Yu Gao

The Star Formation Rate and Dense Molecular Gas in Galaxies

HCN luminosity is a tracer of dense molecular gas, n(H(2)) greater than or similar to3 x 10(4) cm(-3), associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies, including nine ultraluminous infrared galaxies (ULIGs, L(IR) greater than or similar to 10(12) L(circle dot)), 22 luminous infrared galaxies (LIGs, 10(11) L(circle dot) < L(IR) less than or similar to 10(12) L(circle dot)), and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity, and the observations are reported in Paper I. This paper analyzes the relationships between the total far-IR luminosity (a tracer of the star formation rate), the global HCN line luminosity (a measure of the total dense molecular gas content), and the CO luminosity (a measure of the total molecular content). We find a tight linear correlation between the IR and HCN luminosities L(IR) and L(HCN) (in the log-log plot) with a correlation coefficient R = 0.94, and an almost constant average ratio L(IR)/L(HCN) = 900 L(circle dot) (K km s(-1) pc(2))(-1). The IR-HCN linear correlation is valid over 3 orders of magnitude including ULIGs, the most luminous objects in the local universe. The direct consequence of the linear IR-HCN correlation is that the star formation law in terms of dense molecular gas content has a power-law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas.

HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies

We report systematic HCN J = 1-0 (and CO) observations of a sample of 53 infrared (IR) and/or CO-bright and/or luminous galaxies, including seven ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more than a dozen of the nearest normal spiral galaxies. This is the largest and most sensitive HCN survey of galaxies to date. All galaxies observed so far follow the tight correlation between the IR luminosity LIR and the HCN luminosity LHCN initially proposed by Solomon, Downes, & Radford, which is detailed in a companion paper. We also address here the issue of HCN excitation. There is no particularly strong correlation between LHCN and the 12 ?m luminosity; in fact, of all the four IRAS bands, the 12 ?m luminosity has the weakest correlation with the HCN luminosity. There is also no evidence of stronger HCN emission or a higher ratio of HCN and CO luminosities LHCN/LCO for galaxies with excess 12 ?m emission. This result implies that mid-IR radiative pumping, or populating, of the J = 1 level of HCN by a mid-IR vibrational transition is not important compared with the collisional excitation by dense molecular hydrogen. Furthermore, large velocity gradient calculations justify the use of HCN J = 1-0 emission as a tracer of high-density molecular gas (3 ? 104/? cm-3) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, LHCN may be used as a measure of the total mass of dense molecular gas, and the luminosity ratio LHCN/LCO may indicate the fraction of molecular gas that is dense.

Connecting dense gas tracers of star formation in our galaxy to high-z star formation

Observations have revealed prodigious amounts of star formation in starburst galaxies as traced by dust and molecular emission, even at large redshifts. Recent work shows that for both nearby spiral galaxies and distant starbursts, the global star formation rate, as indicated by the infrared luminosity, has a tight and almost linear correlation with the amount of dense gas as traced by the luminosity of HCN. Our surveys of Galactic dense cores in HCN 1-0 emission show that this correlation continues to a much smaller scale, with nearly the same ratio of infrared luminosity to HCN luminosity found over 7-8 orders of magnitude in, with a lower cutoff L(IR) around 10(4.5) L(circle dot) of infrared luminosity. The linear correlation suggests that we may understand distant star, formation in terms of the known properties of local star-forming regions. Both the correlation and the luminosity cutoff can be explained if the basic unit of star formation in galaxies is a dense core, similar to those studied in our Galaxy.

Nonnuclear Hyper/Ultraluminous X-Ray Sources in the Starbursting Cartwheel Ring Galaxy

We report the Chandra/ACIS-S detection of more than 20 ultraluminous X-ray sources (ULXs, L0.5−10keV > �3 × 10 39 ergss −1 ) in the Cartwheel collisional ring galaxy system, of which over a dozen are located in the outer active star-forming ring. A remarkable hyperluminous X-ray source (HLX, L0.5−10keV > �10 41 ergss −1 assuming isotropic radiation), which dominates the X-ray emission from the Cartwheel ring, is located in the same segment of the ring as most ULXs. These powerful H/ULXs appear to be coincident with giant HII region complexes, young star clusters, and radio and midinfrared hot-spots: all strong indicators of recent massive star formation. The X-ray spectra show that H/ULXs have similar properties as those of the most luminous ULXs found in the nearest starbursts and galaxy mergers such as the Antennae galaxies and M82. The close association between the X-ray sources and the starbursting ring strongly suggests that the H/ULXs are intimately associated with the production and rapid evolution of short-lived massive stars. The observations represent the most extreme X-ray luminosities discovered to date associated with star-forming regions—rivaling the X-ray luminosities usually associated with active galactic nuclei. Subject headings: galaxies: individual (VV 784, Cartwheel, ESO 350 G 040) — galaxies: interactions — galaxies: active — galaxies: starburst — X-rays: galaxies

POWERFUL HIGH-VELOCITY DISPERSION MOLECULAR HYDROGEN ASSOCIATED WITH AN INTERGALACTIC SHOCK WAVE IN STEPHAN'S QUINTET

We present the discovery of strong mid-infrared emission lines of molecular hydrogen of apparently high-velocity dispersion (~870 km s-1) originating from a group-wide shock wave in Stephans Quintet. These Spitzer Space Telescope observations reveal emission lines of molecular hydrogen and little else. This is the first time an almost pure H2 line spectrum has been seen in an extragalactic object. Along with the absence of PAH-dust features and very low excitation ionized gas tracers, the spectra resemble shocked gas seen in Galactic supernova remnants, but on a vast scale. The molecular emission extends over 24 kpc along the X-ray-emitting shock front, but it has 10 times the surface luminosity as the soft X-rays and about one-third the surface luminosity of the IR continuum. We suggest that the powerful H2 emission is generated by the shock wave caused when a high-velocity intruder galaxy collides with filaments of gas in the galaxy group. Our observations suggest a close connection between galaxy-scale shock waves and strong broad H2 emission lines, like those seen in the spectra of ultraluminous infrared galaxies where high-speed collisions between galaxy disks are common.

HCN Observations of Dense Star-forming Gas in High-Redshift Galaxies

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 giant molecular cloud (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 4 sigma and appears to be resolved as a double source of less than or similar to 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 emissionline galaxies (EMGs) deviate from the correlation that fits Galactic GMC cores, normal spirals, and luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, respectively). 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.

Molecular Gas and the Modest Star Formation Efficiency in the “Antennae” Galaxies: Arp 244 = NGC 4038/9

We report here a factor of 5.7 higher total CO flux in Arp 244 (the Antennae galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5 × 1010 M☉), based on our fully sampled CO(1-0) observations at the NRAO 12 m telescope. Currently, much of the understanding and modeling of the star formation in Arp 244 has been derived using a much lower molecular gas mass. It is imperative to reconsider everything, as the high molecular gas mass might provide sufficient fuel for ultraluminous extreme starburst in Arp 244 once the merging advances to late stage. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) LIR/M(H2) ≈ 4.2 L☉/M☉, the same as that of giant molecular clouds in the Galactic disk. Additional supportive evidence is the extremely low fraction of the dense molecular gas in Arp 244, revealed by our detections of the HCN(1-0) emission, which traces the active star-forming gas at density 104 cm-3. Using the high-resolution BIMA + NRAO 12 m telescope, full-synthesis CO(1-0) images and our VLA continuum maps at 20 cm, we estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission, down to kiloparsec scale. Remarkably, the local SFE stays roughly the same over the bulk of the molecular gas distribution. Only some localized regions show the highest radio-to-CO ratios that we have identified as the sites of the most intense starbursts with SFE 20 L☉/M☉. Here we have assumed that the 20 cm emission is a fairly good indicator of star formation down to kiloparsec scale in Arp 244 because of the well-known correlation between the FIR and the radio continuum emission. These starburst regions are confined exclusively to the dusty patches seen in the Hubble Space Telescope optical images near the CO and FIR peaks where the violent starbursts are presumably heavily obscured. Nevertheless, recent large-scale star formation is going on throughout the system (e.g., concentrations of numerous super-star clusters and MIR hotspots), yet the measured level is more suggestive of a moderate starburst (SFE 10 L☉/M☉) or a weak to normal star formation (SFE ~ 4 L☉/M☉), not necessarily occurring at the high concentrations of the molecular gas reservoir. The overall starburst from the bulk of the molecular gas is yet to be initiated as most of the gas further condenses into a kiloparsec scale in the final coalescence.

Molecular Gas Depletion and Starbursts in Luminous Infrared Galaxy Mergers

Most luminous infrared galaxies (LIGs) are closely interacting/merging systems that are rich in molecular gas. Here we study the relationship between the stage of the galaxy-galaxy interactions, the molecular gas mass, and the star formation rate as deduced from the infrared luminosity LIR in LIGs. We find a correlation between the CO (1-0) luminosity [a measure of molecular mass M(H2)] and the projected separation of merger nuclei (the indicator of merging stages) in a sample of 50 LIG mergers, which shows that the molecular gas content decreases as merging advances. The starburst is due to enhanced star formation in preexisting molecular clouds and not to the formation of more molecular clouds from atomic gas. Because of the starbursts, the molecular content is being rapidly depleted as merging progresses. This is further supported by an anticorrelation between LIR/M(H2), the global measure of the star formation rate per unit gas mass, and the projected separation that implies an enhanced star formation efficiency in late-stage mergers compared with that of early mergers. This is the first evidence connecting the depletion of molecular gas with starbursts in interacting galaxies.

Detections of CO in Late-Type, Low Surface Brightness Spiral Galaxies

Using the IRAM 30 m telescope, we have obtained (CO)-C-12 J = 1-0 and 2-1 spectral line observations toward the nuclear regions of 15 edge-on, low surface brightness (LSB) spiral galaxies. Our sample comprises extreme late-type LSB spirals with disk-dominated morphologies and rotational velocities V-rot less than or similar to 120 km s(-1). We report detections of four galaxies in at least one transition (greater than or similar to 5 sigma); for the remainder of the sample we provide upper limits on the nuclear CO content. Adopting a standard Galactic I-CO-to-H-2 conversion factor implies molecular gas masses of (3.3-9.8) x 10(6) M-circle dot in the nuclear regions (inner 1.1-1.8 kpc) of the detected galaxies. Combining our new data with samples of late-type spirals from the literature, we find that CO-detected LSB spirals adhere to the same M-H2-far-infrared correlation as more luminous and higher surface brightness galaxies. The amount of CO in the central regions of late-type spirals appears to depend more strongly on mass than on central optical surface brightness, and CO detectability declines significantly for moderate to low surface brightness spirals with V-rot less than or similar to 90 km s(-1); no LSB spirals have so far been detected in CO below this threshold. Metallicity effects alone are unlikely to account for this trend, and we speculate that we are seeing the effects of a decrease in the mean fraction of a galaxy disk able to support giant molecular cloud formation with decreasing galaxy mass.

Nature of Widely Separated Ultraluminous Infrared Galaxies

In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled by D. C. Kim, about 5% consists of widely separated galaxies which are presumably in the early phase of interaction. This fact is contrary to the conventional view that ULIRGs are in the final stages of the merger of two gas-rich disk galaxies. We have undertaken high-resolution CO (J = 1-0) observations for the ultraluminous infrared galaxies that have nuclear separations larger than 20 kpc. We have detected CO emission in five out of six systems, but only in one component of the ULIRG pairs. Four of them have LINER spectral type and one galaxy has Seyfert 2 spectral type. In K-band images these components are also brighter than the other components which have either H II region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few times 1010 M☉, similar to that of the other ultraluminous galaxies. The result indicates that the galaxy containing the molecular gas is also the source of most, if not all, of the huge far-infrared luminosity of the system. The optical and K-band imaging observations and optical spectra suggest multiple merger scenarios for one system. If the remaining systems are in an early stage of a binary tidal interaction, the commonly accepted interpretation of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be reexamined.