1,613 research outputs found
The New Frontier: Galactic-Scale Star Formation
The arena of investigation of star formation and its scaling laws is slowly,
but consistently, shifting from the realm of luminous galaxies to that of faint
ones and to sub--galactic regions, as existing and new facilities enable
investigators to probe regions of the combined parameter space of surface
brightness, wavelength, and angular resolution that were inaccessible until a
few years ago. We summarize what has been accomplished, and what remain as
challenges in the field of galactic--scale star formation.Comment: accepted for publication on PASP, short review for the IYA2009, 12
pages, no figure
Spitzer 70~m Emission as a SFR Indicator for Sub--Galactic Regions
We use Spitzer 24 m, 70 m and ground based H data for a
sample of 40 SINGS galaxies to establish a star formation rate (SFR) indicator
using 70 m emission for sub--galactic ()
line-emitting regions and to investigate limits in application. A linear
correlation between 70 m and SFR is found and a star formation indicator
SFR(70) is proposed for line-emitting sub-galactic regions as $\rm \Sigma(SFR)\
({M_{\odot}\cdot yr^{-1}\cdot kpc^{-2}})=9.4\times10^{-44}\ \Sigma(70)\
\rm{(ergs\cdot s^{-1}\cdot kpc^{-2})}12+\rm{log(O/H)}\gtrsim8.4\rm \Sigma(SFR)\gtrsim10^{-3}\
(M_{\odot}\cdot yr^{-1}\cdot kpc^{-2})\sigma\sim0.16\mu\sim40%\mu$m emission in galaxies, which can be attributed to stellar populations not
involved in the current star formation activity.Comment: 36 pages, 1 table, 18 figures, accepted by Ap
The Brightest Young Star Clusters in NGC 5253
The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the "radio nebula"). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC 5253 with wavelength coverage from 1500 Å to 1.9 μm in 13 filters. These include Hα, Pβ, and Pα, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the nine optically brightest clusters (M_V < −8.8) and the two young radio nebula clusters. The clusters have ages ~1–15 Myr and masses ~1 × 10^4–2.5 × 10^5 M_⊙. The clusters' spatial location and ages indicate that star formation has become more concentrated toward the radio nebula over the last ~15 Myr. The most massive cluster is in the radio nebula; with a mass ~2.5 × 10^5 M_⊙ and an age ~1 Myr, it is 2–4 times less massive and younger than previously estimated. It is within a dust cloud with A_V ~ 50 mag, and shows a clear near-IR excess, likely from hot dust. The second radio nebula cluster is also ~1 Myr old, confirming the extreme youth of the starburst region. These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars
New Constraints on Mass-Dependent Disruption of Star Clusters in M51
We use UBVI,Ha images of the Whirlpool galaxy, M51, taken with the ACS and
WFPC2 cameras on the Hubble Space Telescope (HST) to select star clusters, and
to estimate their masses and ages by comparing their observed colors with
predictions from population synthesis models. We construct the mass function of
intermediate age (1-4x10^8 yr) clusters, and find that it is well described by
a power law, psi(M) propto M^beta, with beta=-2.1 +/- 0.2, for clusters more
massive than approximately 6x10^3 Msun. This extends the mass function of
intermediate age clusters in M51 to masses lower by nearly a factor of five
over previous determinations. The mass function does not show evidence for
curvature at either the high or low mass end. This shape indicates that there
is no evidence for the earlier disruption of lower mass clusters compared with
their higher mass counterparts (i.e., no mass-dependent disruption) over the
observed range of masses and ages, or for a physical upper mass limit Mc with
which clusters in M51 can form. These conclusions differ from previous
suggestions based on poorer-quality HST observations. We discuss their
implications for the formation and disruption of the clusters. Ages of clusters
in two "feathers," stellar features extending from the outer portion of a
spiral arm, show that the feather with a larger pitch angle formed earlier, and
over a longer period, than the other.Comment: 24 pages, 7 figures; to be published in ApJ, 727, 8
Star-Forming or Starbursting? The Ultraviolet Conundrum
Compared to starburst galaxies, normal star forming galaxies have been shown
to display a much larger dispersion of the dust attenuation at fixed reddening
through studies of the IRX-beta diagram (the IR/UV ratio "IRX" versus the UV
color "beta"). To investigate the causes of this larger dispersion and attempt
to isolate second parameters, we have used GALEX UV, ground-based optical, and
Spitzer infrared imaging of 8 nearby galaxies, and examined the properties of
individual UV and 24 micron selected star forming regions. We concentrated on
star-forming regions, in order to isolate simpler star formation histories than
those that characterize whole galaxies. We find that 1) the dispersion is not
correlated with the mean age of the stellar populations, 2) a range of dust
geometries and dust extinction curves are the most likely causes for the
observed dispersion in the IRX-beta diagram 3) together with some potential
dilution of the most recent star-forming population by older unrelated bursts,
at least in the case of star-forming regions within galaxies, 4) we also
recover some general characteristics of the regions, including a tight positive
correlation between the amount of dust attenuation and the metal content.
Although generalizing our results to whole galaxies may not be immediate, the
possibility of a range of dust extinction laws and geometries should be
accounted for in the latter systems as well.Comment: 18 pages, 17 figures, accepted for publication in Ap
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