142 research outputs found
Star formation laws in the Andromeda galaxy: Gas, stars, metals and the surface density of star formation
We use hierarchical Bayesian regression analysis to investigate star formation laws in the Andromeda galaxy (M31) in both local (30, 155 and 750 pc) and global cases. We study and compare the well-known Kennicutt-Schmidt law, the extended Schmidt law and the metallicity/star formation correlation. Using a combination of Hα and 24 μm emission, a combination of far-ultraviolet and 24 μm, and the total infrared emission, we estimate the total star formation rate (SFR) in M31 to be between 0.35 ± 0.04 and 0.4 ± 0.04 M⊙ yr-1. We produce a stellar mass surface density map using IRAC 3.6 μm emission and measured the total stellar mass to be 6.9 × 1010 M⊙. For the Kennicutt-Schmidt law in M31, we find the power-law index N to be between 0.49 and 1.18; for all the laws, the power-law index varies more with changing gas tracer than with SFR tracer. The power-law index also changes with distance from the centre of the galaxy. We also applied the commonly used ordinary least-squares fitting method and showed that using different fitting methods leads to different power-law indices. There is a correlation between the surface density of SFR and the stellar mass surface density, which confirms that the Kennicutt-Schmidt law needs to be extended to consider the other physical properties of galaxies. We found a weak correlation between metallicity, the SFR and the stellar mass surface density
The dustier early-type galaxies deviate from late-type galaxies\u27 scaling relations
Several dedicated surveys focusing on early-type galaxies (ETGs) reveal that significant fractions of them are detectable in all interstellar medium phases studied to date. We select ETGs from the Herschel Reference Survey that have both far-infrared Herschel and either H I or CO detection (or both).We derive their star formation rates (SFRs), stellar masses and dust masses viamodelling their spectral energy distributions.We combine these with literature information on their atomic and molecular gas properties, in order to relate their star formation, total gas mass and dust mass on global scales. The ETGs deviate from the dust mass-SFR relation and the Schmidt-Kennicutt relation that SDSS star-forming galaxies define: compared to SDSS galaxies, ETGs have more dust at the same SFR, or less SFR at the same dust mass. When placing them in the M*-SFR plane, ETGs show a much lower specific SFR as compared to normal star-forming galaxies. ETGs show a large scatter compared to the Schmidt-Kennicutt relation found locally within our Galaxy, extending to lower SFRs and gas mass surface densities. Using an ETG\u27s SFR and the Schmidt-Kennicutt law to predict its gas mass leads to an underestimate. ETGs have similar observed-gas-to-modelled-dust mass ratios to star-forming galaxies of the same stellar mass, as well as they exhibit a similar scatter
Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group
We focus on the resolved stellar populations of one early- and four
transition-type dwarf galaxies in the Sculptor group, with the aim to examine
the potential presence of population gradients and place constraints on their
mean metallicities. We use deep HST images to construct CMDs, from which we
select stellar populations that trace different evolutionary phases in order to
constrain their range of ages and metallicities, as well as to examine their
spatial distribution. In addition, we use the resolved stars in the RGB in
order to derive photometric metallicities. All studied dwarfs contain
intermediate-age stars with ages of ~1Gyr and older as traced by the luminous
asymptotic giant branch and red clump stars, while the transition-type dwarfs
contain also stars younger than ~1Gyr as traced by a young main sequence and
vertical red clump stars. Moreover, the spatial distribution of the stars that
trace different evolutionary phases shows a population gradient in all
transition-type dwarfs. The derived error-weighted mean metallicities, assuming
purely old stellar populations, range from -1.5dex for ESO294-G010 to -1.9dex
for Scl-dE1, and should be considered as lower limits to their true
metallicities. Assuming intermediate-age stellar populations to dominate the
dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex
higher than the metallicities derived assuming purely old populations. We
discuss how photometric metallicity gradients are affected by the
age-metallicity degeneracy, which prevents strong conclusions regarding their
actual presence. Finally, the transition-type dwarfs lie beyond the virial
radius of their closest bright galaxy, as also observed for the LG
transition-type dwarfs. Scl-dE1 is the only dSph in our sample and is an
outlier in a potential morphology-distance relation, similar as the two
isolated dSphs of the LG, Tucana and Cetus.Comment: A&A published; 21 pages, 16 figures, 3 appendices; replaced to match
the published versio
Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group
Aims. We focus on the resolved stellar populations of one early-type and four transition-type dwarf galaxies in the Sculptor group, with the aim to examine the potential presence of population gradients and place constraints on their mean metallicities. Methods. We use deep Hubble Space Telescope images to construct color-magnitude diagrams, from which we select stellar populations that trace different evolutionary phases in order to constrain their range of ages and metallicities, as well as to examine their spatial distribution. In addition, we use the resolved stars in the red giant branch in order to derive photometric metallicities. Results. All studied dwarfs contain intermediate-age stars with ages of ∼1 Gyr and older as traced by the luminous asymptotic giant branch and red clump stars, while the transition-type dwarfs contain also stars younger than ∼1 Gyr as traced by a young main sequence and vertical red clump stars. Moreover, the spatial distribution of the stars that trace different evolutionary phases shows a population gradient in all transition-type dwarfs. The derived error-weighted mean metallicities, assuming purely old stellar populations, range from -1.5 dex for ESO294-G010 to -1.9 dex for Scl-dE1, and should be considered as lower limits to their true metallicities. Assuming intermediate-age stellar populations to dominate the dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex higher than the metallicities derived assuming purely old populations. We discuss how photometric metallicity gradients are affected by the age-metallicity degeneracy, which prevents strong conclusions regarding their actual presence. Finally, the transition-type dwarfs lie beyond the virial radius of their closest bright galaxy, as also observed for the Local Group transition-type dwarfs. Scl-dE1 is the only dwarf spheroidal in our sample and is an outlier in a potential morphology-distance relation, similar as the two isolated dwarf spheroidals of the Local Group, Tucana, and Cetus
The massive star population of the Virgo Cluster galaxy NGC 4535
We analyzed the massive star population of the Virgo Cluster galaxy NGC 4535
using archival Hubble Space Telescope Wide Field Planetary Camera 2 images in
filters F555W and F814W, equivalent to Johnson V and Kron-Cousins I. We
performed high precision point spread function fitting photometry of 24353
sources including 3762 candidate blue supergiants, 841 candidate yellow
supergiants and 370 candidate red supergiants. We estimated the ratio of blue
to red supergiants as a decreasing function of galactocentric radius. Using
Modules for Experiments in Stellar Astrophysics isochrones at solar
metallicity, we defined the luminosity function and estimated the star
formation history of the galaxy over the last 60 Myrs. We conducted a
variability search in the V and I filters using three variability indexes: the
median absolute deviation, the interquartile range and the inverse von-Neumann
ratio. This analysis yielded 120 new variable candidates with absolute
magnitudes ranging from M = 4 to 11 mag. We used the MESA
evolutionary tracks at solar metallicity, to classify the variables based on
their absolute magnitude and their position on the color-magnitude diagram.
Among the new candidate variable sources are eight candidate variable red
supergiants, three candidate variable yellow supergiants and one candidate
luminous blue variable, which we suggest for follow-up observations.Comment: Accepted by A&A, 7 pages, 7 Tables, 53 figure
The first maps of κd - the dust mass absorption coefficient - in nearby galaxies, with DustPedia
The dust mass absorption coefficient, κd is the conversion function used to infer physical dust masses from observations of dust emission. However, it is notoriously poorly constrained, and it is highly uncertain how it varies, either between or within galaxies. Here we present the results of a proof-of-concept study, using the DustPedia data for two nearby face-on spiral galaxies M 74 (NGC 628) and M 83 (NGC 5236), to create the first ever maps of κd in galaxies. We determine κd using an empirical method that exploits the fact that the dust-to-metals ratio of the interstellar medium is constrained by direct measurements of the depletion of gas-phase metals. We apply this method pixel-by-pixel within M 74 and M 83, to create maps of κd. We also demonstrate a novel method of producing metallicity maps for galaxies with irregularly sampled measurements, using the machine learning technique of Gaussian process regression. We find strong evidence for significant variation in κd. We find values of κd at 500 μm spanning the range 0.11-0.25 m^{2 kg^{-1}} in M 74, and 0.15-0.80 m^{2 kg^{-1}} in M 83. Surprisingly, we find that κd shows a distinct inverse correlation with the local density of the interstellar medium. This inverse correlation is the opposite of what is predicted by standard dust models. However, we find this relationship to be robust against a large range of changes to our method - only the adoption of unphysical or highly unusual assumptions would be able to suppress it
HST WFPC2 Observations of the Peculiar Main Sequence of the Double Star Cluster NGC 2011 in the Large Magellanic Cloud
We report the serendipitous discovery of a peculiar main sequence in archived
Hubble Space Telescope WFPC2 observations of the young star cluster NGC 2011 in
the Large Magellanic Cloud. The bright part of this main sequence exhibits a
prominent double, fork-like feature, as if it consists of twin main sequences,
one of them being redder. The color-magnitude diagram, constructed from the
stars found in the only available WFPC2 field of the cluster, is used to
distinguish the stars according to their membership to each of these sequences
and to study their spatial distribution. We find that there are two well
distinguished populations in the sense that the redder main sequence is
dominated by stars that belong to the main body of the cluster, while the stars
of the bluer main sequence belong to the surrounding region. Providing that NGC
2011 is a verified binary cluster, with the second companion unfortunately not
observed, and taking into account the general region where this cluster is
located, we discuss the possible scenarios from both star formation, and early
dynamical evolution point-of-view that might explain this unique discovery.Comment: Astrophysical Journal Letters, in press. Figures in higher resolution
available upon request to D. Gouliermis ([email protected]
Spectroscopic versus Photometric Metallicities: Milky Way Dwarf Spheroidal Companions as a Test Case
Aims. The method of deriving photometric metallicities using red giant branch
stars is applied to resolved stellar populations under the common assumption
that they mainly consist of single-age old stellar populations. We explore the
effect of the presence of mixed-age stellar populations on deriving photometric
metallicities. Methods. We use photometric data sets for the five Galactic
dwarf spheroidals Sculptor, Sextans, Carina, Fornax, and Leo II in order to
derive their photometric metallicity distribution functions from their resolved
red giant branches using isochrones of the Dartmouth Stellar Evolutionary
Database. We compare the photometric metallicities with published spectroscopic
metallicities based on the analysis of the near-infrared Ca triplet (Ca T),
both on the metallicity scale of Carretta & Gratton and on the scale defined by
the Dartmouth isochrones. In addition, we compare the photometric metallicities
with published spectroscopic metallicities based on spectral synthesis and
medium-resolution spectroscopy, and on high resolution spectra where available.
Results. The mean properties of the spectroscopic and photometric metallicity
samples are comparable within the intrinsic scatter of each method although the
mean metallicities of dSphs with pronounced intermediate-age population
fractions may be underestimated by the photometric method by up to a few tenths
of dex in [Fe/H]. The star-by-star differences of the spectroscopic minus the
photometric metallicities show a wide range of values along the fiducial
spectroscopic metallicity range, with the tendency to have systematically lower
photometric metallicities for those dwarf spheroidals with a higher fraction of
intermediate-age populations. Such discrepancies persist even in the case of
the purely old Sculptor dSph, where one would na\"ively expect a very good
match when comparing with medium or low resolution metallicity measurements.
Overall, the agreement between Ca T metallicities and photometric metallicities
is very good in the metallicity range from ~ -2 dex to ~ -1.5 dex. We find that
the photometric method is reliable in galaxies that contain small (less than
15%) intermediate-age stellar fractions. Therefore, in the presence of
mixed-age stellar populations, one needs to quantify the fraction of the
intermediate-age stars in order to assess their effect on determining
metallicities from photometry alone. Finally, we note that the comparison of
spectroscopic metallicities of the same stars obtained with different methods
reveals similarly large discrepancies as the comparison with photometric
metallicities.Comment: 17 pages, 12 figures; A&A accepte
The degeneracy between star-formation parameters in dwarf galaxy simulations and the Mstar-Mhalo relation
We present results based on a set of N-Body/SPH simulations of isolated dwarf
galaxies. The simulations take into account star formation, stellar feedback,
radiative cooling and metal enrichment. The dark matter halo initially has a
cusped profile, but, at least in these simulations, starting from idealised,
spherically symmetric initial conditions, a natural conversion to a core is
observed due to gas dynamics and stellar feedback.
A degeneracy between the efficiency with which the interstellar medium
absorbs energy feedback from supernovae and stellar winds on the one hand, and
the density threshold for star formation on the other, is found. We performed a
parameter survey to determine, with the aid of the observed kinematic and
photometric scaling relations, which combinations of these two parameters
produce simulated galaxies that are in agreement with the observations.
With the implemented physics we are unable to reproduce the relation between
the stellar mass and the halo mass as determined by Guo et al. (2010), however
we do reproduce the slope of this relation.Comment: Accepted for publication in MNRAS | 12 pages, 8 figure
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