SpitzerMid‐Infrared Spectroscopy of Infrared Luminous Galaxies atz∼2. II. Diagnostics

Abstract

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z ∼ 1 -3 and likely L IR ∼ 10 12 -10 13 L ⊙ . Our study aims at quantifying the star-formation and AGN processes in these sources which recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum-(i.e. AGN) dominated sources, but ∼ 60% of these show PAH emission, suggesting the presence of star-formation activity. These sources have redder mid-IR colors than typical optically-selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5 µm hot dust continua. However, their steep 30 µm -to -14 µm slopes suggest that star-formation might dominate the total infrared luminosity. Six of our z ∼ 2 sources have EW 6.2 > ∼ 0.3 µm and L 14µm > ∼ 10 12 L ⊙ (implying L IR > ∼ 10 13 L ⊙ ). At these luminosities, such high EW 6.2 ULIRGs do not exist in the local Universe. We find a median optical depth at 9.7 µm of τ 9.7µm = 1.4. This is consistent with local IRAS-selected ULIRGs, but differs from early results on SCUBAselected z ∼ 2 ULIRGs. Similar to local ULIRGs about 25% of our sample show extreme obscuration (τ 9.7µm > ∼ 3) suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs, but are an order of magnitude more luminous. It is not clear whether our z ∼ 2 ULIRGs are simply scaled-up versions of local ULIRGs, or subject to fundamentally different physical processes.

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

“…9, we plot L 7.7 μm as a function of L IR for different samples. We can see that our sample has L IR comparable to those of the other SMG samples (Pope et al 2008;Menéndez-Delmestre et al 2009), and the PAH-rich sources from Sajina et al (2007). The star-forming galaxies of Shi et al (2009) …”

“…All our sources show L 7.7 μm > ∼ 5×10 10 L and EW 7.7 μm > 2 μm, i.e., well above the EW 7.7 μm > 0.8 μm limit for "strong" PAH sources defined by Sajina et al (2007). All our sources except L10 also show EW 6.2 μm 0.2 μm and L 6.2 μm 2 × 10 10 L .…”

“…Our sample has PAH luminosities on average comparable to those of the PAH-rich sources of Sajina et al (2007) and slightly larger than those of the SMGs observed by Pope et al (2008) and Menéndez-Delmestre et al (2009), and of the "4.5 μm-peakers" of Farrah et al (2008). Such a behavior might be naturally explained by stronger S 24 μm than SMG samples samples and higher redshift than Farrah et al (2008)'s sample.…”

“…It is based on interstellar extinction similar to that of the Milky Way (Smith et al 2007). We note that the silicate absorption is weak on average, τ 9.7 = 0.28 (0.50 if we consider only the sources with τ 9.7 0) vs. 1.09 for the PAH-rich sample of Sajina et al (2007). Indeed, for 44% (7/16) of our sources, PAHFIT provides τ 9.7 = 0.…”

“…(1)). We have also plotted the sample of Sajina et al (2007). To make a true comparison, we have applied a similar reduction and spectral decomposition to the PAH-rich sources from Sajina et al (2007).…”