Tracing the molecular gas in distant submillimetre galaxies via CO(1-0) imaging with the EVLA

By R. Ivison, P. Papadopoulos, I. Smail, T. Greve, A. Thomson, E. Xilouris, Stephen Chapman. 2010

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We report the results of a pilot study with the Expanded Very Large Array (EVLA) of 12 CO J = 1 0 emission from four submillimetre-selected galaxies (SMGs) at z = 2.2‐2.5, each with an existing detection of 12 CO J = 3 2, one of which comprises two distinct spatial components. Using the EVLA’s most compact configuration we d etect strong, broad (medians: 990 km s −1 FWZI; 540 km s −1 FWHM) J = 1 0 line emission from all of our targets ‐ coincident in position and velocity with their J = 3 2 emission. The median line width ratio, �1−0/�3−2 = 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 velo city structure in several cases and typical sizes of �16 kpc FWHM. The median brightness temperature (Tb) ratio is r3−2/1−0 = 0.55 ± 0.05, consistent with local galaxies with LIR > 10 11 L� , noting that our value may be biased high because of the J =3 2-based sample selection. Naively, this suggests gas masses roughly 2× higher than estimates made using higher-J transitions of CO, with the discrepency due entirely to the difference in assumed Tb ratio. We also estimate molecular gas masses using the 12 CO J =1 0 line and the observed global Tb ratios, assuming standard underlying Tb ratios for the non-star-forming and star-forming gas phases as well as a limiting star-formation efficiency (SFE) for the latter in all systems, i.e. without calling upon XCO (� �). Using this new method, we find a median molecular gas mass of (2.5±0.8)×10 10 M� , with a plausible range stretching up to 3× 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) × 10 11 M� , or �6× 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 p opulations. These have previously been interpreted as evidence for different modes of star for mation, but we argue that these differences are to be expected, given the still considerabl e 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/f94e3f8340274a68eec21bf0fbc6e0322270f29b

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Citations in the corpus

REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “…observations of SFGs have generally explicitly targeted either strong SBs (e.g., Solomon et al. 1997, Greve et al. 2005, Riechers et al. 2006, Ivison et al. 2011) or MS galaxies (e.g., Leroy et al. 2008, Daddi et al. 2010a, Tacconi et al. 2010, Geach et al. 2011, Magdis et al. 2012a, Tacconi…”
ALMA REDSHIFTS OF MILLIMETER-SELECTED GALAXIES FROM THE SPT SURVEY: THE REDSHIFT DISTRIBUTION OF DUSTY STAR-FORMING GALAXIES (2013, background): “…diameter) has been found in some DSFGs out to redshift z≈4 (e.g., Genzel et al. 2003; Ivison et al. 2010; Younger et al. 2010; Carilli et al. 2011; Ivison et al. 2011; Riechers et al. 2011b) while the molecular gas distribution in IR luminous AGN host galaxies, which have been measured out to…”
CO excitation of normal star forming galaxies out to $z=1.5$ as regulated by the properties of their interstellar medium (2014, background): “Comparing to SMGs, for which Ivison et al. 2011 report R31 = 0.55± 0.05 and Bothwell et al 2013 R31 = 0.52± 0.09, we see that the normal MS galaxies atz = 1.5 appear to have somewhat less excited CO SLEDs (R31= 0.42± 0.07).”
A survey of molecular gas in luminous sub-millimetre galaxies (2012, methodology): “Also shown in the plot are the 12CO (1–0) observations for SMGs by Ivison et al. (2011).”
Cool Gas in High-Redshift Galaxies (2013, background): “4.3.3 Other fine structure lines There have been only a few detections of fine structure lines other than [C II] at high redshift to date, mostly in strongly lensed systems using principally the PdBI and Herschel.”
OBSERVATIONS OF Arp 220 USING HERSCHEL-SPIRE: AN UNPRECEDENTED VIEW OF THE MOLECULAR GAS IN AN EXTREME STAR FORMATION ENVIRONMENT (2011, background): “More recently, EVLA observations of five high-z quasar host galaxies (Riechers et al. 2011; Ivison et al. 2011) indicate that SMGs with AGN show no cold gas component; i.e., the CO J = 1-0 line strengths in these five systems are consistent with warm, highly excited gas that is also seen in the…”
ISM MASSES AND THE STAR FORMATION LAW AT Z = 1 TO 6: ALMA OBSERVATIONS OF DUST CONTINUUM IN 145 GALAXIES IN THE COSMOS SURVEY FIELD (2015, unknown): “…line fluxes from: (1 Harris et al. 2012), (2 Riechers et al. 2011), (3 Lestrade et al. 2011), (4 Thomson et al. 2015), (5 Aravena et al. 2013), (6 Ivison et al. 2011), (7 Carilli et al. 2011) , (8 Greve et al. 2003),(9 Thomson et al. 2012), (10 Harris et al. 2010),(11 Ivison et al. 2013) and…”
HerMES: CANDIDATE GRAVITATIONALLY LENSED GALAXIES AND LENSING STATISTICS AT SUBMILLIMETER WAVELENGTHS (2012, methodology): “The data were taken as part of a larger follow-up program of lensed Herschel SMGs (program 11A-182; PI: R. Ivison) and were reduced using aips following the procedures described in Ivison et al. (2011), to achieve a resolution of < 0.25′′ (FWHM) in both cases.”
THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, methodology): “The source has exquisite photometric coverage from rest-frame UV to radio including SPIRE broadband data (Swinbank et al. 2010 Table 1 and Ivison et al. 2010 Table 1).”
The CO-to-H2 Conversion Factor (2013, background): “In their SMG sample, Ivison et al. (2011) find XCO,20 ∼ 0.9 − 2.3 to be compatible with the observed star formation histories and dynamical and stellar masses.”
The stellar masses and specific star-formation rates of submillimetre galaxies (2011, methodology): “…while for the remaining objects this number has been inferred from the higher-order CO-line dynamical masses by multiplying them by the average correction factor for the aforementioned 4 SMGs for which the dynamical masses of Ivison et al. (2011) and Engel et al. (2010) can be directly compared.”
SUBMILLIMETER GALAXIES AS PROGENITORS OF COMPACT QUIESCENT GALAXIES (2014, result): “We note however that studies of lower-J CO lines suggest that some SMGs may have more extended CO-disks (Ivison et al. 2011; Riechers et al. 2011a).”
GRAVITATIONAL LENS MODELS BASED ON SUBMILLIMETER ARRAY IMAGING OF HERSCHEL-SELECTED STRONGLY LENSED SUB-MILLIMETER GALAXIES AT z > 1.5 (2013, background): “…are sub-arcsecond observations of CO emission in a handful of SMGs which show that a significant fraction of SMGs that are spatially resolved have multiple, similar mass components—i.e., they are major mergers (Tacconi et al. 2008; Engel et al. 2010; Ivison et al. 2011; Riechers et al. 2011b).”
GAS AND DUST IN A SUBMILLIMETER GALAXY AT z = 4.24 FROM THE HERSCHEL ATLAS (2011, background): “Additional CO transitions, in particular the low-J transitions, should be observed to evaluate more precisely the density and the temperature of the molecular gas and check whether an excess of cold molecular gas could be present (Ivison et al. 2011).”
THE INTERSTELLAR MEDIUM IN DISTANT STAR-FORMING GALAXIES: TURBULENT PRESSURE, FRAGMENTATION, AND CLOUD SCALING RELATIONS IN A DENSE GAS DISK AT z = 2.3 (2011, methodology): “The disk is massively unstable, with a toomre parameter, Q = 0.50±0.15, which is lower than the compact, gas-rich disks residing in local ULIRGs where Q is closer to unity (Downes & Solomon 1998).”
EVIDENCE FOR A CLUMPY, ROTATING GAS DISK IN A SUBMILLIMETER GALAXY AT z = 4 (2012, background): “Indeed, there is direct evidence for multiple CO components and/or disturbed kinematics in some SMGs, supporting the merger picture (e.g., Tacconi et al. 2008; Engel et al. 2010; Ivison et al. 2011; Riechers et al. 2011b).”
KILOPARSEC-SCALE DUST DISKS IN HIGH-REDSHIFT LUMINOUS SUBMILLIMETER GALAXIES (2016, background): “This last theory may be at odds with claims that normal (BM/BX, BzK) high–redshift star–forming galaxies seem to follow a different sequence than SMGs on the Mgas/LIR plane (Genzel et al. e.g., 2010; Daddi et al. e.g., 2010; Hodge et al. e.g., 2015; although see Ivison et al. 2011).”
Evolution of Interstellar Medium, Star Formation, and Accretion at High Redshift (2017, result): “…estimates are consistent with existing studies using the CO lines (Tacconi et al. 2010; Daddi et al. 2010; Genzel et al. 2010; Riechers et al. 2011; Ivison et al. 2011; Magdis et al. 2012b; Saintonge et al. 2013; Carilli & Walter 2013; Tacconi et al. 2013; Bolatto et al. 2015; Genzel et al. 2015),…”
Cosmic evolution of the atomic and molecular gas contents of galaxies (2011, background): “However, recent observations have shown different brightness temperatures for different CO transitions in some high-redshift galaxies (Danielson et al. 2010; Ivison et al. 2011).”