246 research outputs found
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)
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
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
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
Imaging the Molecular Gas in A z=3.9 Quasar Host Galaxy at 0\farcs3 Resolution: A Central, Sub-Kilparsex Scale Star Formation Reservoir in APM 08279+5255
We have mapped the molecular gas content in the host galaxy of the strongly lensed high-redshift quasar APM 08279+5255 (z = 3.911) with the Very Large Array at 0\farcs3 resolution. The CO(J = 1➝0) emission is clearly resolved in our maps. The CO(J = 1➝0) line luminosity derived from these maps is in good agreement with a previous single-dish measurement. In contrast to previous interferometer-based studies, we find that the full molecular gas reservoir is situated in two compact peaks separated by ≲0\farcs4. Our observations reveal, for the first time, that the emission from cold molecular gas is virtually co-spatial with the optical/near-infrared continuum emission of the central active galactic nucleus (AGN) in this source. This striking similarity in morphology indicates that the molecular gas is situated in a compact region close to the AGN. Based on the high-resolution CO maps, we present a revised model for the gravitational lensing in this system, which indicates that the molecular gas emission is magnified by only a factor of 4 (in contrast to previously suggested factors of 100). This model suggests that the CO is situated in a circumnuclear disk of ~550 pc radius that is possibly seen at an inclination of ≲25°, i.e., relatively close to face-on. From the CO luminosity, we derive a molecular gas mass of Mgas = 1.3x10^11 M☉ for this galaxy. From the CO structure and linewidth, we derive a dynamical mass of M dyn sin^2 i = 4.0x10^10 M☉. Based on a revised mass estimate for the central black hole of Mbh = 2.3x10^10 M☉ and the results of our molecular line study, we find that the mass of the stellar bulge of APM 08279+5255 falls short of the local M BH-σbulge relationship of nearby galaxies by more than an order of magnitude, lending support to recent suggestions that this relation may evolve with cosmic time and/or change toward the high-mass end
Extended Cold Molecular Gas Reservoirs in z~3.4 Submillimeter Galaxies
We report the detection of spatially resolved CO(1-0) emission in the z~3.4
submillimeter galaxies (SMGs) SMM J09431+4700 and SMM J13120+4242, using the
Expanded Very Large Array (EVLA). SMM J09431+4700 is resolved into the two
previously reported millimeter sources H6 and H7, separated by ~30kpc in
projection. We derive CO(1-0) line luminosities of L'(CO 1-0) = (2.49+/-0.86)
and (5.82+/-1.22) x 10^10 K km/s pc^2 for H6 and H7, and L'(CO 1-0) =
(23.4+/-4.1) x 10^10 K km/s pc^2 for SMM J13120+4242. These are ~1.5-4.5x
higher than what is expected from simple excitation modeling of higher-J CO
lines, suggesting the presence of copious amounts of low-excitation gas. This
is supported by the finding that the CO(1-0) line in SMM J13120+4242, the
system with lowest CO excitation, appears to have a broader profile and more
extended spatial structure than seen in higher-J CO lines (which is less
prominently seen in SMM J09431+4700). Based on L'(CO 1-0) and excitation
modeling, we find M_gas = 2.0-4.3 and 4.7-12.7 x 10^10 Msun for H6 and H7, and
M_gas = 18.7-69.4 x 10^10 Msun for SMM J13120+4242. The observed CO(1-0)
properties are consistent with the picture that SMM J09431+4700 represents an
early-stage, gas-rich major merger, and that SMM J13120+4242 represents such a
system in an advanced stage. This study thus highlights the importance of
spatially and dynamically resolved CO(1-0) observations of SMGs to further
understand the gas physics that drive star formation in these distant galaxies,
which becomes possible only now that the EVLA rises to its full capabilities.Comment: 6 pages, 4 figures, to appear in ApJL (EVLA Special Issue; accepted
May 19, 2011
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