Showing papers in "The Astronomical Journal in 2000"

The Sloan Digital Sky Survey: Technical summary

Abstract: The Sloan Digital Sky Survey (SDSS) will provide the data to support detailed investigations of the distribution of luminous and nonluminous matter in the universe: a photometrically and astrometrically calibrated digital imaging survey of π sr above about Galactic latitude 30° in five broad optical bands to a depth of g' ~ 23 mag, and a spectroscopic survey of the approximately 106 brightest galaxies and 105 brightest quasars found in the photometric object catalog produced by the imaging survey. This paper summarizes the observational parameters and data products of the SDSS and serves as an introduction to extensive technical on-line documentation.

2MASS Extended Source Catalog: Overview and Algorithms

Abstract: The 2 Micron All-Sky Survey (2MASS)will observe over one-million galaxies and extended Galactic sources covering the entire sky at wavelenghts between 1 and 2 m. Most of these galaxies, from 70 to 80%, will be newly catalogued objetcs.

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

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

Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars

Abstract: We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H] ≤ -0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H] ≤-2.2, which likely represent a pure halo component, are characterized by a radially elongated velocity ellipsoid (σU, σV, σW) = (141 ± 11, 106 ± 9, 94 ± 8) km s-1 and small prograde rotation V = 30 to 50 km s-1, consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we find a decrease in V with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (ΔV/Δ|Z| = -52 ± 6 km s-1 kpc-1), which may represent the signature of a dissipatively formed flattened inner halo. Unlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e, with no apparent correlation between [Fe/H] and e. This demonstrates, clearly and convincingly, that the evidence offered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identified a small concentration of high-e stars at [Fe/H] ~ -1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We find no evidence for an additional thick disk component for stellar abundances [Fe/H] ≤ -2.2. The kinematics of the intermediate-abundance stars close to the Galactic plane are, in part, affected by the presence of a rapidly rotating thick disk component with V 200 km s-1 (with a vertical velocity gradient on the order of ΔV/Δ|Z| = -30 ± 3 km s-1 kpc-1) and velocity ellipsoid (σU, σV, σW) = (46 ± 4, 50 ± 4, 35 ± 3) km s-1. The fraction of low-metallicity stars in the solar neighborhood that are members of the thick disk population is estimated as ~10% for -2.2 < [Fe/H] ≤ -1.7 and ~30% for -1.7 < [Fe/H] ≤ -1. We obtain an estimate of the radial scale length of the metal-weak thick disk of 4.5 ± 0.6 kpc. We also analyze the global kinematics of the stars constituting the halo component of the Galaxy. The outer part of the halo, which we take to be represented by local stars on orbits reaching more than 5 kpc from the Galactic plane, exhibits no systematic rotation. In particular, we show that previous suggestions of the presence of a counter-rotating high halo are not supported by our analysis. The density distribution of the outer halo is nearly spherical and exhibits a power-law profile that is accurately described as ρ ∝ R-3.55±0.13. The inner part of the halo is characterized by a prograde rotation and a highly flattened density distribution. We find no distinct boundary between the inner and outer halo. We confirm the clumping in angular-momentum phase space of a small number of local metal-poor stars noted in 1999 by Helmi et al. We also identify an additional elongated feature in angular-momentum phase space extending from the clump to regions with high azimuthal rotation. The number of members in the detected clump is not significantly increased from that reported by Helmi et al., even though the total number of the sample stars we consider is almost triple that of the previous investigation. We conclude that the fraction of halo stars that may have arisen from the precursor object of this clump may be smaller than 10% of the present Galactic halo, as previously suggested. The implications of our results for the formation of the Galaxy are discussed, in particular in the context of the currently favored cold dark matter theory of hierarchical galaxy formation.

The Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters

Abstract: This paper commences a series of investigations into the stellar populations of local elliptical galaxies as determined from their integrated spectra. The goal of the series is to determine the star formation and chemical evolution histories of present-day elliptical galaxies. The primary galaxy sample analyzed is that of Gonzalez, which consists of 39 elliptical galaxies drawn primarily from the local field and nearby groups, plus the bulge of Messier 31. Single-burst stellar population (SSP)–equivalent ages, metallicities, and abundance ratios are derived from Hβ, Mg b, and Fe line strengths using an extension of the Worthey models that incorporates nonsolar line-strength "response functions" by Tripicco & Bell. These functions account for changes in the Lick/IDS indices caused by nonsolar abundance ratios, allowing us to correct the Worthey models for the enhancements of Mg and other α-like elements relative to the Fe-peak elements. SSP-equivalent ages of the Gonzalez elliptical galaxies are found to vary widely, 1.5 Gyr t 18 Gyr, while metallicities [Z/H] and enhancement ratios [E/Fe] are strongly peaked around [Z/H] = +0.26 and [E/Fe] = +0.20 (in an aperture of radius re/8). The enhancement ratios [E/Fe] are milder than previous estimates because of the application of nonsolar abundance corrections to both Mg b and Fe for the first time. While [E/Fe] is usually greater than zero, it is not the "E" elements that are actually enhanced but rather the Fe-peak elements that are depressed; this serves not only to weaken Fe but also to strengthen Mg b, accounting for the overall generally mild enhancements. Based on index strengths from the Lick/IDS galaxy library (Trager et al.), C is not depressed with Fe but rather seems to be on a par with other elements such as Mg in the E group. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age increasing by 25%, metallicity decreasing by [Z/H] = 0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius re/2 for nearly all systems. Our ages have an overall zero-point uncertainty of at least ~25% because of uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of [E/Fe] ≠ 0 on the isochrones, and other unknowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in ages appears to be real and cannot be explained by contamination of Hβ by blue stragglers or hot horizontal-branch stars, or by fill-in of Hβ by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers.

A New Method For Galaxy Cluster Detection. I. The Algorithm

Abstract: Numerous methods for finding clusters at moderate to high redshifts have been proposed in recent years, at wavelengths ranging from radio to X-rays. In this paper we describe a new method for detecting clusters in two-band optical/near-IR imaging data. The method relies upon the observation that all rich clusters, at all redshifts observed so far, appear to have a red sequence of early-type galaxies. The emerging picture is that all rich clusters contain a core population of passively evolving elliptical galaxies that are coeval and formed at high redshifts. The proposed search method exploits this strong empirical fact by using the red sequence as a direct indicator of overdensity. The fundamental advantage of this approach is that with appropriate filters, cluster elliptical galaxies at a given redshift are redder than all normal galaxies at lower redshifts. A simple color cut thus virtually eliminates all foreground contamination, even at significant redshifts. In this paper, one of a series of two, we describe the underlying assumptions and basic techniques of the method in detail and contrast the method with those used by other authors. We provide a brief demonstration of the effectiveness of the technique using a real photometric sample with redshift data, and from this we conclude that the method offers a powerful yet simple way of identify galaxy clusters. We find that the method can reliably detect structures to masses as small as groups with velocity dispersions of only ~300 km s-1, with redshifts for all detected structures estimated to an accuracy of ~10%.

67 Additional L Dwarfs Discovered by the Two Micron All Sky Survey

Abstract: We present JHKs photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the latest L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Analysis of these new discoveries shows that Hα emission has yet to be convincingly detected in any L dwarf later than type L4.5, indicating a decline or absence of chromospheric activity in the latest L dwarfs. Further analysis shows that 16 (and possibly four more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until type ~L6.5 V, then declines for later types. This disappearance may be the first sign of depletion of atomic lithium as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements, we develop spectrophotometric relations to estimate distances to the other L dwarfs. Of the 67 new discoveries, 21 have photometric distances placing them within 25 pc of the Sun. A table of all known L and T dwarfs believed to lie within 25 pc—53 in total — is also presented. Using the distance measurement of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350 K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition objects have yet been uncovered. This evidence, combined with model fits to low-resolution spectra of late M and early L dwarfs, indicates that L-class objects span the range 1300 K Teff 2000 K. The near-infrared color-color diagram shows that L dwarfs fall along a natural, redder extension of the well-known M dwarf track. These near-infrared colors get progressively redder for later spectral types, with the L dwarf sequence abruptly ending near (J-H, H-Ks, J-Ks) ≈ (1.3, 0.8, 2.1).

New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence

Abstract: We have combined 2MASS and POSS II data in a search for nearby ultracool (later than M6.5) dwarfs with Ks 6 A) ultracool M dwarfs are consistent with an ordinary old disk stellar population, while the kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen-burning limit is near spectral types L2–L4. We use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and temperature and that at a given (M8 or later) spectral type, the younger field (brown) dwarfs are less active than many of the older, more massive field stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in field ultracool dwarfs can be an indication of comparative youth and substellar mass.

Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis

Abstract: We describe observations and abundance analysis of a high-resolution, high signal-to-noise ratio survey of 168 stars, most of which are metal-poor dwarfs. We follow a self-consistent LTE analysis technique to determine the stellar parameters and abundances, and we estimate the effects of random and systematic uncertainties on the resulting abundances. Element-to-iron ratios are derived for key α-, odd-Z, Fe-peak, and r- and s-process elements. Effects of non-LTE on the analysis of Fe I lines are shown to be very small on average. Spectroscopically determined surface gravities are derived that are quite close to those obtained from Hipparcos parallaxes.

9286 Stars: An Agglomeration of Stellar Polarization Catalogs

Abstract: We present an agglomeration of stellar polarization catalogs with results for 9286 stars. We have endeavored to eliminate errors, provide accurate (approximately arcsecond) positions, weight multiple observations of the same star sensibly, and provide reasonable distances. The catalog is available by anonymous ftp from the author.

The Frequency of Barred Spiral Galaxies in the Near-Infrared*

Abstract: We have determined the fraction of barred galaxies in the H-band for a statistically well-defined sample of 186 spirals drawn from the Ohio State University Bright Spiral Galaxy Survey. We find 56% of our sample to be strongly barred in the H band while another 16% is weakly barred. Only 27% of our sample is unbarred in the near-infrared. The RC3 and the Carnegie Atlas of Galaxies both classify only about 30% of our sample as strongly barred. Thus strong bars are nearly twice as prevalent in the near-infrared as in the optical. The frequency of genuine optically hidden bars is significant but lower than many claims in the literature: 40% of the galaxies in our sample that are classified as unbarred in the RC3 show evidence for a bar in the H band while the Carnegie Atlas lists this fraction as 66%. Our data reveal no significant trend in bar fraction as a function of morphology in either the optical or H band. Optical surveys of high-redshift galaxies may be strongly biased against finding bars, as bars are increasingly difficult to detect at bluer rest wavelengths.

The Galactic Thick Disk Stellar Abundances

Abstract: We present first results from a program to measure the chemical abundances of a large (N > 30) sample of thick disk stars with the principal goal of investigating the formation history of the Galactic thick disk. We have obtained high-resolution, high signal-to-noise spectra of 10 thick disk stars with the HIRES spectrograph on the 10 m Keck I telescope. Our analysis confirms previous studies of O and Mg in the thick disk stars, which reported enhancements in excess of the thin disk population. Furthermore, the observations of Si, Ca, Ti, Mn, Co, V, Zn, Al, and Eu all argue that the thick disk population has a distinct chemical history from the thin disk. With the exception of V and Co, the thick disk abundance patterns match or tend toward the values observed for halo stars with [Fe/H] ≈ -1. This suggests that the thick disk stars had a chemical enrichment history similar to the metal-rich halo stars. With the possible exception of Si, the thick disk abundance patterns are in excellent agreement with the chemical abundances observed in the metal-poor bulge stars, suggesting the two populations formed from the same gas reservoir at a common epoch. The principal results of our analysis are as follows. (1) All 10 stars exhibit enhanced α/Fe ratios with O, Si, and Ca showing tentative trends of decreasing overabundances with increasing [Fe/H]. In contrast, the Mg and Ti enhancements are constant. (2) The light elements Na and Al are enhanced in these stars. (3) With the exception of Ni, Cr, and possibly Cu, the iron-peak elements show significant departures from the solar abundances. The stars are deficient in Mn, but overabundant in V, Co, Sc, and Zn. (4) The heavy elements Ba and Y are consistent with solar abundances, but Eu is significantly enhanced. If the trends of decreasing O, Si, and Ca with increasing [Fe/H] are explained by the onset of Type Ia SN, then the thick disk stars formed over the course of 1 Gyr. We argue that this formation time-scale would rule out most dissipational collapse scenarios for the formation of the thick disk. Models which consider the heating of an initial thin disk—either through "gradual" heating mechanisms or a sudden merger event—are favored. These observations provide new tests of theories of nucleosynthesis in the early universe. In particular, the enhancements of Sc, V, Co, and Zn may imply overproduction during an enhanced α-rich freeze out fueled by neutrino-driven winds. Meanwhile, the conflicting trends for Mg, Ti, Ca, Si, and O pose a difficult challenge to our current understanding of nucleosynthesis in Type Ia and Type II SN. The Ba/Eu ratios favor r-process dominated enrichment for the heavy elements, consistent with the ages (tage > 10 Gyr) expected for these stars. Finally, we discuss the impact of the thick disk abundances on interpretations of the abundance patterns of the damped Lyα systems. The observations of mildly enhanced Zn/Fe imply an interpretation for the damped systems which includes a dust depletion pattern on top of a Type II SN enrichment pattern. We also argue that the S/Zn ratio is not a good indicator of nucleosynthetic processes.

NICMOS Imaging of Infrared-Luminous Galaxies

Abstract: We present near-infrared images obtained with the Hubble Space Telescope NICMOS camera for a sample of nine luminous [LIGs: LIR(8?1000 ?m) ? 1011 L?] and 15 ultraluminous (ULIGS: LIR ? 1012 L?) infrared galaxies. The sample includes representative systems classified as warm (f25 ?m/f60 ?m > 0.2) and cold (f25 ?m/f60 ?m ? 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as H II (i.e., starburst), QSO, Seyfert, and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters. The colors and the luminosities of the observed clusters are consistent with them being young (107?108 yr), formed as a result of galactic interactions, and having masses much greater than those of Galactic globular clusters. In NGC 6090 and VV 114, they are preferentially situated along the area of overlap of the two galactic disks. With the exception of IR 17208-0018, all of the ULIGs have at least one compact (2.2??m FWHM ? 200 pc) nucleus. Analysis of the near-infrared colors (i.e., m1.1?1.6 vs. m1.6?2.2) derived from 11 diameter apertures suggests that the warm galaxies have near-infrared colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In addition, the cold ULIG UGC 5101 (and possibly three others) have near-infrared colors suggesting additional active galactic nucleus?like near-infrared components in their nuclei. In a 2 kpc diameter aperture measurement, the global colors of all of the cold galaxies except UGC 5101 are consistent with starlight with a few magnitudes of visual extinction. The general dichotomy of the near-infrared properties of the warm and the cold galaxies are further supported by the light distributions: seven of the eight warm galaxies have unresolved nuclear emission that contributes significantly (i.e., ?30%?40%) to the total near-infrared luminosity. The smooth, more extended light observed in all of the galaxies is most likely composed of giant and supergiant stars, but evidence at longer wavelengths suggests that these stars contribute little to the high 8?1000 ?m luminosity of these galaxies. Finally, light profiles of nine of the 24 systems were fitted well by an r1/4 law (and not so well by an exponential disk profile). Whether these star systems eventually become massive central bulges or giant elliptical galaxies will depend on how efficiently the present ISM is converted into stars.

M31 Globular Clusters: Colors and Metallicities* ** ***

Abstract: We present a new catalog of photometric and spectroscopic data on M31 globular clusters (GCs). The catalog includes new optical and near-infrared photometry for a substantial fraction of the 435 clusters and cluster candidates. We use these data to determine the reddening and intrinsic colors of individual clusters, and we find that the extinction laws in the Galaxy and M31 are not significantly different. There are significant (up to 0.2 mag in V-K) offsets between the clusters' intrinsic colors and simple stellar population colors predicted by population synthesis models; we suggest that these are due to systematic errors in the models. The distributions of M31 clusters' metallicities and metallicity-sensitive colors are bimodal, with peaks at [Fe/H] ≈ -1.4 and -0.6. The distribution of V-I is often bimodal in elliptical galaxies' globular cluster systems, but it is not sensitive enough to metallicity to show bimodality in M31 and Galactic cluster systems. The radial distribution and kinematics of the two M31 metallicity groups imply that they are analogs of the Galactic halo and disk/bulge cluster systems. The globular clusters in M31 have a small radial metallicity gradient, suggesting that some dissipation occurred during the formation of the globular cluster system. The lack of correlation between cluster luminosity and metallicity in M31 GCs shows that self-enrichment is not important in GC formation.

2MASS Observations of the Perseus, Orion A, Orion B, and Monoceros R2 Molecular Clouds

Abstract: We use the 2MASS Second Incremental Release Point Source Catalog to investigate the spatial distribution of young stars in the Perseus, Orion A, Orion B, and MonR2 molecular clouds. After subtracting a semiempirical model of the field star contamination from the observed star counts, stellar surface density maps are used to identify compact clusters and any stellar population found more uniformly distributed over the molecular cloud. Each cloud contains between two and seven clusters, with at least half of the cluster population found in a single, rich cluster. In addition, a distributed stellar population is inferred in the Orion A and MonR2 molecular clouds within the uncertainties of the field star subtraction with a surface density between 0.013 and 0.083 arcmin-2. Sensitivity calculations suggest, however, that the number of stars in the distributed population may be underestimated by a factor of 2 or more if stars have been forming with a Miller-Scalo IMF at a constant star formation rate for longer than 10 Myr. After considering the possible evolutionary status of the distributed population, the global star formation efficiency implied by the sum of the distributed and cluster populations ranges between 1% and 9% among the four clouds. The fraction of the total stellar population contained in clusters for the nominal extinction model ranges from ≈50% to 100% if the distributed population is relatively young (<10 Myr), to ≈25%–70% if it is relatively old (≈100 Myr). The relatively high fraction of stars contained in clusters regardless of the age of the distributed population, in conjunction with the young ages generally inferred for embedded clusters in nearby molecular clouds, indicates that a substantial fraction of the total stellar population in these regions has formed within the past few million years in dense clusters. This suggests that either the star formation rate in each these clouds has recently peaked if one assumes clouds have ages greater than 10 Myr or molecular clouds are younger than typically thought if one assumes that the star formation rate has been approximately constant in time.

Structural and photometric classification of galaxies. i. calibration based on a nearby galaxy sample

Abstract: In this paper we define an observationally robust, multiparameter space for the classification of nearby and distant galaxies. The parameters include luminosity, color, and the image-structure parameters: size, image concentration, asymmetry, and surface brightness. Based on an initial calibration of this parameter space using the normal Hubble types surveyed in 1996 by Frei et al., we find that only a subset of the parameters provide useful classification boundaries for this sample. Interestingly, this subset does not include distance-dependent scale parameters such as size or luminosity. The essential ingredient is the combination of a spectral index (e.g., color) with parameters of image structure and scale: concentration, asymmetry, and surface brightness. We refer to the image structure parameters (concentration and asymmetry) as indices of form. We define a preliminary classification based on spectral index, form, and surface brightness (a scale) that successfully separates normal galaxies into three classes. We intentionally identify these classes with the familiar labels of early, intermediate, and late. This classification, or others based on the above four parameters, can be used reliably to define comparable samples over a broad range in redshift. The size and luminosity distribution of such samples will not be biased by this selection process except through astrophysical correlations between spectral index, form, and surface brightness.

Mapping the Evolution of High Redshift Dusty Galaxies with Submillimeter Observations of a Radio-Selected Sample

Abstract: Direct submillimeter imaging has recently revealed the 850 μm background to be mostly composed of a population of distant ultraluminous infrared galaxies, but identifying the optical/near-infrared (NIR) counterparts to these sources has proved difficult because of the poor submillimeter spatial resolution. However, the proportionality of both centimeter and submillimeter data to the star formation rate suggests that high-resolution radio continuum maps with subarcsecond positional accuracy can be exploited to locate submillimeter sources. In this paper we present results from a targeted SCUBA survey of microjansky radio sources in the flanking fields of the Hubble Deep Field. The sources were selected from the uniform (8 μJy at 1 σ) 1.4 GHz VLA image of Richards. Even with relatively shallow SCUBA observations (a 3 σ detection limit of 6 mJy at 850 μm), we were successful at making submillimeter detections of optical/NIR-faint (I 24 and K 21–22) radio sources, and our counts closely match the bright counts from submillimeter surveys. An important corollary is that a large fraction of the bright (>6 mJy) submillimeter sources in untargeted submillimeter surveys have extremely faint optical/NIR counterparts and hence are inaccessible to optical imaging and spectroscopy. However, redshift estimates can be made from the ratio of the submillimeter flux to the radio flux across the 100 GHz break in the spectral energy distribution. This procedure, which we refer to as millimetric redshift estimation, places the bright submillimeter population at z = 1–3, where it forms the high-redshift tail of the faint radio population. The star formation rate density (SFRD) due to ultraluminous infrared galaxies increases by more than 2 orders of magnitude from z ~ 0 to z ~ 1–3. The SFRD at high-redshift inferred from our >6 mJy submillimeter observations is comparable with that observed in the ultraviolet/optical.

Disks, Microjets, Windblown Bubbles, and Outflows in the Orion Nebula*

Abstract: New deep narrowband images of the Orion Nebula obtained with WFPC2 on the Hubble Space Telescope (HST) and spectra taken with the HIRES spectrometer at the Keck Observatory are presented. We report eight new circumstellar disks seen in silhouette against the background nebular light and about 30 dark disks embedded within the bright proplyds rimmed by ionization fronts. Deep narrowband λ6300 A images reveal skins of glowing [O I] emission associated with several disks embedded within bright proplyds. [O I] emission also surrounds one dark disk not surrounded by an ionization front; this object may be embedded within the photon-dominated, mostly neutral region behind the ionization front of the Orion Nebula. The intensity and morphology of the [O I] emission provides support for the photon-dominated–region models of externally irradiated circumstellar disks in which soft UV powers photoablation of the disk surface. Dozens of outflows powered by young stars have been discovered on the new images. More than 20 stellar jets emerge from the externally irradiated circumstellar disks or their associated young stars embedded within the Nebula. Most are one-sided (monopolar) subarcsecond-scale microjets, too small to be seen on ground-based images against the bright background nebular light. Additionally, wide-angle winds from 10 young stars in the outskirts of the Nebula power large-scale bow shocks facing the Trapezium OB stars. These shocks may be produced by wind-wind interactions where the T-Tauri winds interact with the outflow of plasma from the core of M42. The largest such structure, associated with the star LL Ori, contains a number of compact high–proper-motion clumps moving almost tangentially to the bow shock. The new data are combined with older HST images to determine proper motions for many nebular features. Neither the LL Ori type bow shocks in the outskirts of the nebula nor the Hα + [O III] arcs that surround many proplyds near the Trapezium show measurable proper motions and are therefore stationary structures. However, most other bow-shaped features not centered on young stars exhibit large proper motions, with velocities ranging from 50 to 300 km s-1. The sources of many of these moving features remain unknown. The proper-motion survey of the nebular core reveals the presence of about a dozen new large-scale (>0.1 pc) outflow complexes. Many of these new outflows originate from the vicinity of the high-luminosity OMC-1S infrared and submillimeter source complex located southwest of the Trapezium. These supersonic features provide evidence that stellar outflows inject large amounts of kinetic energy into the nebula. However, a quantitative analysis indicates that their total power is small compared with the power in the plasma flowing away from the main nebular ionization front.

WFPC2 Observations of the Hubble Deep Field South

Abstract: The Hubble Deep Field South (HDF-S) observations targeted a high Galactic latitude field near QSO J2233-606. We present Wide Field Planetary Camera 2 observations of the field in four wide bandpasses centered at roughly 300, 450, 606, and 814 nm. Observations, data reduction procedures, and noise properties of the final images are discussed in detail. A catalog of sources is presented, and the number counts and color distributions of the galaxies are compared with a new catalog of the original Hubble Deep Field (HDF-N) that has been constructed in an identical manner. The two fields are qualitatively similar, with the galaxy number counts for the two fields agreeing to within 20%. The HDF-S has more candidate Lyman break galaxies at z > 2 than the HDF-N. The star formation rate per unit volume computed from the HDF-S, based on the UV luminosity of high-redshift candidates, is a factor of 1.9 higher than from the HDF-N at z ~ 2.7, and a factor of 1.3 higher at z ~ 4.

Improved Color-Temperature Relations and Bolometric Corrections for Cool Stars

Abstract: We present new grids of colors and bolometric corrections for F-K stars having 4000 K ≤ Teff ≤ 6500 K, 0.0 ≤ log g ≤ 4.5, and -3.0 ≤ [Fe/H] ≤ 0.0. A companion paper extends these calculations into the M giant regime (3000 K ≤ Teff ≤ 4000 K). Colors are tabulated for Johnson U-V and B-V, Cousins V-R and V-I, Johnson-Glass V-K, J-K, and H-K, and CIT/CTIO V-K, J-K, H-K, and CO. We have developed these color-temperature relations by convolving synthetic spectra with the best-determined, photometric filter transmission profiles. The synthetic spectra have been computed with the SSG spectral synthesis code using MARCS stellar atmosphere models as input. Both of these codes have been improved substantially, especially at low temperatures, through the incorporation of new opacity data. The resulting synthetic colors have been put onto the observational systems by applying color calibrations derived from models and photometry of field stars that have effective temperatures determined by the infrared flux method. These color calibrations have zero points that change most of the original synthetic colors by less than 0.02 mag, and the corresponding slopes generally alter the colors by less than 5%. The adopted temperature scale, that of Bell & Gustafsson, is confirmed by the extraordinary agreement between the predicted and observed angular diameters of these field stars, indicating that the differences between the synthetic colors and the photometry of the field stars are not due to errors in the effective temperatures adopted for these stars. Thus, we have derived empirical color-temperature relations from the field star photometry, which we use as one test of our calibrated, theoretical, solar-metallicity color-temperature relations. Except for the coolest dwarfs (Teff < 5000 K), our calibrated model colors are found to match these relations, as well as the empirical relations of others, quite well, and our calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to color-magnitude diagrams of M67. We regard this as evidence that our calibrated colors can be applied to many astrophysical problems, including modeling the integrated light of galaxies. Because there are indications that the dwarfs cooler than 5000 K may require different optical color calibrations than the other stars, we present additional colors for our coolest dwarf models that account for this possibility.

The Chemical Evolution of the Globular Cluster ω Centauri (NGC 5139)

Abstract: We present abundances for 22 chemical elements in 10 red giant members of the massive Galactic globular cluster ω Centauri. The spectra are of relatively high spectral resolution and signal-to-noise. Using these abundances plus published literature values, abundance trends are defined as a function of the standard metallicity indicator iron. The lowest metallicity stars in ω Cen have [Fe/H] ~ -1.8, and the initial abundance distribution in the cluster is established at this metallicity. The stars in the cluster span a range of [Fe/H] ~ -1.8 to -0.8. At the lowest metallicity, the heavy-element abundance is found to be well characterized by a scaled solar system r-process distribution, as found in other stellar populations at this metallicity. As iron increases, the s-process heavy-element abundances increase dramatically. Comparisons of the s-process increases with recent stellar models finds that s-process nucleosynthesis in 1.5–3 M⊙ asymptotic giant branch stars (AGB) fits well the heavy-element abundance distributions. In these low-mass AGB stars, the dominant neutron source is 13C(α, n)16O. A comparison of the Rb/Zr abundance ratios in ω Cen finds that these ratios are consistent with the 13C source. The reason ω Cen displays such a large s-process component is possibly due to the fact that in such a relatively low-mass stellar system, AGB ejecta, because of their low velocity winds, are more efficiently retained in the cluster relative to the much faster moving Type II supernova ejecta. Significant s-process enrichment relative to Fe, from the lower mass AGB stars, would require that the cluster was active in star formation for quite a long interval of time, of the order of 2–3 Gyr. The AGB ejecta were mixed with the retained fraction of Type II supernova ejecta and with the residual gas of initial composition. The analysis of α-rich elements shows that no significant amounts of Type Ia supernova debris were retained by the cluster. In this context, interpretation of the low and constant observed [Cu/Fe] ~ -0.6 (derived here for the first time in this cluster) finds a plausible interpretation.

The discovery of a luminous z = 5.80 quasar from the sloan digital sky survey

Abstract: We present observations of SDSSp J104433.04-012502.2, a luminous quasar at z = 5.80 discovered from Sloan Digital Sky Survey (SDSS) multicolor imaging data. This object was selected as an i'-band dropout object, with i* = 21.8 ± 0.2 and z* = 19.2 ± 0.1. It has an absolute magnitude M1450 = -27.2 (H0 = 50 km s-1 Mpc-1, q0 = 0.5). The spectrum shows a strong and broad Lyα emission line, strong Lyα forest absorption lines with a mean continuum decrement DA = 0.91 and a Lyman limit system at z = 5.72. The spectrum also shows strong O I and Si IV emission lines similar to those of quasars at z 5, suggesting that these metals were produced at a redshift beyond 6. The lack of a Gunn-Peterson trough in the spectrum indicates that the universe is already highly ionized at z ~ 5.8. Using a high-resolution spectrum in the Lyα forest region, we place a conservative upper limit on the optical depth because of the Gunn-Peterson effect of τ < 0.5 in regions of minimum absorption. The Lyα forest absorption in this object is much stronger than that in quasars at z 5. The object is unresolved in a deep image with excellent seeing, implying that it is unlensed. The black hole mass of this quasar is ~3 × 109 M⊙ if we assume no lensing amplification and that it is radiating at the Eddington luminosity, implying that it resides in a very massive dark matter halo. The discovery of one quasar at M1450 < -27 in a survey area of 600 deg2 is consistent with an extrapolation of the observed luminosity function at lower redshifts. The abundance and evolution of such quasars can provide sensitive tests for models of quasar and galaxy formation.

Candidate RR Lyrae stars found in Sloan Digital Sky Survey commissioning data

Abstract: We present a sample of 148 candidate RR Lyrae stars selected from Sloan Digital Sky Survey (SDSS) commissioning data for about 100 deg2 of sky surveyed twice with ?t = 1.9946 days. Although the faint-magnitude limit of the SDSS allows us to detect RR Lyrae stars to large Galactocentric distances (~100 kpc, or r* ~ 21), we find no candidates fainter than r* ~ 20, i.e., farther than ~65 kpc from the Galactic center. On the assumption that all 148 candidates are indeed RR Lyrae stars (contamination by other species of variable star is probably less than 10%), we find that their volume density has roughly a power-law dependence on Galactocentric radius, R-2.7?0.2, between 10 and 50 kpc and drops abruptly at R ~ 50?60 kpc, possibly indicating a sharp edge to the stellar halo as traced by RR Lyrae stars. The Galactic distribution of stars in this sample is very inhomogeneous and shows a clump of over 70 stars at about 45 kpc from the Galactic center. This clump is also detected in the distribution of nonvariable objects with RR Lyrae star colors. When sources in the clump are excluded, the best power-law fit becomes consistent with the R-3 distribution found from surveys of bright RR Lyrae stars. These results imply that the halo contains clumpy overdensities inhomogeneously distributed within a smooth R-3 background, with a possible cutoff at ~50 kpc.

Photometry of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: The Age-Metallicity Relation

Abstract: We present deep VI-band photometry of the globular cluster M54, a nearby field in the Sagittarius dwarf galaxy, and a control field. The color-magnitude diagrams reach well below the oldest main-sequence turnoffs, thus enabling an analysis of the galaxy's age-metallicity relation with unprecedented clarity. We also study the variable stars in the direction of M54. From 67 RR Lyrae variables, we confirm and improve on our previous estimates of the cluster horizontal branch magnitude, foreground reddening, and horizontal branch morphology. These values are used in determining the ages of M54 and the Sagittarius field populations. We confirm our previous result that M54 is the same age as Galactic globular clusters of similar metallicity. We also derive ages on a self-consistent scale for the other three globular clusters in Sagittarius. We find strong evidence for multiple episodes of star formation (or continuous star formation with a variable rate) in the field of Sagittarius. We characterize the principal episodes with the ages 11, 5, and 0.5 through 3 Gyr and with [Fe/H] values of -1.3, -0.7, and -0.4, respectively. On this scale, M54 has an age of 15 Gyr. Surprisingly, the age-metallicity relation we have derived for the galaxy as a whole is described quite well by a closed-box chemical evolution model. We also find that the populations associated with the Sgr field are clumped spatially around M54, and we consider several explanations for this phenomenon. We again speculate that Sagittarius is a nucleated dwarf elliptical galaxy with M54 as its nucleus.

High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies*

Abstract: Observations of ultraluminous infrared galaxies (ULIRGs) with an achieved resolution approaching the diffraction limit in the mid-infrared from 8 to 25 μm using the Keck Telescopes are reported. We find extremely compact structures, with spatial scales of less than 0."3 (diameter) in six of the seven ULIRGs observed. These compact sources emit between 30% and 100% of the mid-infrared energy from these galaxies. We have utilized the compact mid-infrared structures as a diagnostic of whether an AGN or a compact (100–300 pc) starburst is the primary power source in these ULIRGs. In Markarian 231, the upper limit on the diameter of the 12.5 μm source, 0."13, shows that the size of the infrared source must increase with increasing wavelength, consistent with AGN models. In IRAS 05189-2524 and IRAS 08572+3915 there is strong evidence that the source size increases with increasing wavelength. This suggests heating by a central source rather than an extended luminosity source, consistent with the optical classification as an AGN. The compact mid-infrared sources seen in the other galaxies cannot be used to distinguish the ultimate luminosity source. If these ULIRGs are powered by compact starbursts, the star formation rates seen in the central few hundred parsecs far exceed the global rates seen in nearby starburst galaxies, and approach the surface brightness of individual clusters in nearby starburst galaxies.

Detailed analysis of early to late-time spectra of supernova 1993j

Abstract: We present a detailed study of line structure in early to late-time spectra of supernova (SN) 1993J. Spectra during the nebular phase, but within the first two years after explosion, exhibit small-scale structure in the emission lines of some species, notably oxygen and magnesium, showing that the ejecta of SN 1993J are clumpy. On the other hand, a lack of structure in emission lines of calcium implies that the source of calcium emission is uniformly distributed throughout the ejecta. These results are interpreted as evidence that oxygen emission originates in clumpy, newly synthesized material, while calcium emission arises from material preexisting in the atmosphere of the progenitor. Spectra spanning the range 433–2454 days after the explosion show boxlike profiles for the emission lines, clearly indicating circumstellar interaction in a roughly spherical shell. This is interpreted within the Chevalier & Fransson model for SNe interacting with mass lost during prior stellar winds. At very late times, the emission lines have a two-horned profile, implying the formation of a somewhat flattened or disklike structure that is a significant source of emission. The very high signal-to-noise ratio spectra are used to demonstrate the potential significance of misinterpretation of telluric absorption lines in the spectra of bright SNe.

What are these blue metal-poor stars?

Abstract: The radial velocity behavior and chemical compositions of sixty-two blue metal-poor (BMP) stars have been established from more than 1200 echelle spectra obtained at Las Campanas Observatory from 1992 through 1999. Analysis of survey spectra provides abundances for this sample, which we use to calibrate the K line versus B-V relation. Forty-four of the stars have [Fe/H] < -1, while eighteen lie on -1 < [Fe/H] < 0. One star, the SX Phe variable CS 22966-043, appears to be the most extreme example of a rare abundance class characterized by α-element deficiencies, high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and [Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities, while 42 are definite or probable members of binary systems. The binary fraction of BMP stars appears to be independent of chemical composition. The high binary fraction fBMP ~ 0.6 of BMP stars compared with that found for the F- and G-type stars near the Sun, the systematically low mass functions of these binaries, and the paucity of double-lined binaries among them lead us to suggest that at least half of the BMP binaries are blue stragglers and that these blue stragglers are formed by McCrea mass transfer rather than by the various merger processes that are currently believed to produce most blue stragglers in globular clusters. This conclusion is supported by the abnormally high proportion of BMP binaries with long periods and small orbital eccentricities, properties these binaries share with McClure's carbon star binaries. The great majority of field blue stragglers (BSs) probably are created by Roche-lobe overflow during red giant branch evolution. Primaries of more widely separated binaries that survive this phase of stellar evolution may engage in mass transfer during subsequent asymptotic giant branch evolution to form s-process abundance enhanced carbon stars. Our result requires a major downward revision of the fraction of BMP stars attributed to a captured intermediate-age population of metal-poor field stars. The high original estimate of the size of this component probably arose from improper use of the globular cluster BS specific frequency, SBS = n(BS)/n(HB)~1, to estimate the halo BS space density. We use a simple model to calculate the specific frequency of BSs produced by McCrea mass transfer in an old metal-poor population with a given primordial binary fraction fB. Our model calculations return values of SBS ~ 5 for fB = 0.15, much more like our value for the field blue stragglers. We suggest that globular clusters either destroy the primordial binaries that produce long period BS binaries like those in the Galactic field reported here, or they never possessed them.

Constraints on the structure of dark matter halos from the rotation curves of low surface brightness galaxies

Abstract: We reexamine the disk-halo decompositions of the rotation curves of low surface brightness (LSB) galaxies with Vmax ≥ 80 km s-1, taking full account of the effects of beam smearing. We show that the spatial resolution of the data is not sufficient to put any meaningful constraints on the density profiles of the dark halos, or on cosmological parameters. This is in strong contrast to claims made in the literature that these LSB rotation curves are only consistent with dark matter halos with shallow central cusps, and it has important implications regarding the halos of LSB galaxies, such as the self-similarity of their rotation curves, and their inconsistency with certain cosmological models or with cold dark matter altogether. Only in one case are the data of sufficient spatial resolution to obtain reliable constraints on the slope of the central density distribution of the dark matter halo. For this single case, we find a central cusp ρ ∝ r-α with 0.55 < α < 1.26 at the 99.73% confidence level. This contrasts strongly with the results for two dwarf galaxies (Vmax < 70 km s-1) that we analyze, which yield α < 0.5 at the same level of confidence. This possibly suggests that halos with constant-density cores are restricted to low-mass systems. We show that violent outflows of baryonic matter by supernova feedback can reproduce this mass dependence of halo cusp slopes.

Formation of a tidal dwarf galaxy in the interacting system Arp 245 (NGC 2992/93)

Abstract: Original article can be found at: http://www.journals.uchicago.edu/AJ/--Copyright American Astronomical Society

The Canada-UK Deep Submillimeter Survey. IV. The Survey of the 14 Hour Field

Abstract: We have used SCUBA to survey an area of simeq 50 arcmin2, detecting 19 sources down to a 3 σ sensitivity limit of ~3.5 mJy at 850 μm. Monte Carlo simulations have shown that the fluxes of sources in this and similar SCUBA surveys are biased upward by the effects of source confusion and noise, leading to an overestimate by a factor of ~1.4 in the fraction of the 850 μm background that has been resolved by SCUBA. Once a correction is made for this effect, about 20% of the background has been resolved. The simulations have also been used to quantify the effects of confusion on source positions. Of the 19 SCUBA sources, five are microjansky radio sources, and two are ISO 15 μm sources. The radio/submillmeter flux ratios imply that the dust in these galaxies is being heated by young stars rather than active galactic nuclei. The upper limit to the average 450 μm/850 μm flux ratio implies either that the SCUBA galaxies are at z G 2 or, if they are at lower redshifts, that the dust is generally colder than in ULIRGs. We have used simple evolution models to address the major questions about the SCUBA sources: (1) What fraction of the star formation at high redshift is hidden by dust? (2) Does the submillimeter luminosity density reach a maximum at some redshift? (3) If the SCUBA sources are protoellipticals, when exactly did ellipticals form? We show, however, that the observations are not yet good enough to answer these questions. There are, for example, acceptable models in which 10 times as much high-redshift star formation is hidden by dust as is seen at optical wavelengths, but also acceptable ones in which the amount of hidden star formation is less than that seen optically. There are acceptable models in which very little star formation occurred before a redshift of 3 (as might be expected in models of hierarchical galaxy formation), but also ones in which 30% of the stars have formed by this redshift. The key to answering these questions are measurements of the dust temperatures and redshifts of the SCUBA sources.