The Mid‐Infrared Properties of Starburst Galaxies fromSpitzer‐IRS Spectroscopy

Abstract

We present 5 − 38µm mid-infrared spectra at a spectral resolution of R ≈ 65 − 130 of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph IRS on board the Spitzer Space Telescope. The spectra show a vast range in starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an optical depth of τ 9.8µm ∼ 5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15µm and 30µm enable a remarkably accurate estimate of the total infrared luminosity of the starburst. We find that the PAH equivalent width is independent of the total starburst luminosity L IR as both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2µm feature scales with L IR and can be used to approximate the total infrared luminosity of the starburst. Although our starburst sample covers about a factor of ten difference in the [Ne III] / [Ne II] ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7µm / 11.3µm PAH ratio. These results are based on spatially integrated diagnostics over an entire starburst region, and local variations may be "averaged out". It is presumably due to this effect that unresolved starburst nuclei with significantly different global properties appear spectrally as rather similar members of one class of objects.are expected to depend on numerous parameters such as the initial stellar mass function (IMF), the duration and epoch of the individual starburst(s), the metallicity of the ISM, the size and distribution of the dust grains, the strength of the magnetic fields, gas pressure and temperature of the ISM, galactic shear, total luminosity, and total mass. Furthermore, nearby starbursts, for which high resolution imaging is possible, have revealed complex substructures -in both stellar distributions and ISM -ranging from ultra-compact H ii regions (UCHIIR) to large complexes of super star clusters (SSC), suggesting small-scale variations of the observables across a starburst region.We use the low resolution mode of the Infrared Spectrograph 3 (IRS) ) on board the Spitzer Space Telescope (Werner et al. 2004) to observe the central regions of 22 starburst galaxies. Our objects represent a sample of "classical" starbursts for which a wealth of literature exists. The sample includes both purely starburst and starbursts with weak AGN activity (as determined from X-ray, optical, or radio observations). The summary in Table 1 lists the observed targets, their general properties, the classifications we adopt, and the references from which they are derived. The continuous 5 − 38µm IRS spectra include the silicate bands around 10µm and 18µm, a large number of PAH emission features, and information on the slope of the spectral con-

Mid-infrared spectroscopy ofSpitzer-selected ultra-luminous starbursts atz ~ 2

“…We also note that the best fit of all these samples is consistent with the relations found by Pope et al (2008) and Menéndez-Delmestre et al (2009) for SMGs and local starbursts (Brandl et al 2006). The AGNdominated sample of Coppin et al (2010) shows L IR and L 7.7 μm greater than our sources, but their ratio is not very different from our sample.…”

“…This is likely due to a stronger AGN contribution in the sample of Sajina et al (2007). Note also that the ratio L 7.7 μm /ν L ν (5.8 μm) is very consistent with that found for the local starbursts of Brandl et al (2006).…”

“…We have computed ν L ν (1.4 GHz) for our sample and those of Sajina et al (2007), Pope et al (2008), Shi et al (2009), andBrandl et al (2006). For our sample, we have used α as computed in F09 and reported in Table 1.…”

“…This is justified by the good correlation that has been found between the luminosity of PAH features (mostly at 6.2 and 7.7 μm) and the total IR or far-IR luminosity, which in the absence of an AGN is taken to be a good tracer of starforming activity (Kennicutt 1998). Such a correlation has been checked for starburst galaxies at both low and high redshift (e.g., Brandl et al 2006;Pope et al 2008;Menéndez-Delmestre et al 2009). It is the origin of the correlation found between the ISO 7.0 μm-or Spitzer 8.0 μm-band flux and the star formation rate (e.g., Elbaz et al 2002;Wu et al 2006;Alonso-Herrero et al 2006;Farrah et al 2007) in low-z luminous infrared galaxies.…”

The Circumnuclear Environment of IRAS 20551-4250: A Case Study of AGN/Starburst Connection forJWST

“…The simultaneous presence of star formation and AGN signatures in the mid-IR makes this a really favourable band to disentangle the AGN and SB components and explore their environment. In particular, (i) the available spectra of bona fide starburst-dominated and, respectively, unobscured AGN-dominated sources are widely different, and show little 2 Advances in Astronomy dispersion within the separate classes [7][8][9][10]. This allowed us to reproduce the AGN/SB contributions with fixed templates, especially over the the 3-8 μm spectral interval.…”