A Search for Dense Molecular Gas in High‐Redshift Infrared‐Luminous Galaxies (original) (raw)
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HCN Observations of Dense Star-forming Gas in High-Redshift Galaxies
The Astrophysical Journal, 2007
We present here the sensitive HCN(1-0) observations made with the VLA of two submillimeter galaxies and two QSOs at high-redshift. HCN emission is the signature of dense molecular gas found in GMC cores, the actual sites of massive star formation. We have made the first detection of HCN in a submillimeter galaxy, SMM J16359+6612. The HCN emission is seen with a signal to noise ratio of 4σ and appears to be resolved as a double-source of < ∼ 2 ′′ separation. Our new HCN observations, combined with previous HCN detections and upper limits, show that the FIR/HCN ratios in these high redshift sources lie systematically above the FIR/HCN correlation established for nearby galaxies by about a factor of 2. Even considering the scatter in the data and the presence of upper limits, this is an indication that the FIR/HCN ratios for the early Universe molecular emission line galaxies (EMGs) deviate from the correlation that fits Galactic giant molecular cloud cores, normal spirals, LIRGs, and ULIRGs. This indicates that the star formation rate per solar mass of dense molecular gas is higher in the high-z objects than in local galaxies including normal spirals LIRGs and ULIRGs. The limited HCN detections at high-redshift show that the HCN/CO ratios for the high-z objects are high and are comparable to those of the local ULIRGs rather than those of normal spirals. This indicates that EMGs have a high fraction of dense molecular gas compared to total molecular gas traced by CO emission.
The Astrophysical Journal, 2014
We report HCN J = 4 → 3, HCO + J = 4 → 3, and CS J = 7 → 6 observations in 20 nearby star-forming galaxies with the Atacama Pathfinder EXperiment 12 m telescope. Combined with four HCN, three HCO + , and four CS detections from the literature, we probe the empirical link between the luminosity of molecular gas (L gas ) and that of infrared emission (L IR ), up to the highest gas densities (∼10 6 cm −3 ) that have been probed so far. For nearby galaxies with large radii, we measure the IR luminosity within the submillimeter beam size (14 -18 ) to match the molecular emission. We find linear slopes for L CS J =7-6 -L IR and L HCN J =4-3 -L IR , and a slightly super-linear slope for L HCO + J =4-3 -L IR . The correlation of L CS J =7-6 -L IR even extends over eight orders of luminosity magnitude down to Galactic dense cores, with a fit of log(L IR ) = 1.00(±0.01)×log(L CS J =7-6 ) + 4.03(±0.04). Such linear correlations appear to hold for all densities >10 4 cm −3 , and indicate that star formation rate is not related to the free-fall timescale for dense molecular gas.
Is HCN a True Tracer of Dense Molecular Gas in Luminous and Ultraluminous Infrared Galaxies?
The Astrophysical Journal, 2006
We present the results of the first HCO + survey probing the dense molecular gas content of a sample of 16 luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). Previous work, based on HCN(1-0) observations, had shown that LIRGs and ULIRGs posses a significantly higher fraction of dense molecular gas compared to normal galaxies. While the picture issued from HCO + partly confirms this result, we have discovered an intriguing correlation between the HCN(1-0)/HCO + (1-0) luminosity ratio and the IR luminosity of the galaxy (L IR). This trend casts doubts on the use of HCN as an unbiased quantitative tracer of the dense molecular gas content in LIRGs and ULIRGs. A plausible scenario explaining the observed trend implies that X-rays coming from an embedded AGN may play a dominant role in the chemistry of molecular gas at L IR ≥ 10 12 L ⊙. We discuss the implications of this result for the understanding of LIRGs, ULIRGs and high redshift gas-rich galaxies.
Dense gas in luminous infrared galaxies
Astronomy and Astrophysics, 2008
Aims. Molecules that trace the high-density regions of the interstellar medium have been observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate multiple-molecule multiple-transition studies to evaluate the physical and chemical environment of the nuclear medium and its response to the ongoing nuclear activity. Methods. The HCN(1−0), HNC(1−0), HCO + (1−0), CN(1−0) and CN(2−1), CO(2−1), and CS(3−2) transitions were observed in sources covering three decades of infrared luminosity including sources with known OH megamaser activity. The data for the molecules that trace the high-density regions have been augmented with data available in the literature. Results. The integrated emissions of high-density tracer molecules show a strong relation to the far-infrared luminosity. Ratios of integrated line luminosities have been used for a first order diagnosis of the integrated molecular environment of the evolving nuclear starbursts. Diagnostic diagrams display significant differentiation among the sources that relate to initial conditions and the radiative excitation environment. Initial differentiation has been introduced between the FUV radiation field in photon-dominated-regions and the X-ray field in X-ray-dominated-regions. The galaxies displaying OH megamaser activity have line ratios typical of photon-dominated regions.
Dense Gas in Ultra-Luminous Far-Infrared Galaxies
2007
Aims. Molecules that trace the high-density regions of the interstellar medium have been observed in (Ultra-)Luminous Infrared Galaxies, in order to initiate multiple-molecule multiple-transition studies to evaluate the physical and chemical environment of the nuclear medium and its response to the ongoing nuclear activity. Methods. The HCN(1−0), HNC(1−0), HCO(1−0), CN(1−0) and CN(2−1), CO(2−1), and CS(3−2) transitions were observed in sources covering three decades of infrared luminosity including sources with known OH megamaser activity. The data for the molecules that trace the high-density regions have been augmented with data available in the literature. Results. The integrated emissions of high-density tracer molecules show a strong relation to the infrared luminosity. Ratios of integrated line luminosities have been used for a first order diagnosis of the integrated molecular environment of the evolving nuclear starbursts. Diagnostic diagrams display significant differentiati...
Search for dense molecular gas in two QSO host galaxies
Astronomy and Astrophysics, 2006
Context. The HCN(1-0) rotational line transition traces the dense (n H 2 > 10 4 cm −3) fraction of the molecular gas typically located in starforming (SF) regions. In addition, an abnormally high HCN/CO line ratio close to AGNs may indicate then conditions of an X-ray-dominated region. Observed correlations between the CO-, HCN-, and FIR luminosities in nearby non-active, starburst, and low-luminosity active galaxies represent the physical connection between star formation and molecular gas as its fuel. HCN(1-0) has hardly been investigated in nearby high-luminosity AGN within this context. Aims. The aim of this study is to compare the HCN luminosity with published CO and IR luminosities to investigate the role of SF in the observed QSO host galaxies. Methods. We used the IRAM 30 m for the first time to search for the HCN(1-0) transition in two standard QSO host galaxies at z ∼ 0.1. Results. Our upper limits on L HCN agree with the known correlations and do not show strong excess abundance or excitation of the HCN due to the luminous active quasar nucleus. The starburst origin of the far-infrared luminosity in the observed QSO hosts cannot be proven unambiguously by the upper limits. We found that the IR/FIR ratio indicates independently of L IR if a significant amount of AGN heated dust is present.
The Astrophysical Journal, 2004
The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium as well as the evolution of the galaxies. We observed near-infrared H 2 emission lines: v = 1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from the central ∼ 1 kpc regions of the archetypical starburst galaxies, M82 and NGC 253, and the less dramatic but still vigorously star-forming galaxies, NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the near-infrared H 2 emission luminosity can directly trace the amount of star formation activity because the H 2 emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H 2 line ratios show that both thermal and non-thermal excittion are responsible for the emission lines, but that the great majority of the near-infrared H 2 line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g., far-ultraviolet radiation field and gas density, from [C II] and [O I] lines and farinfrared continuum observations when used as inputs to photodissociation models, also explain the luminosity of the observed H 2 1-0 S(1) line. The ratio of the H 2 1-0 S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities; L H2 /L F IR ≃ 10 −5 , in normal late-type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: L F IR > 10 11 L ⊙). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface.
Warm Molecular Gas in Luminous Infrared Galaxies
The Astrophysical Journal, 2014
We present our initial results on the CO rotational spectral line energy distribution (SLED) of the J to J−1 transitions from J = 4 up to 13 from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey (GOALS). The observed SLEDs change on average from one peaking at J ≤ 4 to a broad distribution peaking around J ∼ 6−7 as the IRAS 60-to-100 µm color, C(60/100), increases. However, the ratios of a CO line luminosity to the total infrared luminosity, L IR , show the smallest variation for J around 6 or 7. This suggests that, for most LIRGs, ongoing star formation (SF) is also responsible for a warm gas component that emits CO lines primarily in the mid-J regime (5 J 10). As a result, the logarithmic ratios of the CO line luminosity summed over CO (5−4), (6−5), (7−6), (8−7) and (10−9) transitions to L IR , log R midCO , remain largely independent of C(60/100), and show a mean value of −4.13 (≡ log R SF midCO) and a sample standard deviation of only 0.10 for the SF-dominated galaxies. Including additional galaxies from the literature, we show, albeit with small number of cases, the possibility that galaxies, which bear powerful interstellar shocks unrelated to the current SF, and galaxies, in which an energetic active galactic nucleus contributes significantly to the bolometric luminosity, have their R midCO higher and lower than R SF midCO , respectively.
Molecular gas in blue compact dwarf galaxies
Monthly Notices of the Royal Astronomical Society, 2000
Blue compact dwarf galaxies (BCDGs) are currently undergoing strong bursts of star formation. Nevertheless, only a few of them have been clearly detected in CO, which is thought to trace the`fuel' of star formation, H 2. In this paper we present a deep search for CO J 1 3 0 and J 2 3 1 emission lines in a sample of eight BCDGs and two companions. Only two of them (Haro 2 and UM 465) are detected. For the other galaxies we have obtained more stringent upper limits on the CO luminosity than in previously published values. We could not confirm the previously reported`detection' of CO for the galaxies UM 456 and UM 462. We analyse a possible relation between metallicity, CO luminosity and absolute blue magnitude of the galaxies. We use previously determined relations between X ; NH 2 aI CO and the metallicity to derive molecular cloud masses or upper limits for them. With these`global' X CO values we find that, for the galaxies that we detect in CO, the molecular gas mass is similar to the H i mass, whereas for the non-detections, the upper limits on the molecular gas masses are significantly lower than on the H i mass. Using an LVG (large velocity gradient) model we show that X CO depends not only on metallicity but also on other physical parameters, such as volume density and kinetic temperature, which raises the question about the validity of`global' X CO factors.
Publications of the Astronomical Society of Japan, 2010
We report the results of HCN(J=4-3) and HCO + (J=4-3) observations of two luminous infrared galaxies (LIRGs), NGC 4418 and Arp 220, made using the Atacama Submillimeter Telescope Experiment (ASTE). The ASTE wide-band correlator provided simultaneous observations of HCN(4-3) and HCO + (4-3) lines, and a precise determination of their flux ratios. Both galaxies showed high HCN(4-3) to HCO + (4-3) flux ratios of >2, possibly due to AGN-related phenomena. The J = 4-3 to J = 1-0 transition flux ratios for HCN (HCO + ) are similar to those expected for fully thermalized (sub-thermally excited) gas in both sources, in spite of HCN's higher critical density. If we assume collisional excitation and neglect an infrared radiative pumping process, our non-LTE analysis suggests that HCN traces gas with significantly higher density than HCO + . In Arp 220, we separated the double-peaked HCN(4-3) emission into the eastern and western nuclei, based on velocity information. We confirmed that the eastern nucleus showed a higher HCN(4-3) to HCN(1-0) flux ratio, and thus contained a larger amount of highly excited molecular gas than the western nucleus.