Driving the Gaseous Evolution of Massive Galaxies in the Early Universe

Studies of the molecular interstellar medium that fuels star formation and supermassive black hole growth in galaxies at cosmological distances have undergone tremendous progress over the past few years. Based on the detection of molecular gas in >120 galaxies at z=1 to 6.4, we have obtained detailed insight on how the amount and physical properties of this material in a galaxy are connected to its current star formation rate over a range of galaxy populations. Studies of the gas dynamics and morphology at high spatial resolution allow us to distinguish between gas-rich mergers in different stages along the "merger sequence" and disk galaxies. Observations of the most massive gas-rich starburst galaxies out to z>5 provide insight into the role of cosmic environment for the early growth of present-day massive spheroidal galaxies. Large-area submillimeter surveys have revealed a rare population of extremely far-infrared-luminous gas-rich high-redshift objects, which is dominated by strongly lensed, massive starburst galaxies. These discoveries have greatly improved our understanding of the role of molecular gas in the evolution of massive galaxies through cosmic time.Comment: 8 pages, 6 figures, invited talk paper, to appear in ASP Conference Series, "Galaxy Mergers in an Evolving Universe", 23-28 October 2011, Hualien, Taiwa

Constraints on the Star-Forming Interstellar Medium in Galaxies Back to the First Billion Years of Cosmic Time

Constraints on the molecular gas content of galaxies at high redshift are crucial to further our understanding of star formation and galaxy evolution through cosmic times, as molecular gas is the fuel for star formation. Since its initial detection at large cosmic distances almost two decades ago, studies of molecular gas in the early universe have come a long way. We have detected CO emission from >100 galaxies, covering a range of galaxy populations at z>1, reaching out to z>6, down to sub-kpc scale resolution, and spanning ~2 orders of magnitude in gas mass (aided by gravitational lensing). Recently, it has even become possible to directly identify distant galaxies through their molecular emission lines without prior knowledge of their redshifts. The new generation of powerful long wavelength interferometers such as the Expanded Very Large Array (EVLA) and Atacama Large (sub)Millimeter Array (ALMA) thus hold the promise to liberate studies of molecular gas in high redshift galaxies from their heavy pre-selection. This will enable more systematic studies of the molecular gas content in star-forming galaxies back to the earliest cosmic times.Comment: 12 pages, 6 figures, invited talk paper, to appear in ASP Conference Series, "Galaxy Evolution: Infrared to Millimeter Wavelength Perspective", 25-29 October 2010, Guilin, Chin

Molecular Gas in Lensed z >2 Quasar Host Galaxies and the Star Formation Law for Galaxies with Luminous Active Galactic Nuclei

We report the detection of luminous CO(J = 2→1), CO(J = 3→2), and CO(J = 4→3) emission in the strongly lensed high-redshift quasars B1938+666 (z = 2.059), HE 0230-2130 (z = 2.166), HE 1104-1805 (z = 2.322), and B1359+154 (z = 3.240), using the Combined Array for Research in Millimeter-wave Astronomy. B1938+666 was identified in a "blind" CO redshift search, demonstrating the feasibility of such investigations with millimeter interferometers. These galaxies are lensing-amplified by factors of μ_L ≃ 11-170, and thus allow us to probe the molecular gas in intrinsically fainter galaxies than currently possible without the aid of gravitational lensing. We report lensing-corrected intrinsic CO line luminosities of L'_(CO) = 0.65-21×10^9 K km s^(-1) pc^2, translating to H_2 masses of M(H_2) = 0.52-17 × 10^9 (α_(CO)/0.8) M_☉. To investigate whether or not the active galactic nucleus (AGN) in luminous quasars substantially contributes to L FIR, we study the L'_(CO)-L_(FIR) relation for quasars relative to galaxies without a luminous AGN as a function of redshift. We find no substantial differences between submillimeter galaxies and high-z quasars, but marginal evidence for an excess in L_(FIR) in nearby low-L FIR AGN galaxies. This may suggest that an AGN contribution to L_(FIR) is significant in systems with relatively low gas and dust content, but only minor in the most far-infrared-luminous galaxies (in which L_(FIR) is dominated by star formation)

Dense Molecular Gas Excitation in Nuclear Starbursts at High Redshift: HCN, HNC, and HCO+(J=6-5) Emission in the z=3.91 Quasar Host of APM08279+5255

We report the detection of surprisingly strong HCN, HNC, and HCO+(J=6-5) emission in the host galaxy of the z=3.91 quasar APM08279+5255 through observations with CARMA. HCN, HNC, and HCO+ are typically used as star formation indicators, tracing dense molecular hydrogen gas [n(H2) > 10^5,cm^-3] within star-forming molecular clouds. However, the strength of their respective line emission in the J=6-5 transitions in APM08279+5255 is extremely high, suggesting that they are excited by another mechanism besides collisions in the dense molecular gas phase alone. We derive J=6-5 line luminosities of L'(HCN)=(4.9+/-0.6), L'(HNC)=(2.4+/-0.7), and L'(HCO+)=(3.0+/-0.6)x10^10 (mu_L)^-1 K km/s pc^2 (where mu_L is the lensing magnification factor), corresponding to L' ratios of ~0.23-0.46 relative to CO(J=1-0). Such high line ratios would be unusual even in the respective ground-state (J=1-0) transitions, and indicate exceptional, collisionally and radiatively driven excitation conditions in the dense, star-forming molecular gas in APM08279+5255. Through an expansion of our previous modeling of the HCN line excitation in this source, we show that the high rotational line fluxes are caused by substantial infrared pumping at moderate opacities in a ~220K warm gas and dust component. This implies that standard M_dense/L' conversion factors would substantially overpredict the dense molecular gas mass M_dense. We also find a HCN J=6-5/5-4 L' ratio greater than 1 (1.36+/-0.31) - however, our models show that the excitation is likely not `super-thermal', but that the high line ratio is due to a rising optical depth between both transitions. These findings are consistent with the picture that the bulk of the gas and dust in this source is situated in a compact, nuclear starburst, where both the highly active galactic nucleus and star formation contribute to the heating.Comment: 8 pages, 5 figures, to appear in ApJ (accepted October 8, 2010

Blind Detections of CO J = 1–0 in 11 H-ATLAS Galaxies at z = 2.1–3.5 with the GBT/Zpectrometer

We report measurements of the carbon monoxide ground state rotational transition (^(12)C^(16)O J = 1-0) with the Zpectrometer ultrawideband spectrometer on the 100 m diameter Green Bank Telescope. The sample comprises 11 galaxies with redshifts between z = 2.1 and 3.5 from a total sample of 24 targets identified by Herschel-ATLAS photometric colors from the SPIRE instrument. Nine of the CO measurements are new redshift determinations, substantially adding to the number of detections of galaxies with rest-frame peak submillimeter emission near 100 μm. The CO detections confirm the existence of massive gas reservoirs within these luminous dusty star-forming galaxies (DSFGs). The CO redshift distribution of the 350 μm selected galaxies is strikingly similar to the optical redshifts of 850 μm-selected submillimeter galaxies in 2.1 ≤ z ≤ 3.5. Spectroscopic redshifts break a temperature-redshift degeneracy; optically thin dust models fit to the far-infrared photometry indicate characteristic dust temperatures near 34 K for most of the galaxies we detect in CO. Detections of two warmer galaxies, and statistically significant nondetections, hint at warmer or molecule-poor DSFGs with redshifts that are difficult to determine from Herschel-SPIRE photometric colors alone. Many of the galaxies identified by H-ATLAS photometry are expected to be amplified by foreground gravitational lenses. Analysis of CO linewidths and luminosities provides a method for finding approximate gravitational lens magnifications μ from spectroscopic data alone, yielding μ ~ 3-20. Corrected for magnification, most galaxy luminosities are consistent with an ultraluminous infrared galaxy classification, but three are candidate hyper-LIRGs with luminosities greater than 10^(13) L_☉

Dense Molecular Gas Excitation at High Redshift: Detection of HCO+(J=4-3) Emission in the Cloverleaf Quasar

We report the detection of HCO+(J=4-3) emission in the Cloverleaf Quasar at z=2.56, using the IRAM Plateau de Bure Interferometer. HCO+ emission is a star formation indicator similar to HCN, tracing dense molecular hydrogen gas (n(H2) ~= 10^5 cm^-3) within star-forming molecular clouds. We derive a lensing-corrected HCO+(J=4-3) line luminosity of L'(HCO+(4-3)) = (1.6+/-0.3) x 10^9 (mu_L/11)^-1 K km/s pc^2, which corresponds to only 48% of the HCO+(J=1=0) luminosity, and <~4% of the CO(J=3-2) luminosity. The HCO+ excitation thus is clearly subthermal in the J=4-3 transition. Modeling of the HCO+ line radiative transfer suggests that the HCO+ emission emerges from a region with physical properties comparable to that exhibiting the CO line emission, but 2x higher gas density. This suggests that both HCO+ and CO lines trace the warm, dense molecular gas where star formation actively takes place. The HCO+ lines have only ~2/3 the width of the CO lines, which may suggest that the densest gas is more spatially concentrated. In contrast to the z=3.91 quasar APM08279+5255, the dense gas excitation in the Cloverleaf is consistent with being purely collisional, rather than being enhanced by radiative processes. Thus, the physical properties of the dense gas component in the Cloverleaf are consistent with those in the nuclei of nearby starburst galaxies. This suggests that the conditions in the dense, star-forming gas in active galactic nucleus-starburst systems at early cosmic times like the Cloverleaf are primarily affected by the starburst itself, rather than the central active black hole.Comment: 4 pages, 3 figures, to appear in ApJ (accepted November 3, 2010

SMM J04135+10277: A Candidate Early-stage "Wet-Dry" Merger of Two Massive Galaxies at z = 2.8

We report interferometric imaging of CO(J = 3→2) emission toward the z = 2.846 submillimeter-selected galaxy SMM J04135+10277, using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). SMM J04135+10277 was previously thought to be a gas-rich, submillimeter-selected quasar, with the highest molecular gas mass among high-z quasars reported in the literature. Our maps at ~6× improved linear resolution relative to earlier observations spatially resolve the emission on ~1."7 scales, corresponding to a (lensing-corrected) source radius of ~5.2 kpc. They also reveal that the molecular gas reservoir, and thus, likely the submillimeter emission, is not associated with the host galaxy of the quasar, but with an optically faint gas-rich galaxy at 5."2, or 41.5 kpc projected distance from the active galactic nucleus (AGN). The obscured gas-rich galaxy has a dynamical mass of M_(dyn) sin^2i = 5.6 × 10^(11) M_☉, corresponding to a gas mass fraction of ≃21%. Assuming a typical M_(BH)/M_* ratio for z ≳2 quasars, the two galaxies in this system have an approximate mass ratio of ~1.9. Our findings suggest that this quasar-starburst galaxy pair could represent an early stage of a rare major, gas-rich/gas-poor ("wet-dry") merger of two massive galaxies at z = 2.8, rather than a single, gas-rich AGN host galaxy. Such systems could play an important role in the early buildup of present-day massive galaxies through a submillimeter-luminous starburst phase, and may remain hidden in larger numbers among rest-frame far-infrared-selected quasar samples at low and high redshift

Formation of a Quasar Host Galaxy through a Wet Merger 1.4 Billion Years after the Big Bang

We present high-resolution Very Large Array imaging of the molecular gas in the host galaxy of the high redshift quasar BRI 1335-0417 (z=4.41). Our CO(2-1) observations have a linear resolution of 0.15" (1.0 kpc) and resolve the molecular gas emission both spatially and in velocity. The molecular gas in BRI 1335-0417 is extended on scales of 5 kpc, and shows a complex structure. At least three distinct components encompassing about two thirds of the total molecular mass of 9.2 x 10^10 M_sun are identified in velocity space, which are embedded in a structure that harbors about one third of the total molecular mass in the system. The brightest CO(2-1) line emission region has a peak brightness temperature of 61+/-9 K within 1 kpc diameter, which is comparable to the kinetic gas temperature as predicted from the CO line excitation. This is also comparable to the gas temperatures found in the central regions of nearby ultra-luminous infrared galaxies, which are however much more compact than 1 kpc. The spatial and velocity structure of the molecular reservoir in BRI 1335-0417 is inconsistent with a simple gravitationally bound disk, but resembles a merging system. Our observations are consistent with a major, gas-rich (`wet') merger that both feeds an accreting supermassive black hole (causing the bright quasar activity), and fuels a massive starburst that builds up the stellar bulge in this galaxy. Our study of this z>4 quasar host galaxy may thus be the most direct observational evidence that `wet' mergers at high redshift are related to AGN activity.Comment: 5 pages, 4 figures, to appear in ApJL (accepted August 27, 2008