Submillimeter Galaxies atz∼ 2: Evidence for Major Mergers and Constraints on Lifetimes, IMF, and CO‐H2Conversion Factor (original) (raw)
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The ratio of CO to total gas mass in high-redshift galaxies
Monthly Notices of the Royal Astronomical Society, 2013
have recently identified the J=6-5, 5-4, and 2-1 CO rotational emission lines, and [CII] fine-structure emission line from the star-forming interstellar medium in the high-redshift submillimeter source HDF 850.1, at z = 5.183. We employ large velocity gradient (LVG) modeling to analyze the spectra of this source assuming the [CII] and CO emissions originate from (i) separate virialized regions, (ii) separate unvirialized regions, (iii) uniformly mixed virialized regions, and (iv) uniformly mixed unvirialized regions. We present the best fit set of parameters, including for each case the ratio α between the total hydrogen/helium gas mass and the CO(1-0) line luminosity. We also present computations of the ratio of H 2 mass to [CII] line-luminosity for optically thin conditions, for a range of gas temperatures and densities, for direct conversion of [CII] line-luminosities to " CO-dark" H 2 masses. For HDF 850.1 we find that a model in which the CO and C + are uniformly mixed in gas that is shielded from UV radiation, requires a cosmic-ray or X-ray ionization rate of ζ ≈ 3× 10 −14 s −1 , plausibly consistent with the large star-formation rate (∼ 10 3 M ⊙ yr −1 ) observed in this source. Enforcing the cosmological constraint posed by the abundance of dark matter halos in the standard ΛCDM cosmology and taking into account other possible contributions to the total gas mass, we find that the two models in which the virialization condition is enforced can be ruled out at the 2σ level while the model assuming mixed unvirialized regions is less likely. We conclude that modeling HDF 850.1's ISM as a collection of unvirialized molecular clouds with distinct CO and C + layers, for which α = 1.2 M ⊙ (K km s −1 pc 2 ) −1 for the CO to H 2 mass-to-luminosity ratio, (similar to the standard ULIRG value), is most consistent with the ΛCDM cosmology.
An interferometric CO survey of luminous submillimetre galaxies
Monthly Notices of the Royal Astronomical Society, 2005
In this paper, we present results from an Institut de Radio Astronomie Millimétrique (IRAM) Plateau de Bure millimetre-wave Interferometer (PdBI) survey for carbon monoxide (CO) emission towards radio-detected submillimetre galaxies (SMGs) with known optical and nearinfrared spectroscopic redshifts. Five sources in the redshift range z ∼ 1-3.5 were detected, nearly doubling the number of SMGs detected in CO. We summarize the properties of all 12 COdetected SMGs, as well as six sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the submillimetre galaxy (SMG) population as a whole. The median CO line luminosity of the SMGs is L CO = (3.8 ± 2.0) × 10 10 K km s −1 pc 2. Using a CO-to-H 2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass M(H 2) = (3.0 ± 1.6) × 10 10 M within an ∼ 2 kpc radius, approximately 4 times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high-redshift radio galaxies (HzRGs) and quasi-sellar objects (QSOs). The median CO FWHM linewidth is broad, FWHM = 780 ± 320 km s −1 , and the SMGs often have double-peaked line profiles, indicative of either a merger or a disc. From their median gas reservoirs (∼ 3 × 10 10 M) and star formation rates (700 M yr −1), we estimate a lower limit on the typical gas-depletion timescale of 40 Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs and suggests that negative feedback processes may play an important role in prolonging the gas consumption timescale. We find a statistically significant correlation between the far-infrared and CO luminosities of the SMGs, which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts. The non-linear nature of the correlation implies that SMGs have higher far-infrared to CO luminosity ratios and possibly higher star formation efficiencies (SFEs), than local ULIRGs. Assuming a typical CO source diameter of θ ∼ 0.5 arcsec (D ∼ 4 kpc), we estimate a median dynamical mass of M dyn (1.2 ± 1.5) × 10 11 M for the SMG sample. Both the total gas and stellar masses imply that SMGs are very massive systems, dominated by baryons in their central regions. The baryonic and dynamical properties of these systems mirror those of local giant ellipticals and are consistent with numerical simulations of the formation of the most massive galaxies. We have been able to impose a lower limit of 5 × 10 −6 Mpc −3 to the comoving number density of massive galaxies in the redshift range
The Astrophysical Journal, 2011
We report the detection of spatially extended CO(J=1→0) and CO(J=5→4) emission in the z=2.49 submillimeter galaxy (SMG) J123707+6214, using the Expanded Very Large Array and the Plateau de Bure Interferometer. The large molecular gas reservoir is spatially resolved into two CO(J=1→0) components (north-east and south-west; previously identified in CO J=3→2 emission) with gas masses of 4.3 and 3.5×10 10 (α CO /0.8) M ⊙ . We thus find that the optically invisible north-east component slightly dominates the gas mass in this system. The total molecular gas mass derived from the CO(J=1→0) observations is 2.5× larger than estimated from CO(J=3→2). The two components are at approximately the same redshift, but separated by ∼20 kpc in projection. The morphology is consistent with that of an early-stage merger. The total amount of molecular gas is sufficient to maintain the intense 500 M ⊙ yr −1 starburst in this system for at least ∼160 Myr. We derive line brightness temperature ratios of r 31 =0.39±0.09 and 0.37±0.10, and r 51 =0.26±0.07 and 0.25±0.08 in the two components, respectively, suggesting that the J≥3 lines are substantially subthermally excited. This also suggests comparable conditions for star formation in both components. Given the similar gas masses of both components, this is consistent with the comparable starburst strengths observed in the radio continuum emission. Our findings are consistent with other recent studies that find evidence for lower CO excitation in SMGs than in high-z quasar host galaxies with comparable gas masses. This may provide supporting evidence that both populations correspond to different evolutionary stages in the formation of massive galaxies.
Astronomy & Astrophysics, 2015
We investigate the CO excitation of normal (near-IR selected BzK) star forming (SF) disk galaxies at z = 1.5 using IRAM Plateau de Bure observations of the CO[2-1], CO[3-2] and CO transitions for 4 galaxies, including VLA observations of CO[1-0] for 3 of them, with the aim of constraining the average state of H 2 gas. Exploiting prior knowledge of the velocity range, spatial extent and size of the CO emission we measure reliable line fluxes with S/N> 4-7 for individual transitions. While the average CO Spectral Line Energy Distribution (SLED) has a sub-thermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO[5-4] emission is four times stronger than assuming MW excitation. This demonstrates the presence of an additional component of more excited, denser and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is however lower than in local (U)LIRGs and high-redshift starbursting SMGs, and appears to correlate closely with the average intensity of the radiation field < U > and with the star formation surface density, but not with the SF efficiency (SFE). This suggests that, through the < U > parameter that is proportional to SFE divided by metallicity, the overall CO excitation is at least indirectly affected by the metallicity of the ISM. The luminosity of the CO[5-4] transition is found to correlate linearly with the bolometric infrared luminosity over 4 orders of magnitudes. For this transition, z = 1.5 BzK galaxies follow the same linear trend as local spirals and (U)LIRGs and high redshift star bursting sub-millimeter galaxies. The CO[5-4] luminosity is thus empirically related to the dense gas, and might be a more convenient way to probe it than standard high-density tracers that are much fainter than CO. We see excitation variations among our sample galaxies, that can be linked to their evolutionary state and clumpiness in optical rest frame images. In one galaxy we see spatially resolved excitation variations, where the more highly excited part of the galaxy corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high excitation CO emission in high redshift disk-like galaxies.
Interferometric CO Observations of Submillimeter-faint, Radio-selected Starburst Galaxies at z ~ 2
The Astrophysical Journal, 2008
High-redshift, dust-obscured galaxies, selected to be luminous in the radio but relatively faint at 850 μm, appear to represent a different population from the ultraluminous submillimeter-bright population. They may be star-forming galaxies with hotter dust temperatures, or they may have lower far-infrared luminosities and larger contributions from obscured active galactic nuclei (AGNs). Here we present observations of three z ~ 2 examples of this population, which we term "submillimeter-faint radio galaxies" (SFRGs; RG J163655, RG J131236, and RG J123711) in CO(3-2) using the IRAM Plateau de Bure Interferometer to study their gas and dynamical properties. We estimate the molecular gas mass in each of the three SFRGs (8.3 × 109, <5.6 × 109, and 15.4 × 109 M⊙, respectively) and, in the case of RG J163655, a dynamical mass by measurement of the width of the CO(3-2) line (8 × 1010csc2i M⊙). While these gas masses are substantial, on average they are 4 times lower than submillimeter-selected galaxies (SMGs). Radio-inferred star formation rates ( = 970 M⊙ yr-1) suggest much higher star formation efficiencies than are found for SMGs and shorter gas depletion timescales (~11 Myr), much shorter than the time required to form their current stellar masses (~160 Myr; ~1011 M⊙). By contrast, star formation rates (SFRs) may be overestimated by factors of a few, bringing the efficiencies in line with those typically measured for other ultraluminous star-forming galaxies and suggesting that SFRGs are more like ultraviolet-selected (UV-selected) star-forming galaxies with enhanced radio emission. A tentative detection of RG J163655 at 350 μm suggests hotter dust temperatures, and thus gas-to-dust mass fractions, similar to the SMGs.
2003
We report IRAM Plateau de Bure, millimeter interferometry of three z=~2.4 to 3.4, SCUBA deep field galaxies. Our CO line observations confirm the rest-frame UV/optical redshifts, thus more than doubling the number of confirmed, published redshifts of the faint submillimeter population and proving their high-z nature. In all three sources our measurements of the intrinsic gas and dynamical mass are large (1e10 to 1e11 Msun). In at least two cases the data show that the submillimeter sources are part of an interacting system. Together with recent information gathered in the X-ray, optical and radio bands our observations support the interpretation that the submm-population consists of gas rich (gas to dynamical mass ratio ~0.5) and massive, composite starburst/AGN systems, which are undergoing a major burst of star formation and are evolving into m*-galaxies.
The Astrophysical …, 2011
We explore the gas-to-dust mass ratio (M gas /M d ) and the CO luminosityto-M gas conversion factor (α CO ) of two well studied galaxies in the GOODS-N field, that are expected to have different star forming modes, the starburst GN20 at z = 4.05 and the normal star-forming galaxy BzK-21000 at z = 1.52. Detailed sampling is available for their Rayleigh-Jeans emission via ground based mm interferometry (1.1 − 6.6 mm) along with Herschel PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li models, as well as a modified black body function, we measure the dust mass (M dust ) of the sources and find (2.0 +0.7 −0.6 × 10 9 ) M ⊙ for GN20 and (8.6 +0.6 −0.9 × 10 8 ) M ⊙ for BzK-21000. The addition of mm data reduces the uncertainties of the derived M dust by a factor of ∼ 2, allowing the use of the local M gas /M d vs metallicity relation to place constraints on the α CO values of the -2two sources. For GN20 we derive a conversion factor of α CO < 1.0 M ⊙ pc −2 (K km s −1 ) −1 , consistent with that of local ULIRGs, while for BzK-21000 we find a considerably higher value, α CO ∼ 4.0 M ⊙ pc −2 (K km s −1 ) −1 , in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ∼ 25 L ⊙ /M ⊙ for BzK-21000, a factor of ∼ 5 − 10 lower than that of GN20. The findings for these two sources support the existence of different disk-like and starburst star-formation modes in distant galaxies, although a larger sample is required to draw statistically robust results.
Astronomy & Astrophysics, 2015
To extend the molecular gas measurements to more typical star-forming galaxies (SFGs) with star formation rates SFR < 40 M yr −1 and stellar masses M * < 2.5 × 10 10 M at z ∼ 1.5−3, we have observed CO emission with the IRAM Plateau de Bure Interferometer and the IRAM 30 m telescope for five strongly lensed galaxies, selected from the Herschel Lensing Survey. These observations are combined with a compilation of CO measurements from the literature. From this, we infer the CO luminosity correction factors r 2,1 = 0.81 ± 0.20 and r 3,1 = 0.57 ± 0.15 for the J = 2 and J = 3 CO transitions, respectively, valid for SFGs at z > 1. The combined sample of CO-detected SFGs at z > 1 shows a large spread in star formation efficiency (SFE) with a dispersion of 0.33 dex, such that the SFE extends well beyond the low values of local spirals and overlaps the distribution of z > 1 submm galaxies. We find that the spread in SFE (or equivalently in molecular gas depletion timescale) is due to the variations of several physical parameters, primarily the specific star formation rate, and also stellar mass and redshift. The dependence of SFE on the offset from the main sequence and the compactness of the starburst is less clear. The possible increase of the molecular gas depletion timescale with stellar mass, now revealed by low M * SFGs at z > 1 and also observed at z = 0, contrasts with the generally acknowledged constant molecular gas depletion timescale and refutes the linearity of the Kennicutt-Schmidt relation. A net rise of the molecular gas fraction (f gas) is observed from z ∼ 0.2 to z ∼ 1.2, followed by a very mild increase toward higher redshifts, as found in earlier studies. At each redshift the molecular gas fraction shows a large dispersion, mainly due to the dependence of f gas on stellar mass, producing a gradient of increasing f gas with decreasing M *. We provide the first measurement of the molecular gas fraction of z > 1 SFGs at the low-M * end between 10 9.4 < M * /M < 10 9.9 , reaching a mean f gas = 0.69 ± 0.18, which shows a clear f gas upturn at these lower stellar masses. Finally, we find evidence for a nonuniversal dust-togas ratio among high-redshift SFGs, high-redshift submm galaxies, local spirals, and local ultraluminous IR galaxies with near-solar metallicities, as inferred from a homogeneous analysis of their rest-frame 850 μm luminosity per unit gas mass. The SFGs with z > 1 show a trend for a lower L ν (850 μm)/M gas mean by 0.33 dex compared to the other galaxy populations.
The Astrophysical Journal, 2013
We present PHIBSS, the IRAM Plateau de Bure high-z blue sequence CO 3-2 survey of the molecular gas properties in massive, main-sequence star-forming galaxies (SFGs) near the cosmic star formation peak. PHIBSS provides 52 CO detections in two redshift slices at z ∼ 1.2 and 2.2, with log(M * (M )) 10.4 and log(SFR(M /yr)) 1.5. Including a correction for the incomplete coverage of the M * -SFR plane, and adopting a "Galactic" value for the CO-H 2 conversion factor, we infer average gas fractions of ∼0.33 at z ∼ 1.2 and ∼0.47 at z ∼ 2.2. Gas fractions drop with stellar mass, in agreement with cosmological simulations including strong star formation feedback. Most of the z ∼ 1-3 SFGs are rotationally supported turbulent disks. The sizes of CO and UV/optical emission are comparable. The molecular-gas-star-formation relation for the z = 1-3 SFGs is near-linear, with a ∼0.7 Gyr gas depletion timescale; changes in depletion time are only a secondary effect. Since this timescale is much less than the Hubble time in all SFGs between z ∼ 0 and 2, fresh gas must be supplied with a fairly high duty cycle over several billion years. At given z and M * , gas fractions correlate strongly with the specific star formation rate (sSFR). The variation of sSFR between z ∼ 0 and 3 is mainly controlled by the fraction of baryonic mass that resides in cold gas.
The Astrophysical Journal, 2003
We report IRAM millimeter interferometry of three z ∼2.4 to 3.4, SCUBA deep field galaxies. Our CO line observations confirm the rest-frame UV/optical redshifts, thus more than doubling the number of confirmed, published redshifts of the faint submm-population and proving their high-z nature. In all three sources our measurements of the intrinsic gas and dynamical mass are large (10 10 to 10 11 M ⊙). In at least two cases the data show that the submm sources are part of an interacting system. Together with recent information gathered in the X-ray, optical and radio bands our observations support the interpretation that the submm-population, at least the radio detected ones, consists of gas rich (gas to dynamical mass ratio ∼0.5) and massive, interacting starburst/AGN systems.