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.

The history of star formation in a Λ cold dark matter universe

Abstract: Employing hydrodynamic simulations of structure formation in a A cold dark matter cosmology, we study the history of cosmic star formation from the 'dark ages' at redshift z ∼20 to the present. In addition to gravity and ordinary hydrodynamics, our model includes radiative heating and cooling of gas, star formation, supernova feedback and galactic winds. By making use of a comprehensive set of simulations on interlocking scales and epochs, we demonstrate numerical convergence of our results on all relevant halo mass scales, ranging from 10 8 to 10 1 5 h - 1 M O .. The predicted density of cosmic star formation, ρ * (z), is broadly consistent with measurements, given the observational uncertainty. From the present epoch, ρ * (z) gradually rises by approximately a factor of 10 to a peak at z ∼5-6, which is beyond the redshift range where it has been estimated observationally. In our model, fully 50 per cent of the stars are predicted to have formed by redshift z ≃ 2.14, and are thus older than 10.4 Gyr, while only 25 per cent form at redshifts lower than z ≃ 1. The mean age of all stars at the present is approximately 9 Gyr. Our model predicts a total stellar density at z = 0 of Ω * = 0.004, corresponding to approximately 10 per cent of all baryons being locked up in long-lived stars, in agreement with recent determinations of the luminosity density of the Universe. We determine the 'multiplicity function of cosmic star formation' as a function of redshift; i.e. the distribution of star formation with respect to halo mass. At redshifts around z ≃ 10, star formation occurs preferentially in haloes of mass 10 8 -10 1 0 h - 1 M O ., while at lower redshifts, the dominant contribution to ρ * (z) comes from progressively more massive haloes. Integrating over time, we find that approximately 50 per cent of all stars formed in haloes less massive than 10 1 1 . 5 h - 1 M O ., with nearly equal contributions per logarithmic mass interval in the range 10 1 0 -10 1 3 . 5 h - 1 M O ., making up ∼70 per cent of the total. We also briefly examine possible implications of our predicted star formation history for reionization of hydrogen in the Universe. According to our model, the stellar contribution to the ionizing background is expected to rise for redshifts z > 3, at least up to redshift z ∼5, in accord with estimates from simultaneous measurements of the H and He opacities of the Lyman-a forest. This suggests that the ultraviolet background will be dominated by stars for z > 4, provided that there are not significantly more quasars at high z than are presently known. We measure the clumping factor of the gas from the simulations and estimate the growth of cosmic H II regions, assuming a range of escape fractions for ionizing photons. We find that the star formation rate predicted by the simulations is sufficient to account for hydrogen reionization by z ∼6, but only if a high escape fraction close to unity is assumed.

MG and Fe absorption features in elliptical galaxies

Abstract: Fe and Mg indices from two homogenous collections of nuclear elliptical galaxy spectra are compared with model indices. In the average giant elliptical, the [Mg/Fe] ratio exceeds that of the most metal-rich stars in the solar neighborhood by ∼0.2-0.3 dex, with a large spread about this mean. This result implies a variable «enrichment effectiveness» of Type II supernovae (SNs) compared with Type Ia SNs in the evolution of ellipticals, caused perhaps by differences in star formation time scales, the initial mass function, or the amount of Fe versus Mg ejected in galactic winds

VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. I. Nucleosynthesis and Abundance Ratios*

Abstract: We have used the Ultraviolet Echelle Spectrograph (UVES) on Kueyen (UT2) of the Very Large Telescope to take spectra of 15 individual red giants in the Sculptor, Fornax, Carina, and Leo I dwarf spheroidal galaxies (dSph's). We measure the abundances of α-, iron peak, first s-process, second s-process, and r-process elements. No dSph giants in our sample show the deep mixing abundance pattern (O and sometimes Mg depleted, while Na and Al are enhanced) seen in nearly all globular clusters. At a given metallicity the dSph giants exhibit lower [el/Fe] abundance ratios for the α-elements than stars in the Galactic halo. The low α abundances at low metallicities can be caused by a slow star formation rate and contribution from Type Ia SNe, and/or a small star formation event (low total mass) and mass-dependent Type II SN yields. In addition, Leo I and Sculptor exhibit a declining even-Z [el/Fe] pattern with increasing metallicity, while Fornax exhibits no significant slope. In contrast, Carina shows a large spread in the even-Z abundance pattern, even over small metallicity ranges, as might be expected from a bursting star formation history. The metal-poor stars in these dSph galaxies ([Fe/H] < -1) have halo-like s- and r-process abundances, but not every dSph exhibits the same evolution in the s- and r-process abundance pattern. Carina, Sculptor, and Fornax show a rise in the s-/r-process ratio with increasing metallicity, evolving from a pure r-process ratio to a solar-like s- and r-process ratio. On the other hand, Leo I, appears to show an r-process–dominated ratio over the range in metallicities sampled. At present, we attribute these differences in the star formation histories of these galaxies. Comparison of the dSph abundances with those of the halo reveals some consistencies with the Galactic halo. In particular, Nissen & Shuster found that their metal-rich, high Rmax high zmax halo stars exhibited low [α/Fe], [Na/Fe] and [Ni/Fe] abundance ratios. In the same abundance range our dSph exhibit the same abundance pattern, supporting their suggestions that disrupted dSph's may explain up to 50% of the metal-rich halo. Unfortunately, similar comparisons with the metal-poor Galactic halo have not revealed similar consistencies, suggesting that the majority of the metal-poor Galactic halo could not have been formed from objects similar to the dSph studied here. We use the dSph abundances to place new constraints on the nucleosynthetic origins of several elements. We attribute differences in the evolution of [Y/Fe] in the dSph stars versus the halo stars to a very weak AGB or SN Ia yield of Y (especially compared with Ba). That a lower and flatter Ba/Y ratio is seen in the halo is most likely a result of the pattern being erased by the large metallicity dispersion in the halo. Also, we find [Cu/Fe] and [Mn/Fe] are flat and halo-like over the metallicity city range -2 < [Fe/H] < -1.2, and that the [Cu/α] ratios are flat. Combining these abundances with knowledge of the age spread in these galaxies suggests that SNe Ia are not the main site for the production of Cu (and Mn) in very metal-poor stars. We suggest that metallicity-dependent SN yields may be more promising.

Hi-GAL: the Herschel infrared Galactic Plane Survey

Abstract: Hi-GAL, the Herschel infrared Galactic Plane Survey, is an Open Time Key Project of the Herschel Space Observatory. It will make an unbiased photometric survey of the inner Galactic Plane by mapping a two-degree wide strip in the longitude range |l|<60 degrees in five wavebands between 70um and 500um. The aim of Hi-GAL is to detect the earliest phases of the formation of molecular clouds and high-mass stars and to use the optimum combination of Herschel wavelength coverage, sensitivity, mapping strategy and speed to deliver a homogeneous census of star-forming regions and cold structures in the interstellar medium. The resulting representative samples will yield the variation of source temperature, luminosity, mass and age in a wide range of Galactic environments at all scales from massive YSOs in protoclusters to entire spiral arms, providing an evolutionary sequence for the formation of intermediate and high-mass stars. This information is essential to the formulation of a predictive global model of the role of environment and feedback in regulating the star-formation process. Such a model is vital to understanding star formation on galactic scales and in the early Universe. Hi-GAL will also provide a science legacy for decades to come with incalculable potential for systematic and serendipitous science in a wide range of astronomical fields, enabling the optimum use of future major facilities such as JWST and ALMA.

Quasar feedback: more bang for your buck

Abstract: We propose a ‘two-stage’ model for the effects of feedback from a bright quasar on the cold gas in a galaxy. It is difficult for winds or other forms of feedback from near the accretion disc to directly impact (let alone blow out of the galaxy) dense molecular clouds at ∼kpc. However, if such feedback can drive a weak wind or outflow in the hot, diffuse interstellar medium (a relatively ‘easy’ task), then in the wake of such an outflow passing over a cold cloud, a combination of instabilities and simple pressure gradients will drive the cloud material to effectively expand in the direction perpendicular to the incident outflow. This shredding/expansion (and the corresponding decrease in density) may alone be enough to substantially suppress star formation in the host. Moreover, such expansion, by even a relatively small factor, dramatically increases the effective cross-section of the cloud material and makes it much more susceptible to both ionization and momentum coupling from absorption of the incident quasar radiation field. We show that even a moderate effect of this nature can dramatically alter the ability of clouds at large radii to be fully ionized and driven into a secondary outflow by radiation pressure. Since the amount of momentum and volume which can be ionized by observed quasar radiation field is more than sufficient to affect the entire cold gas supply once it has been altered in this manner (and the ‘initial’ feedback need only initiate a moderate wind in the low-density hot gas), this reduces by an order of magnitude the required energy budget for feedback to affect a host galaxy. Instead of ∼5 per cent of the radiated energy (∼100 per cent momentum) needed if the initial feedback must directly heat or ‘blow out’ the galactic gas, if only ∼0.5 per cent of the luminosity (∼10 per cent momentum) can couple to drive the initial hot outflow, this mechanism could be efficient. This amounts to hot gas outflow rates from near the accretion disc of only ∼5–10 per cent of the black hole accretion rate.