575 research outputs found
The Star Formation Relation in Nearby Galaxies
I review observational studies of the large-scale star formation process in
nearby galaxies. A wealth of new multi-wavelength data provide an unprecedented
view on the interplay of the interstellar medium and (young) stellar
populations on a few hundred parsec scale in 100+ galaxies of all types. These
observations enable us to relate detailed studies of star formation in the
Milky Way to the zoo of galaxies in the distant universe. Within the disks of
spiral galaxies, recent star formation strongly scales with the local amount of
molecular gas (as traced by CO) with a molecular gas depletion time of ~2 Gyr.
This is consistent with the picture that stars form in giant molecular clouds
that have about universal properties. Galaxy centers and starbursting galaxies
deviate from this normal trend as they show enhanced star formation per unit
gas mass suggesting systematic changes in the molecular gas properties and
especially the dense gas fraction. In the outer disks of spirals and in dwarf
galaxies, the decreasing availability of atomic gas inevitably limits the
amount of star formation, though with large local variations. The critical step
for the gas-stars circle seems therefore the formation of a molecular gas phase
that shows complex dependencies on various environmental properties and are
nowadays investigated by intensive simulational work.Comment: 8 pages; 5 figures; single column. IAUS292 Invited Review Conference
Proceeding
The Metallicity Dependence of the HI Shielding Layers in Nearby Galaxies
We investigate the metallicity dependence of HI surface densities in
star-forming regions along many lines of sight within 70 nearby galaxies,
probing kpc to 50 pc scales. We employ HI, SFR, stellar mass, and metallicity
(gradient) measurements from the literature, spanning a wide range (5 dex) in
stellar and gas mass and (1.6 dex) in metallicity. We consider metallicities as
observed, or rescaled to match the mass-metallicity relation determined for
SDSS galaxies. At intermediate to high metallicities (0.3-2 times solar), we
find that the HI surface densities saturate at sufficiently large total gas
surface density. The maximal HI columns vary approximately inversely with
metallicity, and show little variation with spatial resolution, galactocentric
radius, or among galaxies. In the central parts of massive spiral galaxies the
HI gas is depressed by factors of 2. The observed behavior is naturally
reproduced by metallicity dependent shielding theories for the HI-to-H2
transitions in star-forming galaxies. We show that the inverse scaling of the
maximal HI columns with metallicity suggests that the area filling fraction of
atomic-molecular complexes in galaxies is of order unity, and weakly dependent
on metallicity.Comment: 12 pages, 5 figures, accepted for publication in ApJ. Figure 3 shows
the main resul
Spatially extended and high-velocity dispersion molecular component in spiral galaxies: single-dish vs. interferometric observations
Recent studies of the molecular medium in nearby galaxies have provided
mounting evidence that the molecular gas can exist in two phases: one that is
clumpy and organized as molecular clouds and another one that is more diffuse.
This last component has a higher velocity dispersion than the clumpy one. In
order to investigate these two molecular components further, we compare the
fluxes and line widths of CO in NGC 4736 and NGC 5055, two nearby spiral
galaxies for which high-quality interferometric as well as single-dish data
sets are available. Our analysis leads to two main results: 1) Employing three
different methods, we determine the flux recovery of the interferometer as
compared to the single-dish to be within a range of 35-74% for NGC4736 and
81-92% for NGC5055, and 2) when focusing on high (SNR>5) lines of sight, the
single-dish line widths are larger by ~(40+-20)% than the ones derived from
interferometric data; which is in agreement with stacking all lines of sight.
These results point to a molecular gas component that is distributed over
spatial scales larger than 30"(~1kpc), and is therefore filtered out by the
interferometer. The available observations do not allow us to distinguish
between a truly diffuse gas morphology and a uniform distribution of small
clouds that are separated by less than the synthesized beam size (~3" or
~100pc), as they would both be invisible for the interferometer. This high
velocity dispersion component has a dispersion similar to what is found in the
atomic medium, as traced through observations of the HI line.Comment: 12 pages, 5 figures, Accepted to A
Чеховские реминисценции в пьесах Акунина, Сорокина, Гловацкого и Мэмета (К типологии интертекстуальных приемов)
The chemical evolution of local star forming galaxies: Radial profiles of ISM metallicity, gas mass, and stellar mass and constraints on galactic accretion and winds
The radially averaged metallicity distribution of the ISM and the young
stellar population of a sample of 20 disk galaxies is investigated by means of
an analytical chemical evolution model which assumes constant ratios of
galactic wind mass loss and accretion mass gain to star formation rate. Based
on this model the observed metallicities and their gradients can be described
surprisingly well by the radially averaged distribution of the ratio of stellar
mass to ISM gas mass. The comparison between observed and model predicted
metallicity is used to constrain the rate of mass loss through galactic wind
and accretion gain in units of the star formation rate. Three groups of
galaxies are found: galaxies with either mostly winds and only weak accretion,
or mostly accretion and only weak winds, and galaxies where winds are roughly
balanced by accretion. The three groups are distinct in the properties of their
gas disks. Galaxies with approximately equal rates of mass-loss and accretion
gain have low metallicity, atomic hydrogen dominated gas disks with a flat
spatial profile. The other two groups have gas disks dominated by molecular
hydrogen out to 0.5 to 0.7 isophotal radii and show a radial exponential
decline, which is on average steeper for the galaxies with small accretion
rates. The rates of accretion (<1.0 x SFR) and outflow (<2.4 x SFR) are
relatively low. The latter depend on the calibration of the zero point of the
metallicity determination from the use of HII region strong emission lines.Comment: 19 pages, 17 figure, accepted to MNRA
Футуристическое подражание Горацию
The article presents an analysis of Alexey Kruchenykh's and Velimir Khlebnikov's collective poem Pamiatnik (The Monument), published in the futurist brochure Slovo kak takovoe ("The Word as Such"; 1913). The poem belongs to the "genre" of emulations of Horace's "Exegi monumentum aere perennius…" (Carmina III, 30), quite widespread and important in Russian poetry. Kruchenykh's and Khlebnikov's poem contains, on the one hand, parodistic references to the well-known "monument" poems of Alexander Pushkin and Gavriil Derzhavin. At the same time, the futurist Monument represents an entirely serious continuation of the tradition of the Horace emulations, reflecting the authors' self-confident conviction of their poetic immortality and of the avoidance of the oblivion by means of the proper creative work
An uncertainty principle for star formation -- III. The characteristic emission time-scales of star formation rate tracers
We recently presented a new statistical method to constrain the physics of
star formation and feedback on the cloud scale by reconstructing the underlying
evolutionary timeline. However, by itself this new method only recovers the
relative durations of different evolutionary phases. To enable observational
applications, it therefore requires knowledge of an absolute 'reference
time-scale' to convert relative time-scales into absolute values. The logical
choice for this reference time-scale is the duration over which the star
formation rate (SFR) tracer is visible because it can be characterised using
stellar population synthesis (SPS) models. In this paper, we calibrate this
reference time-scale using synthetic emission maps of several SFR tracers,
generated by combining the output from a hydrodynamical disc galaxy simulation
with the SPS model SLUG2. We apply our statistical method to obtain
self-consistent measurements of each tracer's reference time-scale. These
include H and 12 ultraviolet (UV) filters (from GALEX, Swift, and
HST), which cover a wavelength range 150-350 nm. At solar metallicity, the
measured reference time-scales of H are Myr
with continuum subtraction, and 6-16 Myr without, where the time-scale
increases with filter width. For the UV filters we find 17-33 Myr, nearly
monotonically increasing with wavelength. The characteristic time-scale
decreases towards higher metallicities, as well as to lower star formation rate
surface densities, owing to stellar initial mass function sampling effects. We
provide fitting functions for the reference time-scale as a function of
metallicity, filter width, or wavelength, to enable observational applications
of our statistical method across a wide variety of galaxies.Comment: 24 pages, 18 figures, 7 tables (including Appendices); published in
MNRA
The Molecular Interstellar Medium of Nearby Star-Forming Galaxies
This thesis presents a comprehensive analysis of the relationship between the star forma- tion rate (SFR) and atomic (HI) and molecular (H2) gas surface densities — known as the 'Star Formation Law'. The investigation capitalizes on the HERACLES survey which mapped molecular gas emission with unprecedented resolution and sensitivity across the star-forming disks of 48 nearby spiral and dwarf galaxies. This data is complemented by recent very high quality radio, infrared, and ultraviolet data to form an unmatched multi-wavelength database. We develop a novel method to average spectral data to derive the most sensitive measurements of CO emission to date. In spiral galaxies, we trace >90% of CO which is located in an exponential disk similar to that of young and old stars. In dwarf galaxies, we derive the first sensitive constraints on the total CO luminosity. With these data we explore the limits of the SF Law in three different regimes: (1) In spiral galaxies, SFR is linearly related with H2, even in regions that are dominated by the atomic gas phase. The highly non-linear relation between SFR and total gas (HI+H2) is thus sensitively controlled by the HI–H2 phase transition. (2) The ratio SFR/CO is approx- imately constant for massive galaxies but increases strongly in low-mass, low-metallicity dwarf galaxies which suggests a significant (factor 10−100) change in the CO-to-H2 conversion factor. (3) The SF Law shows considerable scatter on small (~100 pc) scales, corresponding to the spatial scale of individual star-forming regions and is indicative of their evolution
On the time variability of the star formation efficiency
A star formation efficiency per free fall time that evolves over the life
time of giant molecular clouds (GMCs) may have important implications for
models of supersonic turbulence in molecular clouds or for the relation between
star formation rate and H2 surface density. We discuss observational data that
could be interpreted as evidence of such a time variability. In particular, we
investigate a recent claim based on measurements of H2 and stellar masses in
individual GMCs. We show that this claim depends crucially on the assumption
that H2 masses do not evolve over the life times of GMCs. We exemplify our
findings with a simple toy model that uses a constant star formation efficiency
and, yet, is able to explain the observational data.Comment: 5 pages, 2 figures, submitted to APJ
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