Showing papers on "Star formation published in 1996"

New Insight on Galaxy Formation and Evolution From Keck Spectroscopy of the Hawaii Deep Fields

Abstract: We present the results of spectroscopic studies with the LRIS spectrograph on Keck of two of the Hawaii deep survey fields. The 393 objects observed cover an area of 26.2 square arcmin and constitute a nearly complete sample down to K = 20, I = 23, and B = 24.5. The rest-frame K-band luminosity function and its evolution with redshift are described. Comparisons are made with other optically selected (B and I) samples in the literature, and the corresponding rest-frame B-band luminosity function evolution is presented. The B-band counts near B = 24 are shown to be a mixture of normal galaxies at modest redshifts and galaxies undergoing rapid star formation, which have a wide range of masses and which are spread over the redshift interval from z = 0.2 to beyond z = 1.7. The luminosity functions, number counts, and color distributions at optical and IR wavelengths are discussed in terms of a consistent picture of the star-forming history of the galaxy sample. [OII] emission-line diagnostics or rest-frame ultra-violet--infrared color information are used in combination with rest-frame absolute K magnitudes to construct a ``fundamental plane'' in which the evolution of the global star-formation rate with redshift can be shown, and we find that the maximum rest-frame K luminosity of galaxies undergoing rapid star formation has been declining smoothly with decreasing redshift from a value near L* at z > 1. This smooth decrease in the characteristic luminosity of galaxies dominated by star formation can simultaneously account for the high B-band galaxy counts at faint magnitudes and the redshift distribution at z < 1 in both the B- and K-selected samples. Finally, the overall K-band light density evolution is discussed as a tracer of the baryonic mass in stars and compared with the rate of star formation.

Gasdynamics and starbursts in major mergers

Abstract: Using numerical simulation, we study the development of gaseous inflows and triggering of starburst activity in mergers of comparable-mass disk galaxies. In all encounters studied, the galaxies experience strong gaseous inflows and moderate to intense starburst activity. We find that galaxy structure plays a dominant role in regulating activity. The gaseous inflows are strongest when galaxies with dense central bulges are in the final stages of merging, while inflows in bulgeless galaxies are weaker and occur earlier in the interaction. Orbital geometry plays only a relatively modest role in the onset of collisionally-induced activity. Through an analysis of the torques acting on the gas, we show that these inflows are generally driven by gravitational torques from the host galaxy (rather than the companion), and that dense bulges act to stabilize galaxies against bar modes and inflow until the galaxies merge, at which point rapidly varying gravitational torques drive strong dissipation and inflow of gas in the merging pair. The strongest inflows (and associated starburst activity) develop in co-planar encounters, while the activity in inclined mergers is somewhat less intense and occurs slightly later during the merger. The starbursts which develop in mergers of galaxies with central bulges represent an increase in the star formation rate of two orders of magnitude over that in isolated galaxies. We find that the gaseous and stellar morphology and star-forming properties of these systems provide a good match to those of observed ultraluminous infrared galaxies. Our results imply that the internal structure of the merging galaxies, rather than orbital geometry, may be the key factor in producing ultraluminous infrared galaxies.

The Canada-France Redshift Survey: The Luminosity Density and Star Formation History of the Universe to z ~ 1

Abstract: The comoving luminosity density of the universe, (λ), is estimated from the Canada-France Redshift Survey (CFRS) faint galaxy sample in three wave bands (2800 A, 4400 A, and 1 μm) over the redshift range 0 < z < 1. In all three wave bands, increases markedly with redshift. For a (q0 = 0.5, Ω = 1.0) cosmological model, the comoving luminosity density increases as (1 + z)2.1 ± 0.5 at 1 μm, as (1 + z)2.7 ± 0.5 at 4400 A, and as (1 + z)3.9 ± 0.75 at 2800 A, these exponents being reduced by 0.43 and 1.12 for (0.05, 0.1) and (-0.85, 0.1) cosmological models, respectively. The (λ)-τ relation can be reasonably well modeled by an actively evolving stellar population with a Salpeter initial mass function (IMF) extending to 125 M☉, and a star formation rate declining as τ-2.5 with a turn-on of star formation at early epochs. A Scalo IMF extending to the same mass limit produces too many long-lived low-mass stars. This rapid evolution of the star formation rate and comoving luminosity density of the universe is in good agreement with the conclusions of Pei & Fall from their analysis of the evolving metallicity of the universe. One consequence of this evolution is that the physical luminosity density at short wavelengths has probably declined by 2 orders of magnitude since z ~ 1.

The CANADA-FRANCE REDSHIFT SURVEY XIII: The luminosity density and star-formation history of the Universe to z ~ 1

Abstract: The comoving luminosity density of the Universe is estimated from the CFRS faint galaxy sample in three wavebands (2800A, 4400A and 1 micron) over the redshift range 0 < z < 1. In all three wavebands, the comoving luminosity density increases markedly with redshift. For a (q_0 = 0.5, Omega = 1.0) cosmological model, the comoving luminosity density increases as $(1+z)^{2.1 \pm 0.5}$ at 1 micron, as $(1+z)^{2.7 \pm 0.5}$ at 4400A and as $(1+z)^{3.9 \pm 0.75}$ at 2800A, these exponents being reduced by 0.43 and 1.12 for (0.05,0.1) and (-0.85,0.1) cosmological models respectively. The variation of the luminosity density with epoch can be reasonably well modelled by an actively evolving stellar population with a Salpeter initial mass function (IMF) extending to 125 M_sun, a star-formation rate declining with a power 2.5, and a turn-on of star-formation at early epochs. A Scalo (1986) IMF extending to the same mass limit produces too many long-lived low mass stars. This rapid evolution of the star-formation rate and comoving luminosity density of the Universe is in good agreement with the conclusions of Pei and Fall (1995) from their analysis of the evolving metallicity of the Universe. One consequence of this evolution is that the physical luminosity density at short wavelengths has probably declined by two orders of magnitude since z ~ 1.

Cosmological simulations with TreeSPH

Abstract: We describe numerical methods for incorporating gas dynamics into cosmological simulations and present illustrative applications to the cold dark matter (CDM) scenario. Our evolution code, a version of TreeSPH (Hernquist \& Katz 1989) generalized to handle comoving coordinates and periodic boundary conditions, combines smoothed--particle hydrodynamics (SPH) with the hierarchical tree method for computing gravitational forces. The Lagrangian hydrodynamics approach and individual time steps for gas particles give the algorithm a large dynamic range, which is essential for studies of galaxy formation in a cosmological context. The code incorporates radiative cooling for an optically thin, primordial composition gas in ionization equilibrium with a user-specified ultraviolet background. We adopt a phenomenological prescription for star formation that gradually turns cold, dense, Jeans-unstable gas into collisionless stars, returning supernova feedback energy to the surrounding medium. In CDM simulations, some of the baryons that fall into dark matter potential wells dissipate their acquired thermal energy and condense into clumps with roughly galactic masses. The resulting galaxy population is insensitive to assumptions about star formation; we obtain similar baryonic mass functions and galaxy correlation functions from simulations with star formation and from simulations without star formation in which we identify galaxies directly from the cold, dense gas.

The galactic center environment

Abstract: ▪ Abstract The central half kiloparsec region of our Galaxy harbors a variety of phenomena unique to the central environment. This review discusses the observed structure and activity of the interstellar medium in this region in terms of its inevitable inflow toward the center of the Galactic gravitational potential well. A number of dissipative processes lead to a strong concentration of gas into a “Central Molecular Zone” of about 200-pc radius, in which the molecular medium is characterized by large densities, large velocity dispersions, high temperatures, and apparently strong magnetic fields. The physical state of the gas and the resultant star formation processes occurring in this environment are therefore quite unlike those occurring in the large-scale disk. Gas not consumed by star formation either enters a hot X ray–emitting halo and is lost as a thermally driven galactic wind or continues moving inward, probably discontinuously, through the domain of the few parsec-sized circumnuclear disks and ...

Spectroscopic Confirmation of a Population of Normal Star-Forming Galaxies at Redshifts z>3

Abstract: We report the discovery of a substantial population of star--forming galaxies at $3.0 \simlt z \simlt 3.5$. These galaxies have been selected using color criteria sensitive to the presence of a Lyman continuum break superposed on an otherwise very blue far-UV continuum, and then confirmed with deep spectroscopy on the W. M. Keck telescope. The surface density of galaxies brighter than R=25 with 3 3$ galaxies is large enough that we can begin to detail the spectroscopic characteristics of the population as a whole. The spectra of the $z>3$ galaxies are remarkably similar to those of nearby star-forming galaxies, the dominant features being strong low--ionization interstellar absorption lines and high--ionization stellar lines, often with P-Cygni profiles characteristic of Wolf-Rayet and O--star winds. Lyman $\alpha$ emission is generally weak ($ 50% of the galaxies. The star formation rates, measured directly from the far-UV continua, lie in the range 4-25 $h_{50}^{-2}$ M$\sun$ yr$^{-1}$ for $q_0=0.5$. Together with the morphological properties of the $z>3$ galaxy population, which we discuss in a companion paper (Giavalisco \et 1996), all of these findings strongly suggest that we have identified the high-redshift counterparts of the spheroid component of present--day luminous galaxies. In any case, it is clear that massive galaxy formation was already well underway by $z \sim 3.5$. (shortened abstract). arch-ive/yymmnnn

Bipolar molecular outflows from young stars and protostars

Abstract: A violent outflow of high-velocity gas is one of the first manifestations of the formation of a new star. Such outflows emerge bipolarly from the young object and involve amounts of energy similar to those involved in accretion processes. The youngest (proto-)stellar low-mass objects known to date (the Class 0 protostars) present a particularly efficient outflow activity, indicating that outflow and infall motions happen simultaneously and are closely linked since the very first stages of the star formation processes. This article reviews the wealth of information being provided by large millimeter-wave telescopes and interferometers on the small-scale structure of molecular outflows, as well as the most recent theories about their origin. The observations of highly collimated CO outflows, extremely high velocity (EHV) flows, and molecular “bullets” are examined in detail, since they provide key information on the origin and propagation of outflows. The peculiar chemistry operating in the associated shocked molecular regions is discussed, highlighting the recent highsensitivity observations of low-luminosity sources. The classification schemes and the properties of the driving sources of bipolar outflows are summarized with special attention devoted to the recently identified Class 0 protostars. All these issues are crucial for building a unified theory on the mass-loss phenomena in young stars.

Abundances at High Redshifts: the Chemical Enrichment History of Damped Lyman-alpha Galaxies

Abstract: Damped Lyman-alpha absorption systems found in the spectra of high redshift quasars are believed to trace the interstellar gas in high redshift galaxies. In this paper, we study the elemental abundances of C, N, O, Al, Si, S, Cr, Mn, Fe, Ni, and Zn in a sample of 14 damped Lyman-alpha systems using high quality echelle spectra of quasars obtained with the 10m Keck telescope. These abundances are combined with similar measurements in the literature in order to investigate the chemical evolution of damped Lyman-alpha galaxies in the redshift range 0.7

Hubble Space Telescope WFPC2 Imaging of M16: Photoevaporation and Emerging Young Stellar Objects

Abstract: We present Hubble Space Telescope WFPC2 images of elephant trunks in the H II region M16. There are three principle results of this study. First, the morphology and stratified ionization structure of the interface between the dense molecular material and the interior of the H II region is well understood in terms of photoionization of a photoevaporative flow. Photoionization models of an empirical density profile capture the essential features of the observations, including the extremely localized region of [S II] emission at the interface and the observed offset between emission peaks in lower and higher ionization lines. The details of this structure are found to be a sensitive function both of the density profile of the interface and of the shape of the ionizing continuum. Interpretation of the interaction of the photoevaporative flow with gas in the interior of the nebula supports the view that much of the emission from H II regions may arise in such flows. Photoionization of photoevaporative flows may provide a useful paradigm for interpreting a wide range of observations of H II regions. Second, we report the discovery of a population of small cometary globules that are being uncovered as the main bodies of the elephant trunks are dispersed. Several lines of evidence connect these globules to ongoing star formation, including the association of a number of globules with stellar objects seen in IR images of M16 or in the continuum HST images themselves. We refer to these structures as evaporating gaseous globules, or "EGGs." These appear to be the same type of object as the nebular condensations seen previously in M42. The primary difference between the two cases is that in M16 we are seeing the objects from the side, while in M42 the objects are seen more nearly face-on against the backdrop of the ionized face of the molecular cloud. We find that the "evaporating globule" interpretation naturally accounts for the properties of objects in both nebulae, while avoiding serious difficulties with the competing "evaporating disk" model previously applied to the objects in M42. More generally, we find that disk-like structures are relatively rare in either nebula. Third, the data indicate that photoevaporation may have uncovered many EGGs while the stellar objects in them were still accreting mass, thereby freezing the mass distribution of the protostars at an early stage in their evolution. We conclude that the masses of stars in the cluster environment in M16 are generally determined not by the onset of stellar winds, as in more isolated regions of star formation, but rather by disruption of the star forming environment by the nearby O stars.

Reddening and Star Formation in Starburst Galaxies

Abstract: The reddening properties and the star formation history of a sample of 19 starburst galaxies are investigated using multiwavelength spectroscopy and infrared broad band photometry. The difference in reddening between the ionized gas and the stars is explained as difference in the covering factors of the dust in front of the gas and of the stars. A ``template starburst spectrum'', derived by combining the reddening-corrected UV, optical, and infrared data of all the galaxies in the sample, is used to investigate the star formation history. Spectral synthesis models indicate that the observed UV emission can be attributed to a stellar population which is undergoing active star formation at a constant rate since ~ 20 Myr, in agreement with the supernova rates derived from the [FeII] emission line in the infrared. At least two, and probably more, intermediate age populations (age < 2 Gyr) contribute to the optical and infrared emission, while populations older than 2 Gyr do not contribute significantly to the template. Episodic star formation over the last Gyr is suggested, with star formation rates as large as or larger than the present rates. The synthetic stellar populations are generated according to an Initial Mass Function (IMF) with Salpeter slope (alpha=2.35) in the mass range 0.1--100 solar masses, and reproduce a number of observational constraints, such as the spectral energy distribution of the template spectrum, the equivalent width of the atomic hydrogen emission lines, and the mass-to-light ratios; the data, then, do not provide indication for a high-mass-star truncated or a low-mass-star deficient IMF in starburst galaxies.

Abundances of Actinides and Short-lived Nonactinides in the Interstellar Medium: Diverse Supernova Sources for the r-Processes

Abstract: Abundances of ^(244)Pu, ^(235)U, ^(238)U, and ^(232)Th in the early solar system are about those expected for uniform production over most of galactic history. The inferred abundance of 182Hf is also compatible with this model. We here associate production of ^(182)Hf with the same r-process SN sources that produce actinides (SNACS). This requires that r-process nucleosynthesis in SNACS took place rather uniformly over the age of the galaxy until ~10^7 yr prior to solar system formation. The low abundance of ^(107)Pd and ^(129)I in the early solar system indicates that SNACS cannot produce these nuclei at the high yields expected from standard r-process models. We propose that there are distinctive SN sources for different r-process nuclei with a sharp distinction in different SN contributions below and above A ~ 140. Abundances in stars with very low metallicities will vary depending on the type of SN contributing to the local region of star formation. A time scale of ~10^7 yr is much shorter than the 108 yr time usually associated with processes in the galaxy and with the last time of r-process injection accounting for ^(129)I, but may be compatible with the rate of SN occurrence. The hypothesis of a nearby SN polluting the protosolar nebula is critically discussed.

A Theory of the IMF for Star Formation in Molecular Clouds

Abstract: We present models for the initial mass function (IMF) for stars forming within molecular clouds These models use the idea that stars determine their own masses through the action of powerful stellar outflows This concept allows us to calculate a semi-empirical mass formula (SEMF), which provides the transformation between initial conditions in molecular clouds and the final masses of forming stars For a particular SEMF, a given distribution of initial conditions predicts a corresponding IMF We consider several different descriptions for the distribution of initial conditions in star forming molecular clouds We first consider the limiting case in which only one physical variable -- the effective sound speed -- determines the initial conditions In this limit, we use observed scaling laws to determine the distribution of sound speed and the SEMF to convert this distribution into an IMF We next consider the opposite limit in which many different independent physical variables play a role in determining stellar masses In this limit, the central limit theorem shows that the IMF approaches a log-normal form Realistic star forming regions contain an intermediate number of relevant variables; we thus consider intermediate cases between the two limits Our results show that this picture of star formation and the IMF naturally produces stellar mass distributions that are roughly consistent with observations This paper thus provides a calculational framework to construct theoretical models of the IMF

Hubble space telescope imaging of star-forming galaxies at redshifts Z>3

Abstract: We present HST images of star-forming galaxies at redshifts z>3. These galaxies have been color selected for having a Lyman discontinuity in the otherwise flat (in f_ u units) UV spectra of unreddened star formation. The spectroscopic confirmation of these z>3 galaxies is reported in a companion paper (Steidel et al. 1996). The HST images probe the rest-frame UV at 1400--1900 Ang and show that the morphologies of the z>3 galaxies are generally compact, although we find a few cases of more diffuse light profiles and objects comprised of multiple compact structures. Overall, the dispersion of morphologies is relatively narrow, in contrast to the variety found in star-forming galaxies at intermediate redshifts (z~1). The galaxies with compact morphology are typically characterized by a small but resolved ``core'', approximately 3 galaxies are characterized by a relatively high degree of spherical symmetry. Morphology, space density, star-formation rates, masses, and epoch of the star-formation phase all support the hypothesis that we have identified the progenitors of present-day luminous galaxies at the epoch when they were forming the stars of

The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment

Abstract: CCD images of Ha: and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Ha: line emission, and therefore the Ha: surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Ha surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Ha surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al 1986a,b; Sanders et al 1986; Solomon & Sage 1988; Tinney et al 1990), based on the far-inffared luminosity rather than the Ha luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Ha and the IR luminosity in spiral galaxies. © 1996 American Astronomical

Molecular gas and dust around a radio-quiet quasar at redshift 4.69.

Abstract: GALAXIES are believed to have formed a large proportion of their stars in giant bursts of star formation early in their lives, but when and how this took place are still very uncertain. The presence1–6 of large amounts of dust in quasars and radio galaxies at redshifts z > 4 shows that some synthesis of heavy elements had already occurred at this time. This implies that molecular gas—the building material of stars—should also be present, as it is in galaxies at lower redshifts (z ≈ 2.5, refs 7–10). Here we report the detection of emission from dust and carbon monoxide in the radio-quiet quasar BR1202 – 0725, at redshift z = 4.69. Maps of these emissions reveal two objects, separated by a few arc seconds, which could indicate either the presence of a companion to the quasar or gravitational lensing of the quasar itself. Regardless of the precise interpretation of the maps, the detection of carbon monoxide confirms the presence of a large mass of molecular gas in one of the most distant galaxies known, and shows that conditions conducive to huge bursts of star formation existed in the very early Universe.

The impact of star formation on the interstellar medium in dwarf galaxies

Abstract: Images and long-slit echelle spectra of the H? emission from 14 dwarf galaxies and M82 have been used to identify expanding shells of ionized gas. Supershells (radius >300 pc) are found in 12 of the dwarfs. The measured shell sizes and expansion speeds constrain the ages and power requirements of the bubbles. The dynamical age of the larger bubbles is typically about 10 Myr, and ionized shells older than 20 Myr are rare. An energy equivalent to 100-10,000 supernova explosions over this period is needed to drive the shock front that sweeps out the cavity. The current star formation rates are high enough to meet these power requirements. Many of the shells will break through the surrounding layer of H I supersonically, but the projected expansion speeds are typically less than the lower limits on the escape velocity. Some of the shell material may permanently escape from a few galaxies such as NGC 1569. Whether bound to the galaxy or not, these outflows probably play an important role in regulating the star formation rate and are expected to significantly influence the chemical evolution of the galaxies. The shells lift gas out of the disk at rates comparable to, or even greater than, the current galactic star formation rates. They will displace a substantial fraction of the interstellar gas only if their duty cycle is much longer than the rotational period of the disk.

Bipolar Molecular Outflows in Massive Star Formation Regions

Abstract: Bipolar molecular outflows are a basic component of the star formation process. This is true for stars of all masses, although it has not yet been well established how outflows associated with massive stars differ from those associated with low-mass stars. We present results from a project to identify bipolar outflows from massive young stellar objects (YSOs) and determine how they compare with low-mass YSO systems. Ten massive star formation regions with high-velocity 12CO(J = 1 − 0) line wings were mapped with the Kitt Peak 12 m telescope using the On-the-Fly (OTF) mapping technique. Five of the regions have bipolar outflows. We determine accurate mass estimates of the molecular gas in the red-and blueshifted lobes by taking into account variations in the optical depth as a function of velocity in the flow. We find that the molecular outflows have masses between ~16 and 72 M☉ and kinetic energies between ~1045 and 1046 ergs. The outflows have significantly more mass and kinetic energy than those from low-mass YSOs. Of the remaining five regions, two have a clumpy distribution in 12CO with multiple velocity components within the cloud complex, and three sources did not have sufficient signal-to-noise ratio (S/N) to map the high-velocity line wings. We combine our data with 18 additional outflow sources with stellar luminosities that range from 0.6 L☉ to 2.1 × 105 L☉ to predict the luminosity of the star responsible for the outflow. We find that the stellar luminosities of the sources in our sample range from ~102 to 104 L☉, which correspond to mid- to early-B type stars; the precursors of Herbig Be stars. One source, G173.58, has an IRAS source on the flow axis with the appropriate luminosity to drive the observed outflow. For the remaining four sources, there is no detectable ultracompact (UC) H ii region or isolated IRAS source with the appropriate luminosity to produce the observed molecular outflow. The outflows mapped in this work begin to fill in a region of outflow parameter space in which relatively few sources have been studied, and they help to bridge the gap between low-luminosity outflow sources and the few isolated outflows from massive O stars. Our data are consistent with the ideas that (1) in a molecular outflow increases continuously with Lbol of the driving source over a range from ~1 L☉ to ~106 L☉ (2) one can predict the central source luminosity from the measured mass flux in the flow, (3) there is a clear Mflow versus age (td) relationship for stars of all luminosities, and (4) the larger for higher luminosity sources implies that these objects produce massive outflows on short timescales relative to low-mass stars.

Star-forming galaxies at very high redshifts

Abstract: Analysis of the deepest available images of the sky, obtained by the Hubble Space Telescope, reveals a large number of candidate high-redshift galaxies. A catalogue of 1,683 objects is presented, with estimated redshifts ranging from z = 0 to z > 6. The high-redshift objects are interpreted as regions of star formation associated with the progenitors of present-day normal galaxies, at epochs that may reach back 95% of the time to the Big Bang.

Molecular gas and dust around a radio-quiet quasar at redshift 4.7

Abstract: We report the detection of emission from dust and carbon monoxide in the radio-quiet quasar BR1202--0725 at redshift z=4.7. Maps of these emissions reveal two objects separated by a few arc seconds, which could indicate either the presence of a companion to the quasar or gravitational lensing of the quasar itself. Regardless of the precise interpretation of the maps, the detection of carbon monoxide confirms the presence of a large mass of molecular gas in one of the most distant galaxies known, and shows that conditions conducive to huge bursts of star formation existed in the early Universe.

H ii galaxies versus photoionization models for evolving starbursts

Abstract: We have constructed a grid of models representing an H II region produced by an evolving starburst embedded in a gas cloud of the same metallicity The models were produced with the spectral energy distribution from a stellar evolutionary synthesis code as input for a photoionization code that computes the emission-line strengths and equivalent widths Stellar evolution was assumed to proceed according to the models of Maeder The radiation -eld was computed using the Kurucz model atmospheres, supple- mented by the expanding non-LTE atmospheres of Schmutz et al for stellar evolutionary phases with strong winds, making a signi-cant improvement over previous works using classical static, plane-parallel model atmospheres Models for stellar interiors and atmospheres being still in a phase of continuous improvement, our population synthesis models reNect the state of the art in 1995 The models were used to analyze a sample of 100 H II galaxies for which both the Hb equivalent widths and the (O III) j4363 line intensities were available (the latter allowing a direct determination of the oxygen abundances based on measured electron temperatures) Because of these selection criteria, the results of our study are restricted to metal-poor objects with metallicities less than about one-half solar The confrontation of models with observations is presented in six diagnostic diagrams involving hydro- gen and oxygen lines Our approach is in many respects much more constraining for the models than previous studies on H II regions ionized by evolving starbursts We found that the standard starburst model (instantaneous burst of star formation with a Salpeter initial mass function and an upper cuto† mass of 100 reproduces the observational constraints pro- M _ ) vided by the nebular emission lines extremely well if selection e†ects are taken into account Models with a unique initial mass function are consistent with essentially all observational constraints over a metal- licity range from D0025 to D025 In contrast, models with a Salpeter-type initial mass function Z _ truncated at 50 are not consistent with the observations: they violate the observed distribution of M _ Hb equivalent widths The mean e†ective temperature of the ionizing star cluster declines from about 50,000 to 40,000 K during the time when the line (O III) j4363 is strong enough to be measurable W ithin the framework of our models, and in the abundance range where comparisons were made with observations, there is no signi-- cant evidence for a variation of the star cluster mean e†ective temperature with metallicity, other than the one generated by the metallicity-dependent stellar atmospheric and evolutionary models A very narrow range in ionization parameters is required to reproduce the observed line ratios This should set limits on the dynamical evolution of giant H II regions We -nd a large fraction of H II galaxies having (O I) j6300/Hb ratios larger than 002 Even models with the lowest ionization parameters considered do not produce these large ratios An approximate esti- mate of the mechanical energy released by winds and supernovae during later phases of the starburst leads to the suggestion that the (O I) j6300/Hb ratioEin contrast to other line ratios studiedEis signi-- cantly a†ected by shocks The small spread in the free parameters necessary to reproduce the emission-line properties of metal- poor H II galaxies allows us to propose a new indicator of the starburst age: the (O III) j5007 equivalent width is quite robust and can be used up to larger ages than the traditional Hb equivalent width for high signal-to-noise spectra This indicator should also prove useful for low signal-to-noise spectra of star-forming galaxies at higher redshift, because of the large value of (O III) j5007/Hb in starbursts younger than 5 Myr

Optical structure and star formation in blue compact dwarf galaxies. I. Observations and profile decomposition

Abstract: We present new B and R CCD surface photometry for a sample of 4 Blue Compact Dwarf Galaxies (BCDs) and 2 higher luminosity starburst galaxies. We find that the BCD surface brightness profiles (SBPs) can be generally well fitted by the sum of the light of 3 distinct components: 1) an exponential component at large radii, with or without a central flattening which describes the underlying older stellar population; 2) a plateau component at intermediate radii and 3) a gaussian component at small radii. The last 2 components describe the ongoing starburst superimposed on the older stellar component. We apply the 3–component decomposition scheme to 8 more BCDs from the literature and, for the total sample of 14 galaxies, derive structural properties such as central surface brightness and scale length, and perform a deprojection to obtain the luminosity density distribution for each component. We also derive color profiles and find that BCDs show generally a reddening outwards, followed by a blueing beyond a transition radius.

The nature of starburst galaxies

Abstract: Utilizing a large sample of infrared-selected starburst galaxies having optical images and long-slit spectra, we explore the interrelationships between the properties of starbursts and relate these properties to those of the "host" galaxy. We find that the half-light radius of the Hα-emitting region (re, Hα) enters into several correlations that suggest it is physically related to the actual starburst radius. Most suggestively, the effective IR surface brightness (LIR/πr2e, Hα) correlates strongly with the far-IR color temperature. This can be reproduced roughly with an idealized model of a surrounding dust screen whose far-IR emissivity is determined by the local energy density of UV starburst light. Typical values for re, Hα are a few hundred pc to a few kpc (with the Ha emission being significantly more compact than the red starlight). This confirms the "circumnuclear" scales of typical starbursts. We show also that starbursts seem to obey a limiting IR surface brightness of about 1011 L☉ kpc2, corresponding to a maximum star formation rate of about 20 M☉ yr–1 kpc2 for a normal initial mass function. We argue that this upper limit suggests that starbursts are self-regulating in some way.We show that most of these galaxies have relatively normal, symmetric rotation curves. This implies that the galactic disk need not suffer severe dynamical damage in order to "fuel" a typical starburst. We show also that the starbursts occur preferentially in the inner region of solid-body rotation. This may reflect both bar-driven inflow of gas to the region between the inner Lindblad resonances and the dominance of gravitational instability over tidal shear in this region. Most of the starbursts reside in galaxies with rotation speeds of 120--200 km s–1 (compared to 220 km s–1 for a fiducial L* galaxy like the Milky Way). The lack of a correlation between galaxy rotation speed and starburst luminosity means that even relatively modest galaxies (masses ~10% of the Milky Way) can host powerful starbursts. We argue on the basis of causality that the internal velocity dispersion in a starburst sets an upper limit to the star formation rate. The most extreme starbursts approach this limit, but most are well below. Finally, we show that the relative narrowness of the nuclear emission lines in starbursts (relative to the galaxy rotation speed) arises because the gas in the nuclear "bin" usually does not sample fully the solid-body part of the rotation curve. The narrow lines do not necessarily imply that the starburst is not in dynamical equilibrium.

Galaxies: Interactions and Induced Star Formation

Abstract: Induced Star Formation.- Observational Evidence for Interactions and Mergers.- Dynamics of Galaxy Interactions.

Galaxy Evolution in Abell 2390

Abstract: The galaxy population in the intermediate-redshift (z = 0.228) rich cluster Abell 2390 is investigated. We present velocities, colors, and morphological information for an exceptionally large sample of 323 galaxies (216 cluster members) in a 46' ? 7' (6 h?1 Mpc ? 1 h?1 Mpc) strip centered on the cD galaxy. This sample of confirmed cluster members is second only to that for the Coma cluster in terms of sample size and spatial coverage in the cluster rest frame and it is the first to trace the transition between a rich cluster and the field at intermediate redshift. The galaxy population in the cluster changes gradually from a very evolved, early-type population in the inner 0.4 h?1 Mpc of the cluster to a progressively later type population in the extensive outer envelope of the cluster from 1 to 3 h?1 Mpc in radius. Radial gradients in galaxy g - r color, 4000 ? break, H? and [O II] line strengths, and morphology are seen in the cluster and are investigated by comparing the data to models computed with the GISSEL spectral synthesis package. The results suggest that the cluster has been built up gradually by the infall of field galaxies over ~8 Gyr and that star formation has been truncated in infalling galaxies during the accretion process. The morphological composition of the cluster is shown to be consistent with such a scenario. If true for other clusters, infall-truncated star formation as seen in Abell 2390 may explain both the Butcher-Oemler effect and the large fraction of S0 galaxies in clusters. Only 5% of the galaxies observed in Abell 2390 exhibit evidence for star formation at levels stronger than those seen in typical late-type systems. This suggests that starbursts do not play a major role in driving cluster galaxy evolution at the redshift of Abell 2390, although infall-induced starbursts leading to truncated star formation may have played a role in the earlier history of the cluster. Evidence is found for at least one subcomponent on the west side of the cluster, which is likely to be infalling at the epoch of observation.

NGC 2915.II.A Dark Spiral Galaxy With a Blue Compact Dwarf Core

Abstract: Abridged abstract: We present HI synthesis observations of the weak blue compact dwarf (BCD) galaxy NGC~2915. NGC~2915 has the HI properties of a late type spiral galaxy (Sd - Sm), including a double horn global profile, and HI spiral arms. The HI extends out to over five times the Holmberg radius. The optical counterpart corresponds to a central HI bar. The rotation curve is derived and fitted with a mass model consisting of a stellar disk, a neutral gas disk, and a dark matter (DM) halo. The DM halo dominates at nearly all radii. The total mass to blue light ratio is 76 within the last measured point, making NGC~2915 one of the darkest disk galaxies known. The core of the DM halo is unusually dense (rho_0 ~ 0.1, M_sun pc^{-3}) and compact (R_core approx 1 kpc). The neutral gas component, with mass M_g = 1.27x10^9 M_sun is probably more massive than the stellar component. Split and broad HI lines (line of sight velocity dispersion, sigma ~ 35 km/s) are seen in the central region. Pressure support is probably significant, and it is not clear whether the core is in equilibrium. Beyond the Holmberg radius, the average HI sigma is 8 km/s, which is normal for disk galaxies. NGC~2915 does not obey the Tully-Fisher (1977) relation, being underluminous by a factor of nine. A simple HI surface density threshold of about 10^{21} cm^{-2} adequately describes the location of current star formation. The HI properties are compared to "normal" disk galaxies and other BCDS.

CFRS XIV: Spectral Properties Of Field Galaxies Up To z = 1

Abstract: The spectral properties of more than 400 CFRS galaxies and their changes over the redshift interval 0 - 1.3 are investigated. Emission line intensities and equivalent widths for accessible lines have been measured, as well as continuum color indices based on 200A wide spectral regions. Within the CFRS sample, the comoving fraction of galaxies with significant emission lines (EW(OII) > 15A) increases from ~13% locally to over 50% at z > 0.5. The fraction of luminous ( M_{B} 0.5, the latter fraction being similar to that of early type galaxies at that redshift. There is considerable evidence in the data presented here that star formation increases from z = 0 to z > 0.5 in disk galaxies. However, the absence of extremely blue colors and the presence of significant Balmer absorption suggests that the star formation is primarily taking place over long periods of time, rather than in short-duration, high-amplitude ``bursts''. There are several indications that the average metallicity and dust opacity were lower in emission-line galaxies at high redshift than those typically seen in luminous galaxies locally. Beyond z = 0.7, almost all the emission-line galaxies, including the most luminous (at 1mu at rest) ones, have colors approaching those of present-day irregular galaxies, and a third of them have indications (primarily from the strength of the 4000A break) of metallicities significantly less than solar (Z < 0.2 Z_{sun}). (abbreviated)

Evolution of neutral gas at high redshift: implications for the epoch of galaxy formation

Abstract: Although observationally rare, damped Lyα absorption systems dominate the mass density of neutral gas in the Universe. 11 high-redshift damped Lyα systems covering 2.8 ≤z ≤ 4.4 were discovered in 26 QSOs from the APM z > 4 QSO survey, extending these absorption system surveys to the highest redshifts currently possible. Combining our new data set with previous surveys, we find that the cosmological mass density in neutral gas, Ωg, does not rise as steeply prior to z ∼ 2 as indicated by previous studies. There is evidence in the observed Ωg for a flattening at z ∼ 2 and a possible turnover at z ∼ 3. When combined with the decline at z > 3.5 in number density per unit redshift of damped systems with column densities log NHI ≥ 21 atom cm^(−2), these results point to an epoch at z ≳ 3 prior to which the highest column density damped systems are still forming. We find that, over the redshift range 2 < z < 4, the total mass in neutral gas is marginally comparable to the total visible mass in stars in present-day galaxies. However, if one considers the total mass visible in stellar discs alone, i.e. excluding galactic bulges, the two values are comparable. We are observing a mass of neutral gas that is comparable to the mass of visible disc stars. Lanzetta, Wolfe & Turnshek found that Ω(z ≈ 3.5) was twice Ω(z ≈ 2), implying that a much larger amount of star formation must have taken place between z = 3.5 and 2 than is indicated by metallicity studies. This created a ‘cosmic G-dwarf problem’. The more gradual evolution of Ωg that we find alleviates this. These results have profound implications for theories of galaxy formation.