1,119 research outputs found
The Very Massive Star Content of the Nuclear Star Clusters in NGC 5253
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-alpha 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 (VMS), with masses greater
than 100 Msun, 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 VMS are
present. We investigate the origin of the observed nitrogen enrichment in the
circum-cluster 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 VMS 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 cut-offs greater than 100 Msun are crucial for future studies of young
massive star clusters at all redshifts.Comment: 11 pages, 7 figures, accepted for publication in Astrophysical
Journa
Star Formation Rate Indicators in Wide-Field Infrared Survey Preliminary Release
With the goal of investigating the degree to which theMIR luminosity in
theWidefield Infrared Survey Explorer (WISE) traces the SFR, we analyze 3.4,
4.6, 12 and 22 {\mu}m data in a sample of {\guillemotright} 140,000
star-forming galaxies or star-forming regions covering a wide range in
metallicity 7.66 < 12 + log(O/H) < 9.46, with redshift z < 0.4. These
star-forming galaxies or star-forming regions are selected by matching the WISE
Preliminary Release Catalog with the star-forming galaxy Catalog in SDSS DR8
provided by JHU/MPA 1.We study the relationship between the luminosity at 3.4,
4.6, 12 and 22 {\mu}m from WISE and H\alpha luminosity in SDSS DR8. From these
comparisons, we derive reference SFR indicators for use in our analysis. Linear
correlations between SFR and the 3.4, 4.6, 12 and 22 {\mu}m luminosity are
found, and calibrations of SFRs based on L(3.4), L(4.6), L(12) and L(22) are
proposed. The calibrations hold for galaxies with verified spectral
observations. The dispersion in the relation between 3.4, 4.6, 12 and 22 {\mu}m
luminosity and SFR relates to the galaxy's properties, such as 4000 {\deg}A
break and galaxy color.Comment: 10 pages, 3 figure
Arm & Interarm Star Formation in Spiral Galaxies
We investigate the relationship between spiral arms and star formation in the
grand-design spirals NGC 5194 and NGC 628 and in the flocculent spiral NGC
6946. Filtered maps of near-IR (3.6 micron) emission allow us to identify "arm
regions" that should correspond to regions of stellar mass density
enhancements. The two grand-design spirals show a clear two-armed structure,
while NGC 6946 is more complex. We examine these arm and interarm regions,
looking at maps that trace recent star formation - far-ultraviolet (GALEX NGS)
and 24 micron emission (Spitzer, SINGS) - and cold gas - CO (Heracles) and HI
(Things). We find the star formation tracers and CO more concentrated in the
spiral arms than the stellar 3.6 micron flux. If we define the spiral arms as
the 25% highest pixels in the filtered 3.6 micron images, we find that the
majority (60%) of star formation tracers occurs in the interarm regions; this
result persists qualitatively even when considering the potential impact of
finite data resolution and diffuse interarm 24 micron emission. Even with a
generous definition of the arms (45% highest pixels), interarm regions still
contribute at least 30% to the integrated star formation rate tracers. We look
for evidence that spiral arms trigger star or cloud formation using the ratios
of star formation rate (SFR, traced by a combination of FUV and 24 micron
emission) to H_2 (traced by CO) and H_2 to HI. Any enhancement of SFR / M(H_2)
in the arm region is very small (less than 10%) and the grand design spirals
show no enhancement compared to the flocculent target. Arm regions do show a
weak enhancement in H_2/HI compared to the interarm regions, but at a fixed gas
surface density there is little clear enhancement in the H_2/HI ratio in the
arm regions. Thus, it seems that spiral arms may only act to concentrate the
gas to higher densities in the arms.Comment: 11 pages, 9 Figures, accepted by Ap
HST resolves stars in a tiny body falling on the dwarf galaxy DDO 68
We present new Hubble Space Telescope (HST) imaging of a stream-like system
associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at a
distance of D12.65 Mpc from us. The stream, previously identified in deep
Large Binocular Telescope images as a diffuse low surface brightness structure,
is resolved into individual stars in the F606W (broad V) and F814W (I)
images acquired with the Wide Field Camera 3. The resulting V, I
color-magnitude diagram (CMD) of the resolved stars is dominated by old
(age1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip,
we conclude that the stream is at the same distance as DDO 68, confirming the
physical association with it. A synthetic CMD analysis indicates that the large
majority of the star formation activity in the stream occurred at epochs
earlier than 1 Gyr ago, and that the star formation at epochs more recent
than 500 Myr ago is compatible with zero. The total stellar mass of the
stream is , about 1/100 of that of DDO~68. This is a
striking example of hierarchical merging in action at the dwarf galaxy scales.Comment: ApJ in pres
Dust heating sources in galaxies: the case of M33 (HERM33ES)
Dust emission is one of the main windows to the physics of galaxies and to
star formation as the radiation from young, hot stars is absorbed by the dust
and reemitted at longer wavelengths. The recently launched Herschel satellite
now provides a view of dust emission in the far-infrared at an unequaled
resolution and quality up to 500 \mu m. In the context of the Herschel HERM33ES
open time key project, we are studying the moderately inclined Scd local group
galaxy M33 which is located only 840 kpc away. In this article, using Spitzer
and Herschel data ranging from 3.6 \mu m to 500 \mu m, along with HI, H\alpha\
maps, and GALEX ultraviolet data we have studied the emission of the dust at
the high spatial resolution of 150 pc. Combining Spitzer and Herschel bands, we
have provided new, inclination corrected, resolved estimators of the total
infrared brightness and of the star formation rate from any combination of
these bands. The study of the colors of the warm and cold dust populations
shows that the temperature of the former is, at high brightness, dictated by
young massive stars but, at lower brightness, heating is taken over by the
evolved populations. Conversely, the temperature of the cold dust is tightly
driven by the evolved stellar populations.Comment: 29 pages, 12 figures, accepted for publication in A
Properties of compact 250 μm emission and H II regions in M 33 (HERM33ES)
Aims. Within the framework of the HERM33ES key program, using the high resolution and sensitivity of the Herschel photometric
data, we study the compact emission in the Local Group spiral galaxy M33 to investigate the nature of the compact SPIRE emission
sources. We extracted a catalogue of sources at 250 μm in order to investigate the nature of this compact emission. Taking advantage
of the unprecedented Herschel resolution at these wavelengths, we also focus on a more precise study of some striking Hα shells in
the northern part of the galaxy.
Methods. We present a catalogue of 159 compact emission sources in M33 identified by SExtractor in the 250 μm SPIRE band that
is the one that provides the best spatial resolution. We also measured fluxes at 24 μm and Hα for those 159 extracted sources. The
morphological study of the shells also benefits from a multiwavelength approach including Hα, far-ultraviolet from GALEX, and
infrared from both Spitzer IRAC 8 μm and MIPS 24 μm in order to make comparisons.
Results. For the 159 compact sources selected at 250 μm, we find a very strong Pearson correlation coefficient with the MIPS 24 μm
emission (r_(24) = 0.94) and a rather strong correlation with the Hα emission, although with more scatter (r_(Hα) = 0.83). The morphological
study of the Hα shells shows a displacement between far-ultraviolet, Hα, and the SPIRE bands. The cool dust emission from
SPIRE clearly delineates the Hα shell structures.
Conclusions. The very strong link between the 250 μm compact emission and the 24 μm and Hα emissions, by recovering the star formation
rate from standard recipes for H II regions, allows us to provide star formation rate calibrations based on the 250 μm compact
emission alone. The different locations of the Hα and far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to
a dynamical age of a few Myr for the Hα shells and the associated cool dust
Quantifying non-star formation associated 8um dust emission in NGC 628
Combining Ha and IRAC images of the nearby spiral galaxy NGC 628, we find
that between 30-43% of its 8um dust emission is not related to recent star
formation. Contributions from dust heated by young stars are separated by
identifying HII regions in the Ha map and using these areas as a mask to
determine the 8um dust emission that must be due to heating by older stars.
Corrections are made for sub-detection-threshold HII regions, photons escaping
from HII regions and for young stars not directly associated to HII regions
(i.e. 10-100 Myr old stars). A simple model confirms this amount of 8um
emission can be expected given dust and PAH absorption cross-sections, a
realistic star-formation history, and the observed optical extinction values. A
Fourier power spectrum analysis indicates that the 8um dust emission is more
diffuse than the Ha emission (and similar to observed HI), supporting our
analysis that much of the 8um-emitting dust is heated by older stars. The 8um
dust-to-Ha emission ratio declines with galactocentric radius both within and
outside of HII regions, probably due to a radial increase in disk transparency.
In the course of this work, we have also found that intrinsic diffuse Ha
fractions may be lower than previously thought in galaxies, if the differential
extinction between HII regions and diffuse regions is taken into account.Comment: 14 pages, 11 figures, accepted in Ap
Properties of compact 250 \mu m emission and HII regions in M33 (HERM33ES)
Within the framework of the HERM33ES Key Project, using the high resolution
and sensitivity of the Herschel photometric data, we study the compact emission
in the Local Group spiral galaxy M33 to investigate the nature of the compact
SPIRE emission sources. We extracted a catalogue of sources at 250um in order
to investigate the nature of this compact emission. Taking advantage of the
unprecedented Herschel resolution at these wavelengths, we also focus on a more
precise study of some striking Halpha shells in the northern part of the
galaxy. We present a catalogue of 159 compact emission sources in M33
identified by SExtractor in the 250um SPIRE band that is the one that provides
the best spatial resolution. We also measured fluxes at 24um and Halpha for
those 159 extracted sources. The morphological study of the shells also
benefits from a multiwavelength approach including Halpha, far-UV from GALEX,
and infrared from both Spitzer IRAC 8um and MIPS 24um in order to make
comparisons. For the 159 compact sources selected at 250um, we find a very
strong Pearson correlation coefficient with the MIPS 24um emission (r24 = 0.94)
and a rather strong correlation with the Halpha emission, although with more
scatter (rHa = 0.83). The morphological study of the Halpha shells shows a
displacement between far-ultraviolet, Halpha, and the SPIRE bands. The cool
dust emission from SPIRE clearly delineates the Halpha shell structures. The
very strong link between the 250um compact emission and the 24um and Halpha
emissions, by recovering the star formation rate from standard recipes for HII
regions, allows us to provide star formation rate calibrations based on the
250um compact emission alone. The different locations of the Halpha and
far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to
a dynamical age of a few Myr for the Halpha shells and the associated cool
dust.Comment: 4 pages, 3 figures, Accpeted for publication in the A&A Herschel
Special Issu
Star formation and mass assembly in high redshift galaxies
We study the star formation and the mass assembly process of 0.3<=z<2.5
galaxies using their IR emission from MIPS 24um band. We used an updated
version of the GOODS-MUSIC catalog, extended by the addition of mid-IR fluxes.
We compared two different estimators of the Star Formation Rate: the total
infrared emission derived from 24um, estimated using both synthetic and
empirical IR templates, and the multiwavelength fit to the full galaxy SED. For
both estimates, we computed the SFR Density and the Specific SFR. The two SFR
tracers are roughly consistent, given the uncertainties involved. However, they
show a systematic trend, IR-based estimates exceeding the fit-based ones as the
SFR increases. We show that: a) at z>0.3, the SFR is well correlated with
stellar mass, and this relationship seems to steepen with redshift (using
IR-based SFRs); b) the contribution to the global SFRD by massive galaxies
increases with redshift up to ~2.5, more rapidly than for galaxies of lower
mass, but appears to flatten at higher z; c) despite this increase, the most
important contributors to the SFRD at any z are galaxies of about, or
immediately lower than, the characteristic stellar mass; d) at z~2, massive
galaxies are actively star-forming, with a median SFR 300 Msun/yr. During this
epoch, they assemble a substantial part of their final stellar mass; e) the
SSFR shows a clear bimodal distribution. The analysis of the SFRD and the SSFR
seems to support the downsizing scenario, according to which high mass galaxies
have formed their stars earlier and faster than their low mass counterparts. A
comparison with theoretical models indicates that they follow the global
increase in the SSFR with redshift and predict the existence of quiescent
galaxies even at z>1.5, but they systematically underpredict the average SSFR.Comment: Accepted by A&A. Multiwavelength catalog available at
http://lbc.mporzio.astro.it/goods. Corrected typo
100 mum and 160 mum emission as resolved star-formation rate estimators in M33 (HERM33ES)
Over the past few years several studies have provided estimates of the SFR
(star-formation rate) or the total infrared luminosity from just one infrared
band. However these relations are generally derived for entire galaxies, which
are known to contain a large scale diffuse emission that is not necessarily
related to the latest star-formation episode. We provide new relations to
estimate the SFR from resolved star-forming regions at 100 mum and 160 mum. We
select individual star-forming regions in the nearby (840 kpc) galaxy M33. We
estimate the SFR combining the emission in Halpha and at 24 mum to calibrate
the emission at 100 mum and 160 mum as SFR estimators, as mapped with
PACS/Herschel. The data are obtained in the framework of the HERM33ES open time
key project. There is less emission in the HII regions at 160 mum than at 100
mum. Over a dynamic range of almost 2 dex in Sigma(SFR) we find that the 100
mum emission is a nearly linear estimator of the SFR, whereas that at 160 mum
is slightly superlinear. The behaviour of individual star-forming regions is
surprisingly similar to that of entire galaxies. At high Sigma(SFR), star
formation drives the dust temperature, whereas uncertainties and variations in
radiation-transfer and dust-heated processes dominate at low Sigma(SFR).
Detailed modelling of both galaxies and individual star forming regions will be
needed to interpret similarities and differences between the two and assess the
fraction of diffuse emission in galaxies.Comment: 5 pages, 3 figures, accepted for publication in the A&A Herschel
special issu
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