Gregory Alan Shields

Compton heated winds and coronae above accretion disks. I Dynamics

X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.

The Black Hole-Bulge Relationship in Quasars

We use quasi-stellar object (QSO) emission-line widths to examine the MBH-σ* relationship as a function of redshift and to extend the relationship to larger masses. Supermassive black holes in galactic nuclei are closely related to the bulge of the host galaxy. The mass of the black hole MBH increases with the bulge luminosity and with the velocity dispersion of the bulge stars, σ*. An important clue to the origin of this correlation would be an observational determination of the evolution, if any, in the MBH-σ* relationship as a function of cosmic time. The high luminosity of QSOs affords the potential for studies at large redshifts. We derive black hole masses from the continuum luminosity and the width of the broad Hβ line and σ* from the width of the narrow [O III] lines. We find that radio-quiet QSOs conform to the established MBH-σ* relationship up to values of MBH ≈ 1010 M☉, with no discernible change in the relationship out to redshifts of z ≈ 3. These results are consistent with the idea that the growth of supermassive black holes and massive bulges occurred simultaneously.

Interstellar Abundance Gradients in NGC 2403: Comparison to M33*

We present new long-slit spectra of 12 H II regions in the Scd spiral galaxy NGC 2403. Gas-phase element abundances for O, N, S, Ne, and Ar were estimated using direct measurements of electron temperature based on detections of [O III] λ4363, [S III] λ6312, and [O II] λ7320-7330, and from theoretical photoionization analysis. We find abundance gradients for O/H and N/O of -0.102 ± 0.009 dex kpc-1 and -0.032 ± 0.005 dex kpc-1, respectively. The relatively flat N/O gradient suggest a significant source of primary nitrogen. An upper limit for the C/O ratio for one H II region was determined from a Hubble Space Telescope FOS spectrum. A mild outward increasing gradient in S/O is seen at marginal significance (0.03 ± 0.02 dex kpc-1). We compare the abundance gradient and effective oxygen yield deduced for NGC 2403 with those determined for M33, another Scd galaxy with very similar structural parameters. We find close agreement in most of the chemical properties between NGC 2403 and M33. However, the effective yield for oxygen determined from closed box chemical evolution calculations is higher in M33 than in NGC 2403. The higher yield derived for M33 is similar to the case of H I-deficient Virgo spiral galaxies. We also compare NGC 2403 and M33 with a larger sample of unbarred spirals having abundance measurements. The global metallicity of the spirals correlates well with galaxy luminosity, as already noted from earlier investigations. The O/H gradient per unit disk scale length does not correlate with galaxy luminosity with possibly small intrinsic scatter, suggesting that spiral galaxies are homologous with regard to chemical evolution. The correlation between gas abundances and local surface brightness (mass density) in late-type spirals appears to be described well by chemical evolution models incorporating self-regulating star formation. However, the characteristic abundance at a given value of surface brightness correlates with galaxy luminosity. This suggests that an additional parameter which scales with galaxy mass influences the chemical properties of disks.

The Black Hole Mass-Galaxy Bulge Relationship for QSOs in the Sloan Digital Sky Survey Data Release 3

We investigate the relationship between black hole mass, MBH, and host galaxy velocity dispersion, σ*, for QSOs in Data Release 3 of the Sloan Digital Sky Survey. We derive MBH from the broad Hβ line width and continuum luminosity and the bulge stellar velocity dispersion from the narrow [O III] line width (σ[O III]). At higher redshifts, we use Mg II and [O II] in place of Hβ and [O III]. For redshifts z < 0.5, our results agree with the MBH-σ* relationship for nearby galaxies. For 0.5 < z < 1.2, the MBH-σ* relationship appears to show evolution with redshift in the sense that the bulges are too small for their black holes. However, we find that part of this apparent trend can be attributed to observational biases, including a Malmquist bias involving the QSO luminosity. Accounting for these biases, we find ~0.2 dex evolution in the MBH-σ* relationship between now and redshift z ≈ 1.We investigate the relationship between black hole mass, MBH, and host galaxy velocity dispersion, σ∗, for QSOs in Data Release 3 of the Sloan Digital Sky Survey. We derive MBH from the broad Hβ line width and continuum luminosity, and the bulge stellar velocity dispersion from the [O iii] narrow line width (σ[O III]). At higher redshifts, we use Mg ii and [O ii] in place of Hβ and [O iii]. For redshifts z < 0.5, our results agree with the MBH − σ∗ relationship for nearby galaxies. For 0.5 < z < 1.2, the MBH − σ∗ relationship appears to show evolution with redshift in the sense that the bulges are too small for their black holes. However, we find that part of this apparent trend can be attributed to observational biases, including a Malmquist bias involving the QSO luminosity. Accounting for these biases, we find ∼ 0.2 dex evolution in the MBH − σ∗ relationship between now and redshift z ≈ 1. Subject headings: galaxies: active — quasars: general — black hole physics

The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations

We present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III 1666 A and C III 1909 A in our spectra, enabling us to determine C(+2)/O(+2) with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of carbon from intermediate-mass stars. Our results also suggest that it may not be appropiate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver & Woosleys best estimate for the 12C(alpha, gamma) 16O nuclear reaction rate, assuming negligible contanmination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III 1750 A multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma uppr limits on the 1750 A feature indicate that the N(+2)/O(+2) ratios in these objects are not significantly larger than the N(+)/O(+) ratios measured from optical spectra. This behavior is consistent with predictions of photionization models, although better detections of N III are needed to confirm the results.

A SEARCH FOR BINARY ACTIVE GALACTIC NUCLEI: DOUBLE-PEAKED [O III] AGNs IN THE SLOAN DIGITAL SKY SURVEY

We present active galactic nuclei (AGNs) from the Sloan Digital Sky Survey (SDSS) having double-peaked profiles of [O III]λλ5007, 4959 and other narrow emission lines, motivated by the prospect of finding candidate binary AGNs. These objects were identified by means of a visual examination of 21,592 quasars at z < 0.7 in SDSS Data Release 7 (DR7). Of the spectra with adequate signal-to-noise, 148 spectra exhibit a double-peaked [O III] profile. Of these, 86 are Type 1 AGNs and 62 are Type 2 AGNs. Only two give the appearance of possibly being optically resolved double AGNs in the SDSS images, but many show close companions or signs of recent interaction. Radio-detected quasars are three times more likely to exhibit a double-peaked [O III] profile than quasars with no detected radio flux, suggesting a role for jet interactions in producing the double-peaked profiles. Of the 66 broad-line (Type 1) AGNs that are undetected in the FIRST survey, 0.9% show double-peaked [O III] profiles. We discuss statistical tests of the nature of the double-peaked objects. Further study is needed to determine which of them are binary AGNs rather than disturbed narrow line regions, and how many additional binaries may remain undetected because of insufficient line-of-sight velocity splitting. Previous studies indicate that 0.1% of SDSS quasars are spatially resolved binaries, with typical spacings of ~10-100 kpc. If a substantial fraction of the double-peaked objects are indeed binaries, then our results imply that binaries occur more frequently at smaller separations (<10 kpc). This suggests that simultaneous fueling of both black holes is more common as the binary orbit decays through these spacings.

The Black Hole-Bulge Relationship for QSOs at High Redshift

We examine the black hole mass-galaxy bulge relationship in high-redshift QSOs. Black hole masses are derived from broad emission lines, and the host galaxy stellar velocity dispersion σ* is estimated from the widths of the radio CO emission lines. At redshifts z > 3, the CO line widths are narrower than expected for the black hole mass, indicating that these giant black holes reside in undersized bulges by an order of magnitude or more. The largest black holes (MBH > 109 M☉) evidently grow rapidly in the early universe without commensurate growth of their host galaxies. CO line widths offer a unique opportunity to study AGN host galaxy dynamics at high redshift.

Carbon in Spiral Galaxies from HUBBLE SPACE TELESCOPE Spectroscopy

We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O≈-0.8 at log O/H=-4.0 to log C/O≈-0.1 at log O/H=-3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, confirming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemical-evolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much flatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important effect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

The carbon abundance in the Magellanic Clouds from IUE observations of H II regions

Observations of C II), C III), and C IV lines in the ultraviolet spectra of three H II regions in the Small Magellanic Cloud (SMC) and of four H II regions in the Large Magellanic Cloud (LMC) were obtained with the International Ultraviolet Explorer satellite and used to derive the carbon abundance in the Megellanic Clouds by nebular model analyses. Based on absolute emission line fluxes between the IUE and ground-based observations for the individual H II regions, it is found that 12+log C/H = 7.16 +- 0.04 for the SMC and 7.90 +- 0.15 for the LMC (where the errors quoted are statistical; systematic errors due to aperture geometry and positioning differences between the IUE and ground-based observations may be several times larger). The corresponding values for the average C/O ratios are log C/O = -0.89 +- 0.02 for the SMC and -0.48 +- 0.04 for the LMC (statistical errors). For one SMC H II region, N81, the O III) lambda1663 lines were observed in the IUE spectra and permitted direct calculation of the C/O ratio independent of absolute fluxes. The result (log C/O = -0.94) is in excellent agreement with ratio based on absolute fluxes.

Chemical abundances in Virgo spiral galaxies. II. Effects of cluster environment

We present new measurements of chemical abundances in H II regions in spiral galaxies of the Virgo cluster and a comparison of Virgo galaxies and field spirals. With these new data there now exist nine Virgo spirals with abundance measurements for at least four H II regions. Our sample of Virgo galaxies ranges from H I deficient objects near the core of the cluster to galaxies with normal H I properties, far from the cluster core. We investigate the relationship between H I disk characteristics and chemical abundances to determine whether dynamical process that remove gas from the disk, such as ram pressure stripping by the intracluster medium, also affect the chemical abundances.