A Large Mid-Infrared Spectroscopic and Near-Infrared Imaging Survey of Ultraluminous Infrared Galaxies: Their Nature and Evolution

Abstract

We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) z ¹ 0.3) using ISOPHOT-S on board the Infrared (L IR [ 1012L _ , Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 km polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We Ðnd that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, for starburst-dominated ULIRGs is very L PAH /L IR , similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable e †ect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we Ðnd that systems in the early stages of merging may well put out maximum luminosity. We also Ðnd that the total mass of interstellar gas, as estimated from the CO (1 ] 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we Ðnd that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

Ice features in the mid-IR spectra of galactic nuclei

“…3; Fig. 6 of Rigopoulou et al 1999) and its ascent steep. In contrast, absorption dominated spectra tend to show a wider, less well defined peak around 8 µm (e.g.…”

“…PAH dominated spectra exhibit relatively narrow 7.7 and 8.6 µm emissions (e.g. Helou et al 2000;Rigopoulou et al 1999). Also, they show a very steep rise between 7.2 and 7.7 µm.…”

N‐Band Imaging of Seyfert Nuclei and the Mid‐Infrared–X‐Ray Correlation

“…Midinfrared (MIR) spectroscopy with the Infrared Space Observatory (ISO) has shown that some galaxies, which were optically classiÐed as AGNs, are, in fact, dominated by starbursts. Conversely, the presence of a heavily obscured AGN was revealed in some systems previously classiÐed as starburst galaxies Lutz, Veilleux, & Genzel 1999b ;Rigopoulou et al 1999).…”

HERUS: A CO ATLAS FROM SPIRE SPECTROSCOPY OF LOCAL ULIRGs

“…In the local universe, ULIRGs are invariably mergers hosting compact (<kpc) star-forming regions and active galactic nuclei (AGNs; e.g., Melnick & Mirabel 1990;Clements et al 1996; Rigopoulou et al 1999;Soifer et al 2000;Farrah et al 2001), but at higher redshifts ULIRGs may host more extended star-forming regions, and may be associated both with interacting and isolated galaxies (e.g., Pope et al 2006;Farrah et al 2008;Magdis et al 2011;Symeonidis et al 2013;Béthermin et al 2014). The highredshift ULIRGs thus lie on the galaxy mass-specific star formation rate density relation (the so-called "main sequence"), making them typical rather than extreme at these redshifts (e.g., Elbaz et al 2011).…”