Abstract
We present Atacama Large Millimeter Array C i(1 − 0) (rest frequency 492 GHz) observations for a sample of 13 strongly lensed dusty star-forming galaxies (DSFGs) originally discovered at ...1.4 mm in a blank-field survey by the South Pole Telescope (SPT). We compare these new data with available C i observations from the literature, allowing a study of the interstellar medium (ISM) properties of ∼30 extreme DSFGs spanning a redshift range 2 < z < 5. Using the C i line as a tracer of the molecular ISM, we find a mean molecular gas mass for SPT-DSFGs of 6.6 × 1010 M⊙. This is in tension with gas masses derived via low-J
12CO and dust masses; bringing the estimates into accordance requires either (a) an elevated CO-to-H2 conversion factor for our sample of αCO ∼ 2.5 and a gas-to-dust ratio ∼200, or (b) an high carbon abundance
$X_{\rm C\,\small {I}} \sim 7\times 10^{-5}$
. Using observations of a range of additional atomic and molecular lines (including C i, C iiand multiple transitions of CO), we use a modern photodissociation region code (3d-pdr) to assess the physical conditions (including the density, UV radiation field strength and gas temperature) within the ISM of the DSFGs in our sample. We find that the ISM within our DSFGs is characterized by dense gas permeated by strong UV fields. We note that previous efforts to characterize photodissociation region regions in DSFGs may have significantly under-estimated the density of the ISM. Combined, our analysis suggests that the ISM of extreme dusty starbursts at high redshift consists of dense, carbon-rich gas not directly comparable to the ISM of starbursts in the local Universe.
We present subkiloparsec-scale mapping of the 870 m ALMA continuum emission in six luminous (LIR ∼ 5 × 1012 L ) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field ...South. Our high-fidelity 0 07-resolution imaging (∼500 pc) reveals robust evidence for structures with deconvolved sizes of 0.5-1 kpc embedded within (dominant) exponential dust disks. The large-scale morphologies of the structures within some of the galaxies show clear curvature and/or clump-like structures bracketing elongated nuclear emission, suggestive of bars, star-forming rings, and spiral arms. In this interpretation, the ratio of the "ring" and "bar" radii (1.9 0.3) agrees with that measured for such features in local galaxies. These potential spiral/ring/bar structures would be consistent with the idea of tidal disturbances, with their detailed properties implying flat inner rotation curves and Toomre-unstable disks (Q < 1). The inferred one-dimensional velocity dispersions ( r 70-160 km s−1) are marginally consistent with the limits implied if the sizes of the largest structures are comparable to the Jeans length. We create maps of the star formation rate density ( SFR) on ∼500 pc scales and show that the SMGs are able to sustain a given (galaxy-averaged) SFR over much larger physical scales than local (ultra)luminous infrared galaxies. However, on 500 pc scales, they do not exceed the Eddington limit set by radiation pressure on dust. If confirmed by kinematics, the potential presence of nonaxisymmetric structures would provide a means for net angular momentum loss and efficient star formation, helping to explain the very high star formation rates measured in SMGs.
ABSTRACT We explore the recoverability of gas physical conditions with the Large Velocity Gradient (LVG) model, using the public code RADEX and the molecules HCN and CO. Examining a wide parameter ...range with a series of models of increasing complexity, we use both grid and Monte Carlo Markov Chain methods to recover the input conditions, and quantify the inherent and noise-induced uncertainties in the model results. We find that even with the benefit of generous assumptions, the LVG models struggle to recover any parameter better than to within half a dex, although we find no evidence of systemic offsets. Examining isotopologue lines, we demonstrate that it is always preferable to model the isotopologue abundance ratio as a free parameter due to large biases introduced in all other parameters when an incorrect ratio is assumed. Finally, we explore the effects of the background radiation temperature on CO and HCN line ratios, with an emphasis on the effect of the CMB at , and show that while the effect on the line ratios is minor, the effect on the spectral line energy distribution peak is significant and that the CO(1-0) line luminosity to H2 mass conversion factor ( CO) needs to be altered to account for the loss of contrast against the hotter CMB as redshift increases.
We present the results from a survey of 12CO emission in 40 luminous sub-millimetre galaxies (SMGs), with 850-μm fluxes of S
850 μm = 4-20 mJy, conducted with the Plateau de Bure Interferometer. We ...detect 12CO emission in 32 SMGs at z ∼ 1.2-4.1, including 16 SMGs not previously published. Using multiple 12CO line (J
up = 2-7) observations, we derive a median spectral line energy distribution for luminous SMGs. We report the discovery of a fundamental relationship between 12CO FWHM and 12CO line luminosity in high-redshift starbursts, which we interpret as a natural consequence of the baryon-dominated dynamics within the regions probed by our observations. We use far-infrared luminosities to assess the star formation efficiency in our SMGs, finding that the slope of the L′CO-L
FIR relation is close to linear. We derive molecular gas masses, finding a mean gas mass of (5.3 ± 1.0) × 1010 M. Combining these with dynamical masses, we determine the redshift evolution of the gas content of SMGs, finding that they do not appear to be significantly more gas rich than less vigorously star-forming galaxies at high redshifts. Finally, we collate X-ray observations, and study the interdependence of gas and dynamical properties of SMGs with their AGN activity and supermassive black hole masses (M
BH), finding that SMGs lie significantly below the local M
BH-σ relation.
Understanding how super-massive black holes form and grow in the early Universe has become a major challenge
since it was discovered that luminous quasars existed only 700 million years after the Big ...Bang
. Simulations indicate an evolutionary sequence of dust-reddened quasars emerging from heavily dust-obscured starbursts that then transition to unobscured luminous quasars by expelling gas and dust
. Although the last phase has been identified out to a redshift of 7.6 (ref.
), a transitioning quasar has not been found at similar redshifts owing to their faintness at optical and near-infrared wavelengths. Here we report observations of an ultraviolet compact object, GNz7q, associated with a dust-enshrouded starburst at a redshift of 7.1899 ± 0.0005. The host galaxy is more luminous in dust emission than any other known object at this epoch, forming 1,600 solar masses of stars per year within a central radius of 480 parsec. A red point source in the far-ultraviolet is identified in deep, high-resolution imaging and slitless spectroscopy. GNz7q is extremely faint in X-rays, which indicates the emergence of a uniquely ultraviolet compact star-forming region or a Compton-thick super-Eddington black-hole accretion disk at the dusty starburst core. In the latter case, the observed properties are consistent with predictions from cosmological simulations
and suggest that GNz7q is an antecedent to unobscured luminous quasars at later epochs.
ABSTRACT We study the global star formation law-the relation between gas and star formation (SF) rates-in a sample of 181 local galaxies with infrared (IR) luminosities spanning almost five orders of ...magnitude ( ), which includes 115 normal spiral galaxies and 66 (ultra)luminous IR galaxies ((U)LIRGs, ). We derive their atomic, molecular gas, and dense molecular gas masses using newly available Hi, CO, and HCN data from the literature, and SF rates are determined both from total IR ( ) and 1.4 GHz radio continuum (RC) luminosities. In order to derive the disk-averaged surface densities of gas and SF rates, we have taken a novel approach and used high-resolution RC observations to measure the radio sizes for all 181 galaxies. In our sample, we find that the surface density of dense molecular gas (as traced by HCN) has the tightest correlation with that of SF rates ( SFR), and is linear in log-log space (power-law slope of N = 1.01 0.02) across the full galaxy sample. The correlation between surface densities of molecular gas ( , traced by CO) and SFR is sensitive to the adopted value of the CO-to- conversion factor ( CO) used to infer molecular gas masses from CO luminosities. For a fixed Galactic value of CO, a power law index of 1.14 0.02 is found. If instead we adopt values for CO of 4.6 and 0.8 for disk galaxies and (U)LIRGs, respectively, we find the two galaxy populations separate into two distinct SFR versus relations. Finally, applying a continuously varying CO to our sample, we recover a single SFR- relation with slope of 1.60 0.03. The SFR is a steeper function of total gas gas (molecular gas with atomic gas) than that of molecular gas , and are tighter among low-luminosity galaxies. We find no correlation between global surface densities of SFRs and atomic gas (H i).
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 near-infrared 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 CO-detected 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) × 1010 K km s-1 pc2. Using a CO-to-H2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass 〈M(H2)〉= (3.0 ± 1.6) × 1010 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 × 1010 M⊙) and star formation rates (≳ 700 M⊙ yr−1), we estimate a lower limit on the typical gas-depletion time-scale 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 time-scale. 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 〈Mdyn〉≃ (1.2 ± 1.5) × 1011 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 z∼ 2–3.5, which is in agreement with results from recent spectroscopic surveys and the most recent model predictions.
ABSTRACT We present high-resolution (0 16) 870 m Atacama Large Millimeter/submillimeter Array (ALMA) imaging of 16 luminous ( ) submillimeter galaxies (SMGs) from the ALESS survey of the Extended ...Chandra Deep Field South. This dust imaging traces the dust-obscured star formation in these galaxies on ∼1.3 kpc scales. The emission has a median effective radius of Re = 0 24 0 02, corresponding to a typical physical size of 1.8 0.2 kpc. We derive a median Sérsic index of n = 0.9 0.2, implying that the dust emission is remarkably disk-like at the current resolution and sensitivity. We use different weighting schemes with the visibilities to search for clumps on 0 12 (∼1.0 kpc) scales, but we find no significant evidence for clumping in the majority of cases. Indeed, we demonstrate using simulations that the observed morphologies are generally consistent with smooth exponential disks, suggesting that caution should be exercised when identifying candidate clumps in even moderate signal-to-noise ratio interferometric data. We compare our maps to comparable-resolution Hubble Space Telescope -band images, finding that the stellar morphologies appear significantly more extended and disturbed, and suggesting that major mergers may be responsible for driving the formation of the compact dust disks we observe. The stark contrast between the obscured and unobscured morphologies may also have implications for SED fitting routines that assume the dust is co-located with the optical/near-IR continuum emission. Finally, we discuss the potential of the current bursts of star formation to transform the observed galaxy sizes and light profiles, showing that the descendants of these SMGs are expected to have stellar masses, effective radii, and gas surface densities consistent with the most compact massive ( 1-2 × 1011 ) early-type galaxies observed locally.
We report the results of a pilot study with the Expanded Very Large Array (EVLA) of 12CO J= 1-0 emission from four submillimetre-selected galaxies at z= 2.2-2.5, each with an existing detection of ...12CO J= 3-2, one of which comprises two distinct spatial components. Using the EVLA's most compact configuration, we detect strong, broad medians: 990 km s−1 full width at zero intensity; 540 km s−1 full width at half-maximum (FWHM)J= 1-0 line emission from all of our targets - coincident in position and velocity with their J= 3-2 emission. The median linewidth ratio, σ1-0/σ3-2= 1.15 ± 0.06, suggests that the J= 1-0 is more spatially extended than the J= 3-2 emission, a situation confirmed by our maps which reveal velocity structure in several cases and typical sizes of ∼16 kpc FWHM. The median brightness temperature (T
b) ratio is r
3-2/1-0= 0.55 ± 0.05, consistent with local galaxies with L
IR > 1011 L⊙, noting that our value may be biased high because of the J= 3-2 based sample selection. Naively, this suggests gas masses roughly two times higher than estimates made using higher J transitions of CO, with the discrepancy due entirely to the difference in assumed T
b ratio. We also estimate molecular gas masses using the 12CO J= 1-0 line and the observed global T
b ratios, assuming standard underlying T
b ratios for the non-star-forming and star-forming gas phases as well as a limiting star formation efficiency for the latter in all systems, i.e. without calling upon X
CO (≡α). Using this new method, we find a median molecular gas mass of (2.5 ± 0.8) × 1010 M⊙, with a plausible range stretching up to three times higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass within R∼ 7 kpc for our sample is (2.3 ± 1.4) × 1011 M⊙ or ∼6 times more massive than UV-selected galaxies at this epoch. We examine the Schmidt-Kennicutt (S-K) relation for all the distant galaxy populations for which CO J= 1-0 or J= 2-1 data are available, finding small systematic differences between galaxy populations. These have previously been interpreted as evidence for different modes of star formation, but we argue that these differences are to be expected, given the still considerable uncertainties, certainly when considering the probable excitation biases due to the molecular lines used, and the possibility of sustained S-K offsets during the evolution of individual gas-rich systems. Finally, we discuss the morass of degeneracies surrounding molecular gas mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts.
The South Pole Telescope (SPT) has systematically identified 81 high-redshift, strongly gravitationally lensed, dusty star-forming galaxies (DSFGs) in a 2500 square degree cosmological ...millimeter-wave survey. We present the final spectroscopic redshift survey of this flux-limited (S870 m > 25 mJy) sample, initially selected at 1.4 mm. The redshift survey was conducted with the Atacama Large Millimeter/submillimeter Array across the 3 mm spectral window, targeting carbon monoxide line emission. By combining these measurements with ancillary data, the SPT sample is now spectroscopically complete, with redshifts spanning 1.9 < z < 6.9 and a median of . We present the millimeter through far-infrared photometry and spectral energy density fits for all sources, along with their inferred intrinsic properties. Comparing the properties of the SPT sources to the unlensed DSFG population, we demonstrate that the SPT-selected DSFGs represent the most extreme infrared-luminous galaxies, even after accounting for strong gravitational lensing. The SPT sources have a median star formation rate of and a median dust mass of . However, the inferred gas depletion timescales of the SPT sources are comparable to those of unlensed DSFGs, once redshift is taken into account. This SPT sample contains roughly half of the known spectroscopically confirmed DSFGs at z > 5, making this the largest sample of high-redshift DSFGs to date, and enabling the "high-redshift tail" of extremely luminous DSFGs to be measured. Though galaxy formation models struggle to account for the SPT redshift distribution, the larger sample statistics from this complete and well-defined survey will help inform future theoretical efforts.
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