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.

Mass, metal, and energy feedback in cosmological simulations

Abstract: Using GADGET-2 cosmological hydrodynamic simulations including an observationally constrained model for galactic outflows, we investigate how feedback from star formation distributes mass, metals, and energy on cosmic scales from z = 6 → 0. We include instantaneous enrichment from Type II supernovae (SNe), as well as delayed enrichment from Type Ia SNe and stellar [asymptotic giant branch (AGB)] mass loss, and we individually track carbon, oxygen, silicon and iron using the latest yields. Following on the success of the momentum-driven wind scalings, we improve our implementation by using an on-the-fly galaxy finder to derive wind properties based on host galaxy masses. By tracking wind particles in a suite of simulations, we find: (1) wind material re-accretes on to a galaxy (usually the same one it left) on a recycling time-scale that varies inversely with galaxy mass (e.g. <1 Gyr for L ∗ galaxies at

Cold gas accretion in galaxies

Abstract: Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: (1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. This suggests ongoing minor mergers and recent arrival of external gas. It may be regarded, therefore, as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M 31. (2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is undoubtedly produced by galactic fountains, it is likely that a part of it is of extragalactic origin. Also the Milky Way has extra-planar gas complexes: the Intermediate- and High-Velocity Clouds (IVCs and HVCs). (3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. (4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The new gas could be added to the halo or be deposited in the outer parts of galaxies and form reservoirs for replenishing the inner parts and feeding star formation. The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean “visible” accretion rate of cold gas in galaxies of at least $${0.2\, M_{\odot} year^{-1}}$$ . In order to reach the accretion rates needed to sustain the observed star formation ( $${\approx 1 M_{\odot} year^{-1}}$$ ), additional infall of large amounts of gas from the IGM seems to be required.

Galaxy evolution in groups and clusters: satellite star formation histories and quenching time-scales in a hierarchical Universe

Abstract: Author(s): Wetzel, AR; Tinker, JL; Conroy, C; van den Bosch, FC | Abstract: Satellite galaxies in groups and clusters aremore likely to have low star formation rates (SFRs) and lie on the 'red sequence' than central ('field') galaxies. Using galaxy group/cluster catalogues from the Sloan Digital Sky Survey Data Release 7, together with a high-resolution, cosmological N-body simulation to track satellite orbits, we examine the star formation histories and quenching time-scales of satellites of Mstar g 5 × 109 M⊙ at z ≈ 0. We first explore satellite infall histories: group preprocessing and ejected orbits are critical aspects of satellite evolution, and properly accounting for these, satellite infall typically occurred at z ~ 0.5, or ~5 Gyr ago. To obtain accurate initial conditions for the SFRs of satellites at their time of first infall, we construct an empirical parametrization for the evolution of central galaxy SFRs and quiescent fractions.With this, we constrain the importance and efficiency of satellite quenching as a function of satellite and host halo mass, finding that satellite quenching is the dominant process for building up all quiescent galaxies at Mstar l 1010M⊙. We then constrain satellite star formation histories, finding a 'delayed-then-rapid' quenching scenario: satellite SFRs evolve unaffected for 2-4 Gyr after infall, after which star formation quenches rapidly, with an e-folding time of l0.8Gyr. These quenching time-scales are shorter for more massive satellites but do not depend on host halo mass: the observed increase in the satellite quiescent fraction with halo mass arises simply because of satellites quenching in a lower mass group prior to infall (group preprocessing), which is responsible for up to half of quenched satellites in massive clusters. Because of the long time delay before quenching starts, satellites experience significant stellar mass growth after infall, nearly identical to central galaxies. This fact provides key physical insight into the subhalo abundance matching method. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

A Hubble Space Telescope Imaging Survey of Nearby Active Glactic Nuclei

Abstract: We obtained 500-second F606W WFPC2 images of 256 of the nearest (z<0.035) Seyfert 1,Seyfert 2, and starburst galaxies. Less than 10% show tidal features or multiple nuclei. The incidence of inner starburst rings is about 10% in both classes of Sy galaxies. In contrast, galaxies with H II region emission line spectra appear substantially more irregular because of their much higher specific rates of star formation. An unresolved central continuum source in our HST images is a virtually perfect indicator of a Sy1 spectrum. 52% of these Sy1 point sources are saturated in our images; we use their wings to estimate their magnitudes. The converse is not however true, as over a third of Sy's with direct spectroscopic evidence for broad Balmer wings show no nuclear point source. Like the Sy2's, they have central surface brightnesses consistent with those expected for the bulges of normal galaxies. The frequency of bars in Sy1's and 2's and non-Sys are the same. The Sy2 galaxies are significantly more likely to show nuclear dust absorption, especially in lanes and patches which are irregular or reach close to the nucleus. The difference cannot be explained by different average redshifts or selection techniques. This is confirmed by our morphology classifications, which show that Sy1 nuclei reside in earlier type galaxies than Sy2 nuclei. This intrinsic difference in host galaxy properties may undermine the strong unification hypothesis for Sy galaxies that they appear different due to the orientation of their central engine. The excess galactic dust we see in Sy2's may cause substantial absorption which obscures their hypothesized broad emission-line regions and central nonstellar continua. This galactic dust could produce much of the absorption in Sy2 nuclei which had instead been attributed to a thick dusty accretion torus.

Toward an Understanding of the Rapid Decline of the Cosmic Star Formation Rate

Abstract: We present a first analysis of deep 24 μm observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0.65 ≤ z < 0.75. We combine the infrared data with redshifts, rest-frame luminosities, and colors from COMBO-17 and with morphologies from Hubble Space Telescope images collected by the Galaxy Evolution from Morphology and SEDs (GEMS) and Great Observatories Origins Deep Survey (GOODS) projects. To characterize the decline in star formation rate (SFR) since z ~ 0.7, we estimate the total thermal IR luminosities, SFRs, and stellar masses for the galaxies in this sample. At z ~ 0.7, nearly 40% of intermediate- and high-mass galaxies (with stellar masses ≥2 × 1010 M☉) are undergoing a period of intense star formation above their past-averaged SFR. In contrast, less than 1% of equally massive galaxies in the local universe have similarly intense star formation activity. Morphologically undisturbed galaxies dominate the total infrared luminosity density and SFR density: at z ~ 0.7, more than half of the intensely star-forming galaxies have spiral morphologies, whereas less than ~30% are strongly interacting. Thus, a decline in major merger rate is not the underlying cause of the rapid decline in cosmic SFR since z ~ 0.7. Physical properties that do not strongly affect galaxy morphology—for example, gas consumption and weak interactions with small satellite galaxies—appear to be responsible.