Wallace L. W. Sargent

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

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

Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

The Keck I telescope and the High Resolution Echelle Spectrometer (HIRES) have been used to obtain spectra for red giant stars belonging to the Draco, Sextans, and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 dSph stars is the most extensive yet assembled for stars in such environments. Our principal findings are summarized as follows. (1) There is unambiguous evidence for a large internal dispersion in metallicity in all three galaxies: our program stars span a range of ?[Fe/H] = 1.53, 1.40, and 0.73 dex in Draco, Sextans, and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that three galaxies have similar nucleosynthetic histories and, presumably, similar initial mass functions. (3) A comparison of the measured element abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 [?/Fe] 0.13 dex, whereas the halo field star sample has [?/Fe] ~ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances that are significantly lower than those measured for halo field stars of similar metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r-process. Taken together, these results suggest that the Galactic halo is unlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity, LV = 3 ? 105LV,?, dSph galaxies studied here. We also note that the measured [Zn/Fe] abundance ratios for the dSph stars are lower than those of damped Ly? systems having low levels of dust depletion by roughly an order of magnitude. The first high-resolution abundance analysis for the distant Galactic globular cluster NGC 2419 is also presented. From a HIRES spectrum of a single red giant, we find a metallicity of [Fe/H] = -2.32 ? 0.11 dex. This is slightly lower than, but still consistent with, published estimates based on low-resolution spectroscopy. With the possible exception of a slight enhancement in the abundances of some heavy elements such as Ce, Nd, Y, and Ba, the observed abundance pattern closely resembles those exhibited by red giants in M92: a nearby, well-studied globular cluster of nearly identical metallicity.

Abundances at High Redshifts: The Chemical Enrichment History of Damped Lyα Galaxies

We study the elemental abundances of C, N, O, Al, Si, S, Cr, Mn, Fe, Ni, and Zn in a sample of 14 damped Lyα systems (galaxies) with H I column density N(H I) ≥ 1020 cm−2, using high-quality spectra quasars obtained with the 10 m Keck telescope. To ensure accuracy, only weak, unsaturated absorption lines are used to derive ion column densities and elemental abundances. Combining these abundance measurements with similar measurements in the literature, we investigate the chemical evolution of damped Lyα galaxies based on a sample of 23 systems in the redshift range 0.7 3 in our sample have (Fe/H) around 1/100 solar or less. In comparison, a large fraction of the damped Lyα galaxies at z < 3 have reached 10 times higher metallicity. This suggests that the time around z = 3 may be the epoch of galaxy formation in the sense that galaxies are beginning to form the bulk of their stars. Several other lines of evidence appear to point to the same conclusion, including the evolution of the neutral baryon content of damped Lyα galaxies, the evolution in the quasar space density, and the morphology of z < 3 galaxies. 4. The relative abundance patterns of the elements studied here clearly indicate that the bulk of heavy elements in these high-redshift galaxies were produced by Type II supernovae; there is little evidence for significant contributions from stellar mass loss of low- to intermediate-mass stars or from Type Ia supernovae. 5. Although earlier studies have attributed the overabundance of Zn relative to Cr or Fe found in damped Lyα galaxies to selective depletion of Cr and Fe by dust grains, such an interpretation is inconsistent with many of the other elemental abundance ratios seen in these galaxies, most notably N/O and Mn/Fe. Several other tests also indicate that thereis no significant evidence for dust depletion in these galaxies. We suggest that the overabundance of Zn relative to Cr in damped Lyα galaxies may be intrinsic to their stellar nucleosynthesis. If this interpretation is correct, it will provide important new information to the theory of stellar nucleosynthesis. 6. The absorption profiles of Al III in damped Lyα galaxies are found to resemble those of the low ionization lines. The profiles of Si IV and C IV absorption, while resembling each other in general, are almost always different from those of the low-ionization absorption lines. These results suggest that Al III is probably produced in the same physical region as the low-ionization species in the absorbing galaxies, while the high-ionization species (Si IV and C IV) mostly likely come from distinct physical regions. 7. We discuss possible ways to obtain information on the history of star formation (i.e., continuous or episodic) in damped Lyα galaxies and on the shapes of the stellar initial mass functions. 8. We review the evidence for, and against, the hypothesis that damped Lyα galaxies are disks or protodisks at high redshifts and discuss the implications. 9. We determine upper limits on the temperature of the cosmic microwave background radiation at several redshifts using absorption from the fine-structure level of the C II ion. These upper limits are consistent with the predicted increase of TCMBwith redshift.

The Opacity of the Lyα Forest and Implications for Ωb and the Ionizing Background

We have measured the distribution function of the flux decrement D = 1 - e-τ caused by Lyα forest absorption from intervening gas in the lines of sight to high-redshift QSOs from a sample of seven high-resolution QSO spectra obtained with the Keck telescope. The observed flux decrement distribution function (FDDF) is compared with the FDDF from two simulations of the Lyα forest: a ΛCDM model (with Ω = 0.4, Λ = 0.6), computed with the Eulerian code of Cen & Ostriker, and a standard cold dark matter (SCDM) model (with Ω = 1), computed with the smoothed particle hydrodynamics code of Hernquist et al. Good agreement is obtained between the shapes of the simulated and observed FDDFs for both simulations after fitting only one free parameter, which controls the mean flux decrement. The difference between the predicted FDDFs from the two simulations is small, and we show that it arises mostly from a different temperature in the low-density gas (caused by different assumptions that were made about the reionization history in the two simulations), rather than differences between the two cosmological models or numerical effects in the two codes, which use very different computational methods. A measurement of the parameter μΩ -->2b h -->3/Γ (where Γ is the H I ionization rate due to the ionizing background) is obtained by requiring the mean flux decrement in the simulations to agree with the observed one. Estimating the lower limit Γ > 7 × 10-13 s-1 from the abundance of known QSOs, we derive a lower limit on the baryonic matter density, Ωbh2 > 0.021 (0.017) for the ΛCDM (SCDM) model. The difference between the lower limits inferred from the two models is again due to different temperatures in the low-density gas. We give general analytical arguments for why this lower limit is unlikely to be reduced for any other models of structure formation by gravitational collapse that can explain the observed Lyα forest. When combined with constraints from big bang nucleosynthesis, the large Ωb we infer is inconsistent with some recent D/H determinations (Rugers & Hogan), favoring a low deuterium abundance as reported by Tytler, Fan & Burles. Adopting a fixed Ωb, the measurement of μ(z) allows a determination of the evolution of the ionizing radiation field with redshift. Our models predict an intensity that is approximately constant with redshift, which is in agreement with the assumption that the ionizing background is produced by known quasars for z < 3, but requires additional sources of ionizing photons at higher redshift given the observed rapid decline of the quasar abundance.

Metallicity of the Intergalactic Medium Using Pixel Statistics. II. The Distribution of Metals as Traced by C IV

We measure the distribution of carbon in the intergalactic medium as a function of redshift z and overdensity δ. Using a hydrodynamical simulation to link the H I absorption to the density and temperature of the absorbing gas, and a model for the UV background radiation, we convert ratios of C IV to H I pixel optical depths into carbon abundances. For the median metallicity this technique was described and tested in Paper I of this series. Here we generalize it to reconstruct the full probability distribution of the carbon abundance and apply it to 19 high-quality quasar absorption spectra. We find that the carbon abundance is spatially highly inhomogeneous and is well described by a lognormal distribution for fixed δ and z. Using data in the range log δ = -0.5-1.8 and z = 1.8-4.1, and a renormalized version of the 2001 Haardt & Madau model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = -3.47 + 0.08(z - 3) + 0.65(log δ - 0.5) and a lognormal scatter of σ([C/H]) = 0.76 + 0.02(z - 3) - 0.23(log δ - 0.5). Thus, we find significant trends with overdensity but no evidence for evolution. These measurements imply that gas in this density range accounts for a cosmic carbon abundance of [C/H] = -2.80 ± 0.13 (ΩC ≈ 2 × 10-7), with no evidence for evolution. The dominant source of systematic error is the spectral shape of the UV background, with harder spectra yielding higher carbon abundances. While the systematic errors due to uncertainties in the spectral hardness may exceed the quoted statistical errors for δ < 10, we stress that UV backgrounds that differ significantly from our fiducial model give unphysical results. The measured lognormal scatter is strictly independent of the spectral shape, provided the background radiation is uniform. We also present measurements of the C III/C IV ratio (which rule out temperatures high enough for collisional ionization to be important for the observed C IV) and of the evolution of the effective Lyα optical depth.

The subluminous spectroscopically peculiar type Ia supernova 1991bg in the elliptical galaxy NGC 4374

We present photometric and spectroscopic observations of SN 1991bg, a very peculiar Type Ia supernova located in the outskirts of the E1 galaxy NGC 4374 (M84) in the Virgo cluster. At maximum brightness SN 1991bg was 1.6 mag subluminous in V and 2.5 mag subluminous in B, compared with normal SNe Ia. The colors were unusually red, but the object was not significantly reddened by dust. The decline from maximum was certainly quite steep; we measure an initial linear V fading of 0.10 mag/d rather than the typical value of 0.06 mag/d for SNe Ia, and a late-time decline of 0.034 mag/d rather than 0.026 mag/d

The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyα Forest*

A sample of eight quasars observed at high resolution and signal-to-noise ratio is used to determine the transmitted flux probability distribution function (TFPDF), and the power spectrum and correlation function of the transmitted flux in the Lyα forest, in three redshift bins centered at z = 2.41, 3.00, and 3.89. All the results are presented in tabular form, with full error covariance matrices, to allow for comparisons with any numerical simulations and with other data sets. The observations are compared with a numerical simulation of the Lyα forest of a ΛCDM model with Ω = 0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the TFPDF if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted TFPDF is found at high fluxes and low redshift, which may be due to the uncertain effects of fitting the spectral continuum. Using the numerical simulation, we show how the flux power spectrum can be used to recover the initial power spectrum of density fluctuations. From our sample of eight quasars, we measure the amplitude of the mass power spectrum to correspond to a linear variance per unit ln k of Δ(k) = 0.72 ± 0.09 at k = 0.04(km s-1)-1 and z = 3, and the slope of the power spectrum near the same k to be np = -2.55 ± 0.10 (statistical error bars). The results are statistically consistent with those of Croft et al., although our value for the rms fluctuation is lower by a factor of 0.75. For the ΛCDM model we use, the implied primordial slope is n = 0.93 ± 0.10, and the normalization is σ8 = 0.68 + 1.16(0.95 - n) ± 0.04.

The Chandra Deep Survey of the Hubble Deep Field-North Area. II. Results from the Caltech Faint Field Galaxy Redshift Survey Area*

A deep X-ray survey of the Hubble Deep Field-North (HDF-N) and its environs is performed using data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Currently a 221.9 ks exposure is available, the deepest ever presented, and here we give results on X-ray sources located in the 86 × 87 area covered by the Caltech Faint Field Galaxy Redshift Survey (the Caltech area). This area has (1) deep photometric coverage in several optical and near-infrared bands; (2) extensive coverage at radio, submillimeter, and mid-infrared wavelengths; and (3) some of the deepest and most complete spectroscopic coverage ever obtained. It is also where the X-ray data have the greatest sensitivity; the minimum detectable fluxes in the 0.5-2 keV (soft) and 2-8 keV (hard) bands are ≈1.3 × 10-16 and ≈6.5 × 10-16 ergs cm-2 s-1, respectively. More than ≈80% of the extragalactic X-ray background in the hard band is resolved. The 82 Chandra sources detected in the Caltech area are correlated with more than 25 multiwavelength source catalogs, and the results of these correlations as well as spectroscopic follow-up results obtained with the Keck and Hobby-Eberly Telescopes are presented. All but nine of the Chandra sources are detected optically with R 26.5. Redshifts are available for 39% of the Chandra sources, including 96% of the sources with R 5.0) objects. A total of 16 of the 67 1.4 GHz μJy sources in the Caltech area are detected in the X-ray band, and the detection rates for starburst-type and AGN-candidate μJy sources are comparable. Only two of the 17 red, optically faint (I > 25) μJy sources are detected in X-rays. While many of the starburst-type μJy sources appear to contain obscured active galactic nuclei (AGNs), the Chandra data are consistent with the majority of the μJy radio sources being powered by star formation. A total of 11 of the ≈100 ISO mid-infrared sources found in and near the HDF-N are detected in X-rays. In the HDF-N itself, where both the infrared coverage and the X-ray coverage are deepest, it is notable that six of the eight Chandra sources are detected by ISO; most of these are known to be AGNs where the X-ray and infrared detections reveal both the direct and indirect accretion power being generated. The high X-ray-to-infrared matching rate bodes well for future sensitive infrared observations of faint X-ray sources. Four of the 33 very red objects that have been identified in the Caltech area are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGNs. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. A stacking analysis of the very red objects not detected individually by Chandra yields a soft-band detection with an average soft-band X-ray flux of ≈1.9 × 10-17 ergs cm-2 s-1; the observed emission may be associated with the hot interstellar media of moderate-redshift elliptical galaxies. Constraints on AGN candidates, extended X-ray sources, and Galactic objects in the Caltech area are also presented.

The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs

An extremely deep X-ray survey (≈1 Ms) of the Hubble Deep Field North (HDF-N) and its environs (≈450 arcmin2) has been performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. This is one of the two deepest X-ray surveys ever performed; for point sources near the aim point, it reaches 0.5–2.0 and 2–8 keV flux limits of ≈3 × 10-17 and ≈2 × 10-16 ergs cm-2 s-1, respectively. Here we provide source catalogs, along with details of the observations, data reduction, and technical analysis. Observing conditions, such as background, were excellent for almost all of the exposure. We have detected 370 distinct point sources: 360 in the 0.5–8.0 keV band, 325 in the 0.5–2.0 keV band, 265 in the 2–8 keV band, and 145 in the 4–8 keV band. Two new Chandra sources in the HDF-N itself are reported and discussed. Source positions are accurate to within 06–17 (at ≈90% confidence), depending mainly on the off-axis angle. We also detect two highly significant extended X-ray sources and several other likely extended X-ray sources. We present basic number count results for sources located near the center of the field. Source densities of 7100 deg-2 (at 4.2 × 10-17 ergs cm-2 s-1) and 4200 deg-2 (at 3.8 × 10-16 ergs cm-2 s-1) are observed in the soft and hard bands, respectively.

The Influence of Bars on Nuclear Activity

Gravitational torques induced by a stellar bar on the interstellar medium of a disk galaxy instigate radial inflow of gas toward the central regions of the galaxy. Accordingly, the presence of a bar should reinforce nuclear star formation activity or the fueling of an active galactic nucleus (AGN). We test this hypothesis by comparing the detection rate and intensity of nuclear H II regions and AGNs among barred and unbarred galaxies in a sample of over 300 spirals selected from our recent optical spectroscopic survey of nearby galaxies. The AGN group includes Seyfert nuclei as well as low-ionization nuclear emission-line regions (LINERs). Among late-type spirals (Sc-Sm), as opposed to early-type spirals (S0/a-Sbc), we observe in the barred group a very marginal increase in the detection rate of H II nuclei and a corresponding decrease in the incidence of AGNs. The minor differences in detection rates, however, are statistically insignificant, most likely stemming from selection effects and not from a genuine influence of the bar. The presence of a bar seems to have no noticeable impact on the likelihood of a galaxy for hosting either nuclear star formation or an AGN. The nuclei of early-type barred spirals (S0/a-Sbc) do exhibit measurably higher star formation rates than do their unbarred counterparts, as indicated by either the luminosity or the equivalent width of Hα emission. By contrast, late-type spirals do not show such an effect. These results agree with previous studies and can be explained most easily in terms of structural differences between bars in early-type and late-type spirals. Nuclear H II regions spanning a wide range of intensities are found regardless of the presence of a bar, suggesting that a bar is neither a necessary nor a sufficient condition for star formation to occur in galactic nuclei. Other factors, such as the availablity of gas, must be equally important. Bars, on the other hand, have a negligible effect on the strength of the AGNs in our sample, regardless of the Hubble type of the host galaxy. This result confirms conclusions reached by other studies based on much smaller samples. Assuming that AGNs are fueled by gas from the interstellar medium of the host galaxy, some inferences concerning the fueling process can be made. We speculate that inner Lindblad resonances, particularly common in barred galaxies with large bulge-to-disk ratios, prevent gas which has been radially transported from large scales from reaching the nucleus. We discuss the feasibility of sustaining the power output of nearby AGNs with debris from tidal disruption of stars by a supermassive black hole as well as with mass loss from evolved stars. We conclude that such processes should be sufficient for fueling the low-luminosity nuclear sources found in many nearby galaxies.