arXiv subjects
Publications and source records attributed to David S. Meier.
ALMA will be able to detect a broad spectrum of molecular lines in galaxies. Current aperture synthesis observations indicate that the molecular line emission from galaxies is remarkably variable, even on kpc scales. Imaging spectroscopy at resolutions of an arcsecond or better will reduce the chemical complexity by allowing regions of physical conditions to be defined and classified.
We present high spectral resolution (v~12-16 km/s) Brackett line spectroscopy of the blue compact dwarf galaxy Henize 2-10 made with NIRSPEC on the Keck Telescope. The spatial resolution is seeing limited, at 1". We detect two distinct kinematic features separated by approximately 3", with heliocentric velocities of ~860 and ~890 km/s. In addition to a narrow core, the line profiles also display a broad, low intensity feature on the blue side of the centroid, which we attribute to an outflow. This may be a sign of aging in the clusters. We compare to archival high resolution Very Large Array (VLA) data at 1.3 cm, and find that the centimeter wavelength emission is resolved into six sources. These radio sources are organized into two larger groups, which we associate with the two kinematic peaks in the Brackett spectrum. We estimate a Lyman continuum rate of at least 7 x 10^52 s^-1, with a corresponding stellar mass of 6 x 10^6 M_sun is required to ionize the nebulae. We also estimate the size of the nebulae from the radio continuum brightness and find that the observed sources probably contain many HII regions in smaller, unresolved clumps. Brackett line profiles have supersonic line widths, but, aside from the blue wing, are comparable to line widths observed in Galactic ultracompact HII regions, which are excited by a single star, or a few stars.
We report subarcsecond-resolution VLA imaging of the centers of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, 7 - 12 compact radio continuum sources were identified within the central 15" x 15". Slightly over half of the compact sources appear to be HII regions with flat or positive spectral indices (alpha >~ -0.1). The HII regions with rising spectra are optically thick at centimeter wavelengths, and thus dense (n_i ~ 10^4 cm^(-3)) and young. The largest of these HII regions require the excitation of 500 - 800 O stars, within regions of only few parsecs extent. These clusters approach the sizes expected for globular clusters.
We report subarcsecond-resolution VLA imaging of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, compact radio continuum sources are identified in the central ~ 15" x 15" region. These compact sources are responsible for 20 - 30 % of the total emission from the central kpc of the host galaxies at 2 cm, but only ~ 5 - 10 % at 6 cm. More than half of the compact sources appear to be HII regions. The HII regions with rising spectra must be fairly dense (n_i ~ 10^4 cm^(-3)) and are presumably very young. The largest of these HII regions require the excitation of 500 - 800 O stars, within regions of only few parsecs extent. These clusters approach the sizes expected for globular clusters. Thermal free-free emission from compact sources contributes more significantly at 2 cm, while diffuse synchrotron emission dominates at 6 cm. The radio HII regions are found near the centers of giant molecular clouds in projection, and do not have obvious visual counterparts.
We have imaged emission from the millimeter lines of eight molecules--C2H, C34S, N2H+, CH3OH, HNCO, HNC, HC3N, and SO--in the central half kpc of the nearby spiral galaxy IC 342. The 5" (~50 pc) resolution images were made with OVRO. Using these maps we obtain a picture of the chemistry within the nuclear region on the sizescales of individual GMCs. Bright emission is detected from all but SO. There are marked differences in morphology for the different molecules. A principal component analysis is performed to quantify similarities and differences among the images. This analysis reveals that while all molecules are to zeroth order correlated, that is, they are all found in dense molecular clouds, there are three distinct groups of molecules distinguished by the location of their emission within the nuclear region. N2H+, C18O, HNC and HCN are widespread and bright, good overall tracers of dense molecular gas. C2H and C34S, tracers of PDR chemistry, originate exclusively from the central 50-100 pc region, where radiation fields are high. The third group of molecules, CH3OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by processing of grains. HC3N is observed to correlate tightly with 3mm continuum emission, demonstrating that the young starbursts are the sites of the warmest and densest molecular gas. We compare our HNC images with the HCN images of Downes et al. (1992) to produce the first high resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity across the nucleus and the correlation of both of these gas tracers with the star formation is excellent. (Abridged).
We present high resolution (~5'') maps of the J = 1 - 0 transitions of ^{13}CO and C^{18}O towards the nucleus of NGC 6946, made with the Owens Valley Millimeter Array. The images are compared with existing ^{12}CO(1-0) maps to investigate localized changes in gas properties across the nucleus. As compared to ^{12}CO, both ^{13}CO and C^{18}O are more confined to the central ring of molecular gas associated with the nuclear star formation; that is, ^{12}CO is stronger relative to ^{13}CO and C^{18}O away from the nucleus and along the spiral arms. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio reaches very high values of >40. We attribute the relative ^{13}CO weakness to a rapid change in the interstellar medium from dense star forming cores in a central ring to diffuse, low density molecular gas in and behind the molecular arms. This change is abrupt, occurring in less than a beamsize (90 pc), about the size of a giant molecular cloud. Column densities determined from ^{13}CO(1-0), C^{18}O(1-0), and 1.4 mm dust continuum all indicate that the standard Galactic conversion factor, X_{CO}, overestimates the amount of molecular gas in NGC 6946 by factors of ~3-5 towards the central ring and potentially even more so in the diffuse gas away from the central starburst. We suggest that the nuclear bar acts to create coherent regions of molecular clouds with distinct and different physical conditions. The ^{12}CO(1-0)/^{13}CO(1-0) line ratio in galactic nuclei can be a signpost of a dynamically evolving ISM.
We report the detection of CO(2-1) and 3.1 mm and 1.3 mm continuum emission towards the extremely young starburst in NGC 5253, with data taken from the Owens Valley Millimeter Array. Faint CO emission originates in five molecular clouds distributed along the prominent dust lane seen in visual images. With the gas, the morphology of NGC 5253 looks much like a dwarf elliptical version of the ``dust-lane ellipticals'' or ``polar-ring'' class of galaxies. The molecular gas resides in GMCs well away from the radio-IR super-star cluster/supernebula seen in the radio and infrared. The millimeter continuum data confirm that the 2 cm flux from the supernebula is optically thick; the Lyman continuum rate derived from the 1.3 mm continuum is N$_{Lyc} \sim 6x10^{52} s^{-1}$ for the central ~20''. CO may underestimate the true molecular column density, as expected for a low metallicity system, although there are regions along the dust lane that appear to have near-Galactic conversion factors. We estimate a total molecular gas mass of $M_{H_{2}}\lsim 10^{7} M_{\odot}$. The molecular gas in the dust lane is falling into the galaxy, supporting an accretion hypothesis. The dust lane gas cannot therefore be causally associated with the current burst of star formation. A relatively small amount, $M_{H_{2}}\lsim 5x10^{5} M_{\odot}$, of molecular gas is associated with the current starburst. We estimate a star formation efficiency of at least 25 % and more likely ~75 %, consistent with the formation of a bound cluster. Despite the extreme youth of the starburst, the specific trigger of the starburst remains elusive, although the infall of gas in the dust lane suggests that there is more star formation to come in NGC 5253.
We present high resolution (~2.5'') CO(1-0) and CO(2-1) images of the central kiloparsec of NGC 3077 made with the Owens Valley Millimeter Array. CO emission is distributed in three major complexes which resolve into at least seven GMCs. Two complexes are associated with the central starburst. A third, more distant complex is not associated with strong star formation. The GMCs are ~70 pc in size and contain ~10^{6} M_{\odot} of molecular gas. The Galactic conversion factor appears applicable to NGC 3077, consistent with its solar metallicity. Galactic rotation in NGC 3077 is detected for the first time in the molecular gas. The molecular gas counterrotates with respect to the large scale HI tidal bridge. The molecular clouds closest to the starburst have super-virial linewidths, possibly related to the turbulence generated by the starburst. The 2.6 mm radio continuum flux indicates that thermal Bremsstrahlung dominates the emission from the starburst region below 5 cm, and that the N_{Lyc} \simeq 3.7\times 10^{52} s^{-1} or ~3000 O7 stars, corresponds to a star formation rate of 0.4 M_{\odot} yr^{-1}. At this rate the amount of molecular gas can sustain star formation for only ~10 Myrs; thus NGC 3077 is a true starburst galaxy. The derived age of the starburst is consistent with the inferred ages of the superbubbles, which suggests that the burst is much younger than the age of the M 81-M 82-NGC 3077 interaction. We suggest that it is caused by gas that was pulled out of NGC 3077 during the interaction with M 81, raining back down onto the galaxy.
We present high resolution maps ($\sim 2^{''}$) maps of the J=1-0 and J=2-1 transitions of C$^{18}$O in the central $\sim 150$ pc of the gas-rich nucleus of IC 342, made with the Owens Valley Millimeter Array. From the C$^{18}$O maps we are able to obtain the most accurate map of N$_{H_{2}}$ to date for IC 342. Due to their low opacities, the transitions of C$^{18}$O give a more reliable estimate of the true molecular gas column density distribution than the more common $^{12}$CO and $^{13}$CO isotopomers. The morphology of the C$^{18}$O emission in the nucleus is a mini-spiral similar to that of the main isotopomer, $^{12}$CO, except it is more symmetric, lacking the enhancements to north. We suggest that the asymmetries present in $^{12}$CO images may reflect the viewing perspective of the starburst region biased by the high optical depths of $^{12}$CO, rather than true asymmetries in the amount of molecular gas present. The giant molecular clouds seen in C$^{18}$O appear to be nonspherical, probably due to tidal arm shearing. Column densities determined from C$^{18}$O observations, 1.3 mm dust continuum, and the virial theorem indicate that the standard Galactic conversion factor, X$_{CO}$, overestimates the amount of molecular gas in the center of IC 342 by a factor of $\sim$2-3 at the molecular cloud peaks, and by $>$3 in the diffuse gas away from the starburst. Revised molecular masses based on this conversion factor imply that star formation efficiencies in the starburst region are very high. From the distribution of gas and star formation it appears that the sites of star formation are dynamically determined, rather than driven by density peaks. Near the central star-forming region, evidence is seen for chemical enrichment of C$^{18}$O due to massive stars.
Eight dwarf starburst galaxies have been observed with the Caltech Submillimeter Observatory (CSO) telescope in the CO J= 3 - 2 transition. The galaxies observed are He 2-10, NGC 5253, NGC 1569, NGC 3077, Haro 2, Haro 3, II Zw 40 and Mrk 86; all but the last two are detected. The central regions of He 2-10 and NGC 5253 were mapped and a CO(2-1) spectrum of NGC 5253 was obtained. The error weighted mean CO(3-2)/CO(1-0) ratio of the detected galaxies is 0.60$\pm$0.06, which is virtually identical to what is found for starbursts in the nuclei of nearby spirals, and suggests that the molecular gas is optically thick, warm (T$_{k}>$20 K), and moderately dense ($n_{H_{2}}\sim 10^{3-4} cm^{-3}$). The CO(3-2)/CO(1-0) ratio peaks at or close to the starburst in all cases. CO emission does not appear to be optically thin in these dwarfs, despite the low metallicity and intense radiation fields, which is probably because in order for CO to exist in detectable amounts it must be self-shielding and hence optically thick. Physical properties of the molecular clouds in these dwarf starbursts appear to be essentially the same as nearby spiral nuclei, with the possible exception that CO is more confined to the cloud cores.