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Register a new userSpatial variations of PAH properties in M17SW revealed by Spitzer/IRS spectral mapping
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Mitsuyoshi Yamagishi
Publication date
2016
fields
Physics
and research's language is
English
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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.
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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.
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 have mapped the superwind/halo region of the nearby starburst galaxy M82 in the mid-infrared with $Spitzer-IRS$. The spectral regions covered include the H$_2 S(1)-S(3)$, [NeII], [NeIII] emission lines and PAH features. We estimate the total warm H$_2$ mass and the kinetic energy of the outflowing warm molecular gas to be between $M_{warm}sim5-17times10^6$ M$_{odot}$ and $E_{K}sim6-20times10^{53}$ erg. Using the ratios of the 6.2, 7.7 and 11.3 micron PAH features in the IRS spectra, we are able to estimate the average size and ionization state of the small grains in the superwind. There are large variations in the PAH flux ratios throughout the outflow. The 11.3/7.7 and the 6.2/7.7 PAH ratios both vary by more than a factor of five across the wind region. The Northern part of the wind has a significant population of PAHs with smaller 6.2/7.7 ratios than either the starburst disk or the Southern wind, indicating that on average, PAH emitters are larger and more ionized. The warm molecular gas to PAH flux ratios (H$_2/PAH$) are enhanced in the outflow by factors of 10-100 as compared to the starburst disk. This enhancement in the H$_2/PAH$ ratio does not seem to follow the ionization of the atomic gas (as measured with the [NeIII]/[NeII] line flux ratio) in the outflow. This suggests that much of the warm H$_2$ in the outflow is excited by shocks. The observed H$_2$ line intensities can be reproduced with low velocity shocks ($v < 40$ km s$^{-1}$) driven into moderately dense molecular gas ($10^2 <n_H < 10^4$ cm$^{-3}$) entrained in the outflow.
Aims: We employ archival Spitzer slit-scan observations of the HH211 outflow in order to investigate its warm gas content, assess the jet mass flux in the form of H2 and probe for the existence of an embedded atomic jet. Methods: Detected molecular and atomic lines are interpreted by means of emission line diagnostics and an existing grid of molecular shock models. The physical properties of the warm gas are compared against other molecular jet tracers and to the results of a similar study towards the L1448-C outflow. Results: We have detected and mapped the v=0-0 S(0) - S(7) H2 lines and fine-structure lines of S, Fe+, and Si+. H2 is detected down to 5 from the source and is characterized by a cool T~300K and a warm T~1000 K component, with an extinction Av ~ 8 mag. The amount of cool H2 towards the jet agrees with that estimated from CO assuming fully molecular gas. The warm component is well fitted by C-type shocks with a low beam filling factor ~ 0.01-0.04 and a mass-flux similar to the cool H2. The fine-structure line emission arises from dense gas with ionization fraction ~0.5 - 5 x 10e-3, suggestive of dissociative shocks. Line ratios to sulfur indicate that iron and silicon are depleted compared to solar abundances by a factor ~10-50. Conclusions: Spitzer spectral mapping observations reveal for the first time a cool H$_2$ component towards the CO jet of HH211 consistent with the CO material being fully molecular and warm at ~ 300 K. The maps also reveal for the first time the existence of an embedded atomic jet in the HH211 outflow that can be traced down to the central source position. Its significant iron and silicon depletion excludes an origin from within the dust sublimation zone around the protostar. The momentum-flux seems insufficient to entrain the CO jet, although current uncertainties on jet speed and shock conditions are too large for a definite conclusion.
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.
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