ABSTRACT
The analysis of star cluster ages in tandem with the morphology of their H ii regions can provide insight into the processes that clear a cluster’s natal gas, as well as the accuracy of ...cluster ages and dust reddening derived from Spectral Energy Distribution (SED) fitting. We classify 3757 star clusters in 16 nearby galaxies according to their H α morphology (concentrated, partially exposed, no emission), using Hubble Space Telescope (HST) imaging from the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). We find: (1) The mean SED ages of clusters with concentrated (1–2 Myr) and partially exposed H ii region morphologies (2–3 Myr) indicate a relatively early onset of gas clearing and a short (1–2 Myr) clearing time-scale. (2) The reddening of clusters can be overestimated due to the presence of red supergiants, which is a result of stochastic sampling of the IMF in low mass clusters. (3) The age-reddening degeneracy impacts the results of the SED fitting – out of 1408 clusters with M* ≥ 5000 M⊙, we find that at least 46 (3 per cent) have SED ages which appear significantly underestimated or overestimated based on H α and their environment, while the total percentage of poor age estimates is expected to be several times larger. (4) Lastly, we examine the dependence of the morphological classifications on spatial resolution. At HST resolution, our conclusions are robust to the distance range spanned by the sample (3–10 Mpc). However, analysis of ground-based H α images shows that compact and partially exposed morphologies frequently cannot be distinguished from each other.
Abstract
We derive the recent star formation histories (SFHs) of 23 active dwarf galaxies using
Hubble Space Telescope
observations from the Legacy Extragalactic UV Survey. We apply a color–magnitude ...diagram (CMD) fitting technique using two independent sets of stellar models, PARSEC-COLIBRI and MIST. Despite the nonnegligible recent activity, none of the 23 star-forming dwarfs show enhancements in the last 100 Myr larger than three times the 100 Myr average. The unweighted mean of the individual SFHs in the last 100 Myr is also consistent with a rather constant activity, irrespective of the atomic gas fraction. We confirm previous results that for dwarf galaxies, the CMD-based average star formation rates (SFRs) are generally higher than the FUV-based SFRs. For half of the sample, the 60 Myr average CMD-based SFR is more than two times the FUV SFR. In contrast, we find remarkable agreement between the 10 Myr average CMD-based SFR and the H
α
-based SFR. Finally, using core helium-burning stars of intermediate mass, we study the pattern of star formation spatial progression over the past 60 Myr and speculate on the possible triggers and connections of the star formation activity with the environment in which these galaxies live. Approximately half of our galaxies show spatial progression of star formation in the last 60 Myr and/or very recent diffuse and off-center activity compared to RGB stars.
Abstract
We use
Hubble Space Telescope
observations from the Legacy Extragalactic UV Survey to reconstruct the recent star formation histories (SFHs) of three actively star-forming dwarf galaxies, ...NGC 4449, Holmberg II, and NGC 1705, from their UV color–magnitude diagrams (CMDs). We apply a CMD fitting technique using two independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the uncertainties related to stellar evolution modeling. Irrespective of the adopted stellar models, all three dwarfs are found to have had almost constant star formation rates (SFRs) in the last 100–200 Myr, with modest enhancements (a factor of ∼2) above the 100 Myr averaged SFR. Significant differences among the three dwarfs are found in terms of the overall SFR, the timing of the most recent peak, and the SFR/area. The initial mass function of NGC 1705 and Holmberg II is consistent with a Salpeter slope down to ≈5
M
⊙
, whereas it is slightly flatter,
s
= −2.0, in NGC 4449. The SFHs derived with the two different sets of stellar models are consistent with each other, except for some quantitative details, attributable to their input assumptions. They also share the drawback that all synthetic diagrams predict a clear separation in color between the upper main-sequence and helium-burning stars, which is not apparent in the data. Since neither differential reddening, which is significant in NGC 4449, nor unresolved binaries appear to be sufficient to fill the gap, we suggest this calls for a revision of both sets of stellar evolutionary tracks.
Mid-infrared observations of the Andromeda galaxy, M31, obtained with the Infrared Array Camera on board the Spitzer Space Telescope are presented. The image mosaics cover areas of approximately 3.7 ...x 1.6 and include the satellite galaxies M32 and NGC 205. The appearance of M31 varies dramatically in the different mid-infrared bands, from the smooth bulge and disk of the old stellar population seen at 3.6 km to the well-known "10 kpc ring" dominating the 8 km image. The similarity of the 3.6 km and optical isophotes and the nearly constant optical-mid-infrared color over the inner 400" confirm that there is no significant extinction at optical wavelengths in M31's bulge. The nuclear colors indicate the presence of dust but not an infrared-bright active galactic nucleus. The integrated 8 km nonstellar luminosity implies a star formation rate of 0.4 M sub( )yr super(-1) consistent with other indicators that show M31 to be a quiescent galaxy.
ABSTRACT
We study the young star cluster populations in 23 dwarf and irregular galaxies observed by the Hubble Space Telescope (HST) Legacy ExtraGalactic Ultraviolet Survey (LEGUS), and examine ...relationships between the ensemble properties of the cluster populations and those of their host galaxies: star formation rate (SFR) density (ΣSFR). A strength of this analysis is the availability of SFRs measured from temporally resolved star formation histories that provide the means to match cluster and host galaxy properties on several time-scales (1–10, 1–100, and 10–100 Myr). Nevertheless, studies of this kind are challenging for dwarf galaxies due to the small numbers of clusters in each system. We mitigate these issues by combining the clusters across different galaxies with similar ΣSFR properties. We find good agreement with a well-established relationship ($M_{V}^{\mathrm{ brightest}}$–SFR), but find no significant correlations between ΣSFR and the slopes of the cluster luminosity function, mass function, nor the age distribution. We also find no significant trend between the fraction of stars in bound clusters at different age ranges (Γ1–10, Γ10–100, and Γ1–100) and ΣSFR of the host galaxy. Our data show a decrease in Γ over time (from 1–10 to 10–100 Myr) suggesting early cluster dissolution, though the presence of unbound clusters in the youngest time bin makes it difficult to quantify the degree of dissolution. While our data do not exhibit strong correlations between ΣSFR and ensemble cluster properties, we cannot rule out that a weak trend might exist given the relatively large uncertainties due to low number statistics and the limited ΣSFR range probed.
Abstract
We present a detailed study of the Magellanic irregular galaxy NGC 4449 based on both archival and new photometric data from the Legacy Extragalactic UV Survey, obtained with the
Hubble ...Space Telescope
Advanced Camera for Surveys and Wide Field Camera 3. Thanks to its proximity (
D
= 3.82 ± 0.27 Mpc), we reach stars 3 mag fainter than the tip of the red giant branch in the F814W filter. The recovered star formation history (SFH) spans the whole Hubble time, but due to the age–metallicity degeneracy of the red giant branch stars, it is robust only over the lookback time reached by our photometry, i.e., ∼3 Gyr. The most recent peak of star formation (SF) is around 10 Myr ago. The average surface density SF rate over the whole galaxy lifetime is 0.01
M
⊙
yr
−1
kpc
−2
. From our study, it emerges that NGC 4449 has experienced a fairly continuous SF regime in the last 1 Gyr, with peaks and dips whose SF rates differ only by a factor of a few. The very complex and disturbed morphology of NGC 4449 makes it an interesting galaxy for studies of the relationship between interactions and starbursts, and our detailed and spatially resolved analysis of its SFH does indeed provide some hints on the connection between these two phenomena in this peculiar dwarf galaxy.
We present new H
I
observations of the nearby massive spiral galaxy M 83 taken with the JVLA at 21″ angular resolution (≈500 pc) of an extended (∼1.5 deg
2
) ten-point mosaic combined with GBT ...single-dish data. We study the super-extended H
I
disk of M 83 (∼50 kpc in radius), in particular disk kinematics, rotation, and the turbulent nature of the atomic interstellar medium. We define distinct regions in the outer disk (
r
gal
> central optical disk), including a ring, a southern area, a southern arm and a northern arm. We examine H
I
gas surface density, velocity dispersion, and noncircular motions in the outskirts, which we compare to the inner optical disk. We find an increase of velocity dispersion (
σ
v
) toward the pronounced H
I
ring, indicative of more turbulent H
I
gas. Additionally, we report over a large galactocentric radius range (until
r
gal
∼ 50 kpc) where
σ
v
is slightly larger than thermal component (i.e., > 8 km s
−1
). We find that a higher star-formation rate (as traced by far UV emission) is not necessarily always associated with a higher H
I
velocity dispersion, suggesting that radial transport could be a dominant driver for the enhanced velocity dispersion. Furthermore, we find a possible branch that connects the extended H
I
disk to the dwarf irregular galaxy UGCA 365 and that deviates from the general direction of the northern arm. Lastly, we compare mass flow rate profiles (based on 2D and 3D tilted ring models) and find evidence for outflowing gas at
r
gal
∼ 2 kpc, inflowing gas at
r
gal
∼ 5.5 kpc, and outflowing gas at
r
gal
∼ 14 kpc. We caution that mass flow rates are highly sensitive to the assumed kinematic disk parameters, in particular to inclination.
We investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where ...typically each star particle represents one 'cluster', for the isolated galaxies we are able to model features we term 'clusters' with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myr) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas on to the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback.
Context. We have obtained Herschel images at five wavelengths from 100 to 500 mu m of a similar to 5.5 x 2.5 degree area centred on the local galaxy M31 (Andromeda), our nearest neighbour spiral ...galaxy, as part of the Herschel guaranteed time project “HELGA”. The main goals of HELGA are to study the characteristics of the extended dust emission, focusing on larger scales than studied in previous observations of Andromeda at an increased spatial resolution, and the obscured star formation. Aims. In this paper we present data reduction and Herschel maps, and provide a description of the far-infrared morphology, comparing it with features seen at other wavelengths. Methods. We used high-resolution maps of the atomic hydrogen, fully covering our fields, to identify dust emission features that can be associated to M31 with confidence, distinguishing them from emission coming from the foreground Galactic cirrus. Results. Thanks to the very large extension of our maps we detect, for the first time at far-infrared wavelengths, three arc-like structures extending out to similar to 21, similar to 26 and similar to 31 kpc respectively, in the south-western part of M31. The presence of these features, hosting similar to 2.2 x 10(6) M-circle dot of dust, is safely confirmed by their detection in HI maps. Overall, we estimate a total dust mass of similar to 5.8 x 107 M-circle dot, about 78% of which is contained in the two main ring-like structures at 10 and 15 kpc, at an average temperature of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a function of the galacto-centric distance, in agreement with the known metallicity gradient, with values ranging from 66 in the nucleus to similar to 275 in the outermost region. Conclusions. Dust in M31 extends significantly beyond its optical radius (similar to 21 kpc) and what was previously mapped in the farinfrared. An annular-like segment, located approximately at R-25, is clearly detected on both sides of the galaxy, and two other similar annular structures are undoubtedly detected on the south-west side even further out.
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