We present 12CO (2-1) data of three Virgo spirals -- NGC 4330, NGC 4402 and NGC 4522 obtained using the Submillimeter Array. These three galaxies show clear evidence of ram pressure stripping due to ...the cluster medium as found in previous H i imaging studies. Using the high-resolution CO data, we investigate how the properties of the inner molecular gas disc change while a galaxy is undergoing H i stripping in the cluster. At given sensitivity limits, we do not find any clear signs of molecular gas stripping. However, both its morphology and kinematics appear to be quite disturbed as those of H i. Morphological peculiarities present in the molecular and atomic gas are closely related with each other, suggesting that the molecular gas can be also affected by strong intracluster medium (ICM) pressure even if it is not stripped. CO is found to be modestly enhanced along the upstream sides in these galaxies, which may change the local star formation activity in the disc. Indeed, the distribution of H alpha emission, a tracer of recent star formation, well coincides with that of the molecular gas, revealing enhancements near the local CO peak or along the CO compression. FUV and H alpha share some properties in common, but FUV is always more extended than CO/H alpha in the three galaxies, implying that the star-forming disc is rapidly shrinking as the molecular gas properties have changed. We discuss how ICM pressure affects dense molecular gas and hence star formation properties while diffuse atomic gas is being removed from a galaxy.
We present new high-resolution ALMA (13CO J = 1−0 and J = 2−1) and CARMA (12CO and 13CO J = 1−0) observations of two luminous infrared galaxies (LIRGs), Arp 55 and NGC 2623. The new data are ...complementary to published and archival submillimeter array observations of 12CO J = 2−1 and J = 3−2. We perform a Bayesian likelihood non-local thermodynamic equilibrium analysis to constrain the molecular gas physical conditions such as temperature, column, and volume densities and the 12CO/13CO abundance ratio. For Arp 55, an early/intermediate-staged merger, the line measurements are consistent with cold (∼10-20 K), dense (> cm−3) molecular gas. For NGC 2623, the molecular gas is warmer (∼110 K) and less dense ( cm−3). Because Arp 55 is an early/intermediate stage merger, while NGC 2623 is a merger remnant, the difference in physical conditions may be an indicator of merger stage. Comparing the temperature and volume density of several LIRGs shows that the molecular gas, averaged over ∼kiloparsec scales, of advanced mergers is in general warmer and less dense than early/intermediate stage mergers. We also find that the 12CO/13CO abundance ratio of NGC 2623 is unusually high (>250) when compared with the Milky Way; however, it follows a trend seen with other LIRGs in the literature. This high 12CO/13CO value is very likely due to stellar nucleosynthesis enrichment of the interstellar medium. On the other hand, Arp 55 has a more Galactic 12CO/13CO value with the most probable 12CO/13CO value being 20-30. We measure the CO-to-H2 conversion factor, , to be ∼0.1 and ∼0.7 (3 × 10−4/ ) M (K km s−1 pc2)−1 for Arp 55 and NGC 2623, respectively. Because Arp 55 is an early/intermediate-stage merger, this suggests that the transition from a Galactic conversion factor to a LIRG value happens at an even earlier merger stage.
We present new data obtained with the Submillimeter Array for a sample of 14 nearby luminous and ultraluminous infrared galaxies. The galaxies were selected to have distances image Mpc and ...far-infrared luminosities image. The galaxies were observed with spatial resolutions of order 1 kpc in the CO image, CO image, super(13)CO image, and HCO super(+) image lines as well as the continuum at 880 mum and 1.3 mm. We have combined our CO and continuum data to measure an average gas-to-dust mass ratio of image (rms deviation 109) in the central regions of these galaxies, very similar to the value of 150 determined for the Milky Way. This similarity is interesting given the more intense heating from the starburst and possibly accretion activity in the luminous infrared galaxies compared to the Milky Way. We find that the peak H sub(2) surface density correlates with the far-infrared luminosity, which suggests that galaxies with higher gas surface densities inside the central kiloparsec have a higher star formation rate. The lack of a significant correlation between total H sub(2) mass and far-infrared luminosity in our sample suggests that the increased star formation rate is due to the increased availability of molecular gas as fuel for star formation in the central regions. In contrast to previous analyses by other authors, we do not find a significant correlation between central gas surface density and the star formation efficiency, as traced by the ratio of far-infrared luminosity to nuclear gas mass. Our data show that it is the star formation rate, not the star formation efficiency, that increases with increasing central gas surface density in these galaxies.
We present high-resolution submillimeter interferometric imaging of two of the brightest high-redshift submillimeter galaxies known: GN 20 and AzTEC1 at 0.8 super(image )and 0.3 super(image ) ...resolution, respectively. Our data-the highest resolution submillimeter imaging of high-redshift sources accomplished to date-were collected in three different array configurations: compact, extended, and very extended. We derive angular sizes of 0.6 super(image ) and 1.0 super(image ) for GN 20 and 0.3 super(image ) and 0.4 super(image ) for AzTEC1 from modeling their visibility functions as a Gaussian and an elliptical disk, respectively. Because both sources are B-band dropouts, they likely lie within a relatively narrow redshift window around image, which indicates their angular extent corresponds to physical scales of 4-8 and 1.5-3 kpc, respectively, for the starburst region. By way of a series of simple assumptions, we find preliminary evidence that these hyperluminous starbursts-with star formation rates >1000 image yrimage -are radiating at or close to their Eddington limit. Should future high-resolution observations indicate that these two objects are typical of a population of high-redshift Eddington-limited starbursts, this could have important consequences for models of star formation and feedback in extreme environments.
We present results from a continuing interferometric survey of high-redshift submillimeter galaxies (SMGs) with the Submillimeter Array, including high-resolution (beam size ~2 arcsec) imaging of ...eight additional AzTEC 1.1 mm selected sources in the COSMOS field, for which we obtain six reliable (peak signal-to-noise ratio (S/N) >5 or peak S/N >4 with multiwavelength counterparts within the beam) and two moderate significance (peak S/N >4) detections. When combined with previous detections, this yields an unbiased sample of millimeter-selected SMGs with complete interferometric follow up. With this sample in hand, we (1) empirically confirm the radio-submillimeter association, (2) examine the submillimeter morphology-including the nature of SMGs with multiple radio counterparts and constraints on the physical scale of the far infrared-of the sample, and (3) find additional evidence for a population of extremely luminous, radio-dim SMGs that peaks at higher redshift than previous, radio-selected samples. In particular, the presence of such a population of high-redshift sources has important consequences for models of galaxy formation-which struggle to account for such objects even under liberal assumptions-and dust production models given the limited time since the big bang.
We present a detailed comparison of the CO (3-2) emitting molecular gas between a local sample of luminous infrared galaxies (U/LIRGs) and a high-redshift sample that comprises submm selected ...galaxies (SMGs), quasars, and Lyman Break Galaxies (LBGs). The U/LIRG sample consists of our recent CO (3-2) survey using the Submillimeter Array while the CO (3-2) data for the high-redshift population are obtained from the literature. We find that the and L FIR relation is correlated over five orders of magnitude, which suggests that the molecular gas traced in CO (3-2) emission is a robust tracer of dusty star formation activity. The near unity slope of 0.93 ± 0.03 obtained from a fit to this relation suggests that the star formation efficiency is constant to within a factor of 2 across different types of galaxies residing in vastly different epochs. The CO (3-2) size measurements suggest that the molecular gas disks in local U/LIRGs (0.3-3.1 kpc) are much more compact than the SMGs (3-16 kpc), and that the size scales of SMGs are comparable to the nuclear separation (5-40 kpc) of the widely separated nuclei of U/LIRGs in our sample. We argue from these results that the SMGs studied here are predominantly intermediate stage mergers, and that the wider line widths arise from the violent merger of two massive gas-rich galaxies taking place deep in a massive halo potential.
We have used the Submillimeter Array to image a flux-limited sample of seven submillimeter galaxies, selected by the AzTEC camera on the JCMT at 1.1 mm, in the COSMOS field at 890 mu m with similar ...to 2" resolution. All of the sources-two radio-bright and five radio-dim-are detected as single point sources at high significance (>6 sigma ), with positions accurate to similar to 0.2" that enable counterpart identification at other wavelengths observed with similarly high angular resolution. All seven have IRAC counterparts, but only two have secure counterparts in deep HST ACS imaging. As compared to the two radio-bright sources in the sample, and those in previous studies, the five radio-dim sources in the sample (1) have systematically higher submillimeter-to-radio flux ratios, (2) have lower IRAC 3.6-8.0 mu m fluxes, and (3) are not detected at 24 mu m. These properties, combined with size constraints at 890 mu m ( theta unk 1.2"), suggest that the radio-dim submillimeter galaxies represent a population of very dusty starbursts, with physical scales similar to local ultraluminous infrared galaxies, with an average redshift higher than radio-bright sources.
We present similar to 2" resolution CO (3-2), HCO super(+)(4-3), and 880 mu m continuum images of the luminous infrared galaxy NGC 6240 obtained at the Submillimeter Array. We find that the spatially ...resolved CO (3-2), HCO super(+) (4-3), and the 880 mu m emission peaks between the two nuclear components that are both known to harbor AGNs. Our large velocity gradient (LVG) analysis performed on each velocity channel suggests that the peak of the molecular gas emission traced in our observations is warm (T = 20-100 K), dense ( unkB sub(2) = 10 super(5.0) -10 super(5.4) cm super(-3)), and moderately optically thin ( tau = 0.2 -2) in the central 1 kpc. We also find large column densities of similar to 10 super(23) cm super(-2). Such extreme conditions are observed over similar to 300 km s super(-1) centered around the CO-derived systemic velocity. The derived molecular gas mass from the CO (3-2) emission and a CO-to-H sub(2) conversion factor commonly used for ULIRGs is (6.9 plus or minus 1.7) x 10 super(9) M unk, and this is consistent with the mass derived from previous CO (2-1) observations. The gas is highly turbulent in the central kpc ( Delta sub( tau FWZI) similar to 1175 km s super(-1)). Furthermore, possible inflow or outflow activity is suggested from the CO (3-2) velocity distribution. We tentatively state that 3.5 x 10 unk M unk of isolated CO (3-2) emission seen west of the northern disk may be associated with outflows from starburst superwinds, but the gas outflow scenario from one of the central AGN is not completely ruled out. Piecing all of the information together, the central region of NGC 6240 harbors 2 AGNs, similar to 10 super(10) M unk of molecular gas mass, 5 x 10 super(7) M unk of dust mass, and has possible evidence of inflow and outflow activity.
We have used high-resolution (~2.3) observations of the local (DL = 46 Mpc) luminous infrared galaxy Arp 299 to map out the physical properties of the molecular gas that provides the fuel for its ...extreme star formation activity. The sup 12CO J = 3-2, sup 12CO J = 2-1, and sup 13CO J = 2-1 lines were observed with the Sub-millimeter Array, and the short spacings of the sup 12CO J = 2-1 and J = 3-2 observations have been recovered using the James Clerk Maxwell Telescope single dish observations. We use the radiative transfer code RADEX to estimate the physical properties (density, column density, and temperature) of the different regions in this system. We estimate the CO-to-Hsub 2 factor, alphasubco to be 0.4 + or - 0.3 M (K km ssup -1 pcsup 2)sup -1 for the overlap region. This value agrees well with values determined previously for more advanced merger systems.
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