M. Longhetti

Extremely compact massive galaxies at z ∼ 1.4

The optical rest-frame sizes of 10 of the most massive (∼5 × 10 11 h −2 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 Kfilters. These massive galaxies are at least a factor of 4 +1.9 (±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.

On the Iron content in rich nearby Clusters of Galaxies

In this paper we study the iron content of a sample of 22 nearby hot clusters observed with BeppoSAX .W e fi nd 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.

The Kormendy relation of massive elliptical galaxies at z∼ 1.5: evidence for size evolution

We present the morphological analysis based on HST-NIC2 (0.075 arcsec pixel -1 ) images in the F160W filter of a sample of nine massive field (>10 11 M ⊙ ) galaxies spectroscopically classified as early-types at 1.2 < z < 1.7. Our analysis shows that all of them are bulge-dominated systems. In particular, six of them are well fitted by a de Vaucouleurs profile (n = 4) suggesting that they can be considered pure elliptical galaxies. The remaining three galaxies are better fitted by a Sersic profile with index 1.9 < n fit < 2.3 suggesting that a disc-like component could contribute up to 30 per cent to the total light of these galaxies. We derived the effective radius R e and the mean surface brightness (SB) (μ) e within R e of our galaxies and we compared them with those of early-types at lower redshifts. We find that the SB (μ) e of our galaxies should get fainter by 2.5 mag from z ∼ 1.5 to ∼0 to match the SB of the local ellipticals with comparable R e , that is, the local Kormendy relation. Luminosity evolution without morphological changes can only explain half of this effect, as the maximum dimming expected for an elliptical galaxy is ∼ 1.6 mag in this redshift range. Thus, other parameters, possibly structural, may undergo evolution and play an important role in reconciling models and observations. Hypothesizing an evolution of the effective radius of galaxies we find that R e should increase by a factor of 1.5 from z ∼ 1.5 to ∼0.

Stellar mass estimates in early-type galaxies: procedures, uncertainties and models dependence

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 M/C 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.

The population of early-type galaxies at 1 < z < 2 - new clues on their formation and evolution

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 5 × 10 11 M� ) 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.

The density of very massive evolved galaxies to z ≃ 1.7

We spectroscopically identified seven massive, evolved galaxies with magnitudes 17.8 2 supporting a high efficiency in the accretion of the stellar mass in massive haloes in the early Universe.

Probing the evolution of the near-infrared luminosity function of galaxies to z≃ 3 in the Hubble Deep Field-South

We present the rest-frame J s - and K s -band luminosity function (LF) of a sample of about 300 galaxies selected in the Hubble Deep Field-South (HDF-S) at K s ≤ 23 (Vega). We use calibrated photometric redshift together with spectroscopic redshift for 25 per cent of the sample. The accuracy reached in the photometric redshift estimate is 0.06 (rms) and the fraction of outliers is 1 per cent. We find that the rest-frame J s -band luminosities obtained by extrapolating the observed J s -band photometry are consistent with those obtained by extrapolating the photometry in the redder H and K s bands closer to the rest-frame J s , at least up to z ∼ 2. Moreover, we find no significant differences among the luminosities obtained with different spectral libraries. Thus, our LF estimate is not dependent either on the extrapolation made on the best-fitting template or on the library of models used to fit the photometry. The selected sample has allowed us to probe the evolution of the LF in the three redshift bins [0; 0.8), [0.8; 1.9) and [1.9; 4) centred at the median redshift z m ≃ [0.6, 1.2, 3] and to probe the LF at z m ≃ 0.6 down to the unprecedented faint luminosities M Js ≃ -13 and M Ks ≃ -14. We find hints of a rise of the faint-end (M Js > -17 and M Ks > -18) near-infrared (near-IR) LF at z m ∼ 0.6: a rise that cannot be probed at higher redshift with our sample. The values of a we estimate are consistent with the local value and do not show any trend with redshift. We do not see evidence of evolution from z = 0 to z m ∼ 0.6 suggesting that the population of local bright galaxies was already formed at z 0.8 suggests that a significant fraction of them increase their stellar mass at 1 3. Thus, our results suggest that the assembly of massive galaxies is spread over a large redshift range and that the increase of their stellar mass has been very efficient also at very high redshift at least for a fraction of them.

On the central stellar mass density and the inside-out growth of early-type galaxies

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 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 Sersic 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.

Looking for obscured QSOs in the X-ray emitting ERO population

We present XMM-Newton data centered on one of the MUNICS Near Infrared Cluster Survey fields (S2F1) and we discuss the X-ray properties of the 6 X-ray emitting EROs found. For one of them we have already obtained the redshift using near-infrared spectroscopic data, while for the remaining 5 EROs the analysis is based on photometric redshifts. We find evidence for the presence of an X-ray obscured QSO in at least 5 out of the 6 X-ray emitting EROs. For these 5 objects we derive intrinsic (2–10 keV) luminosities in excess of 1044 erg s−1 and intrinsic column densities higher than 1022 cm−2. These values have been obtained through a basic X-ray spectral analysis for the three brightest sources and through the analysis of the hardness ratios for the remaining two. All of these 5 X-ray emitting EROs appear extended in the optical/near-infrared bands indicating that the host galaxy emission dominates at these wavelengths. This suggests that the hosted AGNs are likely to be absorbed also in the optical/near-infrared bands: i.e. they are likely X-ray obscured possible type 2 QSOs. For the remaining ERO the presence of an AGN is suggested both by its high 0.5–2 keV luminosity (L0.5−2keV ∼ 1043 erg s−1) and by its X-rayto- optical flux ratio. In this case the quality of the present data prevents us from placing firm constraints on the AGN type hosted. Finally, the near-IR spectrum obtained for one of the 6 EROs classifies the host galaxy as an elliptical at z 1.7 with a stellar mass well in excess of 1011 M⊙. This result corroborates the possible link between the QSO activity and the formation of massive spheroids.

High-z massive galaxies in the Hubble Deep Field South

A census of massive galaxies at redshift increasingly higher than z ∼ 1 may provide strong constraints for the history of mass assembly and star formation. Here we report the analysis of three galaxies selected in the Hubble Deep Field South at Ks ≤ 22 on the basis of their unusually red near-IR color J −K ≥ 3. We have used population synthesis models to constrain their redshifts and their stellar masses. One galaxy (HDFS-1269) is at redshift zphot � 2.4 while the other two (HDFS-822 and HDFS- 850) are at zphot � 2.9−3.0. All three galaxies have already assembled a stellar mass of about 10 11 Mat the observed redshift, placing the possible merging event of their formation at z > 3.5. The inferred mass weighted age of their stellar populations implies that the bulk of the stars formed at zf > 3.5. The resulting co-moving density of Mstars > 10 11 Mgalaxies atz �� 2.7 is ρ = 1.2 ± 0.7 × 10 −4 Mpc −3 , about a factor two higher than the predictions of hierarchical models. The comparison with the local density of galaxies implies that the three galaxies must have already formed most of their stellar mass and that they cannot follow an evolution significantly different from a passive aging. The comparison with the density of local L ≥ L ∗ early types (passively evolved galaxies) suggests that their co-moving density cannot decrease by more than a factor 2.5-3 from z = 0t o z � 3 suggesting that up to 40% of the stellar mass content of bright (L ≥ L ∗ ) local early type galaxies was already in place at z > 2.5.