ABSTRACT The dust attenuation for a sample of ∼10,000 local (z 0.1) star-forming galaxies is constrained as a function of their physical properties. We utilize aperture-matched multiwavelength data ...available from the Galaxy Evolution Explorer and the Sloan Digital Sky Survey to ensure that regions of comparable size in each galaxy are being analyzed. We follow the method of Calzetti et al. and characterize the dust attenuation through the UV power-law index, β, and the dust optical depth, which is quantified using the difference in Balmer emission line optical depth, . The observed linear relationship between β and is similar to the local starburst relation, but the large scatter ( int = 0.44) suggests that there is significant variation in the local universe. We derive a selective attenuation curve over the range 1250 Å < λ < 8320 Å and find that a single attenuation curve is effective for characterizing the majority of galaxies in our sample. This curve has a slightly lower selective attenuation in the UV compared to previously determined curves. We do not see evidence to suggest that a 2175 Å feature is significant in the average attenuation curve. Significant positive correlations are seen between the amount of UV and optical reddening and galaxy metallicity, mass, star formation rate (SFR), and SFR surface density. This provides a potential tool for gauging attenuation where the stellar population is unresolved, such as at high z.
We study the spectral energy distribution (SED) of the radio continuum (RC) emission from the Key Insight in Nearby Galaxies Emitting in Radio (KINGFISHER) sample of nearby galaxies to understand the ...energetics and origin of this emission. Effelsberg multi-wavelength observations at 1.4, 4.8, 8.4, and 10.5 GHz combined with archive data allow us, for the first time, to determine the mid-RC (1-10 GHz, MRC) bolometric luminosities and further present calibration relations versus the monochromatic radio luminosities. The 1-10 GHz radio SED is fitted using a Bayesian Markov Chain Monte Carlo technique leading to measurements for the nonthermal spectral index ( ) and the thermal fraction ( ) with mean values of for the total spectral index) and = (10 9)% at 1.4 GHz. The MRC luminosity changes over ∼3 orders of magnitude in the sample, MRC . The thermal emission is responsible for ∼23% of the MRC on average. We also compare the extinction-corrected diagnostics of the star-formation rate (SFR) with the thermal and nonthermal radio tracers and derive the first star-formation calibration relations using the MRC radio luminosity. The nonthermal spectral index flattens with increasing SFR surface density, indicating the effect of the star-formation feedback on the cosmic-ray electron population in galaxies. Comparing the radio and IR SEDs, we find that the FIR-to-MRC ratio could decrease with SFR, due to the amplification of the magnetic fields in star-forming regions. This particularly implies a decrease in the ratio at high redshifts, where mostly luminous/star-forming galaxies are detected.
ABSTRACT
We present the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey. We fit HST NUV to NIR broad-band and H α fluxes to derive star cluster ages, masses, and extinctions ...and determine the star formation rate (SFR) of six merging galaxies. These systems are excellent laboratories to trace cluster formation under extreme gas physical conditions, rare in the local Universe, but typical for star-forming galaxies at cosmic noon. We detect clusters with ages of 1–500 Myr and masses that exceed 107 M⊙. The recent cluster formation history and their distribution within the host galaxies suggest that systems such as NGC 34, NGC 1614, and NGC 4194 are close to their final coalescing phase, while NGC 3256, NGC 3690, and NGC 6052 are at an earlier/intermediate stage. A Bayesian analysis of the cluster mass function in the age interval 1–100 Myr provides strong evidence in four of the six galaxies that an exponentially truncated power law better describes the observed mass distributions. For two galaxies, the fits are inconclusive due to low number statistics. We determine power-law slopes β ∼ −1.5 to −2.0 and truncation masses, Mc, between 106 and a few times 107 M⊙, among the highest values reported in the literature. Advanced mergers have higher Mc than early/intermediate merger stage galaxies, suggesting rapid changes in the dense gas conditions during the merger. We compare the total stellar mass in clusters to the SFR of the galaxy, finding that these systems are among the most efficient environments to form star clusters in the local Universe.
ABSTRACT The blue compact dwarf galaxy NGC 5253 hosts a very young starburst containing twin nuclear star clusters, separated by a projected distance of 5 pc. One cluster (#5) coincides with the peak ...of the H emission and the other (#11) with a massive ultracompact H ii region. A recent analysis of these clusters shows that they have a photometric age of 1 1 Myr, in apparent contradiction with the age of 3-5 Myr inferred from the presence of Wolf-Rayet features in the cluster #5 spectrum. We examine Hubble Space Telescope ultraviolet and Very Large Telescope optical spectroscopy of #5 and show that the stellar features arise from very massive stars (VMSs), with masses greater than 100 M , at an age of 1-2 Myr. We further show that the very high ionizing flux from the nuclear clusters can only be explained if VMSs are present. We investigate the origin of the observed nitrogen enrichment in the circumcluster ionized gas and find that the excess N can be produced by massive rotating stars within the first 1 Myr. We find similarities between the NGC 5253 cluster spectrum and those of metal-poor, high-redshift galaxies. We discuss the presence of VMSs in young, star-forming galaxies at high redshift; these should be detected in rest-frame UV spectra to be obtained with the James Webb Space Telescope. We emphasize that population synthesis models with upper mass cutoffs greater than 100 M are crucial for future studies of young massive star clusters at all redshifts.
Abstract
We present a study of the effective (half-light) radii and other structural properties of a systematically selected sample of young, massive star clusters (≥5 × 10
3
and ≤200 Myr) in two ...nearby spiral galaxies, NGC 628 and NGC 1313. We use
Hubble Space Telescope
(
HST
) WFC3/UVIS and archival ACS/WFC data obtained by the Legacy Extragalactic UV Survey (LEGUS), an
HST
Treasury Program. We measure effective radii with GALFIT, a two-dimensional image-fitting package, and with a new technique to estimate effective radii from the concentration index of observed clusters. The distribution of effective radii from both techniques spans ∼0.5–10 pc and peaks at 2–3 pc for both galaxies. We find slight positive correlations between effective radius and cluster age in both galaxies, but no significant relationship between effective radius and galactocentric distance. Clusters in NGC 1313 display a mild increase in effective radius with cluster mass, but the trend disappears when the sample is divided into age bins. We show that the vast majority of the clusters in both galaxies are much older than their dynamical times, suggesting they are gravitationally bound objects. We find that about half of the clusters in NGC 628 are underfilling their Roche lobes, based on their Jacobi radii. Our results suggest that the young, massive clusters in NGC 628 and NGC 1313 are expanding, due to stellar mass loss or two-body relaxation, and are not significantly influenced by the tidal fields of their host galaxies.
Interstellar dust and starlight are modeled for the galaxies of the project "Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel." The galaxies were observed by the Infrared Array ...Camera and the Multiband Imaging Photometer for Spitzer on Spitzer Space Telescope, and the Photodetector Array Camera and Spectrometer and the Spectral and Photometric Imaging Receiver on Herschel Space Observatory. With data from 3.6 to 500 m, dust models are strongly constrained. Using a physical dust model, for each pixel in each galaxy we estimate (1) dust surface density, (2) dust mass fraction in polycyclic aromatic hydrocarbons (PAHs), (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in subregions with high starlight intensity. The dust models successfully reproduce the observed global and resolved spectral energy distributions. With the angular resolution of Herschel, we obtain well-resolved maps (available online) for the dust properties. As in previous studies, we find the PAH fraction to be an increasing function of metallicity, with a threshold oxygen abundance Z/Z 0.1, but we find the data to be fitted best with increasing linearly with above a threshold value of 0.15(O/H) . We obtain total dust masses for each galaxy by summing the dust mass over the individual map pixels; these "resolved" dust masses are consistent with the masses inferred from a model fit to the global photometry. The global dust-to-gas ratios obtained from this study are found to correlate with galaxy metallicities. Systems with Z/Z 0.5 have most of their refractory elements locked up in dust, whereas in systems with Z/Z 0.3 most of these elements tend to remain in the gas phase. Within galaxies, we find that is suppressed in regions with unusually warm dust with . With knowledge of one long-wavelength flux density ratio (e.g., f160/f500), the minimum starlight intensity heating the dust ( ) can be estimated to within ∼50%, despite a variation in of more than two orders of magnitude. For the adopted dust model, dust masses can be estimated to within ∼0.2 dex accuracy using the f160/f500 flux ratio and the integrated dust luminosity, and to ∼0.07 dex accuracy using the 500 m luminosity alone. There are additional systematic errors arising from the choice of dust model, but these are hard to estimate. These calibrated prescriptions for estimating starlight heating intensity and dust mass may be useful for studies of high-redshift galaxies.
We update the spectral modeling code MAGPHYS to include a 2175 absorption feature in its UV to near-IR dust attenuation prescription. This allows us to determine the strength of this feature and the ...shape of the dust attenuation curve in ∼5000 star-forming galaxies at in the COSMOS field. We find that a 2175 absorption feature of ∼1/3 the strength of that in the Milky Way is required for models to minimize residuals. We characterize the total effective dust attenuation curves as a function of several galaxy properties and find that the UV slopes of the attenuation curve for COSMOS galaxies show a strong dependence on star formation rate (SFR) and total dust attenuation (AV), such that galaxies with higher SFR and AV have shallower curves and vice versa. These results are consistent with expectations from radiative transfer that attenuation curves become shallower as the effective dust optical depth increases. We do not find significant trends in the strength of the 2175 absorption feature as a function of galaxy properties, but this may result from the high uncertainties associated with this measurement. The updated code is publicly available online (following acceptance).
We present a study of the hierarchical clustering of the young stellar clusters in six local (3-15 Mpc) star-forming galaxies using Hubble Space Telescope broadband WFC3/UVIS UV and optical images ...from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey). We identified 3685 likely clusters and associations, each visually classified by their morphology, and we use the angular two-point correlation function to study the clustering of these stellar systems. We find that the spatial distribution of the young clusters and associations are clustered with respect to each other, forming large, unbound hierarchical star-forming complexes that are in general very young. The strength of the clustering decreases with increasing age of the star clusters and stellar associations, becoming more homogeneously distributed after ∼40-60 Myr and on scales larger than a few hundred parsecs. In all galaxies, the associations exhibit a global behavior that is distinct and more strongly correlated from compact clusters. Thus, populations of clusters are more evolved than associations in terms of their spatial distribution, traveling significantly from their birth site within a few tens of Myr, whereas associations show evidence of disruption occurring very quickly after their formation. The clustering of the stellar systems resembles that of a turbulent interstellar medium that drives the star formation process, correlating the components in unbound star-forming complexes in a hierarchical manner, dispersing shortly after formation, suggestive of a single, continuous mode of star formation across all galaxies.
We characterize the influence that inclination has on the shape and normalization in average dust attenuation curves derived from a sample of ∼10,000 local star-forming galaxies. To do this, we ...utilize aperture-matched multiwavelength data from the Galaxy Evolution Explorer, the Sloan Digital Sky Survey, the United Kingdom Infrared Telescope, and the Two Micron All-sky Survey. We separate our sample into groups according to axial ratio (b/a) and obtain an estimate of their average total-to-selective attenuation . The attenuation curves are found to be shallower at UV wavelengths with increasing inclination, whereas the shape at longer wavelengths remains unchanged. The highest inclination subpopulation ( ) exhibits an NUV excess in its average selective attenuation, which, if interpreted as a 2175 feature, is best fit with a bump strength of 17%-26% of the MW value. No excess is apparent in the average attenuation curve of lower inclination galaxies. The differential reddening between the stellar continuum and ionized gas is found to decrease with increasing inclination. We find that higher inclination galaxies have slightly higher values of RV, although this is poorly constrained given the uncertainties. We outline possible explanations for these trends within a two component dust model (dense cloud+diffuse dust) and find that they can be naturally explained if carriers of the 2175 feature are preferentially destroyed in star-forming regions (UV-bright regions).
We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that ...star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25-0.6 power, and that the maximum size over which star formation is physically correlated ranges from ∼200 pc to ∼1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy's shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.
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