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

Abstract

Context. Spitzer's wide-field surveys and followup capabilities have allowed a new breakthrough in mid-IR spectroscopy up to redshifts ≥2, especially for 24 μm detected sources. Aims. We want to study the mid-infrared properties and the starburst and AGN contributions, of 24 μm sources at z ∼ 2, through analysis of mid-infrared spectra combined with millimeter, radio, and infrared photometry. Mid-infrared spectroscopy allows us to recover accurate redshifts. Methods. A complete sample of 16 Spitzer-selected sources (ULIRGs) believed to be starbursts at z ∼ 2 ("5.8 μm-peakers") was selected in the (0.5 deg 2 ) J1064+56 SWIRE Lockman Hole field ("Lockman-North"). These sources have S 24 μm > 0.5 mJy, a stellar emission peak redshifted to 5.8 μm, and r Vega > 23. The entire sample was observed with the low resolution units of the Spitzer/IRS infrared spectrograph. These sources have 1.2 mm observations with IRAM 30 m/MAMBO and very deep 20 cm observations from the VLA. Nine of our sources also benefit from 350 μm observation and detection from CSO/SHARC-II. All these data were jointly analyzed.Results. The entire sample shows good quality IRS spectra dominated by strong PAH features. The main PAH features at 6.2, 7.7, 8.6, and 11.3 μm have high S /N average luminosities of 2.90 ± 0.31, 10.38 ± 1.09, 3.62 ± 0.27, and 2.29 ± 0.26 × 10 10 L , respectively. Thanks to their PAH spectra, we derived accurate redshifts spanning from 1.750 to 2.284. The average of these redshifts is 2.017 ± 0.038. This result confirms that the selection criteria of "5.8 μm-peakers" associated with a strong detection at 24 μm are reliable to select sources at z ∼ 2. We have analyzed the different correlations between PAH emission and infrared, millimeter, and radio emissions. Practically all our sources are strongly dominated by starburst emission, with only one source showing an important AGN contribution. We have also defined two subsamples based on the equivalent width at 7.7 μm to investigate AGN contributions. Conclusions. Our sample contains strong starbursts and represents a particularly 24 μm-bright class of SMGs. The very good correlation between PAH and far-IR luminosities is now confirmed in high-z starburst ULIRGs. These sources show a small AGN contribution to the mid-IR, around ∼20% or less in most cases.

A New Star Formation Rate Calibration From Polycyclic Aromatic Hydrocarbon Emission Features and Application to High-Redshift Galaxies

“…It also indicates that the low-redshift SFR calibration based on PAH luminosities is appropriate to use for high-redshift ULIRGs, even if it is not appropriate for low-redshift ULIRGs, consistent with other recent studies that have demonstrated this result (i.e., Rujopakarn et al 2013). However, at even higher total IR luminosities (L L 3 10 IR 12  ), there is evidence that the L PAH /L IR ratio declines for the high-redshift samples (e.g., Fiolet et al 2010). We fit this trend seen in Figure 12 as a broken powerlaw and derive a linear fit above this luminosity using the high-redshift samples of Pope et al (2013) …”

Strong Molecular Hydrogen Emission and Kinematics of the Multiphase Gas in Radio Galaxies With Fast Jet-Driven Outflows

“…We note that the predominately radio-quiet Higdon et al (2006) sample of ULIRGs (0.02 < z < 0.93) and the Schweitzer et al (2006) sample of QSOs (z < 0.3) have H 2 -to-PAH luminosity ratios significantly higher than the SINGS star-forming galaxies. Although their H 2 -to-PAH luminosity ratios are compatible with the range of ratios predicted by PDR models, other studies (based on extinction or H 2 -to-CO line ratios) suggest that additional sources of H 2 excitation (other than UV photon heating) are required to explain the H 2 emission (e.g., shocks; see Zakamska 2010;Fiolet et al 2010).…”

Dusty star-forming galaxies at high redshift

“…Desai et al (2008) and Fiolet et al (2010) addresses the redshift distribution of 24µm-selected sources by confirming a set of >400 sources spectroscopically out of ∼600 targeted. They find that the redshift distribution of 24µm galaxies brighter than 300 µJy peaks at z ∼ 0.3 with a possible second peak at z ∼ 0.9 and a tail of galaxies detected out to z ∼ 4.5.…”