Roberto Rampazzo

The star formation history of early‐type galaxies as a function of mass and environment

Using the third data release of the Sloan Digital Sky Survey (SDSS), we have rigorously defined a volume-limited sample of early-type galaxies in the redshift range 0.005 < z≤ 0.1. We have defined the density of the local environment for each galaxy using a method which takes account of the redshift bias introduced by survey boundaries if traditional methods are used. At luminosities greater than our absolute r-band magnitude cut-off of −20.45, the mean density of environment shows no trend with redshift. We calculate the Lick indices for the entire sample and correct for aperture effects and velocity dispersion in a model-independent way. Although we find no dependence of redshift or luminosity on environment, we do find that the mean velocity dispersion, σ, of early-type galaxies in dense environments tends to be higher than in low-density environments. Taking account of this effect, we find that several indices show small but very significant trends with environment that are not the result of the correlation between indices and velocity dispersion. The statistical significance of the data is sufficiently high to reveal that models accounting only for α-enhancement struggle to produce a consistent picture of age and metallicity of the sample galaxies, whereas a model that also includes carbon enhancement fares much better. We find that early-type galaxies in the field are younger than those in environments typical of clusters but that neither metallicity, α-enhancement nor carbon enhancement are influenced by the environment. The youngest early-type galaxies in both field and cluster environments are those with the lowest σ. However, there is some evidence that the objects with the largest σ are slightly younger, especially in denser environments. Independent of environment both the metallicity and α-enhancement grow monotonically with σ. This suggests that the typical length of the star formation episodes which formed the stars of early-type galaxies decreases with σ. More massive galaxies were formed in faster bursts. We argue that the timing of the process of formation of early-type galaxies is determined by the environment, while the details of the process of star formation, which has built up the stellar mass, are entirely regulated by the halo mass. These results suggest that the star formation took place after the mass assembly and favours an anti-hierarchical model. In such a model, the majority of the mergers must take place before the bulk of the stars form. This can only happen if there exists an efficient feedback mechanism which inhibits the star formation in low-mass haloes and is progressively reduced as mergers increase the mass.

Nearby early-type galaxies with ionized gas.IV. Origin and powering mechanism of the ionized gas

Aims. A significant fraction of early-type galaxies (ETGs) show em ission lines in their optical spectra. We aim at understandi ng the powering mechanism and the origin of the ionized gas in ETGs, and its connection with the host galaxy evolution. Methods. We analyzed intermediate-resolution optical spectra of a s ample of 65 ETGs, mostly located in low density environments and biased toward the presence of ISM traces, for which we already derived in the previous papers of the series the stellar population properties. To extract the emission lines from the galaxy sp ectra, we developed a new fitting procedure that properly sub tracts the underlying stellar continuum, and that accounts for the unc ertainties due to the age-metallicity degeneracy. The emis sion line luminosities derived in annuli of increasing galacto-centric d istance were used to constrain the excitation mechanism and the metallicity of the ionized gas. Results. Optical emission lines are detected in 89% of the sample. The detection fraction drops to 57% if only the galaxies with EW(Hα + [NII])>3 A are considered. The incidence and strength of emission do not correlate either with the E/S0 classification, or with the fast/slow rotator classification. Comparing the nuclear r<re/16 emission with the classical [OIII]/Hβ vs [NII]/Hα diagnostic diagram, the galaxy activity is so classified: 72% o f the galaxies with emission are LINERs, 9% are Seyferts, 12% are Composite/Transition objects, and 7% are non-classified. Seyferts hav e young luminosity-weighted ages (. 5 Gyr), and appear, on average, significantly younger than LINERs and Composites. Excluding the Seyfert galaxies from our sample, we find that t he spread in the ([OIII], Hα or [NII]) emission strength increases with the galaxy centr al velocity dispersion σc, low-σc galaxies having all weak emission lines, and high-σc galaxies displaying both weak and strong emission lines. Furthermore, the [NII]/Hα ratio tends to increase withσc. A spatial analysis of the emission line properties within t he individual galaxies reveals that the [NII]/Hα ratio decreases with increasing galacto-centric distance, indi cating either a decrease of the nebular metallicity, or a pro gressive “softening” of the ionizing spectrum. The average nebular oxygen abundance is slightly less than solar. A comparison with the stella r metallicities derived in Paper III shows that the gas oxygen abundance is≈ 0.2 dex lower than that of stars. Conclusions. The stronger nuclear (r< re/16) emission can be explained with photoionization by PAGB stars alone only for≈ 22% of the LINERs/Composite sample. On the other hand, we can not exclude an important role of PAGB star photoionization at larger radii. For the major fraction of the sample, the nuclear emission is consistent with excitation from a low-accretion rate AGN, fast shocks (200 -500 km/s) in a relatively gas poor environment (n. 100 cm −3 ) , or coexistence of the two. The derived [SII]6717/6731 ratios are consistent with the low gas densities required by the sho ck models. The derived nebular metallicities suggest eithe r an external origin of the gas, or an overestimate of the oxygen yields by SN models.

Spitzer irs spectra of virgo early type galaxies: detection of stellar silicate emission

We present high signal-to-noise ratio Spitzer Infrared Spectrograph observations of 17 Virgo early-type galaxies. The galaxies were selected from those that define the color-magnitude relation of the cluster, with the aim of detecting the silicate emission of their dusty, mass-losing evolved stars. To flux calibrate these extended sources, we have devised a new procedure that allows us to obtain the intrinsic spectral energy distribution and to disentangle resolved and unresolved emission within the same object. We have found that 13 objects of the sample (76%) are passively evolving galaxies with a pronounced broad silicate feature that is spatially extended and likely of stellar origin, in agreement with model predictions. The other four objects (24%) are characterized by different levels of activity. In NGC 4486 (M87), the line emission and the broad silicate emission are evidently unresolved, and, given also the typical shape of the continuum, they likely originate in the nuclear torus. NGC 4636 shows emission lines superposed on extended (i.e., stellar) silicate emission, thus pushing the percentage of galaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central unresolved region. A more detailed analysis of our sample, with updated models, will be presented in a forthcoming paper.

The mid-infrared colour–magnitude relation of early-type galaxies in the Coma cluster as measured by Spitzer-IRS

We use 16 µm, Spitzer-IRS, blue peakup photometry of 50 early-type galaxies in the Coma cluster to define the mid-infrared colour-magnitude relation. We compare with recent simple stellar population models that include the mid-infrared emission from the extended, dusty envelopes of evolved stars. The Ks-[16] colour in these models is very sensitive to the relative population of dusty Asymptotic Giant Branch (AGB) stars. We find that the passively evolving early-type galaxies define a sequence of approximately constant age (� 10 Gyr) with varying metallicity. Several galaxies that lie on the optical/near-infrared colour-magnitude relation do not lie on the midinfrared relation. This illustrates the sensitivity of the Ks-[16] colour to age. The fact that a colour-magnitude relation is seen in the mid-infrared underlines the extremely passive nature of the majority (68 %) of early-type galaxies in the Coma cluster. The corollary of this is that 32% of the early-type galaxies in our sample are not ‘passive’, insofar as they are either significantly younger than 10 Gyr or they have had some rejuvenation episode within the last few Gyr.

Nearby early-type galaxies with ionized gas: the UV emission from GALEX observations★

We present GALEX far-ultraviolet (FUV, �eff=1538 u and near-ultraviolet (NUV, �eff=2316 u surface photometry of 40 early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs showing emission lines in their optical spectra. We derive FUV and NUV surface brightness profiles, (FUV-NUV) colour profiles and D25 integrated magnitudes. We extend the photometric study to the optical r band from SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour profiles become redder with galactocentric distance in both rejuvenated (6 4 Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC 3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of the galaxy, suggesting the presence of recent star formation. Although seven of our ETGs show shell systems in their optical image, only NGC 7135 displays shells in the UV bands. We characterize the UV and optical surface brightness profiles, along the major axis, using a Sersic law. The Sersic law exponent, n, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values of n (6 5). The Sersic law exponent n = 4 seems to be a watershed: ETGs with n > 4 tend to have [�/Fe] greater than 0.15, implying a short star-formation time scale. We find a significant correlation between the FUV NUV colour and central velocity dispersions �, with the UV colours getting bluer at larger �. This trend is likely driven by a combined effect of ‘downsizing’ and of the mass-metallicity relation.

Nearby early-type galaxies with ionized gas - VI. The Spitzer-IRS view. Basic data set analysis and empirical spectral classification

Context. A large fraction of early-type galaxies (ETGs) shows emission lines in their optical spectra, mostly with LINER characteristics. Despite the number of studies, the nature of the ioni zation mechanisms is still debated. Many ETGs also show several signs of rejuvenation episodes. Aims. We aim to investigate the ionization mechanisms and the physical processes of a sample of ETGs using mid-infrared spectra. Methods. We present here low resolution Spitzer-IRS spectra of 40 ETGs, 18 of which from our proposed Cycle 3 observations, selected from a sample of 65 ETGs showing emission lines in their optical spectra. We homogeneously extract the mid-infrared (MIR) spectra, and after the proper subtraction of a “passive” ETG template, we derive the intensity of the ionic and molecular lines and of the polycyclic aromatic hydrocarbon (PAH) emission features. We use MIR diagnostic diagrams to investigate the powering mechanisms of the ionized gas. Results. The mid-infrared spectra of early-type galaxies show a variety of spectral characteristics. We empirically sub-divid e the sample into five classes of spectra with common characterist ics. Class-0, accounting for 20% of the sample, are purely passive ETGs with neither emission lines nor PAH features. Class-1 show emission lines but no PAH features, and account for 17.5% of the sample. Class-2, in which 50% of the ETGs are found, as well as having emission lines, show PAH features with unusual ratios, e.g. 7: 7� m= 11: 3� m�2: 3. Class-3 objects have emission lines and PAH features with ratios typical of star-forming galaxies. 7.5% of objects fall in this class, likely to be objects in a starburs t/post-starburst regime. Class-4, containing only 5% of the ETGs, is dominated by a hot dust continuum. The diagnostic diagram [Ne III]15.55� m/[Ne II]12.8� m vs. [S III]33.48� m/[Si II]34.82� m, is used to investigate the different mechanisms ionizing the gas. According to the above diagram, if we exclude NGC 3258 where a starburst seems present, most of our ETGs contain gas ionized via either AGN-like or shock phenomena, or both. Conclusions. Most of the spectra in the present sample are classified as LIN ERs in the optical window. The proposed MIR spectral classes show unambiguously the manifold of the physical processes and ionization mechanisms, from star formation, low level AGN activity, to shocks (H2), present in LINER nuclei.

UNUSUAL PAH EMISSION IN NEARBY EARLY-TYPE GALAXIES: A SIGNATURE OF AN INTERMEDIATE-AGE STELLAR POPULATION?

We present the analysis of Spitzer-IRS spectra of four early-type galaxies (ETGs), NGC?1297, NGC?5044, NGC?6868, and NGC?7079, all classified as LINERs in the optical bands. Their IRS spectra present the full series of H2 rotational emission lines in the range 5-38 ?m, atomic lines, and prominent polycyclic aromatic hydrocarbon (PAH) features. We investigate the nature and origin of the PAH emission, characterized by unusually low 6-9/11.3 ?m interband ratios. After the subtraction of a passive ETG template, we find that the 7-9 ?m spectral region requires dust features not normally present in star-forming galaxies. Each spectrum is then analyzed with the aim of identifying their components and origin. In contrast to normal star-forming galaxies, where cationic PAH emission prevails, our 6-14 ?m spectra seem to be dominated by large and neutral PAH emission, responsible for the low 6-9/11.3 ?m ratios, plus two broad dust emission features peaking at 8.2 ?m and 12 ?m. These broad components, observed until now mainly in evolved carbon stars and usually attributed to pristine material, contribute approximately 30%-50% of the total PAH flux in the 6-14 ?m region. We propose that the PAH molecules in our ETGs arise from fresh carbonaceous material that is continuously released by a population of carbon stars, formed in a rejuvenation episode that occurred within the last few?Gyr. The analysis of the MIR spectra allows us to infer that, in order to maintain the peculiar size and charge distributions biased to large and neutral PAHs, this material must be shocked and excited by the weak UV interstellar radiation field of our ETGs.

TRACING REJUVENATION EVENTS IN NEARBY S0 GALAXIES

With the aim of characterizing rejuvenation processes in early-type galaxies, we analyzed five barred S0 galaxies showing a prominent outer ring in ultraviolet (UV) imaging. We analyzed Galaxy Evolution Explorer far-UV (FUV) and near-UV (NUV), and optical data using stellar population models and estimated the age and the stellar mass of the entire galaxies and the UV-bright ring structures. Outer rings consist of young ({approx}<200 Myr old) stellar populations, accounting for up to 70% of the FUV flux but containing only a few percent of the total stellar mass. Integrated photometry of the whole galaxies places four of these objects on the green valley, indicating a globally evolving nature. We suggest such galaxy evolution is likely driven by bar-induced instabilities, i.e., inner secular evolution, that conveys gas to the nucleus and the outer rings. At the same time, H I observations of NGC 1533 and NGC 2962 suggest external gas re-fueling can play a role in the rejuvenation processes of such galaxies.

Multiwavelength Insights into Mixed-Morphology Binary Galaxies. I. ISOCAM, ISOPHOT, and Hα Imaging*

We present Hα and ISO mid- and far-IR observations for a sample of mixed-morphology galaxy pairs that reveal both the stellar and nonstellar signatures of the interaction process. A mixed-morphology pair is perhaps the simplest form of galaxy-galaxy interaction because it is expected to involve only a single rapidly rotating gas-rich component paired with a gas-poor elliptical or lenticular galaxy. A primary assumption that we address is whether spirals are the only IR emitter in these mixed (E+S) pairs. Our observations reveal that many of the early-type galaxies exhibit weak (low equivalent width) emission, as often observed in field elliptical galaxies. These are the classical mixed-morphology pairs. However, some of the early-type components, especially the lenticular galaxies, show evidence for significant star formation, with Hα equivalent widths and 15 μm luminosities comparable to or exceeding those of their often much larger spiral companions. Our sample contains five Seyfert 2 nuclei, of which three can be described as companions on the end of a spiral arm. The Seyfert nucleus is often accompanied by a starburst region, while other such companions currently show only the starburst component. These pairs are among the best candidates for direct interaction fuelling of both starbursts and active galactic nuclei.

GALEX UV properties of the polar ring galaxy MCG-05-07-001 and the shell galaxies NGC 1210 and NGC 5329

Context. Systems of shells and polar rings in early-type galaxies are considered “bona fide” tracers of mass accretion and/or mergers. Their high frequency in low density environments suggests that these processes could drive the evolution of at least a fraction of the early-type galaxy population. Aims. We investigate the star formation histories of this type of galaxies. Their UV emission is important for testing whether these galaxies host ongoing or recent star formation and how this formation varies across the galaxy. Methods. We used far- and near- ultraviolet, optical, near-infrared images, neutral hydrogen HI maps, and line-strength indices to investigate the nuclear and outer regions of these galaxies as well as the regions where fine structures are present. Results. The GALEX near UV (NUV) and far UV (FUV) images of MCG-05-07-001 and NGC 1210 show complex tidal tails and debris structures. The far UV morphology of both galaxies appears so different from the optical morphology that the early-type classification may not apply. In both GALEX bands, the polar ring of MCG-05-07-001 is the dominant feature, whereas an extended tidal tail dominates the FUV bands of NGC 1210. In MCG-05-07-001 and NGC 1210, there is a strong correlation between structures detected in the FUV and NUV bands and in HI. In contrast, NGC 5329 does not show evidence of shells in the GALEX bands. We try to constrain the age of the accretion episode or merger that produced the shells and polar rings with the aid of composite stellar populations that take the presence of dust into account. The presence of HI in both MCG-05-07-001 and NGC 1210 argues in favour of wet mergers. Models suggest the presence of very young stellar populations in MCG-05-07-001: the observations could be explained in the framework of a conspicuous burst of star formation that occurred ≤1 Gyr ago and involved a large fraction of the galaxy mass. Our models suggest that also the nuclei of NGC 1210 and NGC 5329 could have been rejuvenated by an accretion episode about 2–4 Gyr ago.