Showing papers on "Star formation published in 1998"

The Global Schmidt law in star forming galaxies

Abstract: Measurements of Hα, H I, 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 modeling the SFR in numerical simulations of galaxy formation and evolution.

Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies

Abstract: New CO interferometer data show that the molecular gas in infrared ultraluminous galaxies is in rotating nuclear disks or rings. The CO maps yield disk radii, kinematic major axes, rotation speeds, enclosed dynamical masses, and gas masses. The CO brightness temperatures, the double-peaked CO line profiles, the limits on thermal continuum flux from dust, and the constraint that the gas mass must be less than the dynamical mass all indicate that the CO lines are subthermally excited and moderately opaque (τ = 4 to 10). We fit kinematic models in which most of the CO flux comes from a moderate-density warm intercloud medium, rather than from self-gravitating clouds. Typical ring radii are 300 to 800 pc. We derive gas masses not from a standard CO-to-mass ratio, but from a model of radiative transfer through subthermally excited CO in the molecular disks. This model yields gas masses of ~5 × 109 M☉, ~5 times lower than the standard method, and a ratio M/L ≈ 0.8 M☉ (K km s-1 pc2)-1. In the nuclear disks, we derive a ratio of gas to dynamical mass of Mgas/Mdyn ≈ 1/6, and a maximum ratio of gas to total mass surface density, μ/μtot, of 1/3. For the galaxies VII Zw 31, Arp 193, and IRAS 10565+2448, the CO position-velocity diagrams provide good evidence for rotating molecular rings with a central gap. In addition to the rotating central rings or disks, a new class of star formation region is identified, which we call an extreme starburst. These have a characteristic sizes of only 100 pc, with about 109 M☉ of gas and an IR luminosity of ≈ 3 × 1011 L☉ from recently formed OB stars. Four extreme starbursts are identified in the 3 closest galaxies in the sample, including Arp 220, Arp 193, and Mrk 273. These are the most prodigious star formation events in the local universe, each representing about 1000 times as many OB stars as 30 Doradus. In Mrk 231, the CO (2-1) velocity diagram along the line of nodes shows a 12 diameter inner disk and a 3'' diameter outer disk. The narrow CO line width, the single-peak line profile, the equality of the major and minor axes, and the observed velocity gradients all imply that the molecular disk is nearly face-on, yielding low optical and UV extinction to the active galactic nucleus (AGN). Such a geometry means that the molecular disk cannot be heated by the AGN; the far-infrared (FIR) luminosity of Mrk 231 is powered by a starburst, not the AGN. In Mrk 273, the CO (1-0) maps show long streamers of radius 5 kpc (7'') with velocity gradients north-south, and a nuclear disk of radius 400 pc (06) with velocity gradients east-west. The nuclear disk contains a bright CO core of radius 120 pc (02). In Arp 220, the CO and 1.3 mm continuum maps show the two "nuclei" embedded in a central ring or disk at P.A. 50° and a fainter structure extending 7'' (3 kpc) to the east, normal to the nuclear disk. Models of the CO and dust flux indicate that the two K-band sources contain high-density gas, with n(H2) = 2 × 104 cm-3. There is no evidence that these sources really are the premerger nuclei. They are more likely to be compact extreme starburst regions, containing 109 M☉ of dense molecular gas and new stars, but no old stars. Most of the HCN emission arises in the two nuclei. The luminosity-to-mass ratios for the CO sources in Arp 220 are compatible with the early phases of compact starbursts. There is a large mass of molecular gas currently forming stars with plenty of ionizing photons, and no obvious AGN. The entire bolometric luminosity of Arp 220 comes from starbursts, not an AGN. The CO maps show that the gas in ultraluminous IR galaxies is in extended disks that cannot intercept all the power of central AGNs, even if they exist. We conclude that in ultraluminous IR galaxies—even in Mrk 231, which hosts a quasar—the FIR luminosity is powered by extreme starbursts in the molecular rings or disks, not by dust-enshrouded quasars.

The Star formation history of field galaxies

Abstract: We develop a method for interpreting faint galaxy data which focuses on the integrated light radiated from the galaxy population as a whole. The emission history of the universe at ultraviolet, optical, and near-infrared wavelengths is modeled from the present epoch to z ≈ 4 by tracing the evolution with cosmic time of the galaxy luminosity density, as determined from several deep spectroscopic samples and the Hubble Deep Field (HDF) imaging survey. In a q0 = 0.5, h50 = 1 cosmology, the global spectrophotometric properties of field galaxies can be well fitted by a simple stellar evolution model, defined by a time-dependent star formation rate (SFR) per unit comoving volume and a universal initial mass function (IMF) extending from 0.1 to 125 M☉. While a Salpeter IMF with a modest amount of dust reddening or a somewhat steeper mass function, (m) m-2.7, can both reproduce the data reasonably well, a Scalo IMF produces too much long-wavelength light and is unable to match the observed mean galaxy colors. In the best-fit models, the global SFR rises sharply, by about an order of magnitude, from a redshift of zero to a peak value at z ≈ 1.5 in the range 0.12-0.17 M☉ yr-1 Mpc-3, to fall again at higher redshifts. After integrating the inferred star formation rate over cosmic time, we find a stellar mass density at the present epoch of Ω -->s h -->2500.005, hence a mean stellar mass-to-light ratio 4 in the B-band and 1 in K, consistent with the values observed in nearby galaxies of various morphological types. The models are able to account for the entire background light recorded in the galaxy counts down to the very faint magnitude levels probed by the HDF. Since only ~20% of the current stellar content of galaxies is produced at z > 2, a rather low cosmic metallicity is expected at these early times, in good agreement with the observed enrichment history of the damped Lyα systems. The biggest uncertainty is represented by the poorly constrained amount of starlight that was absorbed by dust and reradiated in the IR at early epochs. A monolithic collapse model, where half of the present-day stars formed at z > 2.5 and were shrouded by dust, can be made consistent with the global history of light, but overpredicts the metal mass density at high redshifts as sampled by quasi-stellar object absorbers.

High-redshift star formation in the Hubble Deep Field revealed by a submillimetre-wavelength survey

Abstract: In the local Universe, most galaxies are dominated by stars, with less than ten per cent of their visible mass in the form of gas. Determining when most of these stars formed is one of the central issues of observational cosmology. Optical and ultraviolet observations of high-redshift galaxies (particularly those in the Hubble Deep Field) have been interpreted as indicating that the peak of star formation occurred between redshifts of 1 and 1.5. But it is known that star formation takes place in dense clouds, and is often hidden at optical wavelengths because of extinction by dust in the clouds. Here we report a deep submillimetre-wavelength survey of the Hubble Deep Field; these wavelengths trace directly the emission from dust that has been warmed by massive star-formation activity. The combined radiation of the five most significant detections accounts for 30–50 per cent of the previously unresolved background emission in this area. Four of these sources appear to be galaxies in the redshift range 2< z < 4, which, assuming these objects have properties comparable to local dust-enshrouded starburst galaxies, implies a star-formation rate during that period about a factor of five higher than that inferred from the optical and ultraviolet observations.

What Powers Ultraluminous IRAS Galaxies

Abstract: We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxies (LIR ≥ 1012 L☉). We combine the survey results with a detailed case study, based on arcsecond resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We find the following: 1. Of the ultraluminous IRAS galaxies in our sample, 70%-80% are predominantly powered by recently formed massive stars, and 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 the strength of the 7.7 μm 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 circumnuclear disk/ring. 3. The mid-IR emitting regions are highly obscured [Av(screen) ~ 5-50 or Av(mixed) ~ 50-1000]. In a model where star-forming regions and dense molecular clouds are fully mixed, the ISO-derived, V-band dust extinctions approach the dust column densities inferred from CO millimeter measurements. After correction for these extinctions, we estimate that the star-forming regions in ultraluminous infrared galaxies have ages between 107 and 108 yr, 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 circumnuclear 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.

Unveiling Dust-enshrouded Star Formation in the Early Universe: a Sub-mm Survey of the Hubble Deep Field

Abstract: The advent of sensitive sub-mm array cameras now allows a proper census of dust-enshrouded massive star-formation in very distant galaxies, previously hidden activity to which even the faintest optical images are insensitive. We present the deepest sub-mm survey of the sky to date, taken with the SCUBA camera on the James Clerk Maxwell Telescope and centred on the Hubble Deep Field. The high source density found in this image implies that the survey is confusion-limited below a flux density of 2 mJy. However, within the central 80 arcsec radius independent analyses yield 5 reproducible sources with S(850um) > 2 mJy which simulations indicate can be ascribed to individual galaxies. We give positions and flux densities for these, and furthermore show using multi-frequency photometric data that the brightest sources in our map lie at redshifts z~3. These results lead to integral source counts which are completely inconsistent with a no-evolution model, and imply that massive star-formation activity continues at redshifts > 2. The combined brightness of the 5 most secure sources in our map is sufficient to account for 30 - 50% of the previously unresolved sub-mm background, and we estimate statistically that the entire background is resolved at about the 0.3 mJy level. Finally we discuss possible optical identifications and redshift estimates for the brightest sources. One source appears to be associated with an extreme starburst galaxy at z~1, whilst the remaining four appear to lie in the redshift range 2 < z < 4. This implies a star-formation density over this redshift range that is at least five times higher than that inferred from the ultraviolet output of HDF galaxies.

Modeling the Effects of Dust on Galactic Spectral Energy Distributions from the Ultraviolet to the Millimeter Band

Abstract: We present models of photometric evolution of galaxies in which the effects of a dusty interstellar medium have been included with particular care. A chemical evolution code follows the star formation rate, the gas fraction, and the metallicity, basic ingredients for the stellar population synthesis. The latter is performed with a grid of integrated spectra of simple stellar populations (SSP) of different ages and metallicities, in which the effects of dusty envelopes around asymptotic giant branch (AGB) stars are included. The residual fraction of gas in the galaxy is divided into two phases: the star-forming molecular clouds and the diffuse medium. The relative amount is a model parameter. The molecular gas is subdivided into clouds of given mass and radius: it is supposed that each SSP is born within the cloud and progressively escapes it. The emitted spectrum of the star-forming molecular clouds is computed with a radiative transfer code. The diffuse dust emission (cirrus) is derived by describing the galaxy as an axially symmetric system, in which the local dust emissivity is consistently calculated as a function of the local field intensity due to the stellar component. Effects of very small grains, subject to temperature fluctuations, as well as polycyclic aromatic hydrocarbons (PAHs) are included. The model is compared and calibrated with available data of normal and starburst galaxies in the local universe, in particular new broadband and spectroscopic Infrared Space Observatory (ISO) observations. It will be a powerful tool to investigate the star formation, the initial mass function (IMF), supernova rate (SNR) in nearby starbursts and normal galaxies, as well as to predict the evolution of luminosity functions of different types of galaxies at wavelengths covering four decades.

Submillimetre-wavelength detection of dusty star-forming galaxies at high redshift

Abstract: Optical surveys of the global star-formation rate in high-redshift galaxies show a strong peak in activity at a redshift of z ≈ 1.5, which implies that most of the star formation1 has already been seen. High-redshift galaxies may, however, emit most of their energy at submillimetre wavelengths, if they contain substantial amounts of dust that absorbs the starlight and reradiates it as far-infrared light. Here we report a deep survey of a blank region of sky, performed at submillimetre wavelengths (450 and 850 μm). We detect luminous sources in the 850-μm band which, if they have similar spectra to low-redshift ultraluminous infrared galaxies and are primarily powered by star formation, must each be converting more than 100 solar masses of gas per year into stars: this is larger than themaximum star-formation rates inferred for most optically selected galaxies2. The total amount of star formation at high redshifts is essentially fixed by the level of background light, but where the peak activity occurs at submillimetre wavelengths is not yet well established. However, the background light inferred from the sources that we have detected is already comparable to that from the optically selected sources. Establishing the main epoch of star formation will therefore require a combination of optical and submillimetre studies.

Dusty star forming galaxies at high redshift

Abstract: The global star formation rate in high redshift galaxies, based on optical surveys, shows a strong peak at a redshift of z=1.5, which implies that we have already seen most of the formation. High redshift galaxies may, however, emit most of their energy at submillimeter wavelengths if they contain substantial amounts of dust. The dust would absorb the starlight and reradiate it as far-infrared light, which would be redshifted to the submillimeter range. Here we report a deep survey of two blank regions of sky performed at submillimeter wavelengths (450 and 850-micron). If the sources we detect in the 850-micron band are powered by star formation, then each must be converting more than 100 solar masses of gas per year into stars, which is larger than the maximum star formation rates inferred for most optically-selected galaxies. The total amount of high redshift star formation is essentially fixed by the level of background light, but where the peak occurs in redshift for the submillimeter is not yet established. However, the background light contribution from only the sources detected at 850-micron is already comparable to that from the optically-selected sources. Establishing the main epoch of star formation will therefore require a combination of optical and submillimeter studies.

A Hubble Space Telescope Imaging Survey of Nearby Active Glactic Nuclei

Abstract: We obtained 500-second F606W WFPC2 images of 256 of the nearest (z<0.035) Seyfert 1,Seyfert 2, and starburst galaxies. Less than 10% show tidal features or multiple nuclei. The incidence of inner starburst rings is about 10% in both classes of Sy galaxies. In contrast, galaxies with H II region emission line spectra appear substantially more irregular because of their much higher specific rates of star formation. An unresolved central continuum source in our HST images is a virtually perfect indicator of a Sy1 spectrum. 52% of these Sy1 point sources are saturated in our images; we use their wings to estimate their magnitudes. The converse is not however true, as over a third of Sy's with direct spectroscopic evidence for broad Balmer wings show no nuclear point source. Like the Sy2's, they have central surface brightnesses consistent with those expected for the bulges of normal galaxies. The frequency of bars in Sy1's and 2's and non-Sys are the same. The Sy2 galaxies are significantly more likely to show nuclear dust absorption, especially in lanes and patches which are irregular or reach close to the nucleus. The difference cannot be explained by different average redshifts or selection techniques. This is confirmed by our morphology classifications, which show that Sy1 nuclei reside in earlier type galaxies than Sy2 nuclei. This intrinsic difference in host galaxy properties may undermine the strong unification hypothesis for Sy galaxies that they appear different due to the orientation of their central engine. The excess galactic dust we see in Sy2's may cause substantial absorption which obscures their hypothesized broad emission-line regions and central nonstellar continua. This galactic dust could produce much of the absorption in Sy2 nuclei which had instead been attributed to a thick dusty accretion torus.

Star Formation in Galaxies Along the Hubble Sequence

Abstract: Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence, and are key probes of the evolutionary properties of galaxies. The focus of this review is on the broad patterns in the star formation properties of galaxies along the Hubble sequence, and their implications for understanding galaxy evolution and the physical processes that drive the evolution. Star formation in the disks and nuclear regions of galaxies are reviewed separately, then discussed within a common interpretive framework. The diagnostic methods used to measure SFRs are also reviewed, and a self-consistent set of SFR calibrations is presented as an aid to workers in the field.

Chemical evolution of star-forming regions

Abstract: Recent advances in the understanding of the chemical processes that occur during all stages of the formation of stars, from the collapse of molecular clouds to the assemblage of icy planetesimals in protoplanetary accretion disks, are reviewed. Observational studies of the circumstellar material within 100‐10,000 AU of the young star with (sub)millimeter single-dish telescopes, millimeter interferometers, and ground-based as well as space-borne infrared observatories have only become possible within the past few years. Results are compared with detailed chemical models that emphasize the coupling of gas-phase and grain-surface chemistry. Molecules that are particularly sensitive to different routes of formation and that may be useful in distinguishing between a variety of environments and histories are outlined. In the cold, low-density prestellar cores, radicals and long unsaturated carbon chains are enhanced. During the cold collapse phase, most species freeze out onto the grains in the high-density inner region. Once young stars ignite, their surroundings are heated through radiation and/or shocks, whereupon new chemical characteristics appear. Evaporation of ices drives a “hot core” chemistry rich in organic molecules, whereas shocks propagating through the dense envelope release both refractory and volatile grain material, resulting in prominent SiO, OH, and H2O emission. The role of future instrumentation in further developing these chemical and temporal diagnostics is discussed.

Circular polarization in star-formation regions: implications for biomolecular homochirality.

Abstract: Strong infrared circular polarization resulting from dust scattering in reflection nebulae in the Orion OMC-1 star-formation region has been observed. Circular polarization at shorter wavelengths might have been important in inducing chiral asymmetry in interstellar organic molecules that could be subsequently delivered to the early Earth by comets, interplanetary dust particles, or meteors. This could account for the excess of L-amino acids found in the Murchison meteorite and could explain the origin of the homochirality of biological molecules.

An H I Aperture Synthesis Mosaic of the Large Magellanic Cloud

Abstract: We present the results of an H I aperture synthesis mosaic of the Large Magellanic Cloud (LMC), made by combining data from 1344 separate pointing centers using the Australia Telescope Compact Array (ATCA). The resolution of the mosaicked images is 10 (15 pc, using a distance to the LMC of 50 kpc). This mosaic, with a spatial resolution 15 times higher than that which had been previously obtained, emphasizes the turbulent and fractal structure of the ISM on the small scale, resulting from the dynamical feedback of the star formation processes with the ISM. The structure of the neutral atomic ISM in the LMC is dominated by H I filaments combined with numerous shells and holes. On the large scale, the H I disk appears to be remarkably symmetric and to have a well-organized and orderly, if somewhat complex, rotational field. The bulk of the H I resides in a disk 7.3 kpc in diameter. The mass of the disk component of the LMC is 2.5 × 109 M☉, and the upper limit to all mass within a radius of 4 kpc is ~3.5 × 109 M☉.

Early star formation and the evolution of the stellar initial mass function in galaxies

Abstract: It has frequently been suggested in the literature that the stellar IMF in galaxies was top-heavy at early times. This would be plausible physically if the IMF depended on a mass-scale such as the Jeans mass that was higher at earlier times because of the generally higher temperatures that were present then. In this paper it is suggested, on the basis of current evidence and theory, that the IMF has a universal Salpeter-like form at the upper end, but flattens below a characteristic stellar mass that may vary with time. Much of the evidence that has been attributed to a top-heavy early IMF, including the ubiquitous G-dwarf problem, the high abundance of heavy elements in clusters of galaxies, and the high rate of formation of massive stars in high-redshift galaxies, can be accounted for with such an IMF if the characteristic stellar mass was several times higher during the early stages of galaxy evolution. However, significant variations in the mass-to-light ratios of galaxies and large amounts of dark matter in stellar remnants are not as easily explained in this way, because they require more extreme and less plausible assumptions about the form and variability of the IMF. Metal-free ‘population III’ stars are predicted to have an IMF that consists exclusively of massive stars, and they could help to account for some of the evidence that has been attributed to a top-heavy early IMF, as well as contributing importantly to the energetics and chemical enrichment of the early Universe.

Discovery of seven T Tauri stars and a brown dwarf candidate in the nearby TW Hydrae Association

Abstract: We report the discovery of five T Tauri star systems, two of which are resolved binaries, in the vicinity of the nearest known region of recent star formation, the TW Hydrae Association. The newly discovered systems display the same signatures of youth (namely high X-ray flux, large Li abundance and strong chromospheric activity) and the same proper motion as the original five members. These similarities firmly establish the group as a bona fide T Tauri association, unique in its proximity to Earth and its complete isolation from any known molecular clouds. At an age of ~10 Myr and a distance of ~50 pc, the association members are excellent candidates for future studies of circumstellar disk dissipation and the formation of brown dwarfs and planets. Indeed, as an example, our speckle imaging revealed a faint, very likely companion 2" north of CoD-33 7795 (TWA 5). Its color and brightness suggest a spectral type ~M8.5 which, at an age of ~10^7 years, implies a mass ~20 M(Jupiter).

A hyperluminous galaxy at z = 2.8 found in a deep submillimetre survey

Abstract: We present a detailed study of SMM 02399–0136, a hyperluminous, active galaxy selected from a submm survey of the distant Universe. This galaxy is the brightest source in the fields of six rich, lensing clusters, with a total area of 0.01 deg2, that we have mapped with a sensitivity of ∼ 2 mJy beam−1 at 850 μm. We identify a compact optical counterpart with an apparent magnitude of B ∼ 23 and a low surface-brightness companion ∼ 3 arcsec away. Our spectroscopy shows that both components have the same redshift; z = 2.803 ± 0.003. The emission line widths, FWHM ≃ 1000–1500 km s−1, and line ratios, along with the compact morphology and high luminosity (MB ≃ −24.0) of the galaxy, indicate that SMM 02399–0136 contains a rare dust-embedded, narrow-line or type-2 active galactic nucleus (AGN). The source is gravitationally lensed by the foreground cluster, amplifying its apparent luminosity by a factor of 2.5, and our detailed lens model allows us to accurately correct for this. Taking the amplification into account, we estimate that SMM 02399–0136 is intrinsically a factor of five times more luminous than IRAS F 10214+4724. Its far-infrared (FIR) and Hα luminosities and low surface-brightness radio emission are indicative of an interaction-induced starburst, and the star formation rate (SFR) could be several thousand solar masses per year. This assumes that the starburst is the dominant source of energy, but we cannot yet determine reliably the relative contributions of the starburst and the buried AGN. A dust mass of 5–7 × 108 M⊙ is indicated by our data for a dust temperature of 40–50 K, independent of the dominant energy source. We estimate the possible space density of such luminous submm sources, and find that while a large population of these obscured sources could be detected in future wide-field submm surveys, they are unlikely to dominate the faint counts in this waveband. Galaxies such as SMM 02399–0136 and F 10214+4724 cannot easily be detected in conventional AGN/QSO surveys, and so estimates of the prevalence of AGN in the early Universe may require significant revision.

Infared Observations of Nebular Emission Lines from Galaxies at z = 3

Abstract: We present the first results from a program of near-infrared spectroscopy aimed at studying the familiar rest-frame optical emission lines from the H II regions of Lyman break galaxies at z = 3. By targeting redshifts which bring the lines of interest into gaps between the strong OH sky emission, we have been successful in detecting Balmer and [O III] emission lines in all five galaxies observed so far with CGS4 on UKIRT. For a Salpeter IMF and a H_0 = 70 km/s/Mpc, q_0 = 0.1 cosmology, the Hbeta luminosities uncorrected for dust extinction imply star formation rates of 20 - 270 solar masses per year. On the basis of the present limited sample it appears that an extinction of 1 - 2 magnitudes at 1500 A may be typical of Lyman break galaxies. This value is consistent with recent estimates of dust obscuration in star forming galaxies at z < 1, and does not require a substantial revision of the broad picture of star formation over the Hubble time proposed by Madau et al. (1996). In four out of five cases the velocity dispersion of the emission line gas is sigma = 70 km/s, while in the fifth the line widths are nearly three times larger. Virial masses in the range from 1 x 10^{10} to 5 x 10^{10} solar masses are suggested, but both velocities and masses could be higher because our observations are only sensitive to the brightest cores of these systems where the line widths may not sample the full gravitational potential. The relative redshifts of interstellar absorption, nebular emission, and Lyman alpha emission lines differ by several hundred km/s and suggest that large-scale outflows may be a common characteristic of Lyman break galaxies.

Infrared Observations of Nebular Emission Lines from Galaxies at z ≃ 3

Abstract: We present the first results from a program of near-infrared spectroscopy aimed at studying the familiar rest-frame optical emission lines from the H II regions of Lyman break galaxies at z 3. By targeting redshifts that bring the lines of interest into gaps between the strong OH sky emission, we have been successful in detecting Balmer and [O III] emission lines in all five galaxies observed so far with CGS4 on UKIRT. The typical line fluxes are a few times 10-17 ergs s-1 cm-2, approximately 1 order of magnitude lower than the limits reached with wide-field narrowband imaging surveys. For a Salpeter initial mass function and a H0 = 70 km s-1 Mpc-1, q0 = 0.1 cosmology, the Hβ luminosities uncorrected for dust extinction imply star formation rates of 20-270 M☉ yr-1; these values are greater than those that may have been deduced from the ultraviolet continuum luminosities at 1500 A by factors of between ~0.7 and ~7. Uncertainties in the shape of the reddening curve and in the intrinsic UV continuum slope do not yet allow us to assess accurately the level of dust extinction; however, on the basis of the present limited sample, it appears that an extinction of 1-2 mag at 1500 A may be typical of Lyman break galaxies. This value is consistent with recent estimates of dust obscuration in star-forming galaxies at z ≤ 1 and does not require a substantial revision of the broad picture of star formation over the Hubble time proposed by Madau and coworkers in 1996. In four out of five cases the velocity dispersion of the emission line gas is σ 70 km s-1, while in the fifth the line widths are nearly three times larger. Virial masses Mvir ≈ (1-5) × 1010 M☉ are suggested, but both velocities and masses could be higher, because our observations are only sensitive to the brightest cores of these systems where the line widths may not sample the full gravitational potential. The relative redshifts of interstellar absorption, nebular emission, and Lyα emission lines differ by several hundred km s-1 and suggest that large-scale outflows may be a common characteristic of Lyman break galaxies. The forthcoming availability of high-resolution infrared spectrographs on large telescopes will soon allow all of these questions to be addressed in much greater detail.

High-z lyalpha emitters. i. a blank-field search for objects near redshift z = 3.4 in and around the hubble deep field and the hawaii deep field ssa 22

Abstract: We present deep narrowband (λ = 5390 A, Δλ = 77 A) and multicolor observations of the Hubble Deep Field and the Hawaii Deep Field SSA 22 obtained with the LRIS instrument at the 10 m Keck II Telescope. It is shown that there is a substantial population of galaxies at z ~ 3.4 that can be selected by Lyα emission. Comparison with color-selected samples shows that the samples selected with these different criteria have substantial, but not complete, overlap and that there is a comparable surface density in the two selected populations. The emission-line–selected samples include objects with strong Lyα, and which are significant contributers to the integrated star formation at these epochs. For a Salpeter initial mass function, we estimate a minimum star formation rate of 0.01 M⊙ Mpc-3 yr-1 at z = 3.4 for H0 = 65 km s-1 Mpc-1 and q0 = 0.5 in the Lyα-selected objects, though the value could be substantially higher if there is significant extinction.

Star Formation in Clusters: A Survey of Compact Millimeter-Wave Sources in the Serpens Core

Abstract: We report the results of a millimeter interferometric survey of compact 3 mm continuum sources in the inner 55 ? 55 region of the Serpens core. We detect 32 discrete sources above 4.0 mJy beam-1, 21 of which are new detections at millimeter wavelengths. By comparing our data with published infrared surveys, we estimate that 26 sources are probably protostellar condensations and derive their mass by assuming optically thin thermal emission from dust grains. The mass spectrum of the clumps, dN/dM~M -->?2.1, is consistent with the stellar initial mass function, supporting the idea that the stellar masses in young clusters are determined by the fragmentation of turbulent cloud cores.

The Density of Lyα Emitters at Very High Redshift

Abstract: We describe narrowband and spectroscopic searches for emission-line star-forming galaxies in the redshift range 3-6 with the 10 m Keck II Telescope. These searches yield a substantial population of objects with only a single strong (EW 100 A) emission line, lying in the 4000-8500 A range. Spectra of the objects found in narrowband-selected samples at λ~5390 and ~6741 A show that these very high equivalent width emission lines are generally redshifted Lyα 1216 A at z~3.4 and 4.5. The density of these emitters above the 5 σ detection limit of 1.5×10−17 ergs cm-2 s-1 is roughly 15,000 deg-2 per unit z at both z~3.4 and 4.5. A complementary deeper (1 σ~10−18 ergs cm-2 s-1) slit spectroscopic search covering a wide redshift range but a more limited spatial area (200 arcsec2) shows that such objects can be found over the redshift range z=3-6, with the currently highest redshift detected being at z=5.64. The Lyα flux distribution can be used to estimate a minimum star formation rate in the absence of reddening of roughly 0.01 M☉ Mpc-3 yr-1 (H0=65 km s-1 Mpc-1, q0=0.5). Corrections for reddening are likely to be no larger than a factor of 2, since observed equivalent widths are close to the maximum values obtainable from ionization by a massive star population. Within the still significant uncertainties, the star formation rate from the Lyα-selected sample is comparable to that of the color-break-selected samples at z~3 but may represent an increasing fraction of the total rates at higher redshifts. This higher z population can be readily studied with large ground-based telescopes.

The Impact of Star Formation on the Interstellar Medium in Dwarf Galaxies. II. The Formation of Galactic Winds

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.

The Density of Lyman-alpha Emitters at Very High Redshift

Abstract: We describe narrowband and spectroscopic searches for emission-line star forming galaxies in the redshift range 3 to 6 with the 10 m Keck II Telescope. These searches yield a substantial population of objects with only a single strong (equivalent width >> 100 Angstrom) emission line, lying in the 4000 - 10,000 Angstrom range. Spectra of the objects found in narrowband-selected samples at lambda ~5390 Angstroms and ~6741 Angstroms show that these very high equivalent width emission lines are generally redshifted Lyman alpha 1216 Angstrom at z~3.4 and 4.5. The density of these emitters above the 5 sigma detection limit of 1.5 e-17 ergs/cm^2/s is roughly 15,000 per square degree per unit redshift interval at both z~3.4 and 4.5. A complementary deeper (1 sigma \~1.0 e-18 ergs/cm^2/s) slit spectroscopic search covering a wide redshift range but a more limited spatial area (200 square arcminutes) shows such objects can be found over the redshift range 3 to 6, with the currently highest redshift detected being at z=5.64. The Lyman alpha flux distribution can be used to estimate a minimum star formation rate in the absence of reddening of roughly 0.01 solar masses/Mpc^3/year (H_0 = 65 km/s/Mpc and q_0 = 0.5). Corrections for reddening are likely to be no larger than a factor of two, since observed equivalent widths are close to the maximum values obtainable from ionization by a massive star population. Within the still significant uncertainties, the star formation rate from the Lyman alpha-selected sample is comparable to that of the color-break-selected samples at z~3, but may represent an increasing fraction of the total rates at higher redshifts. This higher-z population can be readily studied with large ground-based telescopes.

Star formation in clusters: a survey of compact mm-wave sources in the Serpens core

Abstract: We report the results of a millimeter interferometric survey of compact 3 mm continuum sources in the inner 5.5'x5.5' region of the Serpens core. We detect 32 discrete sources above 4.0 mJy/beam, 21 of which are new detections at millimeter wavelengths. By comparing our data with published infrared surveys, we estimate that 26 sources are probably protostellar condensations and derive their mass assuming optically thin thermal emission from dust grains. The mass spectrum of the clumps, dN/dM~M^(-2.1), is consistent with the stellar initial mass function, supporting the idea that the stellar masses in young clusters are determined by the fragmentation of turbulent cloud cores.

High-z Lyman-alpha Emitters. I. A Blank-Field Search for Objects Near Redshift z=3.4 in and around the Hubble Deep Field and the Hawaii Deep Field SSA22

Abstract: We present deep narrow-band (lambda = 5390 Angstroms, delta lambda = 77 Angstroms) and multi-color observations of the Hubble Deep Field and the Hawaii Deep Field SSA22 obtained with the LRIS instrument at the Keck II 10-m telescope. It is shown that there is a substantial population of galaxies at z~3.4 which can be selected by Ly alpha emission. Comparison with color-selected samples shows that the samples selected with these different criteria have substantial, but not complete overlap, and that there is a comparable surface density in the two selected populations. The emission-line selected samples include objects with strong Ly alpha, and which are significant contributers to the integrated star formation at these epochs. For a Salpeter IMF we estimate a minimum star formation rate of 0.01 solar masses per cubic Mpc per yr at z=3.4 for H_0 = 65 km/s/Mpc and q_0 = 0.5 in the Ly alpha-selected objects, though the value could be substantially higher if there is significant extinction.

L1544: A Starless dense core with extended inward motions

Abstract: We present a multiline study of the dense core L1544 in the Taurus molecular complex. Although L1544 does not harbor an embedded star, it presents several characteristics of cores that have already undergone star formation, suggesting that it may be rather advanced in its evolution toward becoming a star-forming core. The spectral lines from L1544 present an interesting dichotomy, with the thick dense gas tracers su†ering very strong self absorption while CO and its isotopes are not being absorbed at all. The presence of the self absorptions allows us to study both the density structure and kinematics of the gas in detail. A simple analysis shows that the core is almost isothermal and that the self absorptions are due to very subthermal excitation of the dense gas tracers in the outer layers. The density has to decrease outward rapidly, and a detailed radiative transfer calculation that simultaneously -ts three iso- topes of CO and two of CS shows that the density approximately follows a r~1.5 power law. The self absorptions, in addition, allow us to measure the relative velocity between the inner and outer layers of the core, and we -nd that there is a global pattern of inward motions (background and foreground approaching each other). The relative speed between the foreground and background changes with posi- tion, and we use a simple two-layer model to deduce that while the foreground gas has a constant veloc- ity, the background material presents systematic velocity changes that we interpret as arising from two velocity components. We explore the origin of the inward motions by comparing our observations with models of gravitational collapse. A model in which the infall starts at the center and propagates outward (as in the inside-out collapse of Shu) is inconsistent with the large extension of the absorption (that sug- gests an advanced age) and the lack of a star at the core center (that suggests extreme youth). Ambipolar di†usion seems also ruled out because of the large amount of the inward speed (up to 0.1 km s~1) and the fact that ionized species move with speeds similar to those of the neutrals. Other infall models seem also to have problems -tting the data, so if L1544 is infalling, it seems to be doing so in a manner not contemplated by the standard theories of star formation. Our study of L1544 illustrates how little is still known about the physical conditions that precede star formation and how detailed studies of starless cores are urgently needed. Subject headings: ISM: individual (L1544) E ISM: kinematics and dynamics E stars: formation

The Physical Properties of the Midcourse Space Experiment Galactic Infrared-dark Clouds

Abstract: The SPIRIT III infrared telescope on the Midcourse Space Experiment (MSX) satellite has provided an unprecedented view of the mid-infrared emission (8-25 μm) of the Galactic plane. An initial analysis of images from MSX Galactic plane survey data reveals dark clouds seen in silhouette against the bright emission from the Galactic plane (Egan et al.). These clouds have mid-infrared extinctions in excess of 2 mag at 8 μm. We probed the physical properties of 10 of these MSX dark clouds using millimeter-wave molecular rotational lines as an indicator of dense molecular gas. All 10 clouds were detected in millimeter spectral lines of H2CO, which confirms the presence of dense gas. The distances to these clouds range from 1 to 8 kiloparsecs and their diameters from 0.4 to 15.0 pc. Excitation analysis of the observed lines indicates that the clouds are cold (T 105 cm-3]. Some of the clouds have nearby H II regions, H2O masers, and other tracers of star formation at comparable spectral line velocities; however, only one cloud contains embedded centimeter or infrared sources. The lack of mid- to far-infrared emission associated with these clouds suggests that they are not currently forming high-mass stars. If star formation is present in these clouds, it is clearly protostellar class 0 or earlier.

Magnetospheric Accretion Models for the Hydrogen Emission Lines of T Tauri Stars

Abstract: Using a statistical equilibrium radiative transfer treatment, we examine the behavior of hydrogen emission lines arising from the magnetospheric infall zones of classical T Tauri stars. Having calculated self-consistent line profiles of the Balmer lines, Pa? and Br?, we explore parameter space, examining the effects of the magnetospheric gas temperature and size on the line fluxes. We compare model and observed line fluxes for the Balmer lines and find a good match using a relatively small range of parameters. We are also able to match the observed Br? line profile of the embedded object WL 16, which supports the use of the infrared lines in studying magnetospheric infall even in the earliest stages of star formation. Finally, we discuss constraints on the physical parameters and the possibility of using the emission lines as accretion rate indicators.

The Nature and Evolution of Ultraluminous Infrared Galaxies:A Mid-Infrared Spectroscopic Survey

Abstract: We report the first results of a low-resolution mid-infrared spectroscopic survey of an unbiased, far-infrared-selected sample of 60 ultraluminous infrared galaxies (ULIRGs) (LIR${r IR}$ -->$t SUBgt {r IR}t/SUBgt $ --> > 10 -->12 L?) using ISOPHOT-S on board the Infrared Space Observatory (ISO). We use the ratio of the 7.7 ?m polycyclic aromatic hydrocarbon (PAH) emission feature to the local continuum as a discriminator between starburst and active galactic nucleus (AGN) activity. About 80% of all ULIRGs are found to be predominantly powered by star formation, but the fraction of AGN-powered objects increases with luminosity. Whereas only about 15% of ULIRGs at luminosities below 2 ? 10 -->12 L? are AGN powered, this fraction increases to about half at higher luminosity. Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9 ?m continuum to the IRAS 60 ?m continuum, confirming suggestions that strong mid-infrared continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-Short Wavelength Spectrograph spectroscopy, which implies significant extinction even in the mid-infrared. We have searched for indications that ULIRGs that are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found among those objects for which near-infrared images are available to assess their likely merger status.