MID-INFRARED PROPERTIES OF NEARBY LUMINOUS INFRARED GALAXIES. I. SPITZER INFRARED SPECTROGRAPH SPECTRA FOR THE GOALS SAMPLE

Abstract

The Great Observatories All-Sky LIRG Survey (GOALS) is a comprehensive, multiwavelength study of luminous infrared galaxies (LIRGs) in the local universe. Here we present low resolution Spitzer Infrared Spectrograph spectra covering 5-38 μm and provide a basic analysis of the mid-IR spectral properties observed for nearby LIRGs. In a companion paper, we discuss detailed fits to the spectra and compare the LIRGs to other classes of galaxies. The GOALS sample of 244 nuclei in 180 luminous (10 11 L IR /L < 10 12 ) and 22 ultraluminous (L IR /L 10 12 ) IR galaxies represents a complete subset of the IRAS Revised Bright Galaxy Sample and covers a range of merger stages, morphologies, and spectral types. The majority (>60%) of the GOALS LIRGs have high 6.2 μm polycyclic aromatic hydrocarbon (PAH) equivalent widths (EQW 6.2 μm > 0.4 μm) and low levels of silicate absorption (s 9.7 μm > −1.0). There is a general trend among the U/LIRGs for both silicate depth and mid-infrared (MIR) slope to increase with increasing L IR . U/LIRGs in the late to final stages of a merger also have, on average, steeper MIR slopes and higher levels of dust obscuration. Together, these trends suggest that as gas and dust is funneled toward the center of a coalescing merger, the nuclei become more compact and more obscured. As a result, the dust temperature increases also leading to a steeper MIR slope. The sources that depart from these correlations have very low PAH equivalent width (EQW 6.2 μm < 0.1 μm) consistent with their emission being dominated by an active galactic nucleus (AGN) in the MIR. These extremely low PAH EQW sources separate into two distinct types: relatively unobscured sources with a very hot dust component (and thus very shallow MIR slopes) and heavily dust obscured nuclei with a steep temperature gradient. The most heavily dust obscured sources are also the most compact in their MIR emission, suggesting that the obscuring (cool) dust is associated with the outer regions of the starburst and not simply a measure of the dust along the line of sight through a large, dusty disk. A marked decline is seen for the fraction of high EQW (star formation dominated) sources as the merger progresses. The decline is accompanied by an increase in the fraction of composite sources while the fraction of sources where an AGN dominates the MIR emission remains low. When compared to the MIR spectra of submillimeter galaxies (SMGs) at z ∼ 2, both the average GOALS LIRG and ULIRG spectra are more absorbed at 9.7 μm and the average GOALS LIRG has more PAH emission. However, when the AGN contributions to both the local GOALS LIRGs and the high-z SMGs are removed, the average local starbursting LIRG closely resembles the starburst-dominated SMGs.

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…This has been done by exploiting 5-8 μm spectroscopy (e.g., Nardini et al 2010) and the characteristics of several features in the L (3-4 μm) and M (4-5 μm) bands (Imanishi & Dudley 2000;Risaliti et al 2006;Sani et al 2008;Risaliti et al 2010): the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature, the bare carbonaceous 3.4 μm absorption feature, and the slope of the continuum. The 6.2 μm (e.g., Stierwalt et al 2013Stierwalt et al , 2014 and 7.7 μm PAH features (e.g., Veilleux et al 2009), the presence of highexcitation MIR lines (e.g., [Ne V] 14.32 μm, Veilleux et al 2009), or the radio properties (e.g., Parra et al 2010;Romero-Cañizales et al 2012a;Vardoulaki et al 2015) have also been used to infer the presence of a buried AGN.…”

“…The source has a star formation rate (SFR) of 41.3 M e yr −1 (Howell et al 2010), is in an early merging stage (i.e., stage B or 2, following the classification of Stierwalt et al 2013), and is interacting with the galaxy NGC 6285 (NGC 6286N) located at a distance of ∼1.5 arcmin (∼33 kpc, projected distance; see Figure 1 and panel four of Figure 2). …”

Shocked gas in IRAS F17207-0014: ISM collisions and outflows

“…Although these systems are relatively rare in the local universe, Le Floc'h et al (2005) have shown that high infrared luminosities may have been the norm at redshifts beyond 0.7. To understand these cosmically important objects and the physical processes that occur in them, the Great Observatory All-Sky LIRG Survey (GOALS; Armus et al 2009) team has compiled data across a number of different wavelength regimes, including Hubble Space Telescope imaging in the optical (Kim et al 2013) and near-infrared (Haan et al 2011), Chandra X-ray observations, (Iwasawa et al 2011), Spitzer spectra in the mid-infrared (Stierwalt et al 2013), GALEX observations in the near and far ultraviolet (Howell et al 2010), and Herschel Space Telescope spectra in the far-infrared (Díaz-Santos et al 2013).…”

AGN and Starbursts in Dusty Galaxy Mergers: Insights from the Great Observatories All-sky LIRG Survey

“…In contrast, the mid-IR spectra of the central components are very diverse, implying that observed variations of the integrated properties of local (U)LIRGs reflect processes that are occuring predominantly in their nuclei. The low-resolution IRS spectra spanning 5-58 μm for 244 nuclei in 180 (U)LIRGs in the GOALS sample are presented by Stierwalt et al (2013), along with the following results: Both silicate depth and mid-IR continuum slope rise with increasing L ir ; on average late-stage mergers have steeper mid-IR slopes and higher dust obscuration than early and intermediate stage mergers; when AGN contributions to local LIRGs and high-z sub-mm galaxies (SMGs) are removed, there is a strong resemblance between the average local LIRG and SMGs dominated by star formation.…”