The recent literature suggests that an evolutionary dichotomy exists for early-type galaxies (Es and S0s, ETGs) whereby their central photometric structure (cuspy versus core central luminosity profiles), and figure of rotation (fast (FR) vs. slow (S
R) rotators), are determined by whether they formed by wet or dry mergers. We consider whether the mid infrared (MIR) properties of ETGs, with their sensitivity to accretion processes in particular in the last few Gyr (on average z < 0.2), can put further constraints on this picture. We investigate a sample of 49 ETGs for which nuclear MIR properties and detailed photometrical and kinematical classifications are available from the recent literature. In the stellar light cuspy/core ETGs show a dichotomy that is mainly driven by their luminosity. However in the MIR, the brightest core ETGs show evidence that accretions have triggered both AGN and star formation activity in the recent past, challenging a dry merger scenario. In contrast, we do find, in the Virgo and Fornax clusters, that cuspy ETGs, fainter than M$_{K_s}=-24$, are predominantly passively evolving in the same epoch, while, in low density environments, they tend to be more active. A significant and statistically similar fraction of both FR (38$^{+18}_{-11}$%) and SR (50$^{+34}_{-21}$%) shows PAH features in their MIR spectra. Ionized and molecular gas are also frequently detected. Recent star formation episodes are then a common phenomenon in both kinematical classes, even in those dominated by AGN activity, suggesting a similar evolutionary path in the last few Gyr. MIR spectra suggest that the photometric segregation between cuspy and core nuclei and the dynamical segregation between FR and SR must have originated before z~0.2.
The molecular phase of the interstellar medium (ISM) in galaxies offers fundamental insight for understanding star-formation processes and how stellar feedback affects the nuclear activity of certain galaxies. We present here Large Millimeter Telesco
pe spectra obtained with the Redshift Search Receiver, a spectrograph that cover simultaneously the 3 mm band from 74 to 111 GHz with a spectral resolution of around 100 km/s. The observed galaxies that have been detected previously in HCN, have different degrees of nuclear activity, one normal galaxy (NGC 6946), the starburst prototype (M 82) and two ultraluminous infrared galaxies (ULIRGs, IRAS 17208-0014 and Mrk 231). We plotted our data in the HCO+/HCN vs. HCN/13CO diagnostic diagram finding that NGC 6946 and M 82 are located close to other normal galaxies; and that both IRAS 17208-0014 and Mrk 231 are close to the position of the well known ULIRG Arp 220 reported by Snell et al. (2011). We found that in Mrk 231 -- a galaxy with a well known active galactic nucleus -- the HCO+/HCN ratio is similar to the ratio observed in other normal galaxies.
We investigate the utility of the Tunable Filters (TFs) for obtaining flux calibrated emission line maps of extended objects such as galactic nebulae and nearby galaxies, using the OSIRIS instrument at the 10.4-m GTC. Despite a relatively large field
of view of OSIRIS (8x8), the change in the wavelength across the field (~80 Ang) and the long-tail of Tunable Filter (TF) spectral response function, are hindrances for obtaining accurate flux calibrated emission-line maps of extended sources. The purpose of this article is to demonstrate that emission-line maps useful for diagnostics of nebula can be generated over the entire field of view of OSIRIS, if we make use of theoretically well-understood characteristics of TFs. We have successfully generated the flux-calibrated images of the nearby, large late-type spiral galaxy M101 in the emission lines of Halpha, [NII]6583, [SII]6716 and [SII]6731. We find that the present uncertainty in setting the central wavelength of TFs (~1 Ang), is the biggest source of error in the emission-line fluxes. By comparing the Halpha fluxes of HII regions in our images with the fluxes derived from Halpha images obtained using narrow-band filters, we estimate an error of ~11% in our fluxes. The flux calibration of the images was carried out by fitting the SDSS griz magnitudes of in-frame stars with the stellar spectra from the SDSS spectral database. This method resulted in an accuracy of 3% in flux calibration of any narrow-band image, which is as good as, if not better, to that is feasible using the observations of spectrophotometric standard stars. Thus time-consuming calibration images need not be taken. A user-friendly script under the IRAF environment was developed and is available on request.
We present the analysis of Spitzer-IRS spectra of four early-type galaxies, 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 ra
nge 5--38 microns, atomic lines, and prominent PAH features. We investigate the nature and origin of the PAH emission, characterized by unusually low 6 -- 9/11.3 microns inter-band ratios. After the subtraction of a passive early type galaxy template, we find that the 7 -- 9 microns 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 microns spectra seem to be dominated by large and neutral PAH emission, responsible for the low 6 -- 9/11.3 microns ratios, plus two broad dust emission features peaking at 8.2 microns and 12 microns. Theses 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 microns region. We propose that the PAH molecules in our ETGs arise from fresh carbonaceous material which is continuously released by a population of carbon stars, formed in a rejuvenation episode which 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 ETG.
We describe radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set of a sample of 31 objects that have well-measured radio spectra up to at least 23 GHz. The radio spectra of these objec
ts below 1.4 GHz show a great variety of forms and are rarely a simple power-law extrapolation of the synchrotron spectra at higher frequencies. Most objects of this class have spectral turn-overs or bends in their radio spectra. We interpret these spectra in terms of free-free absorption in the starburst environment. Several objects show radio spectra with two components having free-free turn-overs at different frequencies (including Arp 220 and Arp 299), indicating that synchrotron emission originates from regions with very different emission measures. In these sources, using a simple model for the supernova rate, we estimate the time for which synchrotron emission is subject to strong free-free absorption by ionized gas, and compare this to expected HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII regions. We discuss the implications of this result and argue that those sources which have a significant radio component with strong free-free absorption are those in which the star formation rate is still increasing with time. We note that if ionization losses modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.
We fit the near-infrared to radio spectral energy distributions of a sample of 30 luminous and ultra-luminous infrared galaxies with models that include both starburst and AGN components. The aim of the work was to determine important physical parame
ters for this kind of objects such as the optical depth towards the luminosity source, the star formation rate, the star formation efficiency and the AGN fraction. We found that although about half of our sample have best-fit models that include an AGN component, only 30 % have an AGN which accounts for more than 10 % of the infrared luminosity whereas all have an energetically dominant starburst. Our models also determine the mass of dense molecular gas. Assuming that this mass is that traced by the HCN molecule, we reproduce the observed linear relation between HCN luminosity and infrared luminosity found by Gao and Solomon (2004). However, our derived conversion factor between HCN luminosity and the mass of dense molecular gas is a factor of 2 smaller than that assumed by these authors. Finally, we find that the star formation efficiency falls as the starburst ages.
We fit the near-infrared to radio spectral energy distributions of 30 luminous and ultra-luminous infrared galaxies with pure starburst models or models that include both starburst and AGN components to determine important physical parameters for thi
s population of objects. In particular we constrain the optical depth towards the luminosity source, the star formation rate, the star formation efficiency and the AGN fraction. We find that although about half of our sample have best-fit models that include an AGN component, only 30% have an AGN which accounts for more than 10% of the infrared luminosity, whereas all have an energetically dominant starburst. Our derived AGN fractions are generally in good agreement other measurements based in the mid-infrared line ratios measured by Spitzer IRS, but lower than those derived from PAH equivalent widths or the mid-infrared spectral slope. Our models determine the mass of dense molecular gas via the extinction required to reproduce the SED. Assuming that this mass is that traced by HCN, we reproduce the observed linear relation between HCN and infrared luminosities found by Gao & Solomon. We also find that the star formation efficiency, defined as the current star formation rate per unit of dense molecular gas mass, is enhanced in the ULIRGs phase. If the evolution of ULIRGs includes a phase in which an AGN contributes an important fraction to the infrared luminosity, this phase should last an order of magnitude less time than the starburst phase. Because the mass of dense molecular gas which we derive is consistent with observations of the HCN molecule,it should be possible to estimate the mass of dense, star-forming molecular gas in such objects when molecular line data are not available.
As a constraint for new starburst/AGN models of IRAS bright galaxies we determine the radio spectra of 31 luminous and ultraluminous IRAS galaxies (LIRGs/ULIRGs). We construct the radio spectra using both new and archival data. From our sample of rad
io spectra we find that very few have a straight power-law slope. Although some sources show a flattening of the radio spectral slope at high frequencies the average spectrum shows a steepening of the radio spectrum from 1.4 to 22.5 GHz. This is unexpected because in sources with high rates of star formation we expect flat spectrum, free-free emission to make a significant contribution to the radio flux at higher radio frequencies. Despite this trend the radio spectral indices between 8.4 and 22.5 GHz are flatter for sources with higher values of the FIR-radio flux density ratio q, when this is calculated at 8.4 GHz. Therefore, sources that are deficient in radio emission relative to FIR emission (presumably younger sources) have a larger thermal component to their radio emission. However, we find no correlation between the radio spectral index between 1.4 and 4.8 GHz and q at 8.4 GHz. Because the low frequency spectral index is affected by free-free absorption, and this is a function of source size for a given mass of ionized gas, this is evidence that the ionized gas in ULIRGs shows a range of densities. The youngest LIRGs and ULIRGs are characterized by a larger contribution to their high-frequency radio spectra from free-free emission. However, the youngest sources are not those that have the greatest free-free absorption at low radio frequencies. The sources in which the effects of free-free absorption are strongest are instead the most compact sources. Although these have the warmest FIR colours, they are not necessarily the youngest sources.
