665 research outputs found
Does feedback help or hinder star formation? The effect of photoionisation on star formation in Giant Molecular Clouds
We investigated the effect of photoionising feedback inside turbulent
star-forming clouds, comparing the resultant star formation in both idealised
profiles and more realistic cloud structures drawn from a global galaxy
simulation. We performed a series of numerical simulations which compared the
effect of star formation alone, photoionisation and photoionisation plus
supernovae feedback. In the idealised cloud, photoionisation suppresses gas
fragmentation at early times, resulting in the formation of more massive stars
and an increase in the star formation efficiency. At later times, the dispersal
of the dense gas causes the radiative feedback effect to switch from positive
to negative as the star formation efficiency drops. In the cloud extracted from
the global simulation, the initial cloud is heavily fragmented prior to the
stellar feedback beginning and is largely structurally unaffected by the late
injection of radiation energy. The result is a suppression of the star
formation. We conclude that the efficiency of feedback is heavily dependent on
the gas structure, with negative feedback dominating when the density is high.Comment: Accepted to MNRA
Environmental dependence of star formation induced by cloud collisions in a barred galaxy
Cloud collision have been proposed as a way to link the small-scale star
formation process with the observed global relation between the surface star
formation rate and gas surface density. We suggest that this model can be
improved further by allowing the productivity of such collisions to depend on
the relative velocity of the two clouds. Our adjustment implements a simple
step function that results in the most successful collisions being at the
observed velocities for triggered star formation. By applying this to a high
resolution simulation of a barred galaxy, we successfully reproduce the
observational result that the star formation efficiency (SFE) in the bar is
lower than that in the spiral arms. This is not possible when we use an
efficiency dependent on the internal turbulence properties of the clouds. Our
results suggest that high velocity collisions driven by the gravitational pull
of the clouds are responsible for the low bar SFE.Comment: 6 pages, 4 figures. Accepted for publication in MNRAS Letter
Do Giant Molecular Clouds Care About the Galactic Structure?
We investigate the impact of galactic environment on the properties of
simulated giant molecular clouds formed in a M83-type barred spiral galaxy. Our
simulation uses a rotating stellar potential to create the grand design
features and resolves down to 1.5 pc. From the comparison of clouds found in
the bar, spiral and disc regions, we find that the typical GMC is environment
independent, with a mass of 5e+5 Msun and radius 11 pc. However, the fraction
of clouds in the property distribution tails varies between regions, with
larger, more massive clouds with a higher velocity dispersion being found in
greatest proportions in the bar, spiral and then disc. The bar clouds also show
a bimodality that is not reflected in the spiral and disc clouds except in the
surface density, where all three regions show two distinct peaks. We identify
these features as being due to the relative proportion of three cloud types,
classified via the mass-radius scaling relation, which we label A, B and C.
Type A clouds have the typical values listed above and form the largest
fraction in each region. Type B clouds are massive giant molecular associations
while Type C clouds are unbound, transient clouds that form in dense filaments
and tidal tails. The fraction of each clouds type depends on the cloud-cloud
interactions, which cause mergers to build up the GMA Type Bs and tidal
features in which the Type C clouds are formed. The number of cloud
interactions is greatest in the bar, followed by the spiral, causing a higher
fraction of both cloud types compared to the disc. While the cloud types also
exist in lower resolution simulations, their identification becomes more
challenging as they are not well separated populations on the mass-radius
relation or distribution plots. Finally, we compare the results for three star
formation models to estimate the star formation rate and efficiency in each
region.Comment: 21 pages, 14 figures. Accepted for publication in MNRA
Proto-clusters in the Lambda CDM Universe
We compare the highly clustered populations of very high redshift galaxies
with proto-clusters identified numerically in a standard CDM universe
() simulation. We evolve 256^3 dark matter
particles in a comoving box of side 150h^{-1}Mpc. By the present day there are
63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We
trace these clusters back to higher redshift finding that their progenitors at
z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with
dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically
1.3--13 times the cosmological mean density. Comparison with the observation of
Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the
observed excess clustering is consistent with that expected for a proto-cluster
region if LAEs typically correspond to massive dark halos of more than
10^{12}h^{-1}Mo. We give a brief discussion on the relation between high
redshift concentration of massive dark halos and present day rich clusters of
galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter
Do giant molecular clouds care about the galactic structure?
We investigate the impact of galactic environment on the properties of simulated giant molecular clouds (GMCs) formed in an M83-type barred spiral galaxy. Our simulation uses a rotating stellar potential to create the grand design features and resolves down to 1.5 pc. From the comparison of clouds found in the bar, spiral and disc regions, we find that the typical GMC is environment independent, with a mass of 5 × 105 M⊙ and radius 11 pc. However, the fraction of clouds in the property distribution tails varies between regions, with larger, more massive clouds with a higher velocity dispersion being found in greatest proportions in the bar, spiral and then disc. The bar clouds also show a bimodality that is not reflected in the spiral and disc clouds except in the surface density, where all three regions show two distinct peaks. We identify these features as being due to the relative proportion of three cloud types, classified via the mass-radius scaling relation, which we label A, B and C. Type A clouds have the typical values listed above and form the largest fraction in each region. Type B clouds are massive giant molecular associations (GMAs) while type C clouds are unbound, transient clouds that form in dense filaments and tidal tails. The fraction of each clouds type depends on the cloud-cloud interactions, which cause mergers to build up the GMA type Bs and tidal features in which the type C clouds are formed. The number of cloud interactions is greatest in the bar, followed by the spiral, causing a higher fraction of both cloud types compared to the disc. While the cloud types also exist in lower resolution simulations, their identification becomes more challenging as they are not well-separated populations on the mass-radius relation or distribution plots. Finally, we compare the results for three star formation models to estimate the star formation rate and efficiency in each galactic regio
Cluster Morphologies as a Test of Different Cosmological Models
We investigate how cluster morphology is affected by the cosmological
constant in low-density universes. Using high-resolution cosmological
N-body/SPH simulations of flat (\Omega_0 = 0.3, \lambda_0 = 0.7, \Lambda CDM)
and open (\Omega_0 = 0.3, \lambda_0 = 0, OCDM) cold dark matter universes, we
calculate statistical indicators to quantify the irregularity of the cluster
morphologies. We study axial ratios, center shifts, cluster clumpiness, and
multipole moment power ratios as indicators for the simulated clusters at z=0
and 0.5. Some of these indicators are calculated for both the X-ray surface
brightness and projected mass distributions. In \Lambda CDM all these
indicators tend to be larger than those in OCDM at z=0. This result is
consistent with the analytical prediction of Richstone, Loeb, & Turner, that
is, clusters in \Lambda CDM are formed later than in OCDM, and have more
substructure at z=0. We make a Kolmogorov-Smirnov test on each indicator for
these two models. We then find that the results for the multipole moment power
ratios and the center shifts for the X-ray surface brightness are under the
significance level (5%). We results also show that these two cosmological
models can be distinguished more clearly at z=0 than z = 0.5 by these
indicators.Comment: 30pages, 6figures, Accepted for publication in Ap
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