Nearby early-type galaxies with ionized gas

Abstract

Aims. The paper is devoted to the study of the underlying stellar population of a sample of 65 nearby early-type galaxies, predominantly located in low density environments, a large fraction of which show emission lines. Methods. Ages, metallicities, and [α/Fe] ratios have been derived through the comparison of Lick indices measured at different galacto-centric distances (7 apertures and 4 gradients) with new simple stellar population (SSP) models that account for the presence of α/Fe-enhancement. The SSPs cover a wide range of ages (10 9 −16 × 10 9 yr), metallicities (0.0004 ≤ Z ≤0.05), and [α/Fe] ratios (0-0.8). To derive the stellar population parameters, we use an algorithm that provides, together with the most likely solution in the (age, Z, [α/Fe]) space, the probability density function along the age-metallicity degeneracy. Results. We derive a large spread in age, with SSP-equivalent ages ranging from a few to 15 Gyr. Age does not show any significant trend with central velocity dispersion σ c , but E galaxies appear on average older than lenticulars. On the contrary, a significant increasing trend of metallicity and [α/Fe] with σ c is observed, testifying that the chemical enrichment was more efficient and the duration of the star formation shorter in more massive galaxies. These latter two relations do not depend on galaxy morphological type. We have also sought possible correlations with the local galaxy density ρ xyz , but neither metallicity nor α-enhancement show clear trends. However, we find that while low density environments (LDE) (ρ xyz ≤ 0.4) contain very young objects (from 1 Gyr to 4 Gyr), none of the galaxies in the higher density environments (HDE) (40% of galaxies with a measured density) is younger than 5 Gyr. Considering the lack of environmental effect on the [α/Fe] ratio and the high value of [α/Fe] in some young massive objects, we argue that young galaxies in LDE are more likely due to recent rejuvenation episodes. By comparing the number of "rejuvenated" objects with the total number of galaxies in our sample, and by means of simple two-SSP component models, we estimate that, on average, the rejuvenation episodes do not involve more than 25% of the total galaxy mass. The good quality of the data also allow us to analyze the gradients of stellar populations. An average negative metallicity gradient ∆ log Z/∆ log(r/r e ) ∼ −0.21 is firmly detected, while the age and α-enhancement spatial distributions within r e /2 appear quite flat. These observations suggest that, in a given galaxy, the star formation proceeded on similar timescales all across the central r e /2 region, but with an increasing efficiency toward the center.

Galaxy evolution in Local Group analogs

“…At odds with cluster counterparts, ETGs in LDEs frequently show significant signatures of relatively recent activity as indicated by the presence of distorted morphologies (Reduzzi et al 1996;Colbert et al 2001) and of kinematical sub-components (Emsellem et al 2004, and reference therein). The analysis of line-strength indices (Trager et al 2000;Longhetti et al 2000;Kuntschner 2000;Thomas et al 2003;Denicoló et al 2005;Annibali et al 2007;Rampazzo et al 2007) shows that ETGs in LDE have a large dispersion in their luminosity weighted age, from a Hubble time to ≈1 Gyr. About 40% of ETGs in the Annibali et al (2007) sample have a luminosity weighted age ≤ 6 Gyr, and 10% ≤ 3 Gyr.…”

“…In the group environment, galaxy encounters may then completely reshape the members' morphology and trigger secondary star formation episodes even in earlytype galaxies (see e.g. Annibali et al 2007, and references therein).…”

Study of the stellar population properties in the discs of ten spiral galaxies

“…The columns show the following: 1, galaxy name; 2, morphological classification from Lyon Extragalactic Database (LEDA); 3, Hubble type (LEDA); 4) apparent isophotal diameters measured at a surface-brightness level of µ B = 25 mag arcsec −2 (LEDA); 5, total observed blue magnitude from LEDA; 6, radial velocity with respect to the cosmic microwave background reference frame (LEDA); 7, distance obtained as in Morelli et al (2008); Pizzella et al (2008); Morelli et al (2012) adopting H 0 = 75 km s −1 Mpc −1 ; 8, absolute total blue magnitude from B T corrected for extinction as done in LEDA and adopting the distance D; 9, disc scalelength; 10, central values of the velocity dispersion; 11, Source of the photometric and spectroscopic data: (1 = Morelli et al (2008) els Vazdekis et al 2010) is the most widely used method to derive the age, metallicity, and [α/Fe] enhancement of unresolved stellar populations in galaxies. However, most of targets are elliptical galaxies (Mehlert et al 2003;Annibali et al 2007) and bulges of disc galaxies (Moorthy & Holtzman 2006;Jablonka et al 2007;Morelli et al 2008Morelli et al , 2012Seidel et al 2015) for which the assumption of SSP is a good approximation. To date, only a few papers focused on the stellar populations of galactic discs have been published because of their low surface brightness and the presence of intense nebular emission lines which make the spectroscopic analysis quite difficult.…”

Galaxy evolution in groups. USGC U268 and USGC U376 in the Leo cloud

“…This is not surprising for galaxies located in the group environment in which several studies indicate that the galaxy life is quite eventful (e.g. Annibali et al, 2007;Marino et al, 2011c,b,a;Rampazzo et al, 2011). : Left: position of the galaxies in each group, in a square of approximately of 2 × 2 Mpc 2 at their respective distances.…”