300 research outputs found
Towards a multi-tracer timeline of star formation in the LMC -- I.\ Deriving the lifetimes of H\,{\sc i} clouds
The time-scales associated with the various stages of the star formation process remain poorly constrained. This includes the earliest phases of star formation, during which molecular clouds condense out of the atomic interstellar medium. We present the first in a series of papers with the ultimate goal of compiling the first multi-tracer timeline of star formation, through a comprehensive set of evolutionary phases from atomic gas clouds to unembedded young stellar populations. In this paper, we present an empirical determination of the lifetime of atomic clouds using the Uncertainty Principle for Star Formation formalism, based on the de-correlation of H and H\,{\sc i} emission as a function of spatial scale. We find an atomic gas cloud lifetime of 48\,Myr. This timescale is consistent with the predicted average atomic cloud lifetime in the LMC (based on galactic dynamics) that is dominated by the gravitational collapse of the mid-plane ISM. We also determine the overlap time-scale for which both H\,{\sc i} and H emission are present to be very short (\,Myr), consistent with zero, indicating that there is a near-to-complete phase change of the gas to a molecular form in an intermediary stage between H\,{\sc i} clouds and H\,{\sc ii} regions. We utilise the time-scales derived in this work to place empirically determined limits on the time-scale of molecular cloud formation. By performing the same analysis with and without the 30 Doradus region included, we find that the most extreme star forming environment in the LMC has little effect on the measured average atomic gas cloud lifetime. By measuring the lifetime of the atomic gas clouds, we place strong constraints on the physics that drives the formation of molecular clouds and establish a solid foundation for the development of a multi-tracer timeline of star formation in the LMC
The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51
We present the first results from the EMPIRE survey, an IRAM large program
that is mapping tracers of high density molecular gas across the disks of nine
nearby star-forming galaxies. Here, we present new maps of the 3-mm transitions
of HCN, HCO+, and HNC across the whole disk of our pilot target, M51. As
expected, dense gas correlates with tracers of recent star formation, filling
the "luminosity gap" between Galactic cores and whole galaxies. In detail, we
show that both the fraction of gas that is dense, f_dense traced by HCN/CO, and
the rate at which dense gas forms stars, SFE_dense traced by IR/HCN, depend on
environment in the galaxy. The sense of the dependence is that high surface
density, high molecular gas fraction regions of the galaxy show high dense gas
fractions and low dense gas star formation efficiencies. This agrees with
recent results for individual pointings by Usero et al. 2015 but using unbiased
whole-galaxy maps. It also agrees qualitatively with the behavior observed
contrasting our own Solar Neighborhood with the central regions of the Milky
Way. The sense of the trends can be explained if the dense gas fraction tracks
interstellar pressure but star formation occurs only in regions of high density
contrast.Comment: 7 pages, 5 figures, ApJL accepte
Full-disc CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H conversion factor
Carbon monoxide (CO) provides crucial information about the molecular gas
properties of galaxies. While CO has been targeted extensively,
isotopologues such as CO have the advantage of being less optically
thick and observations have recently become accessible across full galaxy
discs. We present a comprehensive new dataset of CO(1-0) observations
with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from
the EMPIRE survey at a spatial resolution of 1.5kpc. CO(1-0) is
mapped out to and detected at high signal-to-noise throughout our
maps. We analyse the CO(1-0)-to-CO(1-0) ratio () as a
function of galactocentric radius and other parameters such as the
CO(2-1)-to-CO(1-0) intensity ratio, the 70-to-160m flux
density ratio, the star-formation rate surface density, the star-formation
efficiency, and the CO-to-H conversion factor. We find that varies by
a factor of 2 at most within and amongst galaxies, with a median value of 11
and larger variations in the galaxy centres than in the discs. We argue that
optical depth effects, most likely due to changes in the mixture of
diffuse/dense gas, are favored explanations for the observed variations,
while abundance changes may also be at play. We calculate a spatially-resolved
CO(1-0)-to-H conversion factor and find an average value of
cm (K.km/s) over our sample with a standard
deviation of a factor of 2. We find that CO(1-0) does not appear to be a
good predictor of the bulk molecular gas mass in normal galaxy discs due to the
presence of a large diffuse phase, but it may be a better tracer of the mass
than CO(1-0) in the galaxy centres where the fraction of dense gas is
larger.Comment: accepted for publication in MNRA
Millimeter-Wave Line Ratios and Sub-beam Volume Density Distributions
We explore the use of mm-wave emission line ratios to trace molecular gas
density when observations integrate over a wide range of volume densities
within a single telescope beam. For observations targeting external galaxies,
this case is unavoidable. Using a framework similar to that of Krumholz and
Thompson (2007), we model emission for a set of common extragalactic lines from
lognormal and power law density distributions. We consider the median density
of gas producing emission and the ability to predict density variations from
observed line ratios. We emphasize line ratio variations, because these do not
require knowing the absolute abundance of our tracers. Patterns of line ratio
variations have the prospect to illuminate the high-end shape of the density
distribution, and to capture changes in the dense gas fraction and median
volume density. Our results with and without a high density power law tail
differ appreciably; we highlight better knowledge of the PDF shape as an
important area. We also show the implications of sub-beam density distributions
for isotopologue studies targeting dense gas tracers. Differential excitation
often implies a significant correction to the naive case. We provide tabulated
versions of many of our results, which can be used to interpret changes in
mm-wave line ratios in terms of changes in the underlying density
distributions.Comment: 24 pages, 16 figure, Accepted for publication in the Astrophysical
Journal, two online tables temporarily available at
http://www.astronomy.ohio-state.edu/~leroy.42/densegas_table2.txt and
http://www.astronomy.ohio-state.edu/~leroy.42/densegas_table3.tx
Stellar Feedback and Resolved Stellar IFU Spectroscopy in the nearby Spiral Galaxy NGC 300
We present MUSE Integral Field Unit (IFU) observations of five individual HII regions in two giant (> 100 pc in radius) star-forming complexes in the low-metallicity (Z~0.33 Z) nearby (D ~ 2 Mpc) dwarf spiral galaxy NGC 300. We combine the IFU data with high spatial resolution HST photometry to demonstrate the extraction of stellar spectra and the classification of individual stars from ground-based data at the distance of 2 Mpc. For the two star-forming complexes, in which no O-type stars had previously been identified, we find a total of 13 newly identified O-type stars in the mass range 15-50 M, as well as 4 Wolf-Rayet stars. We use the derived massive stellar content to analyze the impact of stellar feedback on the HII regions. As already found for HII regions in the Magellanic Clouds, the dynamics of the analyzed NGC 300 HII regions are dominated by a combination of the pressure of the ionized gas and stellar winds. By comparing the derived ionized gas mass loading factors to the total gas mass loading factor across the NGC 300 disk, we find that the latter is an order of magnitude higher, either indicating very early evolutionary stages for these HII regions, or being a direct result of the multi-phase nature of feedback-driven bubbles. Moreover, we analyze the relation between the star formation rate and the pressure of the ionized gas as derived from small (<100 pc) scales, as both quantities are systematically overestimated when derived on galactic scales. With the wealth of ongoing and upcoming IFU instruments and programs, this study serves as a pathfinder for the systematic investigation of resolved stellar feedback in nearby galaxies, and it delivers the necessary analysis tools to enable massive stellar content and feedback studies sampling an unprecedented range of HII region properties across entire galaxies in the nearby Universe
PDF_CHEM: fast simulations of the chemical ISM using probability distributions
Interstellar matter and star formatio
Dense Molecular Gas in the Nearby Low Metallicity Dwarf Starburst Galaxy IC 10
Dense molecular gas and star formation are correlated in galaxies. The effect
of low metallicity on this relationship is crucial for interpreting
observations of high redshift galaxies, which have lower metallicities than
galaxies today. However, it remains relatively unexplored because dense
molecular gas tracers like HCN and HCO+ are faint in low metallicity systems.
We present Green Bank Telescope observations of HCN(1-0) and HCO+(1-0) on giant
molecular cloud (34pc) scales in the nearby low metallicity () starburst IC 10 and compare them to those in other galaxies. We
detect HCN and HCO+ in one and three of five pointings, respectively. The
values are within the range seen in other galaxies,
but are most similar to those seen in other low metallicity sources and in
starbursts. The detections follow the fiducial - and
- relationships. These trends suggest that HCN and
HCO+ can be used to trace dense molecular gas at metallicities of 1/4
, to first order. The dense gas fraction is similar to that in spiral
galaxies, but lower than that in U/LIRGs. The dense molecular gas star
formation efficiency, however, is on the upper end of those in normal galaxies
and consistent with those in U/LIRGs. These results suggest that the CO and
HCN/HCO+ emission occupy the same relative volumes as at higher metallicity,
but that the entire emitting structure is reduced in size. Dense gas mass
estimates for high redshift galaxies may need to be corrected for this effect.Comment: Accepted to Ap
A 50 pc Scale View of Star Formation Efficiency across NGC 628
Star formation is a multi-scale process that requires tracing cloud formation and stellar feedback within the local (âČkpc) and global galaxy environment. We present first results from two large observing programs on the Atacama Large Millimeter/submillimeter Array (ALMA)and the Very Large Telescope/Multi Unit Spectroscopic Explorer(VLT/MUSE), mapping cloud scales (1âł = 47 pc) in both molecular gas and star-forming tracers across 90 kpc2 of the central disk of NGC 628 to probe the physics of star formation. Systematic spatial offsets between molecular clouds and H ii regions illustrate the time evolution of star-forming regions. Using uniform sampling of both maps on 50-500 pc scales, we infer molecular gas depletion times of 1-3 Gyr, but also find that the increase of scatter in the star formation relation on small scales is consistent with gas and H ii regions being only weakly correlated at the cloud (50 pc) scale. This implies a short overlap phase for molecular clouds and H ii regions, which we test by directly matching our catalog of 1502 H ii regions and 738 GMCs. We uncover only 74 objects in the overlap phase, and we find depletion times >1 Gyr, significantly longer than previously reported for individual star-forming clouds in the Milky Way. Finally, we find no clear trends that relate variations in the depletion time observed on 500 pc scales to physical drivers (metallicity, molecular and stellar-mass surface density, molecular gas boundedness) on 50 pc scales.We thank the referee for helpful comments that improved
this work. K.K. gratefully acknowledges support from grant
KR 4598/1-2 from the German Research Foundation (DFG)
Priority Program 1573. J.M.D.K. and M.C. gratefully acknowledge funding from the DFG in the form of an Emmy Noether
Research Group (grant No. KR4801/1-1). J.M.D.K. gratefully
acknowledges funding from the European Research Council (ERC) under the European Unionâs Horizon 2020 Research
and Innovation Programme via the ERC Starting Grant
MUSTANG (grant agreement No. 714907). B.G. gratefully
acknowledges the support of the Australian Research Council
as the recipient of a Future Fellowship (FT140101202). F.B.
acknowledges funding from the European Unionâs Horizon
2020 Research and Innovation Programme (grant agreement
No. 726384âEMPIRE). G.B. is supported by CONICYT/
FONDECYT, Programa de IniciaciĂłn, Folio 11150220. A.H.
acknowledges support from the Centre National dâEtudes
Spatiales (CNES). E.R. acknowledges the support of the
Natural Sciences and Engineering Research Council of Canada
(NSERC), funding reference No. RGPIN-2017-03987. R.M.
and E.S. acknowledge funding from the ERC under the
European Unionâs Horizon 2020 Research and Innovation
Programme (grant agreement No. 694343). J.P. acknowledges
support by the Programme National âPhysique et Chimie du
Milieu Interstellaireâ(PCMI) of CNRS/INSU with INC/INP
co-funded by CEA and CNES
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