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.

An Infrared Survey of Brightest Cluster Galaxies. II: Why are Some Brightest Cluster Galaxies Forming Stars?

Abstract: Quillen et al presented an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray-luminous clusters They found that at least half of these sources have signs of excess IR emission Here we discuss the nature of the IR emission and its implications for cool core clusters The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines Excluding the four systems dominated by an AGN, the excess mid-IR emission in the remaining brightest cluster galaxies is likely related to star formation The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star-forming galaxies The gas depletion timescale is about 1 Gyr The physical extent of the IR excess is consistent with that of the optical emission-line nebulae This supports the hypothesis that star formation occurs in molecular gas associated with the emission-line nebulae and with evidence that the emission-line nebulae are mainly powered by ongoing star formation We find a correlation between mass deposition rates () estimated from the X-ray emission and the star formation rates estimated from the IR luminosity The star formation rates are 1/10 to 1/100 of the mass deposition rates, suggesting that the reheating of the intracluster medium is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely

Near-Field Cosmology with Metal-Poor Stars

Abstract: The oldest, most metal-poor stars in the Galactic halo and satellite dwarf galaxies present an opportunity to explore the chemical and physical conditions of the earliest star forming environments in the Universe. We review the fields of stellar archaeology and dwarf galaxy archaeology by examining the chemical abundance measurements of various elements in extremely metal-poor stars. Focus on the carbon-rich and carbon-normal halo star populations illustrates how these provide insight into the Population III star progenitors responsible for the first metal enrichment events. We extend the discussion to near-field cosmology, which is concerned with the formation of the first stars and galaxies and how metal-poor stars can be used to constrain these processes. Complementary abundance measurements in high-redshift gas clouds further help to establish the early chemical evolution of the Universe. The data appear consistent with the existence of two distinct channels of star formation at the earliest times.

The Influence of Bars on Nuclear Activity

Abstract: Gravitational torques induced by a stellar bar on the interstellar medium of a disk galaxy instigate radial inflow of gas toward the central regions of the galaxy. Accordingly, the presence of a bar should reinforce nuclear star formation activity or the fueling of an active galactic nucleus (AGN). We test this hypothesis by comparing the detection rate and intensity of nuclear H II regions and AGNs among barred and unbarred galaxies in a sample of over 300 spirals selected from our recent optical spectroscopic survey of nearby galaxies. The AGN group includes Seyfert nuclei as well as low-ionization nuclear emission-line regions (LINERs). Among late-type spirals (Sc-Sm), as opposed to early-type spirals (S0/a-Sbc), we observe in the barred group a very marginal increase in the detection rate of H II nuclei and a corresponding decrease in the incidence of AGNs. The minor differences in detection rates, however, are statistically insignificant, most likely stemming from selection effects and not from a genuine influence of the bar. The presence of a bar seems to have no noticeable impact on the likelihood of a galaxy for hosting either nuclear star formation or an AGN. The nuclei of early-type barred spirals (S0/a-Sbc) do exhibit measurably higher star formation rates than do their unbarred counterparts, as indicated by either the luminosity or the equivalent width of Hα emission. By contrast, late-type spirals do not show such an effect. These results agree with previous studies and can be explained most easily in terms of structural differences between bars in early-type and late-type spirals. Nuclear H II regions spanning a wide range of intensities are found regardless of the presence of a bar, suggesting that a bar is neither a necessary nor a sufficient condition for star formation to occur in galactic nuclei. Other factors, such as the availablity of gas, must be equally important. Bars, on the other hand, have a negligible effect on the strength of the AGNs in our sample, regardless of the Hubble type of the host galaxy. This result confirms conclusions reached by other studies based on much smaller samples. Assuming that AGNs are fueled by gas from the interstellar medium of the host galaxy, some inferences concerning the fueling process can be made. We speculate that inner Lindblad resonances, particularly common in barred galaxies with large bulge-to-disk ratios, prevent gas which has been radially transported from large scales from reaching the nucleus. We discuss the feasibility of sustaining the power output of nearby AGNs with debris from tidal disruption of stars by a supermassive black hole as well as with mass loss from evolved stars. We conclude that such processes should be sufficient for fueling the low-luminosity nuclear sources found in many nearby galaxies.

DUSTY INFRARED GALAXIES: Sources of the Cosmic Infrared Background

Abstract: ▪ The discovery of the Cosmic Infrared Background (CIB) in 1996, together with recent cosmological surveys from the mid-infrared to the millimeter, have revolutionized our view of star form...

The assembly history of field spheroidals: evolution of mass-to-light ratios and signatures of recent star formation

Abstract: We present a comprehensive catalog of high signal-to-noise ratio spectra obtained with DEIMOS on the Keck II telescope for a sample of F850LP 2 for high-mass spheroidals and zf ~ 1.2 for lower mass systems, a more realistic picture is that most of the stellar mass formed in all systems at z > 2 with subsequent activity continuing to lower redshifts (z < 1.2). The fraction of stellar mass formed at recent times depends strongly on galactic mass, ranging from <1% for masses above 1011.5 M☉ to 20%-40% below 1011 M☉. Independent support for recent activity is provided by spectroscopic ([O II] emission, Hδ) and photometric (blue cores and broadband colors) diagnostics. Via the analysis of a large sample with many independent diagnostics, we are able to reconcile previously disparate interpretations of the assembly history of field spheroidals. We discuss the implications of this measurement for the determination of the evolution of the number density of E+S0 galaxies, suggesting that number density evolution of the morphologically selected population has occurred since z ~ 1.2.