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 te
mplate, 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 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 present the observations of the starburst galaxy M82 taken with the Herschel SPIRE Fourier Transform Spectrometer. The spectrum (194-671 {mu}m) shows a prominent CO rotational ladder from J = 4-3 to 13-12 emitted by the central region of M82. The
fundamental properties of the gas are well constrained by the high J lines observed for the first time. Radiative transfer modeling of these high-S/N 12CO and 13CO lines strongly indicates a very warm molecular gas component at ~500 K and pressure of ~3x10^6 K cm^-3, in good agreement with the H_2 rotational lines measurements from Spitzer and ISO. We suggest that this warm gas is heated by dissipation of turbulence in the interstellar medium (ISM) rather than X-rays or UV flux from the straburst. This paper illustrates the promise of the SPIRE FTS for the study of the ISM of nearby galaxies.
We use 16 micron, 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 f
rom 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 mid-infrared 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.
