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.

ATLASGAL - The APEX Telescope Large Area Survey of the Galaxy at 870 microns

Abstract: (Abridged) Studying continuum emission from interstellar dust is essential to locate and characterize the highest density regions in the interstellar medium. In particular, the early stages of massive star formation are still mysterious. Our goal is to produce a large scale, systematic database of massive pre- and proto-stellar clumps in the Galaxy, in order to better understand how and under what conditions star formation takes place. A well characterized sample of star-forming sites will deliver an evolutionary sequence and a mass function of high-mass star-forming clumps. Such a systematic survey at submm wavelengths also represents a pioneering work in preparation for Herschel and ALMA. The APEX telescope is ideally located to observe the inner Milky Way. The recently commissioned Large APEX Bolometer Camera (LABOCA) is a 295-element bolometer array observing at 870 microns, with a beam of 19". Taking advantage of its large field of view (11.4') and excellent sensitivity, we have started an unbiased survey of the Galactic Plane, with a noise level of 50-70 mJy/beam: the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). As a first step, we have covered 95 sq. deg. These data reveal 6000 compact sources brighter than 0.25 Jy, as well as extended structures, many of them filamentary. About two thirds of the compact sources have no bright infrared counterpart, and some of them are likely to correspond to the precursors of (high-mass) proto-stars or proto-clusters. Other compact sources harbor hot cores, compact HII regions or young embedded clusters. Assuming a typical distance of 5 kpc, most sources are clumps smaller than 1 pc with masses from a few 10 to a few 100 M_sun. In this introductory paper, we show preliminary results from these ongoing observations, and discuss the perspectives of the survey.

A New Photometric Technique for the Joint Selection of Star-forming and Passive Galaxies at 1.4 <~ z <~ 2.5

Abstract: A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1.4 z 2.5 in K-selected samples and classifying them as star-forming or passive systems. The method is calibrated on the highly complete spectroscopic redshift database of the K20 survey, verified with simulations and tested on other data sets. Requiring BzK = (z - K)AB - (B - z)AB > -0.2 allows us to select actively star-forming galaxies at z 1.4, independently of their dust reddening. On the other hand, objects with BzK 2.5 colors include passively evolving galaxies at z 1.4, often with spheroidal morphologies. Simple recipes to estimate the reddening, star formation rates (SFRs), and masses of BzK-selected galaxies are derived and are calibrated on K < 20 galaxies. These K < 20 galaxies have typical stellar masses of ~1011 M☉ and sky and volume densities of ~1 arcmin-2 and ~10-4 Mpc-3, respectively. Based on their UV (reddening-corrected), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR ≈ 200 M☉ yr-1 and median reddening E(B - V) ~ 0.4. This SFR is a factor of 10 higher than that of z ~ 1 dusty extremely red objects, and a factor of 3 higher than found for z ~ 2 UV-selected galaxies, both at similar K limits. Besides missing the passively evolving galaxies, the UV selection appears to miss some relevant fraction of the z ~ 2 star-forming galaxies with K < 20, and hence of the (obscured) SFR density at this redshift. The high SFRs and masses add to other existing evidence that these z = 2 star-forming galaxies may be among the precursors of z = 0 early-type galaxies. A V/Vmax test suggests that such a population may be increasing in number density with increasing redshift. Theoretical models cannot reproduce simultaneously the space density of both passively evolving and highly star-forming galaxies at z = 2. In view of Spitzer Space Telescope observations, an analogous technique based on RJL photometry is proposed to complement the BzK selection and to identify massive galaxies at 2.5 z 4.0. By selecting passively evolving galaxies as well as actively star-forming galaxies (including strongly dust-reddened ones), these color criteria should help in completing the census of the stellar mass and of the SFR density at high redshift.

Radiation Pressure-supported Starburst Disks and Active Galactic Nucleus Fueling

Abstract: We consider the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This assumption is particularly appropriate when the disk is optically thick to its own infrared radiation, as in the central regions of ULIRGs. We argue that because the disk radiates at its Eddington limit (for dust), the "Schmidt law" for star formation changes in the optically thick limit, with the star formation rate per unit area scaling as ∝ Σg/κ, where Σg is the gas surface density and κ is the mean opacity of the disk. Our calculations further show that optically thick starburst disks have a characteristic flux, star formation rate per unit area, and dust effective temperature of F ~ 1013 L☉ kpc-2, ~ 103 M☉ yr-1 kpc-2, and Teff ~ 90 K, respectively. We compare our model predictions with observations of ULIRGs and find good agreement. We extend our model of starburst disks from many hundred parsec scales to subparsec scales and address the problem of fueling AGNs. We assume that angular momentum transport proceeds via global torques (e.g., spiral waves, winds, or a central bar) rather than a local viscosity. We consistently account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little or no fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN, although these disk solutions exhibit a broad mid- to far-infrared peak from star formation. We show that the vertical thickness of the starburst disk on parsec scales can approach h ~ r, perhaps accounting for the nuclear obscuration in some type 2 AGNs. We also argue that the disk of young stars in the Galactic center may be the remnant of such a compact nuclear starburst.

Photodissociation regions in the interstellar medium of galaxies

Abstract: The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations

Abstract: This paper analyzes single stellar population (SSP)–equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. The galaxy sample is drawn from the high-quality spectroscopic surveys of Gonzalez (1993) and Kuntschner (1998). The basic data are central values of SSP-equivalent ages, t, metallicities, [Z/H], and enhancement ratios, [E/Fe], derived in Paper I, together with global structural parameters including velocity dispersions, radii, surface brightnesses, masses, and luminosities. The galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log σ, and [E/Fe]. SSP age, t, and velocity dispersion, σ, can be taken as the two independent parameters that specify a galaxy's location in this hyperplane. The hyperplane can be decomposed into two subrelations: (1) a Z-plane, in which [Z/H] is a linear function of log σ and log t and (2) a relation between [E/Fe] and σ in which [E/Fe] is larger in high-σ galaxies. Velocity dispersion is the only structural parameter that is found to modulate the stellar populations; adding other structural variables such as Ie or re does not predict [Z/H] or [E/Fe] more accurately. Cluster and field ellipticals follow the same hyperplane, but their (σ,t) distributions within it differ. Most Fornax and Virgo cluster galaxies are old, with a only a small sprinkling of galaxies to younger ages. The field ellipticals span a larger range in SSP age, with a tendency for lower σ galaxies to be younger. The present sample thus suggests that the distribution of local ellipticals in the (σ,t) plane may depend on environment. Since the (σ,t) distribution affects all two-dimensional projections involving SSP parameters, many of the familiar scaling laws attributed to ellipticals may also depend on environment. Some evidence for this is seen in the current sample. For example, only Fornax ellipticals show the classic mass-metallicity relation, whereas other subsamples do not. The tight Mg-σ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on. At fixed σ, young age tends to be offset by high [Z/H], preserving Mg nearly constant. The tightness of the Mg-σ relations does not necessarily imply a narrow range of ages at fixed σ. Although SSP parameters are heavily weighted by young stars, modeling them still places tight constraints on the total star formation history of elliptical galaxies. The relation between [E/Fe] and σ is consistent with a higher effective yield of Type II SNe elements at higher σ. This might occur if the IMF is enhanced in massive stars at high σ, or if more SNe II–enriched gas is retained by deeper galactic potential wells. Either way, modulating Type II yields versus σ seems to fit the data better than modulating Type Ia yields. The Z-plane is harder to explain and may be a powerful clue to star formation in elliptical galaxies if it proves to be general. Present data favor a frosting model in which low apparent SSP ages are produced by adding a small frosting of younger stars to an older base population (assuming no change in σ). If the frosting abundances are close to or slightly greater than the base population, simple two-component models run along lines of constant σ in the Z-plane, as required. This favors star formation from well-mixed pre-enriched gas rather than unmixed low-metallicity gas from an accreted object.