Extragalactic Research

Miskey and Bruhweiler have been working closely with other members of the STIS Instrument Definition Team at GSFC to develop sophisticated optimal extraction software to extract individual spectra from the 2-D spectral images of OB stars in the crowded stellar fields of the bright nearby starburst H~II region NGC~604 in the galaxy M33. This software uses a 2-D line-spread-function, which is a function of wavelength and y-position on the detector). This LSF was derived as part of this work. These were derived using STIS calibration data. Using this software, they have successfully extracted high quality spectra of 15 OB stars from a single STIS exposure. This work is being prepared for submission to the Astronomical Journal.

Bruhweiler, Smith Neubig, along with students Holz-Donohue, Eaton, and Tanlyi have analyzed all the available HST spectra of the O and B stars of the Magellanic Clouds and have derived more quantitative diagnostics for determining effective temperature and luminosity. They have found that the derived UV spectral types are in good accord with those derived previously by Smith Neubig and Bruhweiler (1997,1999). These results are being prepared for publication. A second paper is also being prepared analyzing the spectra extracted using this software. These results show a significant number of stars showing He~II 1640 emission characteristic of W-R stars. Many stars appear to be quite young, with ages less than 2~Myr.

These spectra are to be included in an evolving UV spectral library of O and B stars that has been developed to determine age, slope of the Initial Mass Function, and metallicity of the bright starbursts in nearby galaxies. Synthetic UV spectra have been produced using the theoretical evolutionary tracks in conjunction with the spectral library, for data obtained by the International Ultraviolet Explorer (IUE) and spectrographs aboard the HST. In most cases good spectral fits have been obtained, but many starbursts have strong spectral features that are too strong to be fit by the modeling. This suggests that some starbursts have metallicities higher than the solar neighborhood.

Gabel, with Bruhweiler, Kraemer and Crenshaw and Miskey have recently completed a photoionization modeling study of the nearby LINER galaxy NGC~1052. These results show that photoionization modeling, without the need for shock ionization, provides excellent fits to "all" the observed emission lines in the UV through optical spectrum in this object. This agreement was achieved using the deduced intrinsic spectrum based upon the observed radio through X-ray flux distribution. The results of this work will appear in the Astrophysical Journal (Gabel et al.) This work is part of a Ph.D. dissertation by Gabel.

Gabel and Bruhweiler are also modeling the starburst/LINER galaxy, NGC~4569, using the inferred flux distribution characteristic of a starburst. The inferred flux from stars appears insufficient to produce the observed optical emission lines in this galaxy. Further modeling efforts are focusing on incorporating photoionization by X-ray binaries and shocks from supernovae both produced by massive stars. At this point it is unclear if the observed emission requires the presence of a AGN.

Also, Bruhweiler and Boggess plan to use a significant portion of their remaining guaranteed observing time associated with The Space Telescope Imaging Spectrograph aboard the HST to probe active galactic nuclei and starburst galaxies.

Crenshaw and Kraemer have obtained the first observations of a Seyfert galaxy with the echelle gratings on the Space Telescope Imaging Spectrograph (STIS). Five kinematic components of absorption are present in L , C~IV, and N~V at radial velocities of -160 to -1060 km s^-1 with respect to the emission lines. Photoionization models were calculated to match the UV column densities from each of the five components associated with the nucleus. In four of the components, the ionization parameters (U = 0.15 -- 0.80) and effective hydrogen column densities (N$_{eff}$ = 6.0 x 10¹⁸ cm^-2 -- 2.8 x 10²⁰ cm^-2) cannot produce the O~VII and O~VIII absorption edges seen in the X-ray warm absorber. The remaining component is more highly ionized (U = 2.4, N$_{eff}$ = 6.5 x 10²¹ cm^-2) and the model of this component matches the previously observed X-ray absorption columns. This component is therefore likely to be responsible for the X-ray warm absorber. It also has the highest outflow velocity and showed the largest variations in column density.

Crenshaw, Kraemer, Boggess, Maran, Mushotzky (GSFC), and Wu (CSC) have published a survey on intrinsic absorption in Seyfert galaxies, based on previous HST observing programs and HST archive data. The fraction of Seyfert 1 galaxies that show intrinsic absorption associated with their active nuclei is more than one-half (10/17), and there is a one-to-one correspondence between Seyferts that show intrinsic UV absorption and X-ray "warm absorbers". The intrinsic UV absorption is generally characterized by high-ionization: C~IV and N~V are seen in 10 Seyferts, whereas Si~IV is present in only four of these Seyferts, and Mg~II absorption is only detected in NGC~4151. The absorption lines are blueshifted with respect to the narrow emission lines, indicating that the absorbing gas is undergoing net radial outflow. At high resolution, the absorption splits into distinct kinematic components that show a wide range in widths (20 -- 400 km s^-1 FWHM) The strong absorption components have cores that are much deeper than the continuum flux levels, indicating that the regions responsible for these components lie completely outside of the broad emission-line regions. The covering factor of the absorbing gas in the line of sight, relative to the total underlying emission, is C$_{los}$ $\geq$ 0.86, on average. The global covering factor, which is the fraction of emission intercepted by the absorber averaged over all lines of sight, is C$_{global}$ $\geq$ 0.5. The individual absorption components show a wide range in C~IV column densities (0.1 -- 14 x 10¹⁴ cm^-2), and the ratio of N~V to C~IV column density varies significantly from one absorption component to the next, even in the same Seyfert galaxy.

Kraemer, Ho (Carnegie), Crenshaw, Shields (Ohio U.), and Filippenko (UC Berkeley) have combined Hubble Space Telescope/Faint Object Spectrograph, ground-based, and Infrared Space Observatory spectra of the nucleus of NGC 4395, the least luminous and nearest known type 1 Seyfert galaxy. The spectra show emission lines from a wide range of ionization states and critical densities. They have generated multicomponent photoionization models of both the broad and narrow emission-line regions (BLR and NLR) to investigate the physical conditions in the emission-line gas and test the proposition that the source of ionization is the non-stellar continuum radiation emitted by the central source. With a minimum of free parameters, the model predictions match the observed emission-line intensity ratios quite well. The elemental abundances appear to be subsolar, with even greater underabundance of nitrogen. From the size of the BLR predicted by the models, they estimate a central mass of a few x 10⁵ M$_\odot$, in reasonable agreement with estimates from the stellar kinematics. Finally, the results suggest that the covering factor of the emission-line gas is close to unity, and that the observed UV to X-ray continuum is absorbed by intervening NLR gas. This suggests that a high covering factor is responsible for the apparent flattening of the Baldwin relation in low-luminosity active galactic nuclei.

Kraemer, Turner (UMBC), Crenshaw, and George (USRA) have explored the effects of UV absorbing material on the shape of the EUV continuum radiation emitted by the active galactic nucleus, and on the relative strengths of emission lines, formed in the narrow line regions of Seyfert galaxies, excited by this continuum. Within a sample of Seyfert 1.5 galaxies, objects with flatter soft X-ray slopes tend to have lower values of He~II 4686 H , which implies a correlation between the observed spectral energy distribution of the ionizing continuum and the narrow emission line strengths. Objects with the flattest soft X-ray continua tend to possess high column density UV absorption and it is plausible that the differences in narrow emission line ratios among these galaxies are an indication of the effects of absorbing material internal to the narrow line region, rather than intrinsic differences in continuum shape. A set of photoionization models were generated to examine the effect of a range of UV absorbers on the ionizing continuum and, hence, the resulting conditions in a typical narrow line cloud. The results indicate that a low ionization UV absorber with large covering factor will indeed produce the combination of narrow line ratios and soft X-ray spectral characteristics observed in several Seyfert 1.5 galaxies. These results also suggest that low ionization UV absorption may be more common than currently believed.

Planets and Comets of Our Solar System