(From Menedez-Delmestre etal. 2009, ApJ, 699, 667
It is over a decade since the discovery of a population of high-redshift galaxies identiﬁed through their submillimeter (submm) emission (Smail et al. 1997; Barger et al. 1999; Eales et al. 1999; Bertoldi et al. 2000; Cowie et al. 2002; Scott et al. 2002; Borys
et al. 2003; Webb et al. 2003; Coppin et al. 2005; Younger et al. 2007). Their submm selection suggests that these are strongly
star-bursting systems. With implied star formation rates (SFRs) of ∼1001000 M⊙ yr−1, these dust-enshrouded submm galaxies
(hereafter, SMGs) may make a signiﬁcant contribution to the global SFR density at z ~ 23 (Chapman et al. 2005, hereafter C05). The absorbing dust that makes them such prodigious submm emitters also makes them quite optically faint and renders follow-up studies at shorter wavelengths challenging. The study of SMGs has been facilitated by the detection of a large fraction of them as μJy radio sources (Ivison et al. 2002) and more recently through millimeter wave interferometry. This has allowed the subsequent measurement of their spectroscopic redshifts for fairly large samples (C05).
With a mean redshift of ⟨z⟩ ∼ 2.2, the redshift distribution of the radio-identiﬁed SMGs in C05 coincides with the globalnear-IR and X-ray studies. Near-IR spectroscopy by Swinbank et al. (2004) shows that broad Hα lines (FWHM >~ 1000 km s−1 ) are often present in these galaxies, while deep X-ray studies using the sensitive Chandra Deep Field-North (CDF-N) Survey suggest that at least ∼28%50% of SMGs host an obscured AGN (Alexander et al. 2005). A number of SMGs that display no AGN signatures in the rest-frame optical (Swinbank et al. 2004) were classiﬁed as AGNs based on X-ray observations (Alexander et al. 2005). This is likely due to geometrical effects in which the broad-line region of the AGN remains hidden in the optical by intervening obscuring material. At high X-ray energies, direct X-ray emission is detectable through even very high column densities, allowing the direct detection of an AGN at very high obscurations. However, Alexander et al. (2005) ﬁnd that the majority of AGNs in SMGs are more modestly
obscured, with column densities of NH >~ 10^(23) cm−2 . A similar mix of AGNs and starburst activity is seen in local ultraluminous
infrared galaxies (ULIRGs, with total IR luminosities LIR >~ 10^(12) L⊙; Soifer et al. 1987; Sanders et al. 1988), most of which have been shown to be composite AGNstarburst systems (e.g., Armus et al. 2007). The SMGs have IR luminosities which are comparable to ULIRGs at low redshift, prompting the question as to whether SMGs are high-redshift analogs of ULIRGs and hence whether we can learn about the physical processes within SMGs from studies of local ULIRGs (Tacconi et al. 2008).
The potential presence of luminous Compton-thick AGNs (NH >~ 10^(24) cm−2 ) in those SMGs with no X-ray AGN signature remains a signiﬁcant caveat to these results. Furthermore, the samples of SMGs with the necessary ultradeep X-ray observations to reveal the presence of highly obscured AGNs are still small. Hence it is possible that a small fraction of luminous, but Compton thick, AGNs lurk within the SMG population (see Coppin et al. 2008). Rest-frame optical emission provides direct insight into the stellar emission and ionized gas of a galaxy, but suffers from obscuration due to intervening dust. At longer wavelengths, the mid-IR emission arising from the dust itself provides an indirect insight into the dust-enshrouded nature of SMGs and suffers from much less obscuration than the shorter, optical wavelengths. We therefore designed a program to follow up a large sample of 24 SMGs in the mid-IR using Spitzers Infrared Spectrograph (IRS; Houck et al. 2004) in an effort to answer the following questions for a large and representative sample of the SMG population: Are SMGs composite AGNstarburst systems? To what extent does AGN activity contribute to the total infrared output of these galaxies? Is there a spectrum of varying levels of AGN activity across the population? Are local ULIRGs good analogs for the mid-IR emission of SMGs?
The main components contributing to the mid-IR spectrum of a galaxy are: thermal dust continuum, emission from vibrational modes in polycyclic aromatic hydrocarbons (PAHs) and other atomic and molecular lines. The continuum emission at longer mid-IR wavelengths, λ >~ 12 μm, arises from emission by very small grains of dust (VSG; <~ 10 nm) found around obscured AGN or star-forming regions. This is often referred to as the VSG continuum, or the warm dust continuum (TDust >~ 250 K). At shorter wavelengths, λ <~ 6 μm, the continuum traces emission from dust heated to signiﬁcantly hotter temperatures (TDust >~ 500 K) likely due to its close location near an AGN or possibly a hot, nuclear starburst region. This is what we refer to as the hot dust continuum. PAH molecules (<~ few nm) consist of chained benzene rings, associated hydrogen, and other trace elements, such as Si and Mg. The line emission in the mid-IR waveband is dominated by PAH molecules which are excited by the UV photons that are copiously available in star-forming regions. The main PAH emission features arise from the bending and stretching of skeletal CC or peripheral
CH bonds and are observed at rest frame 6.2, 7.7, 8.6, 11.3, 12.7, and 17 μm (e.g., Draine & Li 2007). It has been shown locally that stronger PAH features are associated with regions of intense star formation (Helou 1999).
With the unprecedented sensitivity of Spitzer IRS, it has been possible to explore the mid-IR region of galaxies at high redshift down to continuum levels of S(24) μm >~ 0.1 mJy (e.g., Yan et al. 2005, 2007; Lutz et al. 2005, 2007; Desai et al. 2006; Rigopoulou et al. 2006; Teplitz et al. 2007; Siana et al. 2008). At lower redshifts, investigation of the mid-IR properties of local galaxies with IRS provides for a sample of detailed templates against which high-redshift sources can be compared (e.g., Spoon et al. 2004; Armus et al. 2004, 2006, 2007; Weedman et al. 2005; Brandl et al. 2006; Sturm et al. 2006; Desai et al. 2007). A wide range of mid-IR spectra has been uncovered for high-redshift sources, ranging from continuum-dominated spectra with no PAH features to PAH-dominated spectra (e.g., Weedman & Houck 2008). Between these two extremes, there is a myriad of composite spectra displaying a signiﬁcant continuum with superposed PAH features (e.g., Yan et al. 2007).
We presented the Spitzer IRS results for the ﬁrst ﬁve SMGs observed in the program in Menendez-Delmestre et al. (2007), which complemented earlier IRS results for two SMGs at z ∼ 2.8 presented by Lutz et al. (2005). Since then Valiante et al. (2007) and Pope et al. (2008) obtained IRS spectra of nine SMGs in blank ﬁeld and cluster lens surveys and of 13 SMGs from the GOODS-North Field, respectively.