Far-infrared properties of submillimeter and optically faint radio galaxies
B. Magnelli, D. Lutz, S. Berta, B. Altieri, P. Andreani, H. Aussel, H. Castaneda, A. Cava, J. Cepa, A. Cimatti, E. Daddi, H. Dannerbauer, H. Dominguez, D. Elbaz, N. Forster Schreiber, R. Genzel, A. Grazian, C. Gruppioni, G. Magdis, R. Maiolino, R. Nordon, I. Perez Fournon, I. Perez Garcia, A. Poglitsch, P. Popesso, F. Pozzi, L. Riguccini, G. Rodighiero, A. Saintonge, P. Santini, M. Sanchez-Portal, L. Shao, E. Sturm, L. Tacconi, I. Valtchanov, E.Wieprecht, E. Wiezorrek
aa r X i v : . [ a s t r o - ph . C O ] M a y Astronomy&Astrophysicsmanuscript no. 14616 c (cid:13)
ESO 2018November 1, 2018 L etter to the E ditor Far-infrared properties of submillimeter and optically faint radiogalaxies ⋆ B. Magnelli , D. Lutz , S. Berta , B. Altieri , P. Andreani , , H. Aussel , H. Casta˜neda , , A. Cava , , J. Cepa , , A.Cimatti , E. Daddi , H. Dannerbauer , H. Dominguez , D. Elbaz , N. F ¨orster Schreiber , R. Genzel , A. Grazian , C.Gruppioni , G. Magdis , R. Maiolino , R. Nordon , I. P´erez Fournon , , I. P´erez Garc´ıa , A. Poglitsch , P. Popesso ,F. Pozzi , L. Riguccini , G. Rodighiero , A. Saintonge , P. Santini , M. Sanchez-Portal , L. Shao , E. Sturm , L.Tacconi , I. Valtchanov , E.Wieprecht , and E. Wiezorrek (See online Appendix A for author a ffi liations) Received ??; accepted ??
ABSTRACT
We use deep observations obtained with the Photodetector Array Camera & Spectrometer (PACS) onboard the
Herschel space ob-servatory to study the far-infrared (FIR) properties of submillimeter and optically faint radio galaxies (SMGs and OFRGs). Fromliterature we compiled a sample of 35 securely identified SMGs and nine OFRGs located in the GOODS-N and the A2218 fields.This sample is cross-matched with our PACS 100 µ m and 160 µ m multi-wavelength catalogs based on sources-extraction using priordetections at 24 µ m. About half of the galaxies in our sample are detected in at least the PACS 160 µ m bandpass. The dust temper-atures and the infrared luminosities of our galaxies are derived by fitting their PACS and SCUBA 850 µ m (only the upper limits forthe OFRGs) flux densities with a single modified ( β = .
5) black body function. The median dust temperature of our SMG sampleis T dust = ± T dust = ± Herschel data agree well with previous estimates. In particular, Chapman et al. (2005) found a dust temperature of T dust = ± / radio correlation (i.e., q = log ( L FIR [W] / L . [W Hz − ] / . × )).The agreement between our studies confirms that the local FIR / radio correlation e ff ectively holds at high redshift even though wefind h q i = . ± .
19, a slightly lower value than that observed in local systems. The median infrared luminosities of SMGs andOFRGs are 4 . × L ⊙ and 2 . × L ⊙ , respectively. We note that for both samples the infrared luminosity estimates from the radiopart of the spectral energy distribution (SED) are accurate, while estimates from the mid-IR are considerably ( ∼ ×
3) more uncertain.Our observations confirm the remarkably high luminosities of SMGs and thus imply median star-formation rates of 960 M ⊙ yr − forSMGs with S (850 µ m) > ⊙ yr − for SMGs with S (850 µ m) > Key words.
Galaxies: evolution - Infrared: galaxies - Galaxies: starburst - Submillimeter: galaxies
1. Introduction
Herschel observations probe the rest-frame far-infrared emis-sion of high-redshift galaxies. Thus, they provide for the firsttime robust estimates of the infrared luminosities of thesehigh-redshift galaxies and test previous measurements that werebased on extrapolation from shorter or longer wavelengths. Wehere focus on two populations of high-redshift star-forminggalaxies selected at submillimeter (submm) and radio wave-lengths.Since their discovery in the late 1990s, submillimetergalaxies (SMGs) have become the selection of choice for themost luminous tail of the high-redshift star-forming galaxypopulation. It has been found that SMGs have a typical redshiftof z ∼ M ⋆ ∼ − M ⊙ , Swinbank et al., 2004;Tacconi et al., 2006) and are compact (e.g., Tacconi et al.,2008). Interferometric observations of their CO molecular gassuggest that the most luminous SMGs ( S µ m > Send o ff print requests to : B. Magnelli, e-mail: [email protected] ⋆ Herschel is an ESA space observatory with science instruments pro-vided by European-led Principal Investigator consortia and with impor-tant participation from NASA. merging sytems (Tacconi et al., 2006, 2008) with high star-formation e ffi ciencies compared to typical galaxies of a similarmass (Daddi et al., 2008). Therefore, these SMGs are thoughtto exhibit very intense (SFR ∼ ⊙ yr − ) short-lived star-formation bursts triggered by mergers and to be the high-redshiftprogenitors of local massive early-type galaxies (Daddi et al.,2007a,b; Tacconi et al., 2008; Cimatti et al., 2008).Although SMGs provide a powerful tool to constrain theformation and the evolution of high-redshift dusty star-forminggalaxies, their selection is subject to strong biases. In particular,because submm observations probe the blackbody emissionof dust in the Rayleigh-Jeans regime, they are strongly anti-correlated with the dust temperature ( S ∝ T − . dust ). For a giveninfrared luminosity, galaxies with hot dust might fall belowthe detection limit of current submm instruments. The firstobservational evidence of a missing population of high-redshiftdusty star-forming galaxies with hot dust has been given byChapman et al. (2004) using a selection of radio-detectedbut submm-faint galaxies with UV spectra consistent withhigh-redshift starbursts. These optically faint-radio galaxies(OFRGs) have a comoving volume density (i.e., ∼ − Mpc − at 1 < z <
3, Chapman et al. 2004), stellar masses and sizescomparable to SMGs, and some have a dust temperature of
Magnelli et al.: Far-infrared properties of SMGs and OFRGs
Fig. 1.
Spectral energy distribution (SED) of one SMG ( left ) and one OFRG ( right ). Red diamonds present our PACS measurements while greensquares present the multi-wavelength ancillary data taken from the literature (Pope et al., 2006; Casey et al., 2009a,b). The modified blackbodyemission ( β = .
5) best-fitting the data are shown by solid lines. Dashed lines present the Dale & Helou SED template best-fitting the mid- tofar-infrared observations. In the left panel, the blue dotted-dashed line shows the Dale & Helou SED template best-fitting the submm and radiophotometries. The inset in each panel shows χ vs T dust . ∼
52 K (Casey et al., 2009a,b).While SMGs and OFRGs are an important componentof the high-redshift massive galaxy population, many of theirfundamental properties still rely on indirect measurements.In particular, direct determinations of SMG dust temperatureswere limited (Kov´acs et al., 2006) because they were not doneusing rest-frame far-infrared observations on both sides of thepeak of the SEDs. More importantly, their infrared luminositesare still debated. Indeed, because theoretical simulations ofgalaxy evolution have had great di ffi culties to account for thecurrent inferred luminosities / star-formation rates and numbercounts (Baugh et al., 2005; Dav´e et al., 2010), they still questionwhether these luminosities have been overestimated or whetherthe IMF is significantly more top-heavy than in the localUniverse.Using deep observations by the Photodetector ArrayCamera & Spectrometer (PACS; Poglitsch et al. 2010) onboardthe Herschel space observatory (Pilbratt et al. 2010) we willhave for the first time robust estimates of the dust temperaturesand the infrared luminosities of SMGs and OFRGs. Throughoutthe paper we use a cosmology with H = − Mpc − , Ω Λ = . Ω M = .
2. Observations
In this study we used deep PACS 100 µ m and 160 µ mobservations of the Great Observatories Origins Deep Survey-North (GOODS-N; 12 h m , + ◦ ′ ) and the Abell 2218(16 h m , + ◦ ′ ) fields. These observations were taken as partof the PACS Evolutionary Probe (PEP ) guaranteed time keyprogram. The GOODS-N field covers a region of 10 ′ × ′ (30hours), while the deep part of the A2218 field covers a region of4 ′ × ′ (13 hours).At the resolution of Herschel , all sources in our fields arepoint sources (i.e. FWHM ∼ ′′ [12 ′′ ] at 100 µ m [160 µ m]).Flux densities are hence estimated using a point spread function-fitting technique based on prior source positions detected at24 µ m. The use of priors provides a straightforward associationbetween the IRAC, MIPS and PACS sources. Using Monte http: // / ir / Research / PEP
Carlo simulations we estimate the quality of our PACS 100 µ mand 160 µ m catalogs, i.e. photometric error, completenessand contamination as a function of the flux density. In theGOODS-N field our observations reach a 3 σ limit of ∼ ∼ . µ m and 160 µ m respectively, while in theA2218 field they reach a 3 σ limit of ∼ . ∼ µ m and 160 µ m respectively.A complete description of PEP data reduction and sourcesextraction is given in Appendix A of Berta et al. (2010).
3. Galaxy sample
To obtain a robust measurement of the dust temperature andinfrared luminosity of a given galaxy one needs to have an ac-curate estimate of its redshift. Consequently, we decided to re-strict our study to a sample of SMGs and OFRGs with accu-rate redshift estimates derived from secured radio / mid-infraredidentifications (PACS identifications of SMGs are presentedin Dannerbauer et al. in prep). In the GOODS-N field, ourSMG sample is based on multi-wavelength identifications ofSCUBA and AzTEC sources made by Pope et al. (2006) andChapin et al. (2009), respectively. SMGs with tentative redshiftsdetermined from their IRAC or mid / far-infrared / radio colorswere excluded from our sample. Sources with multiple opticalcounterparts (GN04, GN07, GN19 and GN39) were treated asa single system (i.e. we will use the sum of the radio and mid-infrared flux from the two components when determining theirfar-infrared properties) because they are all thought to be in-teracting galaxies (Pope et al., 2006). All these di ff erent criteriayield an SMG sample containing 29 sources in the GOODS-Nfield. In the A2218 field, our SMG sample is assembled from theliterature (Kneib et al., 2004; Knudsen et al., 2006, 2008) andcontains six lensed sources. Because these galaxies are magni-fied, their mid-to-far infrared fluxes were de-magnified prior tofurther analysis using magnification factors from the above refer-ences. Among these six lensed sources, three correspond to thesame lensed galaxy (SMMJ16359 + For GN05, GN07, GN10, GN20 and GN20.2, we used the spectro-scopic redshifts revised in Pope et al. (2008) and Daddi et al. (2009a,b).agnelli et al.: Far-infrared properties of SMGs and OFRGs 3
Fig. 2.
Dust temperature-luminosity relation. The filled blue squaresand opened blue squares denote SMGs located in the GOODS-N fieldswith spectroscopic and photometric redshift, respectively. The crossesdenote sources which contain an AGN, as indicated by the presence ofhard X-rays. The filled green diamonds present SMGs located in theA2218 field (the three data points with L IR ∼ × L ⊙ correspond tothe same lensed galaxy). The filled red circles are from our OFRGssample. The striped area presents results for SMGs extrapolated byChapman et al. (2005) from radio and submm data. The Chapman et al.(2003) derivation of the median and interquartile range of the T dust - L IR relation observed at z ∼ L ⊙ . and contains nine sources, all situated in the GOODS-N field.We note that all but three sources of our entire sample (i.e. SMGsand OFRGs) have spectroscopic redshifts.The SMG and OFRG samples were cross-matched with ourPACS multi-wavelength catalogs using a matching radius of 3 ′′ .We detected 19 out of 35 SMGs in at least the PACS 160 µ mbandpass (17 out of 33 if not multi-counting the 3-componentlensed source detected in A2218). The PACS sample is slightlybiased towards lower redshift sources because of the positive K- correction : while the median redshift of our parent SMG sampleis z =
2, the median redshift of our PACS detected SMG sam-ple is z = .
7. Five out of nine OFRGs have PACS 100 µ m and160 µ m detections. This sample is also slightly biased towardlower redshift ( z = .
5) because the OFRG situated at the high-est redshift is undetected in our PACS images.We note that our SMG sample contains sources with2 mJy < S µ m < S µ m >
4. Data analysis
In order to infer the dust temperature of our galaxies we fittedtheir PACS and SCUBA photometry (only the upper limit forthe OFRGs) with a modified blackbody function, with a dustemissivity β = . L IR [8-1000 µ m]) were inferred from these best fits using thefar-infrared luminosity definition ( L FIR [40 − µ m]) givenby Helou et al. (1988) and a color-correction term (Dale et al.,2001, L IR = . × L FIR ).We adopted a single dust temperature characterizationbecause studies of IRAS galaxies have demonstrated that thisprovides an accurate diagnostic of the typical heating conditionin their interstellar medium (Desert et al., 1990). While for mostof our galaxies this single dust temperature characterization
Fig. 3.
Submm flux densities as function of the infrared luminosity.The symbols are same as in Fig 2 except for the OFRGs, for which wehave only upper limits. The solid and dashed lines show the linear fitto the S µ m - L IR relation and the 1 σ envelop ( L IR [ L ⊙ ] = . ± . × S . ± . µ m [mJy]). If we remove the lensed-SMGs of A2218 from the fit,we find a weaker correlation ( L IR [ L ⊙ ] = . ± . × S . ± . µ m [mJy]). provides a good description of their far-infrared SED, for 6SMGs, this single dust temperature model yields high χ values(i.e. χ > . + N do f ). All these galaxies appear either to be themore distant ones or to exhibit far-infrared colors typical of verycold systems. In both cases, their PACS 100 µ m flux densitiesmight by contaminated by a hotter dust component. Thereforewe excluded their PACS 100 µ m photometry from the fit andrecomputed their dust temperatures. We note that excludingthe PACS 100 µ m photometry from the fit of all our galaxieschanges their median dust temperature by only ∆ T dust ∼ − β and to our single dust component characterization. Indeed wenote that using β =
2, we found only small di ff erences in thevalues of T dust ( ∆ T dust ∼ + S µ m / S µ m ), or equivalently T dust , whichexcellently agree with those inferred from our blackbodyanalysis.
5. Discussion
Figure 2 depicts the locations of our SMGs and OFRGs onthe T dust - L IR plane and Fig 3 shows their locations on the S µ m - L IR plane. As already mentioned, OFRGs are biasedtowards hot dust temperatures; their median T dust is of 47 ± L IR is 2 . × L ⊙ . In contrast, SMGs havelower dust temperatures with median T dust = ± L IR = . × L ⊙ . We note that lensed-SMGs from A2218 andwith L IR < × L ⊙ exhibit intermediate dust properties andare less biased towards cold dust temperatures than the entireSMG sample. This is because these galaxies would have escapedboth the SMG and OFRG selection method without magnifi-cation. We also note that bright SMGs (i.e. S µ m > L IR = . × L ⊙ )and higher median dust temperatures ( T dust =
38 K) than theentire SMG sample because there is a correlation between S µ m and L IR (Fig 3). These estimates are the first directobservational measurements of the dust temperatures and theinfrared luminosities of SMGs and OFRGs.Our observations reveal that high redshift dusty star-forming Magnelli et al.: Far-infrared properties of SMGs and OFRGs galaxies exhibit a wide range of dust temperatures. In particularat low infrared luminosities ( L IR < × L ⊙ ) the dust temper-ature dispersion observed in our sample might suggest a higher T dust - L IR scatter than that observed by Chapman et al. (2003)at z ∼
0. Nevertheless, this conclusion is most likely driven byselection e ff ects because a significant fraction of the galaxieswith intermediate dust properties were probably missed by ourcurrent sample. Indeed, we note that studying a L IR -selectedsample of galaxies observed with Herschel , Hwang et al. (inprep) find modest changes in the T dust - L IR relation as functionof the redshift : at z > .
5, galaxies with L IR > × L ⊙ areslightly colder ( ∼ T dust - L IR relation slightly increase at high redshift.Though previous estimates of the dust temperaturesof SMGs and OFRGs relied on indirect observations, theyagree relatively well with our measurements. In partic-ular, Chapman et al. (2005) found a dust temperature of T dust = ± / radio correlation. In order to establish thisagreement on a common sample, we applied the same methodas Chapman et al. (2005) to our SMG sample, i.e. we fittedthe radio and 850 µ m photometries with dust SED templatesfrom Dale & Helou (2002) and then translated them into T dust using their R(60,100) to T dust map. With this method we foundhigher T dust (by ∼ L IR ( ∼ × . / radio correlation with h q i = .
34 (Yun et al., 2001), whilein our samples we find h q i = . ± .
19 (see also Ivison et al.2010, this issue). Although this value of h q i is still in line withresults from local systems (which have a dispersion of 0.19 dex),it is also consistent with an evolution of h q i proportional to(1 + z ) − . ± . as found by Ivison et al. (2010).Our results reveal that one can obtain a very reliableestimate of the infrared luminosity of a given galaxy fromits radio flux density ( σ [ log ( L RadioIR / L BlackbobyIR )] ∼ .
18 dex)using h q i = .
17. In contrast, the use of the 24 µ m emissionand of the Chary & Elbaz SED library yields an inaccurateestimate of the infrared luminosity characterized by a largescatter ( σ [ log ( L µ mIR / L blackbodyIR )] ∼ .
48 dex) and a systematicoverestimation ( ∼ × L IR > × L ⊙ ). We thus find that for the very luminousinfrared galaxies studied here, luminosity extrapolations basedon the radio emission are considerably more reliable than thosebased on the mid-infrared emission (see also Elbaz et al. 2010and Nordon et al. 2010).Using h q i = .
17, we can predict the infrared luminositiesof our PACS undetected SMGs. Then, using the Dale & HelouSED templates normalized to these infrared luminosities, wecan fit their 850 µ m photometries. We find that for 13 out of 16undetected SMGs, PACS flux densities inferred using these fitsare below the detection threshold of our observations. Of thethree sources with PACS fluxes predictions above our detectionthreshold, one is known to be contaminated by an AGN andthe other two are suspected to have wrong redshift estimates(Daddi et al., 2009b). The PACS nondetections are thus fullyconsistent with the properties inferred from the detections. Thisanalysis cannot be performed on our PACS undetected OFRGsbecause they are also undetected at the SCUBA wavelength.Our observations unambiguously confirm the re-markably large infrared luminosities of bright SMGs (i.e. S µ m > ⊙ yr − (SFR [M ⊙ yr − ] = × − L IR [L ⊙ ], assuming a ChabrierIMF and no dominant AGN contribution to the far-infraredluminosity). Such high SFRs are di ffi cult to reconcile withsecular evolution (e.g. Dav´e et al., 2010) and could correspondto a merger-driven stage in the evolution of these galaxies. Thishypothesis is supported by CO observations of bright SMGswhich have revealed large CO line-widths and disturbed gasmorphologies (Tacconi et al., 2008). Our observations alsoconfirm that OFRGs exhibit higher dust temperatures than faint
SMGs ( S µ m < faint SMGs (260 M ⊙ yr − and 106 M ⊙ yr − respectively) could be explained by secularevolution, we need to understand why they exhibit di ff erent dusttemperatures and to study a possible link with bright SMGs. Aclear evolutionary picture will require detailed studies of thedust and molecular gas distribution in a sample of high-redshiftstar-forming galaxies unbiased towards any particular dusttemperature. This SFR-selected sample can now be built withour ongoing deep
Herschel observations.
Acknowledgements.
PACS has been developed by a consortium of insti-tutes led by MPE (Germany) and including UVIE (Austria); KU Leuven,CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI / OAA / OAP / OAT, LENS, SISSA (Italy); IAC (Spain). This developmenthas been supported by the funding agencies BMVIT (Austria), ESA-PRODEX(Belgium), CEA / CNES (France), DLR (Germany), ASI / INAF (Italy), andCICYT / MCYT (Spain).
References
Baugh, C. M., Lacey, C. G., Frenk, C. S., et al. 2005, MNRAS, 356, 1191Berta, S., et al. 2010, A&A, this VolumeCasey, C. M., Chapman, S. C., Beswick, R. J., et al. 2009a, MNRAS, 399, 121Casey, C. M., Chapman, S. C., Neri, R., et al. 2009b, ArXiv e-prints 0910.5756Chapin, E. L., Pope, A., Scott, D., et al. 2009, MNRAS, 398, 1793Chapman, S. C., Blain, A. W., Smail, I., & Ivison, R. J. 2005, ApJ, 622, 772Chapman, S. C., Helou, G., Lewis, G. F., & Dale, D. A. 2003, ApJ, 588, 186Chapman, S. C., Smail, I., Blain, A. W., & Ivison, R. J. 2004, ApJ, 614, 671Cimatti, A., Cassata, P., Pozzetti, L., et al. 2008, A&A, 482, 21Daddi, E., Alexander, D. M., Dickinson, M., et al. 2007a, ApJ, 670, 173Daddi, E., Dannerbauer, H., Elbaz, D., et al. 2008, ApJ, 673, L21Daddi, E., Dannerbauer, H., Krips, M., et al. 2009a, ApJ, 695, L176Daddi, E., Dannerbauer, H., Stern, D., et al. 2009b, ApJ, 694, 1517Daddi, E., Dickinson, M., Morrison, G., et al. 2007b, ApJ, 670, 156Dale, D. A. & Helou, G. 2002, ApJ, 576, 159Dale, D. A., Helou, G., Contursi, A., Silbermann, N. A., & Kolhatkar, S. 2001,ApJ, 549, 215Dav´e, R., Finlator, K., Oppenheimer, B. D., et al. 2010, MNRAS, 360Desert, F.-X., Boulanger, F., & Puget, J. L. 1990, A&A, 237, 215Elbaz, D., et al. 2010, A&A, this VolumeHelou, G., Khan, I. R., Malek, L., & Boehmer, L. 1988, ApJS, 68, 151Ivison, R. J., Alexander, D. M., Biggs, A. D., et al. 2010, MNRAS, 402, 245Ivison, R. J., Magnelli, B., Ibar, E. et al. 2010, A&A, this VolumeKneib, J., van der Werf, P. P., Kraiberg Knudsen, K., et al. 2004, MNRAS, 349,1211Knudsen, K. K., Barnard, V. E., van der Werf, P. P., et al. 2006, MNRAS, 368,487Knudsen, K. K., van der Werf, P. P., & Kneib, J. 2008, MNRAS, 384, 1611Kov´acs, A., Chapman, S. C., Dowell, C. D., et al. 2006, ApJ, 650, 592Nordon, R., et al. 2010, A&A, this VolumePilbratt, G., et al. 2010, A&A, this VolumePoglitsch, A., et al. 2010, A&A, this VolumePope, A., Chary, R., Alexander, D. M., et al. 2008, ApJ, 675, 1171Pope, A., Scott, D., Dickinson, M., et al. 2006, MNRAS, 370, 1185Swinbank, A. M., Smail, I., Chapman, S. C., et al. 2004, ApJ, 617, 64Tacconi, L. J., Genzel, R., Smail, I., et al. 2008, ApJ, 680, 246Tacconi, L. J., Neri, R., Chapman, S. C., et al. 2006, ApJ, 640, 228Yun, M. S., Reddy, N. A., & Condon, J. J. 2001, ApJ, 554, 803 agnelli et al.: Far-infrared properties of SMGs and OFRGs , Online Material p 1
Appendix A: Authors’ affiliations Max-Planck-Institut f¨ur Extraterrestrische Physik (MPE), Postfach 1312,85741 Garching, Germany. Laboratoire AIM, CEA / DSM-CNRS-Universit´e Paris Diderot,IRFU / Service d’Astrophysique, Bˆat.709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France. Herschel Science Centre; European Space Astronomy Centre ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany. INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste,Italy. Instituto de Astrof´ısica de Canarias, 38205 La Laguna, Spain. Departamento de Astrof´ısica, Universidad de La Laguna, Spain. Dipartimento di Astronomia, Universit`a di Bologna, Via Ranzani 1, 40127Bologna, Italy. INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127Bologna, Italy. INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 MontePorzio Catone, Italy.11