115 research outputs found
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
A Parameter Study of the Dust and Gas Temperature in a Field of Young Stars
We model the thermal effect of young stars on their surrounding environment
in order to understand clustered star formation. We take radiative heating of
dust, dust-gas collisional heating, cosmic-ray heating, and molecular cooling
into account. Using Dusty, a spherical continuum radiative transfer code, we
model the dust temperature distribution around young stellar objects with
various luminosities and surrounding gas and dust density distributions. We
have created a grid of dust temperature models, based on our modeling with
Dusty, which we can use to calculate the dust temperature in a field of stars
with various parameters. We then determine the gas temperature assuming energy
balance. Our models can be used to make large-scale simulations of clustered
star formation more realistic.Comment: 29 pages, 19 figures. Submitted to Ap
Which feedback mechanisms dominate in the high-pressure environment of the Central Molecular Zone?
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Final published version available at https://doi.org/10.1093/mnras/staa2719.Supernovae (SNe) dominate the energy and momentum budget of stellar feedback, but the efficiency with which they couple to the interstellar medium (ISM) depends strongly on how effectively early, pre-SN feedback clears dense gas from star-forming regions. There are observational constraints on the magnitudes and timescales of early stellar feedback in low ISM pressure environments, yet no such constraints exist for more cosmologically typical high ISM pressure environments. In this paper, we determine the mechanisms dominating the expansion of H ii regions as a function of size-scale and evolutionary time within the high-pressure (P/kB ∼ 107 − 8 K cm−3) environment in the inner 100 pc of the Milky Way. We calculate the thermal pressure from the warm ionised (PHII; 104 K) gas, direct radiation pressure (Pdir), and dust processed radiation pressure (PIR). We find that (1) Pdir dominates the expansion on small scales and at early times (0.01-0.1 pc; 0.1 pc; >1 Myr); (3) during the first ≲ 1 Myr of growth, but not thereafter, either PIR or stellar wind pressure likely make a comparable contribution. Despite the high confining pressure of the environment, natal star-forming gas is efficiently cleared to radii of several pc within ∼ 2 Myr, i.e. before the first SNe explode. This ‘pre-processing’ means that subsequent SNe will explode into low density gas, so their energy and momentum will efficiently couple to the ISM. We find the H ii regions expand to a radius of ∼ 3pc, at which point they have internal pressures equal with the surrounding external pressure. A comparison with H ii regions in lower pressure environments shows that the maximum size of all H ii regions is set by pressure equilibrium with the ambient ISM.Peer reviewe
The Simultaneous Formation of Massive Stars and Stellar Clusters
We show that massive stars and stellar clusters are formed simultaneously,
the global evolution of the forming cluster is what allows the central stars to
become massive. We predict that massive star forming clumps, such as those
observed in Motte et al. 2007, contract and grow in mass leading to the
formation of massive stars. This occurs as mass is continually channeled from
large radii onto the central proto-stars, which can become massive through
accretion. Using SPH simulations of massive star forming clumps in a Giant
Molecular Cloud, we show that clumps are initially diffuse and filamentary, and
become more concentrated as they collapse. Simulated interferometry
observations of our data provide an explanation as to why young massive star
forming regions show more substructure than older ones. The most massive stars
in our model are found within the most bound cluster. Most of the mass accreted
by the massive stars was originally distributed throughout the clump at low
densities, and was later funneled to the star due to global in-fall. Even with
radiative feedback no massive pre-stellar cores are formed. The original cores
are of intermediate mass and gain their additional mass in the proto-stellar
stage. We also find that cores which form low mass stars exist within the
volume from which the high mass stars accrete, but are largely unaffected by
this process.Comment: Accepted for publication in MNRAS. 11 pages, 9 figures and 3 table
An uncertainty principle for star formation - II. A new method for characterising the cloud-scale physics of star formation and feedback across cosmic history
The cloud-scale physics of star formation and feedback represent the main uncertainty in galaxy formation studies. Progress is hampered by the limited empirical constraints outside the restricted environment of the Local Group. In particular, the poorly-quantified time evolution of the molecular cloud lifecycle, star formation, and feedback obstructs robust predictions on the scales smaller than the disc scale height that are resolved in modern galaxy formation simulations. We present a new statistical method to derive the evolutionary timeline of molecular clouds and star-forming regions. By quantifying the excess or deficit of the gas-to-stellar flux ratio around peaks of gas or star formation tracer emission, we directly measure the relative rarity of these peaks, which allows us to derive their lifetimes. We present a step-by-step, quantitative description of the method and demonstrate its practical application. The method's accuracy is tested in nearly 300 experiments using simulated galaxy maps, showing that it is capable of constraining the molecular cloud lifetime and feedback time-scale to dex precision. Access to the evolutionary timeline provides a variety of additional physical quantities, such as the cloud-scale star formation efficiency, the feedback outflow velocity, the mass loading factor, and the feedback energy or momentum coupling efficiencies to the ambient medium. We show that the results are robust for a wide variety of gas and star formation tracers, spatial resolutions, galaxy inclinations, and galaxy sizes. Finally, we demonstrate that our method can be applied out to high redshift () with a feasible time investment on current large-scale observatories. This is a major shift from previous studies that constrained the physics of star formation and feedback in the immediate vicinity of the Sun
An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale
Galactic scaling relations between the (surface densities of) the gas mass and the star formation (SF) rate are known to develop substantial scatter or even change form when considered below a certain spatial scale. We quantify how this behaviour should be expected due to the incomplete statistical sampling of independent star-forming regions. Other included limiting factors are the incomplete sampling of SF tracers from the stellar initial mass function and the spatial drift between gas and stars. We present a simple uncertainty principle for SF, which can be used to predict and interpret the failure of galactic SF relations on small spatial scales. This uncertainty principle explains how the scatter of SF relations depends on the spatial scale and predicts a scale-dependent bias of the gas depletion time-scale when centring an aperture on gas or SF tracer peaks. We show how the scatter and bias are sensitive to the physical size and time-scales involved in the SF process (such as its duration or the molecular cloud lifetime), and illustrate how our formalism provides a powerful tool to constrain these largely unknown quantities. Thanks to its general form, the uncertainty principle can also be applied to other astrophysical systems, e.g. addressing the time evolution of star-forming cores, protoplanetary discs or galaxies and their nuclei
Galaxy Zoo: dust and molecular gas in early-type galaxies with prominent dust lanes
We study dust and associated molecular gas in 352 nearby early-type galaxies
(ETGs) with prominent dust lanes. 65% of these `dusty ETGs' (D-ETGs) are
morphologically disturbed, suggesting a merger origin. This is consistent with
the D-ETGs residing in lower density environments compared to the controls
drawn from the general ETG population. 80% of D-ETGs inhabit the field
(compared to 60% of the controls) and <2% inhabit clusters (compared to 10% of
the controls). Compared to the controls, D-ETGs exhibit bluer UV-optical
colours (indicating enhanced star formation) and an AGN fraction that is more
than an order of magnitude greater (indicating higher incidence of nuclear
activity). The clumpy dust mass residing in large-scale features is estimated,
using the SDSS r-band images, to be 10^{4.5}-10^{6.5} MSun. A comparison to the
total (clumpy + diffuse) dust masses- calculated using the far-IR fluxes of 15%
of the D-ETGs that are detected by the IRAS- indicates that only ~20% of the
dust resides in these large-scale features. The dust masses are several times
larger than the maximum value expected from stellar mass loss, ruling out an
internal origin. The dust content shows no correlation with the blue
luminosity, indicating that it is not related to a galactic scale cooling flow.
No correlation is found with the age of the recent starburst, suggesting that
the dust is accreted directly in the merger rather than being produced in situ
by the triggered star formation. Using molecular gas-to-dust ratios of ETGs in
the literature we estimate that the median current and initial molecular gas
fraction are ~1.3% and ~4%, respectively. Recent work suggests that the merger
activity in nearby ETGs largely involves minor mergers (mass ratios between
1:10 and 1:4). If the IRAS-detected D-ETGs form via this channel, then the
original gas fractions of the accreted satellites are 20%-44%. [Abridged]Comment: 11 pages, 18 figures, 1 table, MNRAS (Accepted for publication- 2012
March 19
What controls star formation in the central 500 pc of the Galaxy?
The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of ≥10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scaleheight, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The H I–H2 phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse – once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments
Association between depressive symptoms and incident cardiovascular diseases
Importance: It is uncertain whether depressive symptoms are independently associated with subsequent risk of cardiovascular diseases (CVD).
Objective: To characterize the association between depressive symptoms and CVD incidence across the spectrum of lower mood.
Design, setting and participants: A pooled analysis of individual-participant-data from the Emerging Risk Factors Collaboration (ERFC; 162,036 participants; 21 cohorts; baseline surveys, 1960-2008; latest follow-up, March 2020) and UK Biobank (UKB; 401,219 participants; baseline surveys, 2006-2010; latest follow-up, March 2020). Eligible participants had information about self-reported depressive symptoms and no CVD history at baseline.
Exposure: Depressive symptoms were recorded using validated instruments. ERFC scores were harmonized across studies to a scale representative of the Centre for Epidemiological Studies Depression scale (CES-D; range 0-60; ≥16 indicates possible depressive disorder). UKB recorded the Patient Health Questionnaire-2 (PHQ-2; range 0-6; ≥3 indicates possible depressive disorder).
Main Outcomes and Measures: Primary outcomes were incident fatal/nonfatal coronary heart disease (CHD), stroke and CVD (composite of CHD and stroke). Hazard ratios (HRs) per 1-SD higher log-CES-D or PHQ-2 adjusted for age, sex, smoking and diabetes were reported.
Results: Among 162,036 participants from the ERFC, 73% were female, mean (SD) age at baseline was 63 (9) years, and 5,078 CHD and 3,932 stroke events were recorded (median follow-up, 9.5-years). Associations with CHD, stroke and CVD were log-linear. HRs (95%CI) per 1SD higher depression score for CHD, stroke and CVD respectively were 1.07 (1.03-1.11), 1.05 (1.01-1.10), and 1.06 (1.04-1.08). This reflects, 36 versus 29 CHD events, 28 versus 25 stroke events, and 63 versus 54 CVD events per 1000 individuals over 10 years in the highest versus lowest quintile of CES-D (geometric mean CES-D score, 19 versus 1). Among 401,219 participants from the UKB, 55% were female, mean baseline age was 56 (8) years, and 4607 CHD and 3253 stroke events were recorded (median follow-up, 8.1-years). HRs per 1SD higher depression score for CHD, stroke and CVD respectively were 1.11 (1.08-1.14), 1.10 (1.06-1.14) and 1.10 (1.08-1.13). This reflects, 21 versus 14 CHD events, 15 versus 10 stroke events, and 36 versus 25 CVD events per 1000 individuals over 10 years in those with PHQ2 ≥4 versus 0. The magnitude and statistical significance of the HRs were not materially changed after adjustment for additional risk factors.
Conclusions and Relevance: In a pooled analysis of 563,255 participants in 22 cohorts, baseline depressive symptoms were associated with CVD incidence, including at symptom levels below the threshold indicative of a depressive disorder. However, the magnitude of associations was modest.Lisa Pennells, Stephen Kaptoge and Sarah Spackman are funded by a British Heart Foundation Programme Grant (RG/18/13/33946). Steven Bell was funded by the National Institute for Health Research Blood and Transplant Research Unit in Donor Health and Genomics (NIHR BTRU-2014-10024). Tom Bolton is funded by the National Institute for Health Research Blood and Transplant Research Unit in Donor Health and Genomics (NIHR BTRU-2014-10024). Angela Wood is supported by a BHF-Turing Cardiovascular Data Science Award and by the EC-Innovative Medicines Initiative (BigData@Heart). John Danesh holds a British Heart Foundation Professorship and a National Institute for Health Research Senior Investigator Award.*
*The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care
A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud
To investigate the fundamental principles of H2 formation in a giant
molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and
Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We
use the far-infrared data from the Improved Reprocessing of the IRAS Survey and
the V-band extinction image provided by the COMPLETE Survey to estimate the
dust column density image of Perseus. In combination with the HI data from the
Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local
dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We
find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and
star-forming regions, suggesting a minimum HI surface density required to
shield H2 against photodissociation. As a result, a remarkably tight and
consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2.
The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2)
~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2
formation in equilibrium, suggesting that turbulence may not be of primary
importance for H2 formation. However, the importance of a warm neutral medium
for H2 shielding, an internal radiation field, and the timescale of H2
formation still remain as open questions. We also compare H2 and CO
distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While
significant spatial variations of f_DG are found, we do not find a clear
correlation with the mean V-band extinction.Comment: updated to match the final version published in April 201
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