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Register a new userStudying the small scale ISM structure with supernovae
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AIMS. In this work we explore the possibility of using the fast expansion of a Type Ia supernova photosphere to detect extra-galactic ISM column density variations on spatial scales of ~100 AU on time scales of a few months. METHODS. We constructed a simple model which describes the expansion of the photodisk and the effects of a patchy interstellar cloud on the observed equivalent width of Na I D lines. Using this model we derived the behavior of the equivalent width as a function of time, spatial scale and amplitude of the column density fluctuations. RESULTS. The calculations show that isolated, small (<100 AU) clouds with Na I column densities exceeding a few 10^11 cm^-2 would be easily detected. In contrast, the effects of a more realistic, patchy ISM become measurable in a fraction of cases, and for peak-to-peak variations larger than ~10^12 cm^-2 on a scale of 1000 AU. CONCLUSIONS. The proposed technique provides a unique way to probe the extra-galactic small scale structure, which is out of reach for any of the methods used so far. The same tool can also be applied to study the sub-AU Galactic ISM structure.
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Studying the environments of 0.4<z<1.2 UV-selected galaxies, as examples of extreme star-forming galaxies (with star formation rates in the range of 3-30 M_sol/yr), we explore the relationship between high rates of star-formation, host halo mass and pair fractions. We study the large-scale and small-scale environments of local Ultraviolet Luminous Galaxies (UVLGs) by measuring angular correlation functions. We cross-correlate these systems with other galaxy samples: a volume-limited sample (ALL), a Blue Luminous Galaxy sample (BLG) and a Luminous Red Galaxy sample (LRG). We determine the UVLG comoving correlation length to be r_0=4.8(+11.6/-2.4) h^-1 Mpc at <z> =1.0, which is unable to constrain the halo mass for this sample. However, we find that UVLGs form close (separation < 30 kpc) pairs with the ALL sample, but do not frequently form pairs with LRGs. A rare subset of UVLGs, those with the highest FUV surface brightnesses, are believed to be local analogs of high redshift Lyman Break Galaxies (LBGs) and are called Lyman Break Analogs (LBAs). LBGs and LBAs share similar characteristics (i.e., color, size, surface brightness, specific star formation rates, metallicities, and dust content). Recent HST images of z~0.2 LBAs show disturbed morphologies, signs of mergers and interactions. UVLGs may be influenced by interactions with other galaxies and we discuss this result in terms of other high star-forming, merging systems.
We compare the structure of molecular gas at $40$ pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into $370$ pc and $1.1$ kpc resolution elements, and within each we estimate the molecular gas depletion time ($tau_{rm Dep}^{rm mol}$), the star formation efficiency per free fall time ($epsilon_{rm ff}$), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, $Sigma$, line width, $sigma$, and $bequivSigma/sigma^2proptoalpha_{rm vir}^{-1}$, a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star formation efficiency per free-fall time, $epsilon_{ff} left( left< Sigma_{40pc} right> right) sim 0.3{-}0.36%$, lower than adopted in many models and found for local clouds. More, the efficiency per free fall time anti-correlates with both $Sigma$ and $sigma$, in some tension with turbulent star formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is $b equiv Sigma / sigma^2$, tracing the strength of self gravity, with $tau_{rm Dep}^{rm mol} propto b^{-0.9}$. The sense of the correlation is that gas with stronger self-gravity (higher $b$) forms stars at a higher rate (low $tau_{rm Dep}^{rm mol}$). The different regions of the galaxy mostly overlap in $tau_{rm Dep}^{rm mol}$ as a function of $b$, so that low $b$ explains the surprisingly high $tau_{rm Dep}^{rm mol}$ found towards the inner spiral arms found by by Meidt et al. (2013).
Observations of extragalactic objects need to be corrected for Galactic absorption and this is often accomplished by using the measured 21 cm HI column. However, within the beam of the radio telescope there are variations in the HI column that can have important effects in interpreting absorption line studies and X-ray spectra at the softest energies. We examine the HI and DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for HI variations in of up to a factor of three in 1 degree fields. Column density enhancements would preferentially absorb soft X-rays in spatially extended objects and we find evidence for this effect in the ROSAT PSPC observations of two bright clusters of galaxies, Abell 119 and Abell 2142. For clusters of galaxies, the failure to include column density fluctuations will lead to systematically incorrect fits to the X-ray data in the sense that there will appear to be a very soft X-ray excess. This may be one cause of the soft X-ray excess in clusters, since the magnitude of the effect is comparable to the observed values.
We study the ionised ISM in NGC 7793 with MUSE/AO, at a spatial resolution of $sim$ 10 pc. The data are complemented with young star clusters (YSCs), O stars and GMCs observed with HST and ALMA. Using a strong-line method, we find a median $12 + log(O/H) sim 8.37$ with a scatter of 0.25 dex, in agreement with previous estimates. The abundance map is rich in substructures, surrounding clusters and massive stars, although clear degeneracies with photoionisation are present. We determine the observed total amount of ionising photons, $Q(H^0)$, from the extinction corrected H$alpha$ luminosity, and compare it to the expected $Q(H^0)$ obtained by summing the contributions of YSCs and massive stars, to obtain an escape fraction ($f_{esc}$). Overall, we find $f_{esc, HII} = 0.67_{-0.12}^{+0.08}$ for the population of HII regions. We also conclude that the sources of ionisation observed within the FoV are more than sufficient to explain the amount of diffuse ionised gas observed in this region of the galaxy. In general, we find that YSCs located in HII regions have a higher probability to be younger, less massive, and to emit a higher number of ionising photons than clusters in the rest of the field. Finally, we study the optical depth of the regions traced by [SII]/[OIII], finding no systematic trend between the resulting ionisation structure and $f_{esc}$. [abridged]
The cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key roles in the ability of gas to form stars. New high fidelity, high resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional cloud properties treatment. However, we argue that our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low-J 12CO data from recent surveys, we characterize the molecular ISM at 60pc resolution in the Antennae, the Large Magellanic Cloud, M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60pc scales and show galaxy-to-galaxy and intra-galaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1sigma width of ~0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.
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