STRONG POLYCYCLIC AROMATIC HYDROCARBON ... - Core

The Astrophysical Journal, 700:1190–1204, 2009 August 1  C 2009.

doi:10.1088/0004-637X/700/2/1190

The American Astronomical Society. All rights reserved. Printed in the U.S.A.

STRONG POLYCYCLIC AROMATIC HYDROCARBON EMISSION FROM z ≈ 2 ULIRGs∗ Vandana Desai1,2 , B. T. Soifer1,2 , Arjun Dey3 , Emeric Le Floc’h4,10 , Lee Armus2 , Kate Brand3,5 , Michael J. I. Brown6 , Mark Brodwin3,7,11 , Buell T. Jannuzi3 , James R. Houck8 , Daniel W. Weedman8 , Matthew L. N. Ashby7 , Anthony Gonzalez9 , Jiasheng Huang7 , Howard A. Smith7 , Harry Teplitz2 , Steve P. Willner7 , and Jason Melbourne1 1

Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA 2 Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA 3 National Optical Astronomy Observatory, Tucson, AZ 85726-6732, USA 4 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA 5 Space Telescope Science Institute, Baltimore, MD 21218, USA 6 School of Physics, Monash University, Clayton, Victoria 3800, Australia 7 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 8 Astronomy Department, Cornell University, Ithaca, NY 14853, USA 9 Department of Astronomy, University of Florida, Gainesville, FL 32611-2055, USA Received 2009 January 9; accepted 2009 May 27; published 2009 July 10

ABSTRACT Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 < λ/δλ < 124), mid-infrared (20–38 μm) spectra of 23 high-redshift ULIRGs detected in the Bo¨otes field of the NOAO Deep Wide-Field Survey. All of the sources were selected to have (1) fν (24 μm) > 0.5mJy; (2) R − [24] > 14 Vega mag; and (3) a prominent rest frame 1.6 μm stellar photospheric feature redshifted into Spitzer’s 3–8 μm IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 < z < 3.0, with a mean of z = 1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 μm stellar bump can be efficiently used to select highly obscured star-forming galaxies at z ≈ 2, and that the fraction of starburstdominated ULIRGs increases to faint 24 μm flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 μm detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously use observed spectral energy distributions to explain the broader redshift distribution of Spitzerdetected ULIRGs that are dominated by an active galactic nucleus (AGN). Finally, we conclude that z ≈ 2 sources with a detectable 1.6 μm stellar opacity feature lack sufficient AGN emission to veil the 7.7 μm PAH band. Key words: galaxies: active – galaxies: evolution – galaxies: formation – galaxies: starburst – infrared: galaxies

et al. 2008) have led to the suggestion that DOGs may be evolutionarily related to both coeval SMGs and the most massive local galaxies. However, the properties of DOGs must be studied more thoroughly before any possible relationships to other populations can be firmly established. One of the most basic unresolved issues is whether the enormous luminosities of DOGs are powered predominantly by star formation or an active galactic nucleus (AGN). The optical through mid-infrared spectral energy distributions (SEDs; Dey et al. 2008) of the DOGs that are brightest at 24 μm tend to resemble power laws, as expected for AGNdominated sources. In contrast, the fainter DOGs include a larger fraction of sources featuring a bump at rest frame 1.6 μm. This bump is characteristic of old stellar populations. Its detectability indicates limited AGN activity because an AGN would result in extra flux at rest frame ≈2.5 μm, thereby masking the bump. Follow-up data for the bright (fν (24 μm) > 0.75 mJy) power-law DOGs support the interpretation that they are AGNdominated. The IRS spectroscopy reveals absorbed power laws in the mid-infrared, as expected for AGN (Houck et al. 2005; Weedman et al. 2006). Near-infrared spectroscopy of a small sample of bright power-law DOGs indicates that they host powerful AGN (Brand et al. 2006). Similarly, the far-infrared SEDs of bright DOGs are similar to the AGN-dominated local ULIRG Mrk 231 (Tyler et al. 2009). The morphologies of bright power-law DOGs have also been examined (Melbourne et al. 2009, 2008; Bussmann et al. 2009; Dasyra et al. 2008), and

1. INTRODUCTION The launch of the Spitzer Space Telescope (Werner et al. 2004a) has allowed the identification and study of significant populations of distant, infrared-bright galaxies. The most extreme of these are exceptionally faint in the optical but readily detected in 24 μm surveys carried out with Spitzer/MIPS (Rieke et al. 2004). For example, Dey et al. (2008) selected a population of Dust-Obscured Galaxies (DOGs) from the Bo¨otes field of the NOAO Deep Wide-Field Survey (NDWFS; Jannuzi & Dey 1999) via the criteria R − [24] > 14 Vega mag and fν (24 μm) > 0.3 mJy (see also Houck et al. 2005; Fiore et al. 2008). Drawing on Spitzer/IRS (Houck et al. 2004) redshifts (this work and Houck et al. 2005), near-infrared ground-based spectra (Brand et al. 2007a), and optical spectra (Desai et al. 2008), Dey et al. (2008) find that DOGs have a broad redshift distribution centered at z = 1.99. These redshifts imply enormous luminosities, similar to and even exceeding those of local ULIRGs and distant submillimeter galaxies. While DOGs are rare (≈2600 are found over the ≈9 deg2 of the Bo¨otes field), Dey et al. (2008) estimate that they contribute a quarter of the infrared luminosity density at z = 2. In addition, their space densities (Dey et al. 2008) and clustering properties (Brodwin ∗ Based on observations made with the Spitzer Space Telescope, operated by the Jet Propulsion Laboratory under NASA contract 1407. 10 Spitzer Fellow. 11 W. M. Keck Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics.

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STRONG PAH EMISSION z ≈ 2 ULIRGs

they tend to be more compact than their fainter counterparts, as expected for AGN versus star-forming regions. Because they are relatively more difficult to observe, there has been less follow-up of the faint (fν (24 μm) < 0.75 mJy) bump DOG population. A stacking analysis shows that their far-infrared properties are consistent with star formation, and similar to SMGs (Pope et al. 2008a). However, X-ray stacking analyses have provided mixed results (Fiore et al. 2008; Pope et al. 2008a). Not many IRS spectra of faint bump DOGs exist. Pope et al. (2008a) found that 12 out of 70 DOGs in the GOODs field have serendipitous IRS spectra. The sources lie in the range 0.2 < fν (24 μm)/mJy < 1.5. Of these, half show polycyclic aromatic hydrocarbon (PAH) features with equivalent widths suggesting that they are dominated by star formation in the mid-infrared. The PAH-rich sources tend to be the fainter ones (fν (24 μm) < 0.7 mJy) and have Spitzer IRAC (Fazio et al. 2004) SEDs that deviate from a power law. In addition, IRS spectra exist for infrared-bright galaxies with a range of 24 μm flux densities that display the 1.6 μm stellar bump, but do not necessarily meet the DOG criterion because they are too bright in the optical (Huang et al. 2009; Farrah et al. 2008; Yan et al. 2007). These spectra indicate that the presence of the bump is correlated to the presence of PAH features in the IRS spectrum, suggesting that bump sources have mid-infrared emission dominated by star formation. Given the potential importance of the DOG population in general, and the fact that the DOG population grows with decreasing 24 μm flux density, detailed study of the faint bump DOGs is necessary. The first step in this process is building a statistically significant sample of bump DOGs with redshifts for follow-up study (for example, with Herschel). Here we present IRS spectra for an additional 23 faint bump DOGs. Twenty of these for which we were able to determine redshifts make up ≈23% of the 86 sources used in the redshift distribution of Dey et al. (2008). This paper is organized as follows. In Section 2, we describe our observational data and selection criteria. In Section 3, we present our results, namely the redshifts of our targets, the composite IRS spectrum, and their bolometric luminosities. We discuss these results in Section 4 and summarize in Section 5. In the following, we use H0 = 70 km s−1 Mpc−1 , Ωm = 0.3, and ΩΛ = 0.7. 2. OBSERVATIONAL DATA AND TARGET SELECTION We targeted galaxies for mid-infrared IRS spectroscopy based on multiwavelength imaging of the 9.3 deg2 Bo¨otes field of the NDWFS. In the following subsections, we describe the survey data, our strategy for selecting high-redshift star-forming ULIRGs, the IRS spectroscopy of these ULIRGs, and follow-up observations at 70 and 160 μm. 2.1. Multiwavelength Imaging of the Bo¨otes Field of the NOAO Deep Wide-Field Survey The 9.3 deg2 Bo¨otes field of the NDWFS12 has been imaged in the BW , R, I, and K bands down to 5σ point-source depths of ≈27.1, 26.1, 25.4, and 19.0 Vega mag, respectively. Additional imaging at the J and Ks bands was obtained for 4.7 deg2 of the Bo¨otes field through the FLAMEX survey (Elston et al. 2006). Approximately 8.5 deg2 of the Bo¨otes field has also 12 See http://www.noao.edu/noao/noaodeep/ for more information regarding the depth and coverage of the NDWFS.

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been mapped (PID 30) with Spitzer IRAC. The 5σ point-source depths of the IRAC Shallow Survey are 6.4, 8.8, 51, and 50 μJy at 3.6, 4.5, 5.6, and 8 μm, respectively (Eisenhardt et al. 2004). Approximately 8.74 deg2 of the Bo¨otes field has been imaged with the Multiband Imaging Photometer for Spitzer/ MIPS (Rieke et al. 2004). The 1σ point-source depths of the MIPS survey are 0.051, 5, and 18 mJy at 24, 70, and 160 μm, respectively. Approximately 7 deg2 have been observed at 1.4 GHz with the Westerbork Synthesis Radio Telescope (WRST). The data are characterized by a 13 × 27 beam and a 1-σRMS limiting sensitivity of 28 μJy (de Vries et al. 2002). Although both a reduced mosaic and a catalog have been made publically available,13 we performed our own photometry on the reduced mosaic to ensure proper deblending for each IRS target. 2.2. Selection of High-Redshift Star-Forming ULIRGs Our goal was to select star-forming ULIRGs at z ≈ 2 for follow-up mid-infrared spectroscopy with the IRS. Based on previous mid-infrared, near-infrared, and optical spectroscopy, we have established that a selection criterion of R − [24] > 14 Vega mag results in sources at z ≈ 2 (Dey et al. 2008; Desai et al. 2008; Brand et al. 2007b; Houck et al. 2005). Given both the sensitivity limits of the IRS and our desire to select high-luminosity sources, we only considered objects satisfying fν (24 μm)  0.5 mJy. Finally, we chose sources that display a rest-frame 1.6 μm stellar photospheric feature in their IRAC SEDs. A prominent 1.6 μm bump suggests a limited contribution from AGN-heated hot dust, implying that the luminosity of the source is primarily generated by star formation. In order to choose the best candidates, we eliminated sources that were undetected in any of the IRAC channels. We adopted Arp 220 as a template for identifying sources with a prominent 1.6 μm stellar bump. We varied only the template normalization and redshift in order to fit our Arp 220 template to the four IRAC flux densities of our sample sources. We also fit a power law to these flux densities. If the best power-law model provided a better fit than the best Arp 220 model, we rejected the candidate. If the best-fitting Arp 220 model corresponded to a photometric redshift less than 1.5, we also rejected the source. For the remaining sources, we tested the dependence of the best-fit photometric redshift on photometry errors by performing Monte Carlo simulations. For each of 500 trials, we randomly perturbed the four observed IRAC flux densities. The sizes of the perturbations were based on Gaussian distributions centered on the observed flux densities and with dispersions equal to the reported 1σ photometric errors. In this way, we generated an approximate photometric redshift probability distribution for each source. The full SEDs and photometric redshift probability distributions for each source were visually inspected. Based on these, a total of 23 sources were selected for follow-up IRS spectroscopy. Their positions are listed in Table 1, their photometric properties are summarized in Table 2, and their SEDs are shown in Figure 1. The first 15 sources were chosen based upon the tightness of their redshift probability distributions (favoring sources with the highest signal-to-noise photometry). The last eight sources had broad photometric redshift probability distributions, but were chosen because they had a significant probability of lying at z > 2.5. 13

http://www.astron.nl./wow/testcode.php?survey=5.

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DESAI ET AL. Table 1 Sample

Num

MIPS Name

z

Δz

Templatea

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

SST24 J142920.1+333023.9 SST24 J143458.9+333437.0 SST24 J143324.3+334239.5 SST24 J143137.1+334501.6 SST24 J143349.6+334601.7 SST24 J143503.2+340243.6 SST24 J142832.4+340849.8 SST24 J142941.1+340915.7 SST24 J142951.2+342042.1 SST24 J143321.8+342502.0 SST24 J143502.9+342658.8 SST24 J142600.6+343452.8 SST24 J143152.4+350030.1 SST24 J142724.9+350824.3 SST24 J143331.9+352027.2 SST24 J143143.4+324943.8 SST24 J143020.5+330344.2 SST24 J143816.6+333700.6 SST24 J143216.8+335231.7 SST24 J143743.3+341049.4 SST24 J143702.0+344630.4 SST24 J142652.5+345506.0 SST24 J142637.4+333025.7

2.01 2.13 1.91 1.77 1.86 1.97 1.84 1.91 1.76 2.10 2.10 ··· 1.50 1.71 1.91 ··· 1.87 1.84 1.76 2.19 3.04 1.91 ···

0.02 0.02 0.03 0.02 0.01 0.02 0.02 0.03 0.01 0.02 0.02 ··· 0.02 0.02 0.02 ··· 0.02 0.04 0.02 0.02 0.02 0.02 ...

NGC 7023 NGC 7023 NGC 7023 NGC 7023 starburst NGC 7714 NGC 7023 starburst NGC 7714 NGC 7714 NGC 7023 ··· NGC 7023 NGC 7023 NGC 7023 ··· NGC 7023 NGC 7023 M82 NGC 7023 starburst starburst ...

Notes. The MIPS name encodes the R.A. and Decl. (J2000) of the 24 μm source. a The local template spectrum that provided the best fit to the observed IRS spectrum. This fit was used to determine the redshift.

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Dey et al. (2008) find that the DOG selection picks out about 50% of the ULIRGs at z ≈ 2. We now explore how our present sample of bump DOGs compares to other z ≈ 2 galaxies that have been selected for follow-up with the IRS. These include sources detected in Bo¨otes (Houck et al. 2005; Weedman et al. 2006), the First Look Survey (FLS; Yan et al. 2005; Sajina et al. 2007), and SWIRE (Farrah et al. 2008; Lonsdale et al. 2009). The Bo¨otes IRS targets cited above satisfy the same R − [24] color cut, but are brighter (fν (24 μm) > 0.75 mJy) than the current sample. Although they were selected without regard to IRAC SED shape, the bright 24 μm flux density cut resulted in the vast majority having power-law SEDs through the IRAC bands. The FLS sample consists of bright (fν (24 μm) > 0.9 mJy) 24 μm sources with less extreme infrared-to-optical flux density ratios than we imposed. Also, > 0.5 to try the FLS sample satisfied a cut of log10 (νS24 /νS8 ) ∼ to select for star-forming, rather than AGN-like, sources. The SWIRE sample was selected to have fν (24 μm) > 0.5 mJy and strong 1.6 μm stellar bumps peaking in IRAC channel 2 (“bump-2” sources). Figure 2 shows log10 (νS24 /νS8 ) versus log10 (νS24 /νSR ) for these different samples. Our sample has similar values of log10 (νS24 /νS8 ) compared to the FLS and SWIRE samples. This is not surprising since all three programs were designed to select star-forming ULIRGs (see also Brand et al. 2006). However, the brighter Bo¨otes samples of Houck et al. (2005) and Weedman et al. (2006) extend to lower values of log10 (νS24 /νS8 ). This is consistent with their IRS spectra being primarily AGN-like. IRAC color–color diagrams have been used by various groups (see, e.g., Lacy et al. 2004; Stern et al. 2005; Sajina et al. 2005) to determine the origin of the mid-infrared luminosity

Table 2 Photometric Properties of IRS Targets Num

BW (mag)

R (mag)

I (mag)

Ja (mag)

Ks a (mag)

Ka (mag)

fν (3.6 μm) fν (4.5 μm) (μJy) (μJy)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

27.2 ± 0.2 25.4 ± 0.1 25.8 ± 0.2 24.4 ± 0.1 26.1 ± 0.3 >26.4 25.8 ± 0.2 >26.7 25.0 ± 0.1 25.8 ± 0.2 25.5 ± 0.1 25.1 ± 0.1 26.9 ± 0.2 >25.4 26.0 ± 0.2 >26.6 >26.6 25.9 ± 0.2 25.9 ± 0.2 >26.7 >26.5 26.4 ± 0.2 >26.0

24.6 ± 0.2 24.6 ± 0.2 24.6 ± 0.2 25.0 ± 0.2 24.7 ± 0.3 25.0 ± 0.3 24.5 ± 0.3 >24.9 24.8 ± 0.3 24.3 ± 0.2 24.5 ± 0.2 24.6 ± 0.3 25.2 ± 0.3 26.3 ± 2.7 24.7 ± 0.1 26.0 ± 0.5 26.8 ± 1.1 24.6 ± 0.1 24.7 ± 0.3 26.4 ± 0.9 27.7 ± 2.1 25.3 ± 0.4 26.7 ± 2.4

24.0 ± 0.1 23.6 ± 0.1 23.3 ± 0.1 23.2 ± 0.2 24.4 ± 0.2 24.3 ± 0.2 23.6 ± 0.1 24.2 ± 0.2 23.5 ± 0.1 23.5 ± 0.1 24.3 ± 0.3 23.1 ± 0.1 23.8 ± 0.1 23.5 ± 0.2 23.8 ± 0.1 24.3 ± 0.2 24.0 ± 0.3 23.7 ± 0.1 24.2 ± 0.2 24.3 ± 0.3 >24.7 >24.6 24.7 ± 0.3

... ... 21.9 ± 0.3 20.4 ± 0.2 21.2 ± 0.4 21.4 ± 0.2 20.5 ± 0.2 22.5 ± 0.5 21.0 ± 0.2 ... ... 21.2 ± 0.3 ... ... ... ... 21.2 ± 0.3 ... 21.9 ± 0.4 21.1 ± 0.3 ... ... ...

... ... 19.2 ± 0.2 19.3 ± 0.1 19.2 ± 0.1 18.9 ± 0.1 19.0 ± 0.1 19.5 ± 0.1 18.8 ± 0.1 ... ... 19.4 ± 0.1 ... ... ... ... 19.2 ± 0.1 ... 19.3 ± 0.1 19.3 ± 0.1 ... ... ...

... ... ... ... ... >18.0 >18.1 >18.1 > 17.6 > 17.9 >18.2 > 18.2 >17.9 >18.3 > 17.8 ... ... ... ... >18.3 ... >18.3 ...

25.2 ± 2.5 42.9 ± 2.7 43.1 ± 2.7 34.0 ± 2.6 43.1 ± 2.7 40.8 ± 2.7 37.1 ± 2.6 30.4 ± 2.6 45.4 ± 2.7 31.9 ± 2.6 56.8 ± 2.8 60.0 ± 2.8 51.1 ± 2.8 43.3 ± 2.7 30.2 ± 2.5 57.7 ± 2.8 38.2 ± 2.6 24.2 ± 2.5 36.8 ± 2.6 16.3 ± 2.4 21.4 ± 2.5 24.2 ± 2.5 2.9 ± 2.3

27.5 ± 3.1 53.1 ± 3.3 51.5 ± 3.3 38.8 ± 3.2 52.3 ± 3.3 56.9 ± 3.4 48.5 ± 3.3 38.0 ± 3.2 58.5 ± 3.4 39.9 ± 3.2 63.7 ± 3.4 71.3 ± 3.5 65.1 ± 3.4 48.9 ± 3.3 38.8 ± 3.2 94.0 ± 3.7 44.5 ± 3.3 24.0 ± 3.1 36.0 ± 3.2 22.1 ± 3.1 25.8 ± 3.1 24.7 ± 3.1 13.0 ± 3.0

fν (5.8 μm) (μJy)

fν (8 μm) (μJy)

53.4 ± 16.6 21.4 ± 14.7 59.4 ± 16.7 57.8 ± 14.8 43.7 ± 16.6 30.8 ± 14.8 42.9 ± 16.5 30.5 ± 14.8 45.7 ± 16.6 33.2 ± 14.8 67.6 ± 16.7 45.1 ± 14.8 60.1 ± 16.7 36.6 ± 14.8 49.6 ± 16.6 42.0 ± 14.8 70.0 ± 16.7 59.4 ± 14.8 49.8 ± 16.6 49.2 ± 14.8 60.2 ± 16.7 56.9 ± 14.8 94.0 ± 16.9 69.3 ± 14.9 78.2 ± 16.8 48.1 ± 14.8 75.5 ± 16.8 50.7 ± 14.8 53.1 ± 16.6 44.7 ± 14.8 182.8 ± 17.6 262.0 ± 15.5 39.9 ± 16.5 53.5 ± 14.8 43.6 ± 16.6 34.0 ± 14.8 42.7 ± 16.5 58.5 ± 14.8 33.0 ± 16.5 30.8 ± 14.8 40.1 ± 16.5 27.3 ± 14.7 34.4 ± 16.5 20.4 ± 14.7 32.5 ± 16.5 81.3 ± 14.9

fν (24 μm) fν (20 cm) (μJy) (μJy) 510 ± 40 573 ± 51 530 ± 37 573 ± 52 529 ± 37 764 ± 58 524 ± 35 586 ± 40 603 ± 36 556 ± 41 502 ± 37 711 ± 35 524 ± 48 507 ± 47 601 ± 48 535 ± 48 540 ± 49 530 ± 36 502 ± 44 501 ± 43 508 ± 60 598 ± 50 636 ± 49