We have detected the 158 μm C II line from 12 galaxies at z ~ 1-2. This is the first survey of this important star formation tracer at redshifts covering the epoch of maximum star formation in the ...universe and quadruples the number of reported high-z C II detections. The line is very luminous, between <0.024% and 0.65% of the far-infrared (FIR) continuum luminosity of our sources, and arises from photodissociation regions on molecular cloud surfaces. An exception is PKS 0215+015, where half of the C II emission could arise from X-ray-dominated regions near the central active galactic nucleus (AGN). The L C II/L FIR ratio in our star formation-dominated systems is ~8 times larger than that of our AGN-dominated systems. Therefore this ratio selects for star formation-dominated systems. Furthermore, the L C II/L FIR and L C II/L (CO(1-0)) ratios in our star-forming galaxies and nearby starburst galaxies are the same, so that luminous star-forming galaxies at earlier epochs (z ~ 1-2) appear to be scaled-up versions of local starbursts entailing kiloparsec-scale starbursts. Most of the FIR and C II radiation from our AGN-dominated sample (excepting PKS 0215+015) also arises from kiloparsec-scale star formation, but with far-UV radiation fields ~8 times more intense than in our star formation-dominated sample. We speculate that the onset of AGN activity stimulates large-scale star formation activity within AGN-dominated systems. This idea is supported by the relatively strong O III line emission, indicating very young stars, that was recently observed in high-z composite AGN/starburst systems. Our results confirm the utility of the C II line, and in particular, the L C II/L (FIR) and L C II/L CO(1-0) ratios as tracers of star formation in galaxies at high redshifts.
We present deep 1.2 mm continuum mapping of a 566 arcmin2 area within the Lockman Hole North (LHN) field, previously a target of the Spitzer Wide-Area Infrared Extragalactic survey and extremely deep ...20 cm mapping with the Very Large Array, which we have obtained using the Max-Planck millimeter bolometer (MAMBO) array on the IRAM 30 m telescope. After filtering, our full map has an rms sensitivity ranging from 0.45 to 1.5 mJy beam--1, with an average of 0.75 mJy beam--1. Using the pixel flux distribution (PFD) in a map made from our best data, we determine the shape, normalization, and approximate flux density cutoff for 1.2 mm number counts well below our nominal sensitivity and confusion limits. After validating our full data set through comparison with this map, we successfully detect 41 1.2 mm sources with signal-to-noise ratio (S/N) > 4.0 and S 1.2 mm 2-5 mJy. We use the most significant of these detections to directly determine the integral number counts down to 1.8 mJy, which are consistent with the results of the PFD analysis. Ninety-three percent of our 41 individual detections have 20 cm counterparts, 49% have Spitzer/MIPS 24 Delta *mm counterparts, and one may have a significant Chandra X-ray counterpart. We resolve 3% of the cosmic infrared background (CIB) at 1.2 mm into significant detections and directly estimate a 0.05 mJy faint-end cutoff for the counts that is consistent with the full intensity of the 1.2 mm CIB. The median redshift of our 17 detections with spectroscopic or robust photometric redshifts is z median = 2.3, and rises to z median = 2.9 when we include redshifts estimated from the radio/far-infrared spectral index. By using a nearest neighbor and angular correlation function analysis, we find evidence that our S/N > 4.0 detections are clustered at the 95% confidence level.
We present SHARC-2 350 μm data on 20 luminous z ~ 2 starbursts with S 1.2 mm > 2 mJy from the Spitzer-selected samples of Lonsdale et al. and Fiolet et al. All the sources were detected, with S 350 ...μm > 25 mJy for 18 of them. With the data, we determine precise dust temperatures and luminosities for these galaxies using both single-temperature fits and models with power-law mass-temperature distributions. We derive appropriate formulae to use when optical depths are non-negligible. Our models provide an excellent fit to the 6 μm-2 mm measurements of local starbursts. We find characteristic single-component temperatures T 1 ~= 35.5 ± 2.2 K and integrated infrared (IR) luminosities around 1012.9 ± 0.1 L sun for the SWIRE-selected sources. Molecular gas masses are estimated at sime4 × 1010 M sun, assuming κ850 μm = 0.15 m2 kg-1 and a submillimeter-selected galaxy (SMG)-like gas-to-dust mass ratio. The best-fit models imply gsim2 kpc emission scales. We also note a tight correlation between rest-frame 1.4 GHz radio and IR luminosities confirming star formation as the predominant power source. The far-IR properties of our sample are indistinguishable from the purely submillimeter-selected populations from current surveys. We therefore conclude that our original selection criteria, based on mid-IR colors and 24 μm flux densities, provides an effective means for the study of SMGs at z ~ 1.5-2.5.
We present CO observations of nine ultra-luminous infrared galaxies (ULIRGs) at z ~ 2 with f ν(24 μm) gsim 1 mJy, previously confirmed with the mid-IR spectra in the Spitzer First Look Survey. All ...targets are required to have accurate redshifts from Keck/GEMINI near-IR spectra. Using the Plateau de Bure millimeter-wave Interferometer at the Institute for Radioastronomy at Millimeter Wavelengths, we detect CO J(3-2) (seven objects) or J(2-1) (one object) line emission from eight sources with integrated intensities Ic ~ 5σ-9σ. The CO-detected sources have a variety of mid-IR spectra, including strong polycyclic aromatic hydrocarbon, deep silicate absorption, and power-law continuum, implying that these molecular gas-rich objects at z ~ 2 could be either starbursts or dust-obscured active galactic nuclei (AGNs). The measured line luminosity L'CO is (1.28-3.77) × 1010 K km/s pc2. The averaged molecular gas mass M_H_2 is 1.7 × 1010 M sun, assuming CO-to-H2 conversion factor of 0.8 M sun (K km/s pc2)-1. Three sources (33%)—MIPS506, MIPS16144, and MIPS8342—have double peak velocity profiles. The CO double peaks in MIPS506 and MIPS16144 show spatial separations of 45 kpc and 10.9 kpc, allowing the estimates of the dynamical masses of 3.2 × 1011 sin-2(i) M sun and 5.4 × 1011 sin-2(i) M sun, respectively. The implied gas fraction, M gas/M dyn, is 3% and 4%, assuming an average inclination angle. Finally, the analysis of the Hubble Space Telescope/NIC2 images, mid-IR spectra, and IR spectral energy distribution revealed that most of our sources are mergers, containing dust-obscured AGNs dominating the luminosities at (3-6) μm. Together, these results provide some evidence suggesting submillimeter galaxies, bright 24 μm, z ~ 2 ULIRGs, and QSOs could represent three different stages of a single evolutionary sequence, however, a complete physical model would require much more data, especially high spatial resolution spectroscopy. Based on observations obtained at the Institute for Radioastronomy at Millimeter Wavelengths (IRAM) Plateau de Bure Interferometer (PdBI). IRAM is funded by the Centre National de la Recherche Scientifique (France), the Max-Planck Gesellschaft (Germany), and the Instituto Geografico Nacional (Spain).
We present millimetre observations of a sample of 12 high-redshift ultraluminous infrared galaxies (ULIRGs) in the extended growth strip (EGS). These objects were initially selected on the basis of ...their observed mid-IR colours (0.0 < 3.6−4.5 < 0.4 and −0.7 < 3.6−8.0 < 0.5) to lie at high redshift 1.5 ≲z≲ 3, and subsequent 20–38 μm mid-IR spectroscopy confirms that they lie in a narrow redshift window centred on z≈ 2. We detect 9/12 of the objects in our sample at high significance (>3σ) with a mean 1200 μm flux of 〈F1200 μm〉= 1.6 ± 0.1 mJy. Our millimetre photometry, combined with existing far-IR photometry from the Far-IR Deep Extragalactic Legacy Survey (FIDELS) and accurate spectroscopic redshifts, places constraints both sides of the thermal dust peak. This allows us to estimate the dust properties, including the far-IR luminosity, dust temperature and dust mass. We find that our sample is similar to other high-z and intermediate-z ULIRGs, and local systems, but has a different dust selection function than submillimeter-selected galaxies. Finally, we use existing 20-cm radio continuum imaging to test the far-IR/radio correlation at high redshift. We find that our sample is consistent with the local relation, implying little evolution. Furthermore, this suggests that our sample selection method is efficient at identifying ultraluminous, starburst-dominated systems within a very narrow redshift range centred at z∼ 2.
ABSTRACTWe present millimetre observations of a sample of 12 high-redshift ultraluminous infrared galaxies (ULIRGs) in the extended growth strip (EGS). These objects were initially selected on the ...basis of their observed mid-IR colours (0.0 < 3.6 - 4.5 < 0.4 and -0.7 < 3.6 - 8.0 < 0.5) to lie at high redshift 1.5 lsim z lsim 3, and subsequent 20-38 mu m mid-IR spectroscopy confirms that they lie in a narrow redshift window centred on z approximately 2. We detect 9-12 of the objects in our sample at high significance (>3 sigma ) with a mean 1200 mu m flux of <F1200 mu m> = 1.6 plus or minus 0.1 mJy. Our millimetre photometry, combined with existing far-IR photometry from the Far-IR Deep Extragalactic Legacy Survey (FIDELS) and accurate spectroscopic redshifts, places constraints both sides of the thermal dust peak. This allows us to estimate the dust properties, including the far-IR luminosity, dust temperature and dust mass. We find that our sample is similar to other high-z and intermediate-z ULIRGs, and local systems, but has a different dust selection function than submillimeter-selected galaxies. Finally, we use existing 20-cm radio continuum imaging to test the far-IR-radio correlation at high redshift. We find that our sample is consistent with the local relation, implying little evolution. Furthermore, this suggests that our sample selection method is efficient at identifying ultraluminous, starburst-dominated systems within a very narrow redshift range centred at z similar to 2.
Context.Spitzer’s wide-field surveys and followup capabilities have allowed a new breakthrough in mid-IR spectroscopy up to redshifts ≥ 2, especially for 24 μm detected sources. Aims. We want to ...study the mid-infrared properties and the starburst and AGN contributions, of 24 μm sources at z ~ 2, through analysis of mid-infrared spectra combined with millimeter, radio, and infrared photometry. Mid-infrared spectroscopy allows us to recover accurate redshifts. Methods. A complete sample of 16 Spitzer-selected sources (ULIRGs) believed to be starbursts at z ~ 2 (“5.8 μm-peakers”) was selected in the (0.5 deg2) J1064+56 SWIRE Lockman Hole field (“Lockman-North”). These sources have S24 μm > 0.5 mJy, a stellar emission peak redshifted to 5.8 μm, and $r'_{\rm Vega} > 23$rVega′>23. The entire sample was observed with the low resolution units of the Spitzer/IRS infrared spectrograph. These sources have 1.2 mm observations with IRAM 30 m/MAMBO and very deep 20 cm observations from the VLA. Nine of our sources also benefit from 350 μm observation and detection from CSO/SHARC-II. All these data were jointly analyzed. Results. The entire sample shows good quality IRS spectra dominated by strong PAH features. The main PAH features at 6.2, 7.7, 8.6, and 11.3 μm have high S/N average luminosities of 2.90 ± 0.31, 10.38 ± 1.09, 3.62 ± 0.27, and 2.29 ± 0.26 × 1010 L⊙, respectively. Thanks to their PAH spectra, we derived accurate redshifts spanning from 1.750 to 2.284. The average of these redshifts is 2.017 ± 0.038. This result confirms that the selection criteria of “5.8 μm-peakers” associated with a strong detection at 24 μm are reliable to select sources at z ~ 2. We have analyzed the different correlations between PAH emission and infrared, millimeter, and radio emissions. Practically all our sources are strongly dominated by starburst emission, with only one source showing an important AGN contribution. We have also defined two subsamples based on the equivalent width at 7.7 μm to investigate AGN contributions. Conclusions. Our sample contains strong starbursts and represents a particularly 24 μm-bright class of SMGs. The very good correlation between PAH and far-IR luminosities is now confirmed in high-z starburst ULIRGs. These sources show a small AGN contribution to the mid-IR, around ~20% or less in most cases.
Context. Wide-field Spitzer surveys allow identification of thousands of potentially high-z submillimeter galaxies (SMGs) through their bright 24 μm emission and their mid-IR colors. Aims. We want to ...determine the average properties of such $z\sim2$ Spitzer-selected SMGs by combining millimeter, radio, and infrared photometry for a representative IR-flux ($\lambda_{\rm rest}\sim 8\,\mu$m) limited sample of SMG candidates. Methods. A complete sample of 33 sources believed to be starbursts (“5.8 μm-peakers”) was selected in the (0.5 deg2) J1046+56 field with selection criteria F24\,\mu m > 400 μJy, the presence of a redshifted stellar emission peak at 5.8 μm, and $r^\prime_{\rm Vega}$ > 23. The field, part of the SWIRE Lockman Hole field, benefits from very deep VLA/GMRT 20 cm, 50 cm, and 90 cm radio data (all 33 sources are detected at 50 cm), and deep 160 μm and 70 μm Spitzer data. The 33 sources, with photometric redshifts ~$1.5{-}2.5$, were observed at 1.2 mm with IRAM-30m/MAMBO to an rms ~0.7–0.8 mJy in most cases. Their millimeter, radio, 7-band Spitzer, and near-IR properties were jointly analyzed. Results. The entire sample of 33 sources has an average 1.2 mm flux density of $1.56 \pm 0.22$ mJy and a median of 1.61 mJy, so the majority of the sources can be considered SMGs. Four sources have confirmed 4σ detections, and nine were tentatively detected at the 3σ level. Because of its 24 μm selection, our sample shows systematically lower $F_{\rm 1.2\,mm}/F_{\rm 24\,\mu m}$ flux ratios than classical SMGs, probably because of enhanced PAH emission. A median FIR SED was built by stacking images at the positions of 21 sources in the region of deepest Spitzer coverage. Its parameters are $T_{\rm dust} = 37 \pm 8$ K, $L_{\rm FIR} = 2.5 \times 10^{12}\,L_{\odot}$, and SFR = 450 $M_{\odot}$ yr-1. The FIR-radio correlation provides another estimate of LFIR for each source, with an average value of $4.1 \times 10^{12}\,L_{\odot}$; however, this value may be overestimated because of some AGN contribution. Most of our targets are also luminous star-forming BzK galaxies which constitute a significant fraction of weak SMGs at $1.7 \lesssim z \lesssim 2.3.$ Conclusions. Spitzer 24 μm-selected starbursts and AGN-dominated ULIRGs can be reliably distinguished using IRAC-24 μm SEDs. Such “5.8 μm-peakers” with F24~\mu m > 400 μJy have $L_{\rm FIR}\,\gtrsim 10^{12}\,L_{\odot}$. They are thus $z \sim 2$ ULIRGs, and the majority may be considered SMGs. However, they have systematically lower 1.2 mm/24 μm flux density ratios than classical SMGs, warmer dust, comparable or lower IR/mm luminosities, and higher stellar masses. About 2000-3000 “5.8 μm-peakers” may be easily identifiable within SWIRE catalogues over 49 deg2.
We have detected the 158 {mu}m C II line from 12 galaxies at z {approx} 1-2. This is the first survey of this important star formation tracer at redshifts covering the epoch of maximum star formation ...in the universe and quadruples the number of reported high-z C II detections. The line is very luminous, between <0.024% and 0.65% of the far-infrared (FIR) continuum luminosity of our sources, and arises from photodissociation regions on molecular cloud surfaces. An exception is PKS 0215+015, where half of the C II emission could arise from X-ray-dominated regions near the central active galactic nucleus (AGN). The L{sub C{sub II}}/L{sub FIR} ratio in our star formation-dominated systems is {approx}8 times larger than that of our AGN-dominated systems. Therefore this ratio selects for star formation-dominated systems. Furthermore, the L{sub C{sub II}}/L{sub FIR} and L{sub C{sub II}}/L{sub (CO(1-0))} ratios in our star-forming galaxies and nearby starburst galaxies are the same, so that luminous star-forming galaxies at earlier epochs (z {approx} 1-2) appear to be scaled-up versions of local starbursts entailing kiloparsec-scale starbursts. Most of the FIR and C II radiation from our AGN-dominated sample (excepting PKS 0215+015) also arises from kiloparsec-scale star formation, but with far-UV radiation fields {approx}8 times more intense than in our star formation-dominated sample. We speculate that the onset of AGN activity stimulates large-scale star formation activity within AGN-dominated systems. This idea is supported by the relatively strong O III line emission, indicating very young stars, that was recently observed in high-z composite AGN/starburst systems. Our results confirm the utility of the C II line, and in particular, the L{sub C{sub II}}/L{sub (FIR)} and L{sub C{sub II}}/L{sub CO(1-0)} ratios as tracers of star formation in galaxies at high redshifts.
![A 158 μm [C II] Line Survey...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-637X/small "A 158 μm [C II] Line Survey of Galaxies at z ~ 1-2: An Indicator of Star Formation in the Early Universe")
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