Star formation

About: Star formation is a research topic. Over the lifetime, 37405 publications have been published within this topic receiving 1808161 citations. The topic is also known as: astrogenesis.

Lyman-Alpha Emission from Galaxies

Abstract: We use new models of stellar population synthesis to compute the Lyα emission from galaxies with different star formation histories and initial mass functions. The models include all phases of stellar evolution and recent advances in the theories of stellar interiors and atmospheres. We find that dust-free galaxies would have Lyα equivalent widths of 50-200 A, i.e., significantly higher than previous estimates, except from a few times 10 7 to 10 9 yr after a burst of star formation. We also consider several other factors that can affect the observed Lyα emission: the contributions by supernova remnants and active galactic nuclei, the orientation of a galaxy, and absorption by dust

Tracing the molecular gas in distant submillimetre galaxies via CO(1–0) imaging with the Expanded Very Large Array

Abstract: We report the results of a pilot study with the Expanded Very Large Array (EVLA) of 12 CO J = 1-0 emission from four submillimetre-selected galaxies at z = 2.2-2.5, each with an existing detection of 12 CO J = 3-2, one of which comprises two distinct spatial components. Using the EVLA's most compact configuration, we detect strong, broad [medians: 990 km s ―1 full width at zero intensity; 540 km s ―1 full width at half-maximum (FWHM)] J = 1-0 line emission from all of our targets - coincident in position and velocity with their J = 3-2 emission. The median linewidth ratio, σ 1―0 /σ 3―2 = 1.15 ± 0.06, suggests that the J = 1-0 is more spatially extended than the J = 3-2 emission, a situation confirmed by our maps which reveal velocity structure in several cases and typical sizes of ∼16 kpc FWHM. The median brightness temperature (T b ) ratio is r 3―2/1―0 = 0.55 ± 0.05, consistent with local galaxies with L IR > 10 11 L ⊙ , noting that our value may be biased high because of the J = 3-2 based sample selection. Naively, this suggests gas masses roughly two times higher than estimates made using higher J transitions of CO, with the discrepancy due entirely to the difference in assumed T b ratio. We also estimate molecular gas masses using the 12 CO J = 1-0 line and the observed global T b ratios, assuming standard underlying T b ratios for the non-star-forming and star-forming gas phases as well as a limiting star formation efficiency for the latter in all systems, i.e. without calling upon X CO (= α). Using this new method, we find a median molecular gas mass of (2.5 ± 0.8) x 10 10 M ⊙ , with a plausible range stretching up to three times higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass within R ∼ 7 kpc for our sample is (2.3 ± 1.4) x 10 11 M ⊙ or ∼6 times more massive than UV-selected galaxies at this epoch. We examine the Schmidt-Kennicutt (S-K) relation for all the distant galaxy populations for which CO J = 1-0 or J = 2-1 data are available, finding small systematic differences between galaxy populations. These have previously been interpreted as evidence for different modes of star formation, but we argue that these differences are to be expected, given the still considerable uncertainties, certainly when considering the probable excitation biases due to the molecular lines used, and the possibility of sustained S-K offsets during the evolution of individual gas-rich systems. Finally, we discuss the morass of degeneracies surrounding molecular gas mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts.

Dusty starburst galaxies in the early Universe as revealed by gravitational lensing

Abstract: In the past decade, our understanding of galaxy evolution has been revolutionized by the discovery that luminous, dusty starburst galaxies were 1,000 times more abundant in the early Universe than at present. It has, however, been difficult to measure the complete redshift distribution of these objects, especially at the highest redshifts (z > 4). Here we report a redshift survey at a wavelength of three millimetres, targeting carbon monoxide line emission from the star-forming molecular gas in the direction of extraordinarily bright millimetre-wave-selected sources. High-resolution imaging demonstrates that these sources are strongly gravitationally lensed by foreground galaxies. We detect spectral lines in 23 out of 26 sources and multiple lines in 12 of those 23 sources, from which we obtain robust, unambiguous redshifts. At least 10 of the sources are found to lie at z > 4, indicating that the fraction of dusty starburst galaxies at high redshifts is greater than previously thought. Models of lens geometries in the sample indicate that the background objects are ultra-luminous infrared galaxies, powered by extreme bursts of star formation.

Variations in the Galactic star formation rate and density thresholds for star formation

Abstract: The conversion of gas into stars is a fundamental process in astrophysics and cosmology. Stars are known to form from the gravitational collapse of dense clumps in interstellar molecular clouds, and it has been proposed that the resulting star formation rate is proportional to either the amount of mass above a threshold gas surface density, or the gas volume density. These star formation prescriptions appear to hold in nearby molecular clouds in our Milky Way Galaxy's disc as well as in distant galaxies where the star formation rates are often much larger. The inner 500 pc of our Galaxy, the Central Molecular Zone (CMZ), contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of star formation prescriptions can be tested. Here, we show that by several measures, the current star formation rate in the CMZ is an order-of-magnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1 degrees several 10(3) cm(-3)) molecular gas - enough to form 1000 Orion-like clusters - but the present-day star formation rate within this gas is only equivalent to that in Orion. In addition to density, another property of molecular clouds must be included in the star formation prescription to predict the star formation rate in a given mass of molecular gas. We discuss which physical mechanisms might be responsible for suppressing star formation in the CMZ.

Identifying the Low-Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores

Abstract: We present the results of a search for all embedded protostars with internal luminosities ≤1.0 L☉ in the full sample of nearby, low-mass star-forming regions surveyed by the Spitzer Space Telescope Legacy Project From Molecular Cores to Planet Forming Disks (c2d). The internal luminosity of a source, Lint, is the luminosity of the central source and excludes luminosity arising from external heating. On average, the Spitzer c2d data are sensitive to embedded protostars with -->Lint ≥ 4 × 10−3(d/140 pc)2 L☉, a factor of 25 better than the sensitivity of the Infrared Astronomical Satellite (IRAS) to such objects. We present a set of selection criteria used to identify candidates from the Spitzer data and examine complementary data to decide whether each candidate is truly an embedded protostar. We find a tight correlation between the 70 μm flux and internal luminosity of a protostar, an empirical result based on both observations and detailed two-dimensional radiative transfer models of protostars. We identify 50 embedded protostars with -->Lint ≤ 1.0 L☉; 15 have -->Lint ≤ 0.1 L☉. The intrinsic distribution of source luminosities increases to lower luminosities. While we find sources down to the above sensitivity limit, indicating that the distribution may extend to luminosities lower than probed by these observations, we are able to rule out a continued rise in the distribution below -->Lint = 0.1 L☉. Between 75% and 85% of cores classified as starless prior to being observed by Spitzer remain starless to our luminosity sensitivity; the remaining 15%-25% harbor low-luminosity, embedded protostars. We compile complete spectral energy distributions for all 50 objects and calculate standard evolutionary signatures (Lbol, Tbol, and Lbol/Lsmm) and argue that these objects are inconsistent with the simplest picture of star formation, wherein mass accretes from the core onto the protostar at a constant rate.