428 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
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
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
PHANGS CO kinematics: disk orientations and rotation curves at 150 pc resolution
We present kinematic orientations and high resolution (150 pc) rotation
curves for 67 main sequence star-forming galaxies surveyed in CO (2-1) emission
by PHANGS-ALMA. Our measurements are based on the application of a new fitting
method tailored to CO velocity fields. Our approach identifies an optimal
global orientation as a way to reduce the impact of non-axisymmetric (bar and
spiral) features and the uneven spatial sampling characteristic of CO emission
in the inner regions of nearby galaxies. The method performs especially well
when applied to the large number of independent lines-of-sight contained in the
PHANGS CO velocity fields mapped at 1'' resolution. The high resolution
rotation curves fitted to these data are sensitive probes of mass distribution
in the inner regions of these galaxies. We use the inner slope as well as the
amplitude of our fitted rotation curves to demonstrate that CO is a reliable
global dynamical mass tracer. From the consistency between photometric
orientations from the literature and kinematic orientations determined with our
method, we infer that the shapes of stellar disks in the mass range of log()=9.0-10.9 probed by our sample are very close to circular
and have uniform thickness.Comment: 19 figures, 36 pages, accepted for publication in ApJ. Table of
PHANGS rotation curves available from http://phangs.org/dat
Deep transfer learning for star cluster classification: I. application to the PHANGSâHST survey
We present the results of a proof-of-concept experiment that demonstrates that deep learning can successfully be used for production-scale classification of compact star clusters detected in Hubble Space Telescope(HST) ultraviolet-optical imaging of nearby spiral galaxies (â DâČ20Mpcâ ) in the Physics at High Angular Resolution in Nearby GalaxieS (PHANGS)âHST survey. Given the relatively small nature of existing, human-labelled star cluster samples, we transfer the knowledge of state-of-the-art neural network models for real-object recognition to classify star clusters candidates into four morphological classes. We perform a series of experiments to determine the dependence of classification performance on neural network architecture (ResNet18 and VGG19-BN), training data sets curated by either a single expert or three astronomers, and the size of the images used for training. We find that the overall classification accuracies are not significantly affected by these choices. The networks are used to classify star cluster candidates in the PHANGSâHST galaxy NGC 1559, which was not included in the training samples. The resulting prediction accuracies are 70âperâcent, 40âperâcent, 40â50âperâcent, and 50â70âperâcent for class 1, 2, 3 star clusters, and class 4 non-clusters, respectively. This performance is competitive with consistency achieved in previously published human and automated quantitative classification of star cluster candidate samples (70â80âperâcent, 40â50âperâcent, 40â50âperâcent, and 60â70âperâcent). The methods introduced herein lay the foundations to automate classification for star clusters at scale, and exhibit the need to prepare a standardized data set of human-labelled star cluster classifications, agreed upon by a full range of experts in the field, to further improve the performance of the networks introduced in this study
Characterization of aluminum, aluminum oxide and titanium dioxide nanomaterials using a combination of methods for particle surface and size analysis
International audienceThe application of appropriate analytical techniques is essential for nanomaterial (NM) characterization. In this study, we compared different analytical techniques for NM analysis. Regarding possible adverse health effects, ionic and particulate NM effects have to be taken into account. As NMs behave quite differently in physiological media, special attention was paid to techniques which are able to determine the biosolubility and complexation behavior of NMs. Representative NMs of similar size were selected: aluminum (Al 0) and aluminum oxide (Al 2 O 3), to compare the behavior of metal and metal oxides. In addition, titanium dioxide (TiO 2) was investigated. Characterization techniques such as dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) were evaluated with respect to their suitability for fast characterization of nanoparticle dispersions regarding a particle's hydrodynamic diameter and size distribution. By application of inductively coupled plasma mass spectrometry in the single particle mode (SP-ICP-MS), individual nanoparticles were quantified and characterized regarding their size. SP-ICP-MS measurements were correlated with the information gained using other characterization techniques, i.e. transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The particle surface as an important descriptor of NMs was analyzed by X-ray diffraction (XRD). NM impurities and their co-localization with biomolecules were determined by ion beam microscopy (IBM) and confocal Raman microscopy (CRM). We conclude advantages and disadvantages of the different techniques applied and suggest options for their complementation. Thus, this paper may serve as a practical guide to particle characterization techniques
The Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy
International audienceContext. The far-infrared (FIR) lines are important tracers of the cooling and physical conditions of the interstellar medium (ISM) and are rapidly becoming workhorse diagnostics for galaxies throughout the universe. There are clear indications of a different behavior of these lines at low metallicity that needs to be explored. Aims. Our goal is to explain the main differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies, and how this translates in ISM properties. Methods. We present Herschel/PACS spectroscopic observations of the [Câii] 157 ÎŒm, [Oâi] 63 and 145 ÎŒm, [Oâiii] 88 ÎŒm, [Nâii] 122 and 205 ÎŒm, and [Nâiii] 57 ÎŒm fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. Results. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of [Oâiii]88/[Nâii]122 and [Nâiii]57/[Nâii]122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the [Câii]157/[Oâi]63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the [Oâi]145/[Oâi]63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The [Oâiii]88/[Oâi]63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found to be ~4 times higher in the dwarfs than in metal-rich galaxies. The high [Câii]/LTIR, [Oâi]/LTIR, and [Oâiii]/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate far-UV fields and a low PDR covering factor. Harboring compact phases of a low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that of metal-rich galaxies
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