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Register a new userThe Extreme Star Formation Activity of Arp299 Revealed by Spitzer IRS Spectral Mapping
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Almudena Alonso-Herrero
Publication date
2009
fields
Physics
and research's language is
English
Authors
Almudena Alonso-Herrero
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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.
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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.
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.
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.
Nuclear star clusters (NSCs) are the densest stellar systems in the Universe and are found in the centres of all types of galaxies. They are thought to form via mergers of star clusters such as ancient globular clusters (GCs) that spiral to the centre as a result of dynamical friction or through in-situ star formation directly at the galaxy centre. There is evidence that both paths occur, but the relative contribution of either channel and their correlation with galaxy properties are not yet constrained observationally. We aim to derive the dominant NSC formation channel for a sample of 25 nucleated galaxies, mostly in the Fornax galaxy cluster, with stellar masses between $M_rm{gal} sim 10^8$ and $10^{10.5} M_odot$ and NSC masses between $M_rm{NSC} sim 10^5$ and $10^{8.5} M_odot$. Using Multi-Unit Spectroscopic Explorer (MUSE) data from the Fornax 3D survey and the ESO archive, we derive star formation histories, mean ages and metallicities of NSCs, and compare them to the host galaxies. In many low-mass galaxies, the NSCs are significantly more metal-poor than the hosts with properties similar to GCs. In contrast, in the massive galaxies, we find diverse star formation histories and cases of ongoing or recent in-situ star formation. Massive NSCs ($> 10^7 M_odot$) occupy a different region in the mass-metallicity diagram than lower mass NSCs and GCs, indicating a different enrichment history. We find a clear transition of the dominant NSC formation channel with both galaxy and NSC mass. We hypothesise that while GC-accretion forms the NSCs of the dwarf galaxies, central star formation is responsible for the efficient mass build up in the most massive NSCs in our sample. At intermediate masses, both channels can contribute. The transition between these formation channels seems to occur at galaxy masses $M_rm{gal} sim 10^9 M_odot$ and NSC masses $M_rm{NSC} sim 10^7 M_odot$.
The optical spectra of Seyfert galaxies are often dominated by emission lines excited by both star formation and AGN activity. Standard calibrations (such as for the star formation rate) are not applicable to such composite (mixed) spectra. In this paper, we describe how integral field data can be used to spectrally and spatially separate emission associated with star formation from emission associated with accretion onto an active galactic nucleus (AGN). We demonstrate our method using integral field data for two AGN host galaxies (NGC 5728 and NGC 7679) from the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7). The spectra of NGC 5728 and NGC 7679 form clear sequences of AGN fraction on standard emission line ratio diagnostic diagrams. We show that the emission line luminosities of the majority (> 85 per cent) of spectra along each AGN fraction sequence can be reproduced by linear superpositions of the emission line luminosities of one AGN dominated spectrum and one star formation dominated spectrum. We separate the Halpha, Hbeta, [N II]$lambda$6583, [S II]$lambda lambda$6716, 6731, [O III]$lambda$5007 and [O II]$lambda lambda$3726, 3729 luminosities of every spaxel into contributions from star formation and AGN activity. The decomposed emission line images are used to derive the star formation rates and AGN bolometric luminosities for NGC 5728 and NGC 7679. Our calculated values are mostly consistent with independent estimates from data at other wavelengths. The recovered star forming and AGN components also have distinct spatial distributions which trace structures seen in high resolution imaging of the galaxies, providing independent confirmation that our decomposition has been successful.
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