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.

Constraining the Low-Mass Slope of the Star Formation Sequence at 0.5 Less than Z Less than 2.5

Abstract: We constrain the slope of the star formation rate (SFR; log psi) to stellar mass (log stellar mass) relation down to log (stellar mass / solar mass) = 8.4 (log (stellar mass / solar mass) = 9.2) at redshift = 0.5 (redshift = 2.5) with a mass-complete sample of 39,106 starforming galaxies selected from the 3D-Hubble Space Telescope photometric catalogs, using deep photometry in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses (log psi proportional to log stellar mass) than at high masses (log psi is proportional to (0.3-0.6) log stellar mass). These steeper low-mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24 micrometer imaging; beta-corrected UV SFRs; and H-alpha SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log (stellar mass / solar mass) is equal to less than 10 the specific SFR (psi / stellar mass) is observed to be roughly self-similar with psi / stellar mass proportional to (1 + redshift) (sup 1.9), whereas more massive galaxies show a stronger evolution with psi / solar mass proportional to (1 + redshift) (sup 2.2-3.5) for log (stellar mass / solar mass) = 10.2-11.2. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations.

Wind from the black-hole accretion disk driving a molecular outflow in an active galaxy

Abstract: Observations of an ultrafast accretion-disk wind in the X-ray spectrum of a nearby ultraluminous infrared galaxy support the theory that such winds affect the evolution of supermassive black holes and their host galaxies. Francesco Tombesi et al. report the detection of a powerful accretion-disk wind with a mildly relativistic velocity in the X-ray spectrum of IRAS F11119+3257, a nearby (z = 0.189) optically classified type 1 ultraluminous infrared galaxy hosting a powerful molecular outflow. The energetics of these winds are consistent with the suggestion that active galactic nuclei winds can provide an efficient way to transfer energy to the interstellar medium and support the theory that such winds affect the evolution of supermassive black holes and their host galaxies. Powerful winds driven by active galactic nuclei are often thought to affect the evolution of both supermassive black holes and their host galaxies, quenching star formation and explaining the close relationship between black holes and galaxies1,2. Recent observations of large-scale molecular outflows3,4,5,6,7,8 in ultraluminous infrared galaxies support this quasar-feedback idea, because they directly trace the gas from which stars form. Theoretical models9,10,11,12 suggest that these outflows originate as energy-conserving flows driven by fast accretion-disk winds. Proposed connections between large-scale molecular outflows and accretion-disk activity in ultraluminous galaxies were incomplete3,4,5,6,7,8 because no accretion-disk wind had been detected. Conversely, studies of powerful accretion-disk winds have until now focused only on X-ray observations of local Seyfert galaxies13,14 and a few higher-redshift quasars15,16,17,18,19. Here we report observations of a powerful accretion-disk wind with a mildly relativistic velocity (a quarter that of light) in the X-ray spectrum of IRAS F11119+3257, a nearby (redshift 0.189) optically classified type 1 ultraluminous infrared galaxy hosting a powerful molecular outflow6. The active galactic nucleus is responsible for about 80 per cent of the emission, with a quasar-like luminosity6 of 1.5 × 1046 ergs per second. The energetics of these two types of wide-angle outflows is consistent with the energy-conserving mechanism9,10,11,12 that is the basis of the quasar feedback1 in active galactic nuclei that lack powerful radio jets (such jets are an alternative way to drive molecular outflows).

The Luminosity and Mass Functions of Low-mass Stars in the Galactic Disk. II. The Field

Abstract: We report on new measurements of the luminosity function (LF) and mass function (MF) of field low-mass dwarfs derived from Sloan Digital Sky Survey Data Release 6 photometry. The analysis incorporates ~15 million low-mass stars (0.1 ), spread over 8400 deg2. Stellar distances are estimated using new photometric parallax relations, constructed from ugriz photometry of nearby low-mass stars with trigonometric parallaxes. We use a technique that simultaneously measures Galactic structure and the stellar LF from 7 < Mr < 16. We compare the LF to previous studies and convert to an MF using the mass-luminosity relations of Delfosse et?al. The system MF, measured over ?1.0< log

Spectral and Photometric Evolution of Young Stellar Populations: the Impact of Gaseous Emission at Various Metallicities

Abstract: We include gaseous continuum and line emission into our GALEV models for the spectral and photometric evolution of Simple Stellar Populations (SSPs) for various metallicities in the range 0.02 <= Z/Zsun <= 2.5. This allows to extend them to significantly younger ages than before. They now cover the age range from 4 Myr all through 14 Gyr. We point out the very important contributions of gaseous emission to broad band fluxes and their strong metallicity dependence during very early evolutionary stages of star clusters, galaxies or subgalactic fragments with vigorous ongoing star formation. Emission-line contributions are commonly seen in these actively star-forming regions. Models without gaseous emission cannot explain their observed colors at all, or lead to wrong age estimates. We use up-to-date Lyman continuum emission rates and decided to use recent empirical determinations of emission line ratios relative to Hbeta for subsolar metallicities. We justify this approach for all situations where no or not enough spectral information is available to determine all the parameters required by photoionization models. The effects of gaseous line and continuum emission on broad band fluxes are shown for different metallicities and as a function of age. In addition to the many filter systems already included in our earlier models, we here also include the HST NICMOS and Advanced Camera for Surveys (= ACS) filter systems.

Multiple stellar populations in the globular cluster ω Centauri as tracers of a merger event

Abstract: The discovery of the Sagittarius dwarf galaxy1, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars2,3, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is ω Centauri, and here we report that ω Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that ω Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that ω Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and ω Centauri appears to be a relict of this era.