at their given redshift. Such a fraction decreases to 1-6% for clusters above a richness of 50, where 3% of the BCGs are 0.5 mag below the red-sequence. A preliminary morphological study suggests that the increase in the blue fraction at lower richnesses may have a non-negligible contribution from spiral galaxies. We show that a colour selection based on the g-r red-sequence or on a cut at colour u-r >2.2 can lead to missing the majority of such blue BCGs. We also extend the colour analysis to the UV range by cross-matching our catalogue with publicly available data from Galex GR4 and GR5. We show a clear correlation between offset from the optical red-sequence and the amount of UV-excess. Finally, we cross-matched our catalogue with the ACCEPT cluster sample (Cavagnolo et al., 2009), and find that blue BCGs tend to be in clusters with low entropy and short cooling times. That is, the blue light is presumably due to recent star formation associated to gas feeding by cooling flows. (abridged)

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arXiv2010

The chemical evolution of elliptical galaxies with stellar and QSO dust production

The presence of dust strongly affects the way we see galaxies and also the chemical abundances we measure in gas. It is therefore important to study he chemical evolution of galaxies by taking into account dust evolution. We aim at performing a detailed study of abundance ratios of high redshift objects and their dust properties. We focus on Lyman-Break galaxies (LBGs) and Quasar (QSO) hosts and likely progenitors of low- and high-mass present-day elliptical galaxies, respectively. We have adopted a chemical evolution model for elliptical galaxies taking account the dust production from low and intermediate mass stars, supernovae Ia, supernovae II, QSOs and both dust destruction and accretion processes. By means of such a model we have followed the chemical evolution of ellipticals of different baryonic masses. Our model complies with chemical downsizing. We made predictions for the abundance ratios versus metallicity trends for models of differing masses that can be used to constrain the star formation rate, initial mass function and dust mass in observed galaxies. We predict the existence of a high redshift dust mass-stellar mass relationship. We have found a good agreement with the properties of LBGs if we assume that they formed at redshift z=2-4. In particular, a non-negligible amount of dust is needed to explain the observed abundance pattern. We studied the QSO SDSS J114816, one of the most distant QSO ever observed (z=6.4), and we have been able to reproduce the amount of dust measured in this object. The dust is clearly due to the production from supernovae and the most massive AGB stars as well as from the grain growth in the interstellar medium. The QSO dust is likely to dominate only in the very central regions of the galaxies and during the early development of the galactic wind.

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arXiv2010

Abundance gradient slopes versus mass in spheroids: predictions by monolithic models

We investigate whether it is possible to explain the wide range of observed gradients in early type galaxies in the framework of monolithic models. To do so, we extend the set of hydrodynamical simulations by Pipino et al. (2008a) by including low-mass ellipticals and spiral (true) bulges. These models satisfy the mass-metallicity and the mass-[alpha/Fe] relations. The typical metallicity gradients predicted by our models have a slope of -0.3 dex per decade variation in radius, consistent with the mean values of several observational samples. However, we also find a few quite massive galaxies in which this slope is -0.5 dex per decade, in agreement with some recent data. In particular, we find a mild dependence from the mass tracers when we transform the stellar abundance gradients into radial variations of the Mg_2 line-strength index, but not in the Mg_b. We conclude that, rather than a mass- slope relation, is more appropriate to speak of an increase in the scatter of the gradient slope with the galactic mass. We can explain such a behaviour with different efficiencies of star formation in the framework of the revised monolithic formation scenario, hence the scatter in the observed gradients should not be used as an evidence of the need of mergers. Indeed, model galaxies that exhibit the steepest gradient slopes are preferentially those with the highest star formation efficiency at that given mass.

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arXiv2010

Signature of cool cores in SZ clusters: a multiwavelength approach

We use the high quality pressure profiles of 239 galaxy clusters made available by the ACCEPT project (Cavagnolo et al. 2009) in order to derive the expected Sunyaev Zeldovich (SZ) signal in a variety of cases that hardly find a counterpart in the simulations. We made use of the Melin et al. (2006) cluster selection function for both the South Pole Telescope (SPT) and Planck instruments. We infer a clear effect of the cool core (CC) on the central Compton parameter y_0. We find that the presence of CCs introduces a small bias in cluster detection, especially around the mass at which the performance of the survey begins to decrease. If the CC were removed, a lower overall fraction of detected clusters would be expected. In order to estimate the presence of such a bias by means of SZ only surveys, we show that the ratio between y_0 and y_int anti-correlates with the cluster central cooling time. If multi-band optical cluster surveys are either available for a cross-match or a follow-up is planned, we suggest that likely CC clusters are those with a Brightest Cluster Galaxy (BCG) at least 0.3 magnitudes bluer than the average. A more robust estimate of the CC presence is given by UV-optical colours of the BCG, like the NUV-r, whose values can be 4 magnitudes off the NUV-r equivalent of the red sequence, in clusters with low excess entropy. We also find correlation of the y_0/y_int ratio with Halpha, IR and radio luminosities. We argue that the analysis of a combined SZ/optical/UV surveys can be also used to shed light on the suggested CC evolution with redshift. (abridged)

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