ABSTRACT The goal of this paper is to test whether the formation rate of star clusters is proportional to the star formation rate (SFR) in galaxies. As a first step, we present the mass functions of ...compact clusters younger than 10 Myr in seven star-forming galaxies of diverse masses, sizes, and morphologies: the Large and Small Magellanic Clouds, NGC 4214, NGC 4449, M83, M51, and the Antennae. These cluster mass functions (CMFs) are well represented by power laws, , with similar exponents , but with amplitudes that differ by factors up to , corresponding to vast differences in the sizes of the cluster populations in these galaxies. We then normalize these CMFs by the SFRs in the galaxies, derived from dust-corrected H luminosities, and find that the spread in the amplitudes collapses, with a remaining rms deviation of only . This is close to the expected dispersion from random uncertainties in the CMFs and SFRs. Thus, the data presented here are consistent with exact proportionality between the formation rates of stars and clusters. However, the data also permit weak deviations from proportionality, at the factor of two level, within the statistical uncertainties. We find the same spread in amplitudes when we normalize the mass functions of much older clusters, with ages in the range 100-400 Myr, by the current SFR. This is another indication of the general similarity among the cluster populations of different galaxies.
We present the highest spatial resolution ( approximately 0."5) CO (3-2) observations to date of the "overlap" region in the merging Antennae galaxies (NGC 4038/39), taken with the Atacama Large ...Millimeter/submillimeter Array. We report on the discovery of a long (3 kpc), thin (aspect ratio 30/1), filament of CO gas that breaks up into roughly 10 individual knots. At the other extreme, we find that the individual clouds in the supergiant molecular cloud 2 region discussed by Wilson and collaborators have a large range of internal velocity dispersions (10 to 80kms super(-1)), and a large dispersion among the ensemble ( approximately 80kms super(-1)). Using the evolutionary framework, we estimate that the maximum age range of clusters in a single GMC is approximately 10Myr, which suggests that the molecular gas is removed over this timescale, resulting in the cessation of star formation and the destruction of the GMC within a radius of about 200 pc.
VERSION 1 OF THE HUBBLE SOURCE CATALOG Whitmore, Bradley C.; Allam, Sahar S.; Budavári, Tamás ...
The Astronomical journal,
06/2016, Letnik:
151, Številka:
6
Journal Article
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Odprti dostop
ABSTRACT The Hubble Source Catalog is designed to help optimize science from the Hubble Space Telescope (HST) by combining the tens of thousands of visit-based source lists in the Hubble Legacy ...Archive (HLA) into a single master catalog. Version 1 of the Hubble Source Catalog includes WFPC2, ACS/WFC, WFC3/UVIS, and WFC3/IR photometric data generated using SExtractor software to produce the individual source lists. The catalog includes roughly 80 million detections of 30 million objects involving 112 different detector/filter combinations, and about 160,000 HST exposures. Source lists from Data Release 8 of the HLA are matched using an algorithm developed by Budavári & Lubow. The mean photometric accuracy for the catalog as a whole is better than 0.10 mag, with relative accuracy as good as 0.02 mag in certain circumstances (e.g., bright isolated stars). The relative astrometric residuals are typically within 10 mas, with a value for the mode (i.e., most common value) of 2.3 mas. The absolute astrometric accuracy is better than 0 1 for most sources, but can be much larger for a fraction of fields that could not be matched to the PanSTARRS, SDSS, or 2MASS reference systems. In this paper we describe the database design with emphasis on those aspects that enable the users to fully exploit the catalog while avoiding common misunderstandings and potential pitfalls. We provide usage examples to illustrate some of the science capabilities and data quality characteristics, and briefly discuss plans for future improvements to the Hubble Source Catalog.
We estimate the fraction of stars that form in compact clusters (bound and unbound), , in a diverse sample of eight star-forming galaxies, including two irregulars, two dwarf starbursts, two spirals, ...and two mergers. The average value for our sample is . We also calculate the fraction of stars in clusters that survive to ages between and , denoted by , and find and , significantly lower than for the same galaxies. We do not find any systematic trends in or with the star formation rate (SFR), the SFR per unit area ( ), or the surface density of molecular gas ( ) within the host galaxy. Our results are consistent with those found previously from the CMF/SFR statistic (where CMF is the cluster mass function), and with the quasi-universal model in which clusters in different galaxies form and disrupt in similar ways. Our results, however, contradict many previous claims that the fraction of stars in bound clusters increases strongly with and . We find that the previously reported trends are largely driven by comparisons that mixed and , where was systematically used for galaxies with higher and , and for galaxies with lower and .
Abstract
Nearly all current simulations predict that outcomes of the star formation process, such as the fraction of stars that form in bound clusters (Γ), depend on the intensity of star formation ...activity (Σ
SFR
) in the host galaxy. The exact shape and strength of the predicted correlations, however, vary from simulation to simulation. Observational results also remain unclear at this time, because most works have mixed estimates made from very young clusters for galaxies with higher Σ
SFR
with those from older clusters for galaxies with lower Σ
SFR
. The three blue compact dwarf (BCD) galaxies ESO 185-IG13, ESO 338-IG04, and Haro 11 have played a central role on the observational side because they have some of the highest known Σ
SFR
and published values of Γ. We present new estimates of Γ for these BCDs in three age intervals (1–10 Myr, 10–100 Myr, 100–400 Myr), based on age-dating, which includes H
α
photometry to better discriminate between clusters younger and older than ≈10 Myr. We find significantly lower values for Γ(1–10 Myr) than published previously. The likely reason for the discrepancy is that previous estimates appear to be based on age–reddening results that underestimated ages and overestimated reddening for many clusters, artificially boosting Γ(1–10 Myr). We also find that fewer stars remain in clusters over time, with ≈15%–39% in 1–10 Myr clusters, ≈5%–7% in 10–100 Myr clusters, and ≈1%–2% in 100–400 Myr clusters. We find no evidence that Γ increases with Σ
SFR
. These results imply that cluster formation efficiency does not vary with star formation intensity in the host galaxy. If confirmed, our results will help guide future assumptions in galaxy-scale simulations of cluster formation and evolution.
Abstract
The PHANGS program is building the first data set to enable the multiphase, multiscale study of star formation across the nearby spiral galaxy population. This effort is enabled by large ...survey programs with the Atacama Large Millimeter/submillimeter Array (ALMA), MUSE on the Very Large Telescope, and the Hubble Space Telescope (HST), with which we have obtained CO(2–1) imaging, optical spectroscopic mapping, and high-resolution UV–optical imaging, respectively. Here, we present PHANGS-HST, which has obtained NUV–
U
–
B
–
V
–
I
imaging of the disks of 38 spiral galaxies at distances of 4–23 Mpc, and parallel
V
- and
I
-band imaging of their halos, to provide a census of tens of thousands of compact star clusters and multiscale stellar associations. The combination of HST, ALMA, and VLT/MUSE observations will yield an unprecedented joint catalog of the observed and physical properties of ∼100,000 star clusters, associations, H
ii
regions, and molecular clouds. With these basic units of star formation, PHANGS will systematically chart the evolutionary cycling between gas and stars across a diversity of galactic environments found in nearby galaxies. We discuss the design of the PHANGS-HST survey and provide an overview of the HST data processing pipeline and first results. We highlight new methods for selecting star cluster candidates, morphological classification of candidates with convolutional neural networks, and identification of stellar associations over a range of physical scales with a watershed algorithm. We describe the cross-observatory imaging, catalogs, and software products to be released. The PHANGS high-level science products will seed a broad range of investigations, in particular, the study of embedded stellar populations and dust with the James Webb Space Telescope, for which a PHANGS Cycle 1 Treasury program to obtain eight-band 2–21
μ
m imaging has been approved.
The Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) have been used to obtain new Hubble Space Telescope images of NGC 4038/4039 ('The Antennae'). These ...new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function (LF) by approximately 2 mag over our previous WFPC2 results, and find that it continues as a single power law, dN/dL L Delta *a with Delta *a = --2.13 ? 0.07, down to the observational limit of MV --7. Similarly, the mass function (MF) is a single power law dN/dM M Delta *b with Delta *b = --2.10 ? 0.20 for clusters with ages <3 X 108 yr, corresponding to lower mass limits that range from 104 to 105 M , depending on the age range of the subsample. Hence, the power-law indices for the luminosity and MFs are essentially the same. The LF for intermediate-age clusters (i.e., ~100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to MV --6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until MV --4. An analysis of individual ~0.5 kpc sized areas over diverse environments shows good agreement between values of Delta *a and Delta *b, similar to the results for the total population of clusters in the system. There is tentative evidence that the values of both Delta *a and Delta *b are flatter for the youngest clusters in some areas, but it is possible that this is caused by observational biases. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with a ~10-50 Myr old cluster of estimated mass ~106 M having apparently triggered the formation of several younger, more massive (up to 5 X 106 M ) clusters along a dust lane. A comparison with new NICMOS observations reveals that only 16% ? 6% of the IR-bright clusters in the Antennae are still heavily obscured, with values of AV >3 mag.
We study star clusters in two fields in the nearby spiral galaxy M83 using broadband and narrowband optical imaging taken with the Wide Field Camera 3 onboard the Hubble Space Telescope. The age ...distributions can be described by a power law, dN/dtau is proportional to taugamma, with gamma approximately -0.84 + or - 0.12 in the inner field, and gamma approximately -0.48 + or - 0.12 in the outer field for tau gap 10 Myr. We bracket the difference, Deltagamma, between the two fields to be in the 0.18 to 0.36 range, based on estimates of the relative star-formation histories. We compare the shapes of the distributions with those predicted by two popular cluster disruption models, and find that both show evidence that the clusters are disrupted at a rate that is approximately independent of their mass, and that observational results do not support the earlier disruption of lower-mass clusters relative to their higher-mass counterparts.
These cluster catalogs provide one of the largest sets of uniform, automatically generated cluster candidates available in the literature at present. A comparison of fitting results based on methods ...that use binned and unbinned data shows good agreement, although there may be a systematic tendency for the unbinned (maximum likelihood) method to give slightly more negative values of alpha for galaxies with steeper LFs. We find that galaxies with high rates of star formation (or equivalently, with the brightest or largest numbers of clusters) have a slight tendency to have shallower values of alpha . We examine the bright end of the LFs and find evidence for a downturn, although it only pertains to about 1% of the clusters. Our uniform database results in a small scatter ( approximately 0.4 to 0.5 mag) in the correlation between the magnitude of the brightest cluster (M sub(brightest)) and log of the number of clusters brighter than M sub(l) = -9 (log N).
We compare the observed bivariate distribution of masses (M) and ages (tau) of star clusters in the Large Magellanic Cloud (LMC) with the predicted distributions g(M, tau) from three idealized models ...for the disruption of star clusters: (1) sudden mass-dependent disruption, (2) gradual mass-dependent disruption, and (3) gradual mass-independent disruption. The model with mass-independent disruption provides a good, first-order description of these cluster populations, with g(M, tau) {proportional_to} M {sup beta}tau{sup g}amma, beta = -1.8 +- 0.2 and gamma = -0.8 +- 0.2, at least for clusters with ages tau {approx}< 10{sup 9} yr and masses M {approx}> 10{sup 3} M{sub sun} (more specifically, tau {approx}< 10{sup 7}(M/10{sup 2} M{sub sun}){sup 1.3} yr). This model predicts that the clusters should have a power-law luminosity function, dN/dL {proportional_to} L {sup -1.8}, in agreement with observations. The first two models, on the other hand, fare poorly when describing the observations, refuting previous claims that mass-dependent disruption of star clusters is observed in the LMC over the studied M-tau domain. Clusters in the SMC can be described by the same g(M, tau) distribution as for the LMC, but with smaller samples and hence larger uncertainties. The successful g(M, tau) model for clusters in the Magellanic Clouds is virtually the same as the one for clusters in the merging Antennae galaxies, but extends the domain of validity to lower masses and to older ages. This indicates that the dominant disruption processes are similar in these very different galaxies over at least tau {approx}< 10{sup 8} yr and possibly tau {approx}< 10{sup 9} yr. The mass functions for young clusters in the LMC are power laws, while that for ancient globular clusters is peaked. We show that the observed shapes of these mass functions are consistent with expectations from the simple evaporation model presented by McLaughlin and Fall.
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