Showing papers on "Star formation published in 1997"

The Global Schmidt Law in Star Forming Galaxies

Abstract: Measurements of H-alpha, HI, and CO distributions in 61 normal spiral galaxies are combined with published far-infrared and CO observations of 36 infrared-selected starburst galaxies, in order to study the form of the global star formation law, over the full range of gas densities and star formation rates (SFRs) observed in galaxies. The disk-averaged SFRs and gas densities for the combined sample are well represented by a Schmidt law with index N = 1.4+-0.15. The Schmidt law provides a surprisingly tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density. An alternative formulation of the star formation law, in which the SFR is presumed to scale with the ratio of the gas density to the average orbital timescale, also fits the data very well. Both descriptions provide potentially useful "recipes" for modelling the SFR in numerical simulations of galaxy formation and evolution.

A Search for “Dwarf'' Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies

Abstract: We have completed an optical spectroscopic survey of the nuclear regions (r < 200 pc) of a large sample of nearby galaxies. Although the main objectives of the survey are to search for low-luminosity active galactic nuclei and to quantify their luminosity function, the database can be used for a variety of other purposes. This paper presents measurements of the spectroscopic parameters for the 418 emission-line nuclei, along with a compilation of the global properties of all 486 galaxies in the survey. Stellar absorption generally poses a serious obstacle to obtaining accurate measurement of emission lines in nearby galactic nuclei. We describe a procedure for removing the starlight from the observed spectra in an efficient and objective manner. The main parameters of the emission lines (intensity ratios, fluxes, profile widths, equivalent widths) are measured and tabulated, as are several stellar absorption-line and continuum indices useful for studying the stellar population. Using standard nebular diagnostics, we determine the probable ionization mechanisms of the emission-line objects. The resulting spectral classifications provide extensive information on the demographics of emission-line nuclei in the local universe. This new catalog contains over 200 objects showing spectroscopic evidence for recent star formation and an equally large number of active galactic nuclei, including 46 which show broad H-alpha emission. These samples will serve as the basis of future studies of nuclear activity in nearby galaxies.

A Deep Submillimeter Survey of Lensing Clusters: A New Window on Galaxy Formation and Evolution

Abstract: We present the first results of a submillimeter survey of distant clusters using the new Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z=0.37 and Cl 2244-02 at z=0.33, at wavelengths of 450 and 850 μm. The resulting continuum maps cover a total area of about 10 arcmin2 to 1 σ noise levels less than 14 and 2 mJy beam-1 at the two wavelengths, 2-3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3-2 as compared with a blank-field survey. A cumulative source surface density of (2.4±1.0)×103 deg-2 is found to a 50% completeness limit of ~4 mJy at 850 μm. The submillimeter spectral properties of these sources indicate that the majority lie at high redshift, z>1. Without correcting for lens amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a nonevolving model using the local IRAS 60 μm luminosity function. The observed source counts thus require a substantial increase in the number density of strongly star-forming galaxies in the high-redshift universe and suggest that optical surveys may have substantially underestimated the star formation density in the distant universe. Deeper submillimeter surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift and so provide strong constraints on the formation of normal galaxies.

What Powers Ultra-luminous IRAS Galaxies?

Abstract: We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultra-luminous IRAS galaxies. We combine the survey results with a detailed case study, based on near-IR and mm imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultra-luminous galaxies to ISO and literature data of thirty starburst and active galactic nuclei (AGN), template galaxies. We find that 1) 70-80% of the ultra-luminous IRAS galaxies in our sample are predominantly powered by recently formed massive stars. 20-30% are powered by a central AGN. These conclusions are based on a new infrared 'diagnostic diagram' involving the ratio of high to low excitation mid-IR emission lines on the one hand, and on the strength of the 7.7um PAH feature on the other hand. 2) at least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring. 3) the mid-infrared emitting regions are highly obscured. After correction for these extinctions, we estimate that the star forming regions in ULIRGs have ages between 10^7 and 10^8 years, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4) in the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time dependent compression of the circum-nuclear interstellar gas, the accretion rate onto the central black hole and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

A Deep Sub-millimeter Survey of Lensing Clusters: A New Window on Galaxy Formation and Evolution

Abstract: We present the first results of a sub-millimeter survey of distant clusters using the new Sub-mm Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z=0.37 and Cl2244-02 at z=0.33, at wavelengths of 450 and 850-um. The resulting continuum maps cover a total area of about 10 sq. arcmin to 1 sigma noise levels less than 14 and 2 mJy per beam at the two wavelengths, 2-3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3--2 as compared with a blank-field survey. A cumulative source surface density of (2.4+/-1.0) x 10^3 per sq. degree is found to a 50% completeness limit of ~4 mJy at 850-um. The sub-mm spectral properties of these sources indicate that the majority lie at high redshift, z>1. Without correcting for lens amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a non-evolving ber density of strongly star-forming galaxies in the high-redshift Universe and suggest that optical surveys may have substantial underestimated the star formation density in the distant Universe. Deeper sub-mm surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift, and so provide strong constraints on the formation of normal galaxies.

The Chemical Evolution of the Galaxy: The Two-Infall Model

Abstract: We present a new chemical evolution model for the Galaxy that assumes two main infall episodes, for the formation of the halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin), but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin disk is much longer than that of the thick disk, implying that the infalling gas forming the thin disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin disk is assumed to be a function of Galactocentric distance, leading to an inside-out picture for the Galaxy's building. The model takes into account the most up-to-date nucleosynthesis prescriptions and adopts a threshold in the star formation process, which naturally produces a hiatus in the star formation rate at the end of the thick-disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighborhood and for the whole disk. Among these constraints, the tightest is the metallicity distribution of the G-dwarf stars, for which new data are now available. Our model fits these new data very well. The model also predicts the evolution of the gas mass, the star formation rate, the supernova rates, and the abundances of 16 chemical elements as functions of time and Galactocentric distance. We show that, in order to reproduce most of these constraints, a timescale of ≤1 Gyr for the (halo) thick disk and of 8 Gyr for the thin disk's formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk (R < 1 R☉) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner gradients steepen during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process are discussed.

Reionization of the Universe and the Early Production of Metals

Abstract: We simulate a plausible cosmological model in considerable physical and numerical detail through the successive phases of reheating (at 10 z 20) and reionization at z ≈ 7. We assume an efficiency of high-mass star formation appropriate to leave the universe, after it becomes transparent, with an ionizing background J21 ≈ 0.4 (at z = 4), near (and perhaps slightly below) the observed value. Since the same stars produce the ionizing radiation and the first generation of heavy elements, a mean metallicity of Z/Z☉ ~ 1/200 is produced in this early phase, but there is a large variation about this mean, with the high density regions having Z/Z☉ ≈ 1/30 and the low density regions (or the Lyα forest with NH I 1013.5 cm2) having essentially no metals. When it occurs, reionization is very rapid (phase change-like), which will leave a signature that may be detectable by very large area meter-wavelength radio instruments. Also, the background UV radiation field will show a sharp drop of ~10-3 from 1 to 4 ryd because of absorption edges. The simulated volume is too small to form L* galaxies, but the smaller objects that are found in the simulation obey the Faber-Jackson relation. In order to explore theoretically this domain of "the end of the dark ages" quantitatively, numerical simulations must have a mass resolution of the order of 104.5 M☉ in baryons, have high spatial resolution (1 kpc) to resolve strong clumping, and allow for detailed and accurate treatment of both the radiation field and atomic/molecular physics.

The universality of the stellar initial mass function

Abstract: We propose that the stellar initial mass function (IMF) is universal in the sense that its functional form arises as a consequence of the statistics of random supersonic flows. A model is developed for the origin of the stellar IMF, that contains a dependence on the average physical parameters (temperature, density, velocity dispersion) of the large scale site of star formation. The model is based on recent numerical experiments of highly supersonic random flows that have a strong observational counterpart. It is shown that a Miller-Scalo like IMF is naturally produced by the model for the typical physical conditions in molecular clouds. A more “massive” IMF in star bursts is also predicted.

The Universality of the Stellar IMF

Abstract: We propose that the stellar initial mass function (IMF) is universal in the sense that its functional form arises as a consequence of the statistics of random supersonic flows. A model is developed for the origin of the stellar IMF, that contains a dependence on the average physical parameters (temperature, density, velocity dispersion) of the large scale site of star formation. The model is based on recent numerical experiments of highly supersonic random flows that have a strong observational counterpart. It is shown that a Miller-Scalo like IMF is naturally produced by the model for the typical physical conditions in molecular clouds. A more ``massive'' IMF in star bursts is also predicted.

X-ray and molecular emission from the nearest region of recent star formation

Abstract: The isolated, young, sunlike star TW Hya and four other young stars in its vicinity are strong x-ray sources. Their similar x-ray and optical properties indicate that the stars make up a physical association that is on the order of 20 million years old and that lies between about 40 and 60 parsecs (between about 130 and 200 light years) from Earth. TW Hya itself displays circumstellar CO, HCN, CN, and HCO+ emission. These molecules probably orbit the star in a solar-system-sized disk viewed more or less face-on, whereas the star is likely viewed pole-on. Being at least three times closer to Earth than any well-studied region of star formation, the TW Hya Association serves as a test-bed for the study of x-ray emission from young stars and the formation of planetary systems around sunlike 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.

The Evolution of Early-Type Galaxies in Distant Clusters

Abstract: We present results from an optical-IR photometric study of early-type galaxies in 19 galaxy clusters out to z=0.9. The galaxy sample is selected on the basis of morphologies determined from HST WFPC2 images, and is photometrically defined in the K-band to minimize redshift-dependent selection biases. The optical-IR colors of the early-type cluster galaxies become bluer with increasing redshift in a manner consistent with the passive evolution of an old stellar population formed at an early cosmic epoch. The degree of color evolution is similar for clusters at similar redshift, and does not depend strongly on the optical richness or X-ray luminosity of the cluster, suggesting that the history of early-type galaxies is relatively insensitive to environment. The slope of the color-magnitude relationship shows no significant change out to z=0.9, providing evidence that it arises from a correlation between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in the optical-IR colors is small and nearly constant with redshift, indicating that the majority of giant, early-type galaxies in clusters share a common star formation history, with little perturbation due to uncorrelated episodes of later star formation. Taken together, our results are consistent with models in which most early-type galaxies in rich clusters are old, formed the majority of their stars at high redshift in a well-synchronized fashion, and evolved quiescently thereafter.

K and evolutionary corrections from uv to ir

Abstract: K and evolutionary corrections are given for the E, Sa and Sc Hubble types for the filters of the Johnson – Bessell & Brett photometric system and the gri filters of the modified Thuan & Gunn system up to the redshift . Their dependence on the time scale of star formation in ellipticals is investigated. The corrections are computed according to an evolutionary synthesis model that reproduces the integrated galaxy spectrum in the range A; such a model makes use of an infrared observed stellar library and its results are compared with nearby galaxies. Evolving spectral energy distributions of the various Hubble types, as well as optical-IR and IR-IR colour evolution and adopted filter response functions are also given.

The galactic distribution of ob associations in molecular clouds

Abstract: Molecular clouds account for half of the mass of the interstellar medium interior to the solar circle and for all current star formation. Using cloud catalogs of two CO surveys of the first quadrant, we have fitted the mass distribution of molecular clouds to a truncated power law in a similar manner as the luminosity function of OB associations in the companion paper to this work. After extrapolating from the first quadrant to the entire inner Galaxy, we find that the mass of cataloged clouds amounts to only 40% of current estimates of the total Galactic molecular mass. Following Solomon & Rivolo, we have assumed that the remaining molecular gas is in cold clouds, and we normalize the distribution accordingly. The predicted total number of clouds is then shown to be consistent with that observed in the solar neighborhood where cloud catalogs should be more complete. Within the solar circle, the cumulative form of the distribution is -->c(>M)=105[(M -->u/M) -->0.6-1], where -->c is the number of clouds, and Mu = 6 ? 106 M? is the upper mass limit. The large number of clouds near the upper cutoff to the distribution indicates an underlying physical limit to cloud formation or destruction processes. The slope of the distribution corresponds to d -->c/dMM -->?1.6, implying that although numerically most clouds are of low mass, most of the molecular gas is contained within the most massive clouds. The distribution of cloud masses is then compared to the Galactic distribution of OB association luminosities to obtain statistical estimates of the number of massive stars expected in any given cloud. The likelihood of massive star formation in a cloud is determined, and it is found that the median cloud mass that contains at least one O star is ~105 M?. The average star formation efficiency over the lifetime of an association is about 5% but varies by more than 2 orders of magnitude from cloud to cloud and is predicted to increase with cloud mass. O stars photoevaporate their surrounding molecular gas, and even with low rates of formation, they are the principal agents of cloud destruction. Using an improved estimate of the timescale for photoevaporation and our statistics on the expected numbers of stars per cloud, we find that 106 M? giant molecular clouds (GMCs) are expected to survive for about 3 ? 107 yr. Smaller clouds are disrupted, rather than photoionized, by photoevaporation. The porosity of H II regions in large GMCs is shown to be of order unity, which is consistent with self-regulation of massive star formation in GMCs. On average, 10% of the mass of a GMC is converted to stars by the time it is destroyed by photoevaporation.

The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field

Abstract: The Hubble Deep Field (HDF) is the deepest set of multicolor optical photometric observations ever undertaken, and it offers a valuable data set with which to study galaxy evolution. Combining the optical WFPC2 data with ground-based near-infrared photometry, we derive photometrically estimated redshifts for HDF galaxies with J 2, and they bridge the redshift gap between those two samples. The overall star formation or metal enrichment rate history is consistent with the theoretical models of White and Frenk and the predictions of Pei and Fall based on the evolving H I content of Lyα QSO absorption line systems.

LBDS 53W091: An Old, Red Galaxy at z = 1.552*

Abstract: The weak radio source LBDS 53W091 is associated with a very faint (R ? 24.5) red (R - K ? 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z = 1.552 ? 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6 V star, and a single-burst old stellar population that matches the IR colors, the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr. We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estimates resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques. The models most consistent with the data tend to yield ages at z = 1.55 of 3.5 Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of ?0, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for ?0 = 1, only models with H0 45 km s-1 Mpc-1 are permitted. For H0 = 50 km s-1 Mpc-1 and ?0 = 0.2, we derive a formation redshift, zf ? 5.

The Metallicity of High Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyman Alpha Systems from z = 0.7 to 3.4

Abstract: We report new observations of ZnII and CrII absorption lines in 10 damped \lya systems (DLAs), mostly at redshift $z_{abs} \simgt 2.5$ . By combining these results with those from our earlier survey (Pettini et al. 1994) and other recent data, we construct a sample of 34 measurements (or upper limits) of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than one third of the total number of DLAs known. The plot of the abundance of Zn as a function of redshift reinforces the two main findings of our previous study. (1) Damped \lya systems are mostly metal-poor, at all redshifts sampled; the column density weighted mean for the whole data set is [Zn/H] $= -1.13 \pm 0.38$ (on a logarithmic scale), or approximately 1/13 of solar. (2) There is a large spread, by up to two orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped \lya systems are drawn from a varied population of galaxies of different morphological types and at different stages of chemical evolution, supporting the idea of a protracted epoch of galaxy formation. At redshifts $z \simgt 2$ the typical metallicity of the damped \lya systems is in agreement with expectations based on the consumption of HI gas implied by the recent measurements of $\Omega_{DLA}$ by Storrie-Lombardi et al. (1996a), and with the metal ejection rates in the universe at these epochs deduced by Madau (1996) from the ultraviolet luminosities of high redshift galaxies revealed by deep imaging surveys. There are indications in our data for an increase in the mean metallicity of the damped \lya systems from $z > 3$ to $\approx 2$, consistent with the rise in the comoving star formation rate indicated by the relative numbers of $U$ and $B$ drop-outs in the Hubble Deep Field. Although such comparisons are still tentative, it appears that these different avenues for exploring the early evolution of galaxies give a broadly consistent picture.

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.

The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field

Abstract: The Hubble Deep Field (HDF) is the deepest set of multicolor optical photometric observations ever undertaken, and offers a valuable data set with which to study galaxy evolution. Combining the optical WFPC2 data with ground-based near-infrared photometry, we derive photometrically estimated redshifts for HDF galaxies with J 2, and bridge the redshift gap between those two samples. The overall star formation or metal enrichment rate history is consistent with the predictions of Pei and Fall (1995) based on the evolving HI content of Lyman-alpha QSO absorption line systems.

Feedback, Disk Self-Regulation, and Galaxy Formation

Abstract: Self-regulation of star formation in disks is controlled by two dimensionless parameters: the Toomre parameter for gravitational instability and the porosity of the interstellar medium to supernova remnant-heated gas. An interplay between these leads to expressions for the gas velocity dispersion, gas fraction, star formation rate and star formation efficiency in disks and to a possible explanation of the Tully-Fisher relation. I further develop feedback arguments that arise from the impact of massive star formation and death on protogalaxies in order to account for the characteristic luminosity of a galaxy and for early winds from forming spheroids.

The Panchromatic Starburst Intensity Limit at Low and High Redshift

Abstract: The integrated bolometric effective surface brightness S_e distributions of starbursts are investigated for samples observed in 1. the rest frame ultraviolet (UV), 2. the far-infrared and H-alpha, and 3. 21cm radio continuum emission. For the UV sample we exploit a tight empirical relationship between UV reddening and extinction to recover the bolometric flux. Parameterizing the S_e upper limit by the 90th percentile of the distribution, we find a mean S_{e,90} = 2.0e11 L_{sun}/kpc^2 for the three samples, with a factor of three difference between the samples. This is consistent with what is expected from the calibration uncertainties alone. We find little variation in S_{e,90} with effective radii for R_e ~ 0.1 - 10 kpc, and little evolution out to redshifts z ~ 3. The lack of a strong dependence of S_{e,90} on wavelength, and its consistency with the pressure measured in strong galactic winds, argue that it corresponds to a global star formation intensity limit (\dot\Sigma_{e,90} ~ 45 M_{sun}/kpc^2/yr) rather than being an opacity effect. There are several important implications of these results: 1. There is a robust physical mechanism limiting starburst intensity. We note that starbursts have S_e consistent with the expectations of gravitational instability models applied to the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral bulges can plausibly be built with maximum intensity bursts, while normal spiral disks can not. 3. The UV extinction of high-z galaxies is significant, implying that star formation in the early universe is moderately obscured. After correcting for extinction, the observed metal production rate at z ~ 3 agrees well with independent estimates made for the epoch of elliptical galaxy formation.

Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers

Abstract: We have observed 150 regions of massive star formation, selected originally by the presence of an H2O maser, in the J = 5 → 4, 3 → 2, and 2 → 1 transitions of CS, and 49 regions in the same transitions of C34S. Over 90% of the 150 regions were detected in the J = 2 → 1 and 3 → 2 transitions of CS, and 75% were detected in the J = 5 → 4 transition. We have combined the data with the J = 7 → 6 data from our original (1992) survey to determine the density by analyzing the excitation of the rotational levels. Using large velocity gradient models, we have determined densities and column densities for 71 of these regions. The gas densities are very high (log n = 5.9), but much less than the critical density of the J = 7 → 6 line. Small maps of 25 of the sources in the J = 5 → 4 line yield a mean diameter of 1.0 pc. Several estimates of the mass of dense gas were made for the sources for which we had sufficient information. The mean virial mass is 3800 M☉. The mean ratio of bolometric luminosity to virial mass (L/M) is 190, about 50 times higher than estimates made using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The depletion time for the dense gas is ~1.3 × 107 yr, comparable to the timescale for gas dispersal around open clusters and OB associations. We find no statistically significant line width-size or density-size relationships in our data. Instead, both line width and density are greater for a given size than would be predicted by the usual relationships. We find that the line width increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5 → 4 transition is 15-23 L☉, considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample.

Morphological studies of the galaxy populations in distant "Butcher-Oemler" clusters with HST. II. AC103, AC118 and AC114 at z=0.31

Abstract: We present new results of a program to study the detailed morphologies of galaxies in intermediate redshift clusters and hence understand the physical origin of the enhanced star formation in these environments at earlier epochs. Deep, high resolution imagery has been obtained of 3 rich clusters, AC103,AC118 & AC114 at z=0.31, using the WFPC2 on HST. For AC103 & AC118, single pointings covering a central 0.5x0.5Mpc have been obtained; for AC114, 4 pointings covering a 1.2x0.7Mpc area have been obtained, allowing the more outer regions of a distant cluster to be studied. Hubble types plus evidence for dynamical interactions and/or structural abnormalities have been determined visually for all galaxies down to R=22.25 in AC103 & AC118 and R=23.0 in AC114.We find the numbers of spirals (Sa-Sdm) in our clusters to be up to 4x higher than that seen in present-day clusters; only in the virialised core of our most massive regular cluster, AC114, do we see morphological fractions approaching those of the present epoch. Dynamical interactions are also widespread throughout the clusters with ~20% of members showing morphological evidence of this phenomenon. The highest incidence is amongst those blue members either undergoing a starburst or seen <0.5Gyr thereafter with the majority being involved in major mergers. These galaxies, however, are of modest luminosity (L~L*+1mag), destined to become dwarfs once they fade. Cluster members with ongoing star formation typical of nearby spirals are largely late Sb-Sdm Hubble types. Galaxies whose star formation ended 1-2Gyr prior to the epoch of observation are all normal early-type (S0-Sb) disk systems. The red H-delta strong galaxies are a mixture of E & S0 galaxies, consistent with them being old dormant systems which have undergone a secondary star formation episode.

The Multiplicity of Pre-Main-Sequence Stars in Southern Star-forming Regions

Abstract: High-resolution studies of young stars in the star-forming regions of Taurus and Ophiuchus have revealed a large population of multiple star systems. To test how applicable this earlier result is for other star-forming regions, we have carried out a K-band (2.2 μm) multiplicity survey of pre-main-sequence stars located in the dark cloud complexes Chameleon, Lupus, and Corona Australis. This survey, which was conducted with both speckle- and direct-imaging techniques, covers a binary star separation range of 01-12'' (15-1800 AU) and identifies 25 companion stars of which nine are new detections. The companion star fraction over the separation range covered by this survey is estimated to be 0.52 ± 0.11, in agreement with Taurus (0.58 ± 0.08) and Ophiuchus (0.50 ± 0.12). A comparison of the direct-imaging portion of this survey with Reipurth & Zinnecker's optical multiplicity study reveals that 4% of the overlap sample have "infrared companions," companions too red to be detected at optical wavelengths. This suggests that infrared surveys will systematically measure a slightly higher companion star fraction compared with optical surveys. The result of combining all K-band surveys of dark cloud complexes, which cover the separation range 15-1800 AU, shows a factor of 2 excess of the companion star fraction for young stars compared with that for the solar-type stars in the solar neighborhood (0.54 ± 0.06 vs. 0.26 ± 0.04).

Observations of the Hubble Deep Field with the Infrared Space Observatory V. Spectral energy distributions starburst models and star formation history

Abstract: We have modelled the spectral energy distributions of the 13 Hubble Deep Field (HDF) galaxies reliably detected by the Infrared Space Observatoiy (ISO). For two galaxies the emission detected by ISO is consistent with being starlight or the infrared 'cirrus' in the galaxies. For the remaining 11 galaxies there is a clear mid-infrared excess, which we interpret as emission from dust associated with a strong starburst. 10 of these galaxies are spirals or interacting pairs, while the remaining one is an elliptical with a prominent nucleus and broad emission lines. We give a new discussion of how the star formation rate can be deduced from the far-infrared luminosity, and derive star formation rates for these galaxies of 8-1000o M? yr-1, where o takes account of the uncertainty in the initial mass function. The HDF galaxies detected by ISO are clearly forming stars at a prodigious rate compared with nearby normal galaxies. We discuss the implications of our detections for the history of star and heavy element formation in the Universe. Although uncertainties in the calibration, reliability of source detection, associations and starburst models remain, it is clear that dust plays an important role in star formation out to redshift 1 at least.

A pair of lensed galaxies at z = 4.92 in the field of CL 1358+62

Abstract: The cluster CL 1358+62 displays a prominent red arc in WFPC2 images obtained with the Hubble Space Telescope. Keck spectra of the arc show Lyα emission at 7204 A, a continuum drop blueward of the line, and several absorption lines to the red. We identify the arc as a gravitationally lensed galaxy at a redshift of z = 4.92. It is the highest redshift object currently known. A gravitational lens model was used to reconstruct images of the high-redshift galaxy. The reconstructed image is asymmetric, containing a bright knot and a patch of extended emission 04 from the knot. The effective radius of the bright knot is 0022 or 130 h−150 pc. The extended patch is partially resolved into compact regions of star formation. The reconstructed galaxy has IAB = 24, giving a bolometric luminosity of about 3 × 1011 L☉. This can be produced by a star formation rate of 36 h−250 M☉ yr-1 (q0 = 0.5) or by an instantaneous starburst of 3 × 108 M☉. The spectral lines show velocity variations on the order of 300 km s-1 along the arc. The Si II line is blueshifted with respect to the Lyα emission, and the Lyα emission line is asymmetric with a red tail. These spectral features are naturally explained by an outflow model, in which the blue side of the Lyα line has been absorbed by outflowing neutral H I. Evidence from other sources indicates that outflows are common in starburst galaxies at high and low redshift. We have discovered a companion galaxy with a radial velocity only 450 km s-1 different than that of the arc. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

Dust in Starburst Galaxies

Abstract: To investigate the nature of starburst dust, we constructed a model of the stars and dust in starburst galaxies and applied it to 30 observed starburst spectral energy distributions (SEDs). The starburst model was constructed by combining two stellar evolutionary synthesis models with a model describing the radiative transfer of stellar photons through dust. The stellar evolutionary synthesis models were used to compute the dust-free SEDs for stellar populations with ages between 1 ? 106 and 15 ? 109 yr. Using a Monte Carlo radiative transfer model, the effects of dust were computed for average Milky Way (MW) and Small Magellanic Cloud (SMC) dust, two different star/dust geometries, and locally homogeneous or clumpy dust. Using color-color plots, the starburst model was used to interpret the behavior of 30 starbursts with aperture-matched UV and optical SEDs (and IR for 19 of the 30) from previous studies. From the color-color plots, it was evident that the dust in starbursts has an extinction curve lacking a 2175 ? bump, like the SMC curve, and a steep far-UV rise, intermediate between the MW and SMC curves. The star/dust geometry that is able to explain the distribution of the 30 starbursts in various color-color plots has an inner dust-free sphere of stars surrounded by an outer star-free shell of clumpy dust. When combined with other work from the literature on the Orion region and the 30 Dor region of the Large Magellanic Cloud, this work implies a trend in dust properties with star formation intensity.

The Nature of Compact Galaxies in the Hubble Deep Field. II. Spectroscopic Properties and Implications for the Evolution of the Star Formation Rate Density of the Universe

Abstract: We present a spectroscopic study of 51 compact field galaxies with redshifts z 0.7 have similar SFRs per unit mass to those at z < 0.7, they are on average ~10 times more massive. Our sample implies a lower limit for the global comoving SFR density of ~0.004 M? yr-1 Mpc-3 at z = 0.55, and ~0.008 M? yr-1 Mpc-3 at z = 0.85 (assuming Salpeter IMF, H0 = 50 km s-1 Mpc-1, and q0 = 0.5). These values, when compared to estimates for a sample of local compact galaxies selected in a similar fashion, support a history of the universe in which the SFR density declines by a factor ~10 from z = 1 to today. From the comparison with the SFR densities derived for magnitude-limited samples of field galaxies, we conclude that compact emission-line galaxies, though only ~20% of the general field population, may contribute as much as ~45% to the global SFR of the universe at 0.4 < z < 1.

Star Formation in Cluster Galaxies at 0.2<z<0.55

Abstract: The rest-frame equivalent width of the [OII] λ3727 emission line, W0(O II), has been measured for cluster and field galaxies in the Canadian Network for Observational Cosmology redshift survey of rich clusters at 0.2 10 A, as expected in a model of cluster formation in which star formation is truncated on infall. Evidence of suppressed star formation relative to the field is present in the whole cluster sample, out to 2R200, so the mechanism responsible for the differential evolution must be acting at a large distance from the cluster center and not just in the core. The mean star formation rate in the cluster galaxies with the strongest emission corresponds to an increase in the total stellar mass of less than about 4% if the star formation is due to a secondary burst lasting 0.1 Gyr.