We combined optical
Hubble Space Telescope
ACS images with mid-infrared
Spitzer
data of the two young star clusters NGC346 and NGC602 in the Small Magellanic Cloud, to study how local and global ...conditions may affect the process of star formation. We found that, while general conditions such as metallicity, or the mass or morphological type of the parent galaxy do not strongly affect the process of star formation, local conditions such as the gas and stellar density can affect both how star formation occurs and propagates, and also the evolution of a star cluster from early times.
Galaxies in the local Universe are thought to require ongoing replenishment of their gas reservoir in order to maintain the observed star formation rates. Cosmological simulations predict that such ...accretion can occur in both a dynamically hot and cold mode. However, until now observational evidence of the accretion required to match the observed star formation histories is lacking. This paper attempts to determine whether galaxies in the local Universe possess a significant reservoir of HI and what would be the accretion rates derived from such reservoirs. We search the vicinity of 22 nearby galaxies for isolated HI clouds or distinct streams in a systematic and automated manner. The HALOGAS observations represent one of the most sensitive and detailed HI surveys to date. These observations typically reach column density sensitivities of 10^19 cm^-2 over a 20 km/s width. We find 14 secure HI cloud candidates without an observed optical counterpart. These cloud candidates appear to be analogues to the most massive clouds detected around the Milky Way and M31. However, on average their numbers seem significantly reduced. We constrain upper limits for HI accretion in the local Universe. The average HI mass currently observed amounts to a rate of 0.05 Msun/yr with a stringent upper limit of 0.22 Msun/yr, confirming previous estimates. This is much lower than the average star formation rate in this sample. Our best estimate, based on GBT detection limits of several galaxies, suggests that another 0.04 Msun/yr could be accreted from undetected clouds and streams. These results show that in nearby galaxies HI is not being accreted at the same rate as stars are currently being formed. Our study can not exclude that other forms of gas accretion are at work. However, these observations also do not reveal extended neutral gas reservoirs around most nearby spiral galaxies.
The Legacy ExtraGalactic UV Survey (LEGUS) is a multiwavelength Cycle 21 Treasury program on the Hubble Space Telescope. It studied 50 nearby star-forming galaxies in 5 bands from the near-UV to the ...I-band, combining new Wide Field Camera 3 observations with archival Advanced Camera for Surveys data. LEGUS was designed to investigate how star formation occurs and develops on both small and large scales, and how it relates to the galactic environments. In this paper we present the photometric catalogs for all the apparently single stars identified in the 50 LEGUS galaxies. Photometric catalogs and mosaicked images for all filters are available for download. We present optical and near-UV color-magnitude diagrams for all the galaxies. For each galaxy we derived the distance from the tip of the red giant branch. We then used the NUV color-magnitude diagrams to identify stars more massive than 14 M , and compared their number with the number of massive stars expected from the GALEX FUV luminosity. Our analysis shows that the fraction of massive stars forming in star clusters and stellar associations is about constant with the star formation rate. This lack of a relation suggests that the timescale for evaporation of unbound structures is comparable or longer than 10 Myr. At low star formation rates this translates to an excess of mass in clustered environments as compared to model predictions of cluster evolution, suggesting that a significant fraction of stars form in unbound systems.
We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) ...Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes and a truncation of a few times 105 . After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find mass-independent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (≤104 ) clusters, suggesting that a mass-dependent component is necessary to fully describe the YSC disruption process in NGC 628.
PDF
