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Register a new userNearby early-type galaxies with ionized gas. The UV emission from GALEX observations
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We present GALEX far-ultraviolet (FUV, $lambda_{eff}$=1538 AA) and near-ultraviolet (NUV, $lambda_{eff}$=2316 AA) 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 D$_{25}$ integrated magnitudes. We extend the photometric study to the optical {it 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 ($leq 4$ 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 $n$ ($leq5$). The Sersic law exponent $n=4$ seems to be a watershed: ETGs with $n>4$ tend to have [$alpha$/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 $sigma$, with the UV colours getting bluer at larger $sigma$. This trend is likely driven by a combined effect of `downsizing and of the mass-metallicity relation.
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We present low resolution Spitzer-IRS spectra of 40 ETGs, 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 emission features. We use MIR diagnostic diagrams to investigate the powering mechanisms of the ionized gas. The mid-infrared spectra of early-type galaxies show a variety of spectral characteristics. We empirically sub-divide the sample into five classes of spectra with common characteristics. 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 {mu}m/11.3 {mu}m leq 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 starburst/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{mu}m/[Ne II]12.8{mu}m vs. [S III]33.48{mu}m/[Si II]34.82{mu}m, is used to investigate the different mechanisms ionizing the gas. 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. Most of the spectra in the present sample are classified as LINERs 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, present in LINER nuclei.
We present a study of the ionized gas in a sample of 65 nearby early-type galaxies, for which we have acquired optical intermediate-resolution spectra. Emission lines are detected in ~89 % of the sample. The incidence of emission appears independent from the E or S0 morphological classes. According to classical diagnostic diagrams, the majority of the galaxies are LINERs. However, the galaxies tend to move toward the Composites region (at lower [NII]/Halpha values) as the emission lines are measured at larger galacto-centric distances. This suggests that different ionization mechanisms may be at work in LINERs.
Strong Balmer absorption lines and the lack of Ha and [OII] emission lines signify that E+As are post-starburst systems. Recent studies suggest that E+As may undergo the transition from the `blue cloud to the `red sequence and eventually migrate to red sequence ETGs. An observational validation of this scenario is to identify the intervening galaxy population between E+As and the red-sequence. Motivated by recent findings with GALEX that a large fraction of ETGs exhibit UV-excess as a sign of RSF, we investigate the possible connection of the UV-excess galaxies to E+As. In particular, we examine the FP scaling relations of the largest sample of ~1,000 E+As selected from the SDSS and ~20,000 morphologically-selected SDSS ETGs with GALEX UV data. The FP parameters, combined with stellar population indicators, reveal a certain group of UV-excess ETGs that bridges between E+As and quiescent red galaxies. The newly identified galaxies are the post-starburst systems characterized by UV-excess but no Ha emission. This is a conceptual generalisation of E+A, in that the Balmer absorption line in the E+A definition is replaced with UV-optical colours that are far more sensitive to RSF than the Balmer lines. We refer to these UV-excess galaxies as E+a galaxies, which stands for elliptical (E) galaxies with a minority of A-type (a) young stars. The species are either (1) galaxies that experienced starbursts weaker than those observed in E+As (1~10% of E+As, mild E+As) or (2) the products of passively evolved E+As after quenching star formation quite a while ago (~1 Gyr, old E+As). We suggest that the latter type of E+a galaxies represents the most recent arrival to the red sequence in the final phase of the E+A to red early-type transition. (Abridged)
Shell galaxies are widely considered the debris of recent accretion/merging episodes. Their high frequency in low density environment suggests that such episodes could be among the driver of the early-type galaxy secular evolution. We present far and near UV (FUV and NUV respectively hereafter) GALEX photometric properties of a sample of shell galaxies.
We present results of optical broad-band and narrow-band Halpha observations of a sample of forty nearby early-type galaxies. The majority of sample galaxies are known to have dust in various forms viz. dust lanes, nuclear dust and patchy/filamentary dust. A detailed study of dust was performed for 12 galaxies with prominent dust features. The extinction curves for these galaxies run parallel to the Galactic extinction curve, implying that the properties of dust in these galaxies are similar to those of the Milky-Way. The ratio of total to selective extinction (Rv) varies between 2.1 and 3.8, with an average of 2.9 +/- 0.2, fairly close to its canonical value of 3.1 for our Galaxy. The average relative grain size <a>/a_Gal of dust particles in these galaxies turns out to be 1.01 +/- 0.2, while dust mass estimated using optical extinction lies in the range 10^2 to 10^4 M(sun) . The Halpha emission was detected in 23 out of 29 galaxies imaged through narrow- band filters with the Halpha luminosities in the range 10^38 - 10^41 erg s^-1. The mass of the ionized gas is in the range 10^3-10^5 M(sun). The morphology and extent of ionized gas is found similar to those of dust, indicating possible coexistence of dust and ionized gas in these galaxies. The absence of any apparent correlation between blue luminosity and normalized IRAS dust mass is suggestive of merger related origin of dust and gas in these galaxies.
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