The aim of the present paper is to quantify the dependence of the estimates of luminosities and stellar mass content of early-type galaxies on the different models and model parameters which can be ...used to analyse the observational data. The paper is organized in two parts. The first one analyses the dependence of the ratios and of the k-corrections in different bands on model parameters (initial mass function, metallicity, star formation history, age), assuming some among the most popular spectrophotometric codes usually adopted to study the evolutionary status of galaxies: Bruzual & Charlot (BC03), Charlot & Bruzual (CB08), Maraston (Ma05), Fioc & Rocca-Volmerange (PEGASE), Silva et al. (GRASIL). The second part of our work is dedicated to quantify the reliability and systematics affecting the mass and luminosity estimates obtained by means of the best-fitting technique applied to the photometric spectral energy distributions (SEDs) of early-type galaxies at 1 < z < 2. To this end, we apply the best-fitting technique to some mock catalogues built on the basis of a wide set of models of early-type galaxies. We then compare the luminosity and the stellar mass estimated from the SED fitting with the true known input values. The goodness of the mass estimate is found to be dependent on the mass estimator adopted to derive it, but masses cannot anyhow be retrieved better than within a factor of 2–3, depending on the quality of the available photometric data and/or on the distance of the galaxies since more distant galaxies are fainter on average and thus affected by larger photometric errors. Finally, we present a new empirical mass estimator based on the K-band apparent magnitude and on the observed(V−K) colour. We show that the reliability of the stellar mass content derived with this new estimator for early-type galaxies and its stability are even higher than those achievable with the best classic estimators, with the not negligible advantage that it does not need any multiwavelength data fitting.
Recent theoretical and observational studies on the assembly of early-type galaxies (ETGs) point towards an inside-out growth of their stellar mass characterized by extended low-mass-density haloes ...grown around compact and dense cores. Models can form ETGs at high-z as compact spheroids that then grow in size through dry minor mergers. Dry mergers would affect mainly the outskirts of the galaxy, enlarging the size (i.e. the effective radius), keeping the inner parts and the total stellar mass nearly unchanged. Hence, the central stellar mass density will not change with time, in contrast to the stellar mass density within the effective radius, which should decrease with time as the effective radius increases. Some previous observations are interpreted as supporting inside-out growth, as the central stellar mass density of high-z ETGs is found to be similar to that of local ETGs. In this paper we derive the central stellar mass density within a fixed radius and the effective stellar mass density within the effective radius for a complete sample of 34 ETGs morphologically selected at 0.9 < z
spec < 2 and compare them with those derived for a sample of ∼900 local ETGs in the same mass range. We find that the central stellar mass density of high-z ETGs spans just an order of magnitude and is similar to that of local ETGs, as found in previous studies. However, we find that the effective stellar mass density of high-z ETGs spans three orders of magnitude, exactly as the local ETGs, and that it is similar to the effective stellar mass density of local ETGs, showing that it has not changed since z∼ 1.5, in the last 9-10 Gyr. Thus, the wide spread of the effective stellar mass density observed up to z∼ 1.5 must originate earlier, at z > 2. Furthermore, we show that the small scatter of the central mass density of ETGs compared with the large scatter of the effective mass density is simply a peculiar feature of the Sérsic profile and hence is independent of redshift and of any assembly history experienced by galaxies. Thus, it has no connection with the possible inside-out growth of ETGs. Finally, we show a tight correlation between the central stellar mass density and the total stellar mass of ETGs in the sense that the central mass density increases with mass as
. This implies that the fraction of the central stellar mass of ETGs decreases with the mass of the galaxy. These correlations are valid for the whole population of ETGs considered, independently of their redshift, suggesting that they originate in the early phases of their formation.
We present a study based on a sample of 62 early-type galaxies (ETGs) at 0.9 < z
spec < 2 aimed at constraining their past star formation and mass assembly histories. The sample is composed of normal ...ETGs having effective radii comparable to the mean radius of local ones and of compact ETGs having effective radii from two to six times smaller. We do not find evidence of a dependence of the compactness of ETGs on their stellar mass. The best fit to their spectral energy distribution at known redshift has allowed us to constrain the epoch at which the stellar mass formed. We find that the stellar mass of normal ETGs formed at z
form≲ 3, while the stellar content of compact ETGs formed over a wider range of redshift (2 < z
form < 10) with a large fraction of them characterized by z
form > 5. Earlier stars, those formed at z
form > 5, are assembled in compact and more massive (
M⊙) ETGs, while stars formed later (z
form≲ 3) or resulting from subsequent episodes of star formation are assembled both in compact and in normal ETGs. Thus, the older the stellar population, the higher the mass of the hosting galaxy but not vice versa. This suggests that the epoch of formation may play a role in the formation of massive ETGs rather than the mass itself. We show that the possible general scheme in which normal ETGs at 〈z〉≃ 1.5 are descendants of compact spheroids assembled at higher redshift is not compatible with the current models. Indeed, we find that the number of dry mergers expected in a hierarchical model is almost two orders of magnitude lower than that needed to enlarge a compact ETG up to a normal-size ETG. Moreover, we do not find evidence supporting a dependence of the compactness of galaxies on their redshift of assembly, a dependence expected in the hypothesis that the compactness of a galaxy is due to the higher density of the Universe at earlier epochs. Finally, we propose a simple scheme of formation and assembly of the stellar mass of ETGs based on dissipative gas-rich merger, which can qualitatively account for the coexistence of normal and compact ETGs observed at 〈z〉≃ 1.5 in spite of the same stellar mass, the lack of normal ETGs with high z
form and the absence of correlation between compactness, stellar mass and formation redshift.
Many of the early-type galaxies (ETGs) observed so far at z > 1 turned out to have smaller radii with respect to that of a typical present day ETG with comparable mass. This has generated the ...conviction that in the past ETGs were more compact, hence denser, and that as a consequence, they should have increased their radius across the time to reconcile with the present day ones. However, observations have not yet established whether the population of early types in the early universe was fully represented by compact galaxies nor if they were so much more numerous than in the present day Universe to require an evolution of their sizes. Here we report the results of a study based on a complete sample of 34 ETGs at 0.9 < zspec < 1.92. We find a majority (62 per cent) of normal ETGs, similar to typical local ones, co-existing with compact early types from ∼ two to ∼ six times smaller in spite of the same mass and redshift. The co-existence of normal and compact ETGs at 〈z〈 ≃ 1.5 suggests that their build-up taken place in the first 3–4 Gyr, followed distinct paths. Furthermore, we find that the number density of compact early types at 〈z〈 ≃ 1.5 is consistent with the lower limits of the local number density of compact early types derived from local clusters of galaxies. The similar number of compact early types found in the early and in the present day Universe frustrates the hypothesized effective radius evolution while provides evidence that also compact ETGs were as we see them today 9–10 Gyr ago. Finally, the fact that (at least) most of the compact ETGs at high z are accounted for by compact early types in local cluster of galaxies implies that the former are the direct progenitors of the compact early-type cluster galaxies establishing a direct link between environment and early phases of assembly of ETGs.
ABSTRACT
We analyse publicly available, individual spectra of four massive ($M\gt 10^{11}\, \mathrm{M}_{\odot }$) early-type galaxies with redshifts in the range 1.4 ≤ z ≤ 2 to determine their ...stellar content, extending our previous work up to z ∼ 2. The wide wavelength range of the VLT/X-Shooter spectroscopic data in the UV–Optical–NIR arms along with the availability of spectro-photometry allows us to explore different techniques to obtain the stellar population properties, namely through age/metallicity-sensitive spectral indices, full spectral fitting, and broad-band photometric fitting. Moreover, together with the widely used optical Lick indices, we consider further indices in the UV rest frame, and demonstrate that UV indices significantly help the accuracy of the resulting population parameters. We find galaxy ages ranging from 0.2 to 4 Gyr, where the oldest galaxy is found at the lowest redshift, with an excellent agreement between ages determined via indices, full spectral fitting, or broad-band colours. These ages are in perfect agreement with ages of local galaxies at the same velocity dispersion when we assume pure passive evolution. Total metallicities derived from indices show some scatter (between less than half-solar to very high values, Z/H ∼ 0.6). We speculate on possible mechanisms explaining these values, but given the sample size and low S/N of the spectra no conclusion can be made. Indices in the UV rest frame generally lead to similar conclusions as optical indices. For the oldest galaxy (4 Gyr), we show that its UV indices can only be explained by stellar population models including a UV contribution from old stellar populations, suggesting that old, UV bright populations start to inhabit mature galaxies of a few Gyr of age. This is the highest redshift (z ∼ 1.4) detection of the UV upturn up to date.
Nebular emission from star-forming galaxies Charlot, Stéphane; Longhetti, Marcella
Monthly notices of the Royal Astronomical Society,
06/2001, Letnik:
323, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We present a new model for computing consistently the line and continuum emission from galaxies, based on a combination of recent population synthesis and photoionization codes. We use effective ...parameters to describe the H ii regions and the diffuse gas ionized by single stellar generations in a galaxy, among which the most important ones are the zero-age effective ionization parameter, the effective gas metallicity and the effective dust-to-heavy element ratio. We calibrate the nebular properties of our model using the observed O iii/Hβ, O ii/O iii, S ii/Hα and N ii/S ii ratios of a representative sample of nearby spiral and irregular, starburst and H ii galaxies. To compute whole (line plus continuum) spectral energy distributions, we include the absorption by dust in the neutral interstellar medium (ISM) using a recent simple prescription, which is consistent with observations of nearby starburst galaxies. Our model enables us to interpret quantitatively the observed optical spectra of galaxies in terms of stars, gas and dust parameters. We find that the range of ionized-gas properties spanned by nearby galaxies implies factors of 3.5 and 14 variations in the Hα and O ii luminosities produced per unit star formation rate (SFR). When accounting for stellar Hα absorption and absorption by dust in the neutral ISM, the actual uncertainties in SFR estimates based on the emergent Hα and O ii luminosities are as high as several decades. We derive new estimators of the SFR, the gas-phase oxygen abundance and the effective absorption optical depth of the dust in galaxies. We show that, with the help of other lines such as O ii, Hβ, O iii, N ii or S ii, the uncertainties in SFR estimates based on Hα can be reduced to a factor of only 2–3, even if the Hα line is blended with the adjacent N ii lines. Without Hα, however, the SFR is difficult to estimate from the O ii, Hβ and O iii lines. The reason for this is that the absorption by dust in the neutral ISM and the ionized-gas parameters are then difficult to constrain independently. This suggests that, while insufficient by itself, the Hα line is essential for estimating the star formation rate from the optical emission of a galaxy.
Extremely compact massive galaxies at z ∼ 1.4 Trujillo, I.; Feulner, G.; Goranova, Y. ...
Monthly notices of the Royal Astronomical Society,
November 2006, Letnik:
373, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The optical rest-frame sizes of 10 of the most massive (∼5 × 1011 h−270 M⊙) galaxies found in the near-infrared MUNICS survey at 1.2 < z < 1.7 are analysed. Sizes are estimated in both the J and K′ ...filters. These massive galaxies are at least a factor of 4+1.9−1.0 (±1σ) smaller in the rest-frame V-band than local counterparts of the same stellar mass. Consequently, the stellar mass density of these objects is (at least) 60 times larger than that of massive ellipticals today. Although the stellar populations of these objects are passively fading, their structural properties are rapidly changing since that redshift. This observational fact disagrees with a scenario where the more massive and passive galaxies are fully assembled at z∼ 1.4 (i.e. a monolithic scenario) and points towards a dry merger scenario as the responsible mechanism for the subsequent evolution of these galaxies.
In this paper we study the iron content of a sample of 22 nearby hot clusters observed with BeppoSAX. We find that the global iron mass of clusters is tightly related to the cluster luminosity and ...that the relatively loose correlation between the iron mass and the cluster temperature follows from the combination of the iron mass vs. luminosity and luminosity vs. temperature correlations. The iron mass is found to scale linearly with the intracluster gas mass, implying that the global iron abundance in clusters is roughly constant. This result suggests that enrichment mechanisms operate at a similar rate in all clusters. By employing population synthesis and chemical enrichment models, we show that the iron mass associated with the abundance excess which is always found in the centre of cool core clusters can be entirely produced by the brightest cluster galaxy (BCG), which is always found at the centre of cool core clusters. The iron mass associated with the excess, the optical magnitude of the BCG and the temperature of the cluster are found to correlate with one another suggesting a link between the properties of the BCG and the hosting cluster. These observational facts lends strength to current formation theories which envisage a strong connection between the formation of the giant BCG and its hosting cluster.
We present the morphological analysis based on Hubble Space Telescope HST-NICMOS (Near-Infrared Camera and Multi-Object Spectrometer) observations in the F160W filter (λ≃ 1.6 μm) of a sample of 32 ...early-type galaxies (ETGs) at 1 < z < 2 with spectroscopic confirmation of their redshift and spectral type. The 32 ETGs at 〈z〉∼ 1.5 are placed on the (〈μ〉e, Re) plane according to the Kormendy relation (KR) with the same slope of the local one but with a different zero-point, which accounts for the evolution they undergo from z∼ 1.5–2 to z= 0. The best fitting of their spectral energy distribution shows that ETGs at 1 < z < 2 are composed of two distinct populations, an older population (oETGs) and a younger population (yETGs) whose mean ages differ by about 1.5–2 Gyr. Young ETGs are not denser than local ones since they follow the size–mass relation of local ETGs, and luminosity evolution brings them on to the local KR and size–luminosity relations without the need of size evolution. Old ETGs do not follow the size–mass relation of local ETGs, and luminosity evolution does not account for the discrepancy they show with respect to the local size–luminosity relation and KR. An increase in their Re by a factor of 2.5–3 (a density decrease by a factor of 15–30) from z∼ 1.5–2 to z∼ 0 is required to bring these galaxies on to the local scaling relations. The different properties and the different behaviour shown by the two populations with respect to the scaling relations imply different formation and evolution scenarios. The older population of ETGs must have formed at a higher z in a sort of dissipative gas-rich collapse able to produce remnants which at z∼ 2 are old and compact, a scenario which can be fitted qualitatively by some recent hydrodynamic simulations of gas-rich mergers. Given the typical time-scale of merging and the old age of their stellar population, oETGs should exist as they are up to z≳ 3–3.5. The size evolution they must experience from z∼ 2 to ∼0 must leave unchanged their mass to not exceed the local number of high-mass ETGs. Thus, major merging cannot fit this requirement. Satellite merging, close encounters and interactions can help at least qualitatively in solving this problem. The younger population of ETGs can be formed later through subsequent episodes of merging which increased progressively their size and assembled their mass down to z∼ 2. At z < 2, they evolve purely in luminosity since episodes of major merging would bring them far from the local scaling relations.
Context. The upcoming new generation of optical spectrographs on four-meter-class telescopes, with their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength ...coverage, will provide invaluable information for reconstructing the history of star formation in individual galaxies up to redshifts of about 0.7. Aims. We aim at defining simple but robust and meaningful physical parameters that can be used to trace the coexistence of widely diverse stellar components: younger stellar populations superimposed on the bulk of older ones. Methods. We produced spectra of galaxies closely mimicking data from the forthcoming Stellar Populations at intermediate redshifts Survey (StePS), a survey that uses the WEAVE spectrograph on the William Herschel Telescope. First, we assessed our ability to reliably measure both ultraviolet and optical spectral indices in galaxies of different spectral types for typically expected signal-to-noise ratios. We then analyzed such mock spectra with a Bayesian approach, deriving the probability density function of r- and u-band light-weighted ages as well as of their difference. Results. We find that the ultraviolet indices significantly narrow the uncertainties in estimating the r- and u-band light-weighted ages and their difference in individual galaxies. These diagnostics, robustly retrievable for large galaxy samples even when observed at moderate signal-to-noise ratios, allow us to identify secondary episodes of star formation up to an age of ∼0.1 Gyr for stellar populations older than ∼1.5 Gyr, pushing up to an age of ∼1 Gyr for stellar populations older than ∼5 Gyr. Conclusions. The difference between r-band and u-band light-weighted ages is shown to be a powerful diagnostic to characterize and constrain extended star-formation histories and the presence of young stellar populations on top of older ones. This parameter can be used to explore the interplay between different galaxy star-formation histories and physical parameters such as galaxy mass, size, morphology, and environment.
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