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Register a new userResolved Giant Molecular Clouds in Nearby Spiral Galaxies: Insights from the CANON CO (1-0) Survey
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Jennifer Donovan Meyer
Publication date
2013
fields
Physics
and research's language is
English
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We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 $times$ 10$^{5}$ Msun in the inner disks of five large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and $^{12}$CO (J=1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H$_{2}$ mass (or xcons) for each galaxy is 1-2 xcounits, all within a factor of two of the Milky Way disk value ($sim$2 xcounits). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-xco trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
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We present results of the $^{12}$CO (1--0) mosaic observations of the nearby barred-spiral galaxy M83 obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). The total flux is recovered by combining the ALMA data with single-dish data obtained using the Nobeyama 45-m telescope. The combined map covers a $sim$13 kpc$^{2}$ field that includes the galactic center, eastern bar, and spiral arm with a resolution of timeform{2.03} $times$ timeform{1.1} ($sim$45 pc $times$ $sim$25 pc). With a resolution comparable to typical sizes of giant molecular clouds (GMCs), the CO distribution in the bar and arm is resolved into many clumpy peaks that form ridge-like structures. Remarkably, in the eastern arm, the CO peaks form two arc-shaped ridges that run along the arm and exhibit a distinct difference in the activity of star formation: the one on the leading side has numerous HII regions associated with it, whereas the other one on the trailing side has only a few. To see whether GMCs form stars with uniform star formation efficiency (SFE) per free-fall time (SFEff), GMCs are identified from the data cube and then cross-matched with the catalog of HII regions to estimate the star formation rate for each of them. 179 GMCs with a median mass of 1.6 $times$ 10$^{6}$ $M_{odot}$ are identified. The mass-weighted average SFEff of the GMCs is $sim$9.4 $times$ 10$^{-3}$, which is in agreement with models of turbulence regulated star formation. Meanwhile, we find that SFEff is not universal within the mapped region. In particular, one of the arm ridges shows a high SFEff with a mass-weighted value of $sim$2.7 $times$ 10$^{-2}$, which is higher by more than a factor of 5 compared to the inter-arm regions. This large regional variation in SFEff favors the recent interpretation that GMCs do not form stars at a constant rate within their lifetime.
We present a CO(1-0) survey for cold molecular gas in a representative sample of 13 high-z radio galaxies (HzRGs) at 1.4<z<2.8, using the Australia Telescope Compact Array. We detect CO(1-0) emission associated with five sources: MRC 0114-211, MRC 0152-209, MRC 0156-252, MRC 1138-262 and MRC 2048-272. The CO(1-0) luminosities are in the range $L_{rm CO} sim (5 - 9) times 10^{10}$ K km/s pc$^{2}$. For MRC 0152-209 and MRC 1138-262 part of the CO(1-0) emission coincides with the radio galaxy, while part is spread on scales of tens of kpc and likely associated with galaxy mergers. The molecular gas mass derived for these two systems is M$_{rm H2} sim 6 times 10^{10}, {rm M}_{odot}$ (M$_{rm H2}$/$L_{rm CO}$=0.8). For the remaining three CO-detected sources, the CO(1-0) emission is located in the halo (~50-kpc) environment. These three HzRGs are among the fainter far-IR emitters in our sample, suggesting that similar reservoirs of cold molecular halo gas may have been missed in earlier studies due to pre-selection of IR-bright sources. In all three cases the CO(1-0) is aligned along the radio axis and found beyond the brightest radio hot-spot, in a region devoid of 4.5$mu$m emission in Spitzer imaging. The CO(1-0) profiles are broad, with velocity widths of ~ 1000 - 3600 km/s. We discuss several possible scenarios to explain these halo reservoirs of CO(1-0). Following these results, we complement our CO(1-0) study with detections of extended CO from the literature and find at marginal statistical significance (95% level) that CO in HzRGs is preferentially aligned towards the radio jet axis. For the eight sources in which we do not detect CO(1-0), we set realistic upper limits of $L_{rm CO} sim 3-4 times 10^{10}$ K km/s pc$^{2}$. Our survey reveals a CO(1-0) detection rate of 38%, allowing us to compare the CO(1-0) content of HzRGs with that of other types of high-z galaxies.
We investigate how star formation quenching proceeds within central and satellite galaxies using spatially resolved spectroscopy from the SDSS-IV MaNGA DR15. We adopt a complete sample of star formation rate surface densities ($Sigma_{rm SFR}$), derived in Bluck et al. (2020), to compute the distance at which each spaxel resides from the resolved star forming main sequence ($Sigma_{rm SFR} - Sigma_*$ relation): $Delta Sigma_{rm SFR}$. We study galaxy radial profiles in $Delta Sigma_{rm SFR}$, and luminosity weighted stellar age (${rm Age_L}$), split by a variety of intrinsic and environmental parameters. Via several statistical analyses, we establish that the quenching of central galaxies is governed by intrinsic parameters, with central velocity dispersion ($sigma_c$) being the most important single parameter. High mass satellites quench in a very similar manner to centrals. Conversely, low mass satellite quenching is governed primarily by environmental parameters, with local galaxy over-density ($delta_5$) being the most important single parameter. Utilising the empirical $M_{BH}$ - $sigma_c$ relation, we estimate that quenching via AGN feedback must occur at $M_{BH} geq 10^{6.5-7.5} M_{odot}$, and is marked by steeply rising $Delta Sigma_{rm SFR}$ radial profiles in the green valley, indicating `inside-out quenching. On the other hand, environmental quenching occurs at over-densities of 10 - 30 times the average galaxy density at z$sim$0.1, and is marked by steeply declining $Delta Sigma_{rm SFR}$ profiles, indicating `outside-in quenching. Finally, through an analysis of stellar metallicities, we conclude that both intrinsic and environmental quenching must incorporate significant starvation of gas supply.
We present a $^{13}mathrm{CO} (J = 1 rightarrow 0)$ mapping survey of 12 nearby galaxies from the CARMA STING sample. The line intensity ratio $mathcal{R} equiv I[^{12}mathrm{CO} (J = 1 rightarrow 0)]/I[^{13}mathrm{CO} (J = 1 rightarrow 0)]$ is derived to study the variations in molecular gas properties. For 11 galaxies where it can be measured with high significance, the spatially resolved $mathcal{R}$ on (sub-)kiloparsec scales varies by up to a factor of 3--5 within a galaxy. Lower $mathcal{R}$ values are usually found in regions with weaker $^{12}rm CO$. We attribute this apparent trend to a bias against measuring large $mathcal{R}$ values when $^{12}rm CO$ is weak. Limiting our analysis to the $^{12}rm CO$ bright regions that are less biased, we do not find $mathcal{R}$ on (sub)kpc scales correlate with galactocentric distance, velocity dispersion or the star formation rate. The lack of correlation between SFR and $mathcal{R}$ indicates that the CO optical depth is not sensitive to stellar energy input, or that any such sensitivity is easily masked by other factors. Extending the analysis to all regions with $rm ^{12}CO$ emission by spectral stacking, we find that 5 out of 11 galaxies show higher stacked $mathcal{R}$ for galactocentric radii of $gtrsim 1$ kpc and $Sigma_{mathrm{SFR}} lesssim 0.1 rm M_{sun} yr^{-1} kpc^{-2}$, which could result from a greater contribution from diffuse gas. Moreover, significant galaxy-to-galaxy variations are found in $mathcal{R}$, but the global $mathcal{R}$ does not strongly depend on dust temperature, inclination, or metallicity of the galaxy.
New-generation spectroscopic surveys of the Milky Way plane have been revealing the structure of the interstellar medium, allowing the simultaneous study of dense structures from single star-forming objects or systems to entire spiral arms. We present the catalogue of molecular clouds extracted from the $^{13}$CO(1-0) data cubes of the Forgotten Quadrant Survey, which mapped the Galactic plane in the range 220deg<l<240deg, and -2.5deg<b<0deg in $^{12}$CO(1-0) and $^{13}$CO(1-0).The catalogue contains 87 molecular clouds for which the main physical parameters such as area, mass, distance, velocity dispersion, and virial parameter were derived. These structures are overall less extended and less massive than the molecular clouds identified in the $^{12}$CO(1-0) data-set because they trace the brightest and densest part of the $^{12}$CO(1-0) clouds. Conversely, the distribution of aspect ratio, equivalent spherical radius, velocity dispersion, and virial parameter in the two catalogues are similar. The mean value of the mass surface density of molecular clouds is 87$pm$55 M$_{odot}$ pc$^{-2}$ and is almost constant across the galactocentric radius, indicating that this parameter, which is a proxy of star formation, is mostly affected by local conditions.In data of the Forgotten Quadrant Survey, we find a good agreement between the total mass and velocity dispersion of the clouds derived from $^{12}$CO(1-0) and $^{13}$CO(1-0). This is likely because in the surveyed portion of the Galactic plane, the H$_2$ column density is not particularly high, leading to a CO emission with a not very high optical depth. This mitigates the effects of the different line opacities between the two tracers on the derived physical parameters. This is a common feature in the outer Galaxy, but our result cannot be readily generalised to the entire Milky Way.
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