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Register a new userSpitzer IRS Spectral Mapping of the Toomre Sequence: Spatial Variations of PAH, Gas, and Dust Properties in Nearby Major Mergers
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We have mapped the key mid-IR diagnostics in eight major merger systems of the Toomre Sequence (NGC4676, NGC7592, NGC6621, NGC2623, NGC6240, NGC520, NGC3921, and NGC7252) using the Spitzer Infrared Spectrograph (IRS). With these maps, we explore the variation of the ionized-gas, PAH, and warm-gas (H_2) properties across the sequence and within the galaxies. While the global PAH interband strength and ionized gas flux ratios ([Ne III]/[Ne II]) are similar to those of normal star forming galaxies, the distribution of the spatially resolved PAH and fine structure line flux ratios is significant different from one system to the other. Rather than a constant H_2/PAH flux ratio, we find that the relation between the H_2 and PAH fluxes is characterized by a power law with a roughly constant exponent (0.61+/-0.05) over all merger components and spatial scales. While following the same power law on local scales, three galaxies have a factor of ten larger integrated (i.e. global) H_2/PAH flux ratio than the rest of the sample, even larger than what it is in most nearby AGNs. These findings suggest a common dominant excitation mechanism for H_2 emission over a large range of global H_2/PAH flux ratios in major mergers. Early merger systems show a different distribution between the cold (CO J=1-0) and warm (H_2) molecular gas component, which is likely due to the merger interaction. Strong evidence for buried star formation in the overlap region of the merging galaxies is found in two merger systems (NGC6621 and NGC7592) as seen in the PAH, [Ne II], [Ne III], and warm gas line emission, but with no apparent corresponding CO (J=1-0) emission. Our findings also demonstrate that the variations of the physical conditions within a merger are much larger than any systematic trends along the Toomre Sequence.
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We present $Spitzer$/IRS mid-infrared spectral maps of the Galactic star-forming region M17 as well as IRSF/SIRIUS Br$gamma$ and Nobeyama 45-m/FOREST $^{13}$CO ($J$=1--0) maps. The spectra show prominent features due to polycyclic aromatic hydrocarbons (PAHs) at wavelengths of 6.2, 7.7, 8.6, 11.3, 12.0, 12.7, 13.5, and 14.2 $mu$m. We find that the PAH emission features are bright in the region between the HII region traced by Br$gamma$ and the molecular cloud traced by $^{13}$CO, supporting that the PAH emission originates mostly from photo-dissociation regions. Based on the spatially-resolved $Spitzer$/IRS maps, we examine spatial variations of the PAH properties in detail. As a result, we find that the interband ratio of PAH 7.7 $mu$m/PAH 11.3 $mu$m varies locally near M17SW, but rather independently of the distance from the OB stars in M17, suggesting that the degree of PAH ionization is mainly controlled by local conditions rather than the global UV environments determined by the OB stars in M17. We also find that the interband ratios of the PAH 12.0 $mu$m, 12.7 $mu$m, 13.5 $mu$m, and 14.2 $mu$m features to the PAH 11.3 $mu$m feature are high near the M17 center, which suggests structural changes of PAHs through processing due to intense UV radiation, producing abundant edgy irregular PAHs near the M17 center.
Spitzer and AKARI observations have found that polycyclic aromatic hydrocarbons (PAHs) are present in nearby elliptical galaxies, but their spatial distributions are still unknown. In order to investigate their distributions, we performed deep spectral mapping observations of the PAH-detected elliptical galaxy NGC4589, a merger remnant with a minor-axis optical dust lane. As a result, we obtain clear evidence that the PAH 11.3 um emission comes predominantly from the dust lane of the galaxy. We also detect molecular hydrogen line emissions from the dust lane. The PAH 17 um emission is distributed differently from the PAH 11.3 um emission, and more similarly to the dust continuum emission. From their distinctive distributions, we suggest that the PAHs responsible for the 11.3 um feature are secondary products through the evolution of the ISM brought in by the merger.
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 Spitzer/IRS spectral mapping observations of the luminous infrared galaxy (LIRG) Arp299 (IC694 + NGC3690) covering the central 45arcsec ~ 9kpc. The integrated mid-IR spectrum of Arp299 is similar to that of local starbursts despite its strongly interacting nature and high infrared luminosity, L_IR ~ 6x10^11 Lsun. This is explained because the star formation (probed by e.g. high [NeIII]15.56micron/[NeII]micron line ratios) is spread across at least 6-8kpc. Moreover, a large fraction of this star formation is taking place in young regions of moderate mid-IR optical depths such as the C+C complex in the overlap region between the two galaxies and in HII regions in the disks of the galaxies. It is only source A, the nuclear region of IC694, that shows the typical mid-IR characteristics of ultraluminous infrared galaxies (ULIRGs, L_IR > 10^12 Lsun), that is, very compact (less than 1kpc) and dust-enshrouded star formation resulting in a deep silicate feature and moderate equivalent widths of the PAHs. The nuclear region of NGC3690, known as source B1, hosts a low-luminosity AGN and is surrounded by regions of star formation. Although the high excitation [NeV]14.32micron line typical of AGN is not detected in B1, its upper limit is consistent with the value expected from the X-ray luminosity. The AGN emission is detected in the form of a strong hot dust component that accounts for 80-90% of the 6micron luminosity of B1. The similarity between the Arp299 integrated mid-IR spectrum and those of high-z ULIRGs suggests that Arp299 may represent a local example, albeit with lower IR luminosity and possibly higher metallicity, of the star-formation processes occurring at high-z.
We present a low-resolution (R = 90), 5.5-38 micron spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H_2O at 6.27 microns, CH_4 at 7.65 microns, and NH_3 at 10.5 microns and are relatively featureless at lambda > 15 microns. Three spectral indices that measure the strengths of these bands are presented; H_2O absorption features are present throughout the MLT sequence while the CH_4 and NH_3 bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 microns. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5-7.5 microns relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 micron spectra and find good agreement with the values of Golimowski et al. who use L- and M-band photometry and to account for the flux emitted at lambda > 2.5 microns.
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