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.

On the Origin of [OII] Emission in Red Sequence and Post-starburst Galaxies

Abstract: We investigate the emission-line properties of galaxies with red rest-frame colors using spectra from SDSS DR4. Emission lines are detected in more than half of the red galaxies. We focus on the relationship between two emission lines commonly used as star formation rate indicators: Ha 6563 and [OII] 3727. There is a strong bimodality in [OII]/Ha ratio in the full SDSS sample which closely corresponds to the bimodality in rest-frame color. Nearly all of the line-emitting red galaxies have line ratios typical of various types of AGN -- most commonly LINERs, a small fraction of transition objects and, more rarely, Seyferts. Only ~6% of red galaxies display star-forming line ratios. A straight line in the [OII]-Ha equivalent width plane separates LINER-like galaxies from other categories. Quiescent galaxies with no detectable emission lines and LINER-like galaxies combine to form a single, tight red sequence in color-magnitude-concentration space. [OII] EWs in LINER- and AGN-like galaxies can be as large as in star-forming galaxies. Thus, unless objects with AGN/LINER-like line ratios are excluded, [OII] emission cannot be used directly as a proxy for star formation rate. Lack of [OII] emission is generally used to indicate lack of star formation when post-starburst galaxies are selected at high redshift. Our results imply, however, that these samples have been cut on AGN properties as well as star formation, and therefore may provide seriously incomplete sets of post-starburst galaxies. Furthermore, post-starburst galaxies identifed in SDSS by requiring minimal Ha EW generally exhibit weak but nonzero line emission with ratios typical of AGNs; few of them show residual star formation. This suggests that most post-starbursts may harbor AGNs/LINERs.

The SCUBA Local Universe Survey II. 450 Micron Data - Evidence for Cold Dust in Bright IRAS Galaxies

Abstract: This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper we provided 850 micron flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60, 100 micron (IRAS) and 850 micron (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present 450 micron data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted with an isothermal dust model - two temperature components are now required. Using our 450 micron data and fluxes from the literature, we find that the 450/850 micron flux ratio for the galaxies is remarkably constant and this holds from objects in which the star formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGS) such as Arp 220. The only possible explanation for this is if the dust emissivity index for all of the galaxies is ~2 and the cold dust component has a similar temperature in all galaxies (20-21 K). The dust masses estimated using the new temperatures are higher by a factor ~2 than those determined previously using a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the submm.

Dwarf galaxies in voids: suppressing star formation with photoheating

Abstract: We study the structure formation in cosmological void regions using high-resolution hydrody-namical simulations. Despite being significantly underdense, voids are populated abundantly with small dark matter haloes which should appear as dwarf galaxies if their star formation is not suppressed significantly. We here investigate to which extent the cosmological ultraviolet (UV) background reduces the baryon content of dwarf galaxies, and thereby limits their cooling and star formation rates. Assuming a Haardt & Madau UV background with reionization at redshift z = 6, our samples of simulated galaxies show that haloes with masses below a characteristic mass of M c (z = 0) = 6.5 x 10 9 h -1 M ⊙ are baryon-poor, but in general not completely empty, because baryons that are in the condensed cold phase or are already locked up in stars resist evaporation. In haloes with mass M? M c , we find that photoheating suppresses further cooling of gas. The redshift- and UV-background-dependent characteristic mass M c (z) can be understood from the equilibrium temperature between heating and cooling at a characteristic overdensity of δ ≃ 1000. If a halo is massive enough to compress gas to this density despite the presence of UV-background radiation, gas is free to 'enter' the condensed phase and cooling continues in the halo, otherwise it stalls. By analysing the mass accretion histories of dwarf galaxies in voids, we show that they can build up a significant amount of condensed mass at early times before the epoch of reionization. Later on, the amount of mass in this phase remains roughly constant, but the masses of the dark matter haloes continue to increase. Consequently, photoheating leads to a reduced baryon fraction in void dwarf galaxies, endows them with a rather old stellar population, but still allows late star formation to some extent. We estimate the resulting stellar mass function for void galaxies. While the number of galaxies at the faint end is significantly reduced due to photoheating, additional physical feedback processes may be required to explain the apparent paucity of dwarfs in observations of voids.

Protostellar Turbulence Driven by Collimated Outflows

Abstract: We investigate the global properties of the outflow-driven protostellar turbulence through 3D MHD simulations The simulations show that the turbulence in regions of active cluster formation is quickly transformed by the forming stars through protostellar outflows, and that strongly influences and perhaps controls protostellar turbulence cluster formation We find that collimated outflows are more efficient in driving turbulence than spherical outflows that carry the same amounts of momentum This is because collimated outflows can propagate farther away from their sources, effectively increasing the turbulence driving length; turbulence driven on a larger scale decays more slowly Gravity plays an important role in shaping the turbulence, generating infall motions that balance the outward motions driven by outflows The resulting quasi-equilibrium state is maintained through a slow rate of star formation, with a fraction of the total mass converted into stars per free-fall time as low as a few percent Magnetic fields are dynamically important even in magnetically supercritical clumps, provided that their initial strengths are not far below the critical value for static cloud support They contain an energy comparable to the turbulent energy and can significantly reduce the rate of star formation The mass-weighted probability distribution function (PDF) of the volume density of the protostellar turbulence is often, although not always, approximately lognormal The PDFs of the column density deviate more strongly from lognormal distributions There is a prominent break in the power spectrum, which may provide a way to distinguish it from other types of turbulence

An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters

Abstract: We present a simple analytical description of the disruption of star clusters in a tidal field. The cluster disruption time, defined as tdis = {dln M/dt} −1 , depends on the mass M of the cluster as tdis = t0(M/M� ) γ with γ = 0.62 for clusters in a tidal field, as shown by empirical studies of cluster samples in different galaxies and by N-body simulations. Using this simple description we derive an analytic expression for the way in which the mass of a cluster decreases with time due to stellar evolution and disruption. The result agrees very well with those of detailed N-body simulations for clusters in the tidal field of our galaxy. The analytic expression can be used to predict the mass and age histograms of surviving clusters for any cluster initial mass function and any cluster formation history. The method is applied to explain the age distribution of the open clusters in the solar neighbourhood within 600 pc, based on a new cluster sample that appears to be unbiased within a distance of about 1 kpc. From a comparison between the observed and predicted age distributions in the age range between 10 Myr to 3 Gyr we find the following results: (1) The disruption time of a 10 4 Mcluster in the solar neighbourhood is about 1.3 ± 0.5 Gyr. This is a factor of 5 shorter than that derived from N-body simulations of clusters in the tidal field of the galaxy. Possible reasons for this discrepancy are discussed. (2) The present star formation rate in bound clusters within 600 pc of the Sun is 5.9 ± 0.8 × 10 2 MMyr −1 , which corresponds to a surface star formation rate of bound clusters of 5.2 ± 0.7 × 10 −10 Myr −1 pc −2 . (3) The age distribution of open clusters shows a bump between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher than before and after. (4) The present star formation rate in bound clusters is about half that derived from the study of embedded clusters. The difference suggests that about half of the clusters in the solar neighbourhood become unbound within about 10 Myr. (5) The most massive clusters within 600 pc had an initial mass of about 3 × 10 4 M� . This is in agreement with the statistically expected value based on a cluster initial mass function with a slope of −2, even if the physical upper mass limit for cluster formation is as high as 10 6 M� .