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002.tex 644 34 12 5434 5555513720 5323 % Session 10 -- Gamma Ray Sources

Display presentation, Monday, 30, 1994, 9:20-6:30

[10.02] Cosmology with Ground-Based VHE Gamma-Ray Detectors

Active Galactic Nuclei (AGN) and Superclusters of Galaxies, considered to be possible sources for the observed flux of extremely high energy cosmic rays ($E \geq 10^{19} \, \rm eV$), may well have $\gamma$-ray emission extending into the VHE (very high energy, $E_\gamma>100$ GeV) domain. Because VHE $\gamma$-rays are absorbed by pair production on the intergalactic background radiation fields, much of this emission may not be directly visible. The electromagnetic cascade initiated by the absorbed VHE $\gamma$-rays, however, is observable. Since, the velocities of $e^{+}e^{-}$ pairs produced in the cascade are most probably isotropized by an ambient random magnetic field, extended `halos' ($R > 1 \, \rm Mpc$) of pairs will be formed around AGN with VHE emission. The cascade radiation from these pair halos is emitted almost isotropically and should be observable at energies below a few TeV. The $\gamma$-ray flux from a halo depends mainly on the total luminosity and duty cycle of the source at energies $\geq 10$ TeV, but not on the source geometry, in particular the direction and beaming/opening angle of a $\gamma$-ray emmiting jet. (The halo emission is not beamed.) The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This depends on the distance to the source and the level of the $IR/O$ background of the local enviroment. Thus, the investigation of the angular and spectral distribution of halo radiation could provide almost model-independent and unambiguous cosmological information about the universal $IR/O$ background, the Hubble constant, and the VHE power of AGN. Next-generation ground-based systems (arrays) of low threshold ($E \geq 100 \, \rm GeV$) imaging Cherenkov telescopes will be able to probe the VHE apparent luminosities of extragalactic sources down to $L \approx 10^{43}(d/1000 \, \rm Mpc)^{2} \, \rm erg/s$, which is two orders of magnitude lower than the minimum luminosity detectable by EGRET at GeV energies. The excellent angular resolution expected from these systems ($\approx 0.1 \deg$) will allow us to image pair halos of even cosmologically distant ($z \geq 1$) sources.

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S1003.tex 644 34 12 3304 5555513475 5325 % Session 10 -- Gamma Ray Sources

Display presentation, Monday, 30, 1994, 9:20-6:30

[10.03] ROSAT Observations of the May 1 1992 Gamma Ray Burst Source

The gamma-ray burst of May 1 1992 was observed and rapidly localized by the 3rd Interplanetary Network, and a 2000 s ROSAT TOO observation was carried out on the small error box. A weak x-ray source was detected, but with too few photons to determine the spectral and temporal characteristics of the source. We have recently carried out a 30 ksec PSPC observation of this source, resulting in the detection of 140 photons. We find that the spectrum is consistent with thermal bremsstrahlung from a 7 million degree plasma with about $10^{22} cm^{-2}$ column depth. In addition, analysis of the photon arrival times indicates that the source is variable. The distance implied by the column depth makes it unlikely that this source is coronal emission from a field star. Optical and HRI observations of this region are now planned or underway. We discuss the possible identification of this object and its implication for the GRB source. ing jet. (The halo emission is not beamed.) The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This depends on the distance to the source and the level

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the $IR/O$ background of the local enviroment. Thus, the investigatS1004.tex 644 34 12 3743 5555513543 5331 % Session 10 -- Gamma Ray Sources

Display presentation, Monday, 30, 1994, 9:20-6:30

[10.04] A Search for Short Duration Bursts of Gamma Rays above 10$^{14}$ eV using CASA-MIA

CASA-MIA is a ground based array which detects the air-showers produced by gamma-rays at 100 TeV and above. The array's ability to detect muons allows much of the cosmic-ray background to be discarded. Data from 1990 to 1993 has been used to search for short duration gamma-ray bursts from arbitrary direction. No evidence for 0.035s bursts has been found. This result has been used to set a limit on the rate-density of evaporating primordial black holes. Additional point source searches with longer time windows of 1s, 10s and 100s are also investigated. ays above

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$^{14}$ eV using S1005.tex 644 34 12 2743 5555512306 5325 % Session 10 -- Gamma Ray Sources

Display presentation, Monday, 30, 1994, 9:20-6:30

[10.05] Comments on the Accuracy of Angular Clustering Tests Used to Study Repeating Gamma-Ray Bursts in BATSE Data

Comparisons are made of various techniques used to identify gamma-ray burst repeaters in BATSE data. The effectiveness of each test is examined when BATSE positional errors and incomplete sky sampling are folded into the data analysis. Techniques studied include the two-point angular correlation function, the nearest neighbor statistic, and the farthest neighbor statistic. Comments are made concerning the observed incidence of repeaters in BATSE's First Burst Catalog. This work has been supported by NASA grants NAG8-192 and NRA 92-OSSA-17.

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rays at 100 TeV and above. S1006.tex 644 34 12 2553 5555512307 5326 % Session 10 -- Gamma Ray Sources

Display presentation, Monday, 30, 1994, 9:20-6:30

[10.06] Integral Moment Analysis of the BATSE Burst Intensity Distribution: Comments on Two-Population (Local Disk/Halo Plus Extended Halo) Models

Moment analysis of the gamma-ray burst log(N>Fp) vs. log(Fp) distribution observed by BATSE is used to place additional constraints on Galactic coronal/extended halo source distributions. It is found that the allowed parameter space of such models is smaller than identified previously (Hakkila et al. 1994, Proc. 1993 Hunts. Workshop) due to the more efficient tolerances impose by this technique. This work has been supported by NASA grant NAG8-192. Burst Catalog. This work has been supported by NASA grants NAG8-192 and NRA 92-OSSA-17.

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rays at 100 TeV and above. S101.tex 644 34 12 3112 5555513401 5225 % Session 1 -- Brouwer Prize Lecture

Oral presentation, Monday, 30, 1994, 8:30-9:20

[1.01] What Happened to M51 and M81?

On this occasion, I plan to review the evidence that has accumulated from observations and numerical experiments during especially the past decade suggesting that each of these two famous spiral galaxies has been influenced decisively by recent tidal interplay with their respective close neighbors. It is no secret that I am biased: Like a few other astronomers, I have long suspected on dynamical grounds that the magnificent spiral structures which have brought these two galaxies into nearly every introductory textbook (and the latter also onto the cover of the Hubble Atlas) are largely transient wavelike relics of those tidal interactions --- meaning that we would probably have favored some quite different examples had mankind come along just 200 million years earlier or later. But I will try to keep such biases well under control in this review of the recent developments achieved almost entirely by workers other than myself. In other words, this talk will focus not only those portions of the tidal stories which seem well established, but also on some parts which remain shaky.

iscuss the possible identification of this object and its implication for the GRB source. ing jet. (The halo emission is not beamed.) The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This depends on the distance to the source and the level

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the $IR/O$ background of the local enviroment. Thus, the investigatS1101.tex 644 34 12 4347 5555513502 5323 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.01] The Evolving Radio Jet in BL Lacerta

During the past 15 years, there have been at least ten outbursts at centimeter wavelengths in this extragalactic object. We describe here the flux density and linear polarization variations observed at 4.8, 8.0 and 14.5~GHz with the Michigan 26-meter telescope during the series of outbursts since 1987. The recent bursts are somewhat different from the highly polarized bursts in the early 1980s, which provided a successful quantitative test of a source model based on propagating transverse shocks in a relativistic jet. The most notable change is that the polarization position angle during polarized outbursts has increased by approximately 10 degrees. This is approximately the same shift as found in a comparison of VLBI maps taken during the same time periods by Mutel, Denn and Dryer (1994, NRAO Workshop on Compact Extragalactic Radio Sources , ed. Zensus and Kellermann, p. 191), and supports their conclusion that the orientation of the radio emitting jet in BL~Lac has changed over time. Our preliminary analysis further suggests that a simple scaling of the physical parameters which gave quantitatively good fits to both the 1982 and 1983 bursts will not accurately describe the recent, relatively isolated, burst in 1991. We also note that while some bursts (e.g. in 1982, 1983 and 1991) exhibit degrees of linear polarization in excess of ten percent, other bursts (e.g. in 1980 and 1990) exhibit a very low degree of linear polarization. A common characteristic of the low polarization events is that they all exhibit high internal synchrotron self absorption (as indicated by the flux density spectra of the outbursts).

This research has been supported in part by NSF grant AST-9120224. beamed.) The halo radiation can be distinguished by its characteristic variation in spectrum and intensity with angular distance from the central source. This depends on the distance to the source and the level

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the $IR/O$ background of the local enviroment. Thus, the investigatS1102.tex 644 34 12 2114 5555513305 5313 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.02] EUVE spectrographic results on the BL Lac Object PKS 2155-304

We present spectra of PKS 2155--304 obtained by the Extreme Ultraviolet Explorer (EUVE) satellite during two long ($>100,000$~s) observations taken three weeks apart. Each spectrum shows pronounced, but different, absorption features. We discuss mechanisms that might produce these features. We use these results to rule out various models for the object that have been suggested based on X-ray results. This work has been supported by NASA contract NAS5-30180. meters which gave quantitatively good fits to both the 1982 and 1983 bursts will not accurately describe the recent, relatively isolated, burst in 1991. We also note that while some bursts (e.g. in 1982, 1983 and 1991) exhibit degrees of linear polarization in excess of ten percent, other bursts (e.g. in 1980 and 1990) exhibit a very low degree of linear polarization. A common characteristic of the low

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larization evenS1103.tex 644 34 12 3757 5555513745 5342 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.03] Pictor A: Discovery of Strong Balmer Line Variations

We have detected strong changes in Balmer line flux and profile shape for the Broad Line Radio Galaxy Pictor A (0518--458). Comparison of spectra obtained in 1983 and 1993 reveal that strong broad line component has appeared with FWHM$\sim$400 \AA. The Balmer lines observed in 1993 are reminiscent of the rare kind (extremely broad profiles with shifted ``humps'') observed in Arp 102B, 3C390.3 and a handful of other BLRGs. Earlier observations of Pic A show much narrower and weaker broad line emission, with a centrally peaked profile and with monotonically falling line wings.

The emission line profile widths and redshift are consistent with emission from an inclined relativistic accretion disk but the relative strength of the red and blue ``humps'' are not. Considerable excess flux is observed on the red side of the profiles. It might be possible to reconcile the observations with a disk model if we assume that a ``hot spot'' co-rotating with the disk contributes to the line emission.

The rarity of Pic A--like profiles however suggests that the radial velocity spread is maximized if the orientation is such that the gas is predominantly moving along the line of sight. We show that a system of outflowing clouds (possibly a double shell) in a bi-conical geometry can reproduce the profile and the profile variations.

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the investigatS1104.tex 644 34 12 2647 5555513465 5337 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.04] 3cm Observations of the Extended NW radio Jet in Pictor A

We present high--resolution, 3\,cm radio synthesis images of Pictor A constructed from four 12--hour sets of observations with the ATCA (Australia Telescope Compact Array). We find that the kiloparsec--scale radio jet located between the nucleus and the NW radio--optical hotspot, which lies more than 100\,kpc from the central galaxy, has an unresolved width ($\sim$ 1 \arcsec \, or 500 $h^{-1}$\,pc). This jet is not completely straight, but changes direction between RA 5h\,20m\,35s and 5h\,20m\,40s. The SE and NW hotspots are not alligned with the nucleus. The eastern segment of the unresolved jet points towards the nucleus while the western segment, if extended to the east, intersects the SE hotspot. Near the central host galaxy, the 3\,cm continuum structure seems to be closely related to loops of ionized, H\,II gas (on a 2 to 4\,kpc scale).

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uthor{S1105.tex 644 34 12 4567 5555513150 5332 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.05] Observations of `Flicker' in Extragalactic Radio Sources.

We present the initial results of The University of Michigan Radio Astronomy Observatory's `flicker' program. The `flicker' sources are compact or very compact QSOs and BL Lac objects that have been found to vary by 10 -- 20\% in flux, on time scales of hours to days (Quirrenbach, et al., Astr. Ap., 226, L1 1989), in contrast to the month to year time scale of variability traditionally thought to characterize these objects. The short timescales of this variability can help to provide us with information on structure currently beyond the reach of VLBI. This variability must also be taken into account when making synthesis maps of sources which may vary significantly in flux over the time period when they are observed.

We have made closely spaced observations at 4.8, 8.0 and 14.5 GHz using The University of Michigan's 26-meter radio telescope so as to extend the variability program started in the 1960s down to time scales of hours to days. The emphasis in our program has been to confirm the variability on time scales of hours to days as well as to better define variability on scales of days to weeks. The goal is to attempt to determine if the nature of the variations on these intermediate time scales is different for the `flicker' sources (where the variations appear to peak at lower frequencies) as compared to the traditional sources (where variations peak at higher frequencies and can be explained by a shocked jet model). We have also done Monte Carlo type simulations to simulate flicker phenomena. We hope that the simulations will help us to better understand what features of the light curves are due to small scale turbulent structure, and determine to what extent single dish observations can be used to test quantitatively models of such flows.

This research has been supported in part by NSF grant AST-9120224 HE apparent luminosities of extragalactic sources down to $L \approx 10^{43}(d/1000 \, \rm Mpc)^{2} \, \rm erg/s$, which is two orders

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S1106.tex 644 34 12 4521 5555513021 5316 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.06] Spectropolarimetry of the Broad Absorption Line QSO 1524+5147

Broad absorption line QSOs (BALQSOs) comprise approximately 10\% of all optically-selected quasars, and as a class are the only such objects to exhibit strong ($>3$\%), static linear polarization. Observations of the emission line and continuum properties of BALQSOs suggest that all radio-quiet QSOs are potential BALQSOs, being observed as such if the line of sight intercepts an absorbing cloud. The unique insight which polarimetry lends to the geometry of a source of radiation therefore motivates intense study of the accessible BALQSO candidates.

Optical linear spectropolarimetry is presented of the high-redshift (z = 2.86) BALQSO 1524+5147, an object selected for its brightness (V = 17.0) and strong white-light polarization (P = 2.7\%). In the rest frame of the object (1070 - 1940\AA), the degree of continuum polarization is nearly constant with wavelength, and the position angle is undeviated through the spectrum. The broad emission lines (BELs) provide pure dilution of the polarized component, while BAL features primarily absorb this component.

These conditions indicate a single, nuclear source of polarized radiation, but constant polarimetric properties over a 3.5 yr baseline suggest that synchrotron emission is not responsible. Instead, we propose that the polarization arises in an asymmetric distribution of scatterers which are coincident with and/or interior to the BAL and BEL regions, and the latter two need not be distinct. The bluer spectrum of polarized vs. total flux suggests either that the scatterers are dust grains or that an additional, red component of unpolarized light contributes to the overall continuum. These properties are discussed in terms of competing models of the nuclear morphology of AGN.

This research is supported by NSF grant AST 91-14087. 20224 HE apparent luminosities of extragalactic sources down to $L \approx 10^{43}(d/1000 \, \rm Mpc)^{2} \, \rm erg/s$, which is two orders

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S1107.tex 644 34 12 4627 5555512761 5340 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.07] Time Variable Associated Absorption in the QSO UM 675

We discuss dramatic changes in the $z_a \approx z_e$ absorption system of the $z_e = 2.15$ QSO UM 675 (Q0150$-$203). The C IV $\lambda$1550 and N V $\lambda$1240 doublets at $z_a = 2.1344$ strengthened by a factor of $\sim$3 between the observations of Sargent, Boksenberg and Steidel (1988, ApJS, 68, 539; measured November 1981) and our earliest measurements (November and December 1990). During this time, C IV in the $z_a = 2.0083$ system may also have strengthened. The variability of other lines in these systems is unknown. Continued monitoring is in progress.

We consider several models of the $z_a \approx z_e$ absorption environment, and conclude that the absorbing clouds are close to the QSO and photoionized by the QSO continuum. The variability timescale ($\lap$2.9 yrs rest) requires gas densities $\gap$4000 cm$^{-3}$ to allow changes in the ionization balance. This minimum density, and the high ionization needed to produce the Ne VIII $\lambda$774 and O VI $\lambda$1035 absorptions reported previously (E. M. Burbidge et al. , 1993, BAAS, 24, 1135), requires clouds $\lap$200 pc from the QSO. The full range of absorption line ionizations (including C III $\lambda$977 and N III $\lambda$989) implies that the clouds are segregated, spanning a factor of $\gap$10 in distance or $\gap$100 in density. Across these regions the H I fraction varies from $\sim$10$^{-3}$ to $\sim$10$^{-6}$. The total hydrogen column ranges from a few times $10^{18}$ cm$^{-2}$ in the low ionization gas to $\sim$10$^{20}$ cm$^{-2}$ where the Ne VIII lines form. The Lyman continuum is expected to be optically thin throughout, consistent with the measured absence of a Lyman edge. The metal abundances are roughly solar or above. Implications of these results are discussed.

This work is supported by NASA grant NAG 5-1630.

ion angle is undeviated through the spectrum. The broad emission lines

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ELs) provide pure dilution of tS1108.tex 644 34 12 3421 5555512727 5332 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.08] Optical-UV-Soft-X-ray Spectra of AGN

The physical origin of the much studied optical/UV ``Big Bump'' in quasars is unknown. Even its suggestive connections with ``soft X-ray excesses'' is uncertain. We have investigated several models for the blue bump and the soft X-ray excess: (a) free-free emission from an optically thin plasma (see Barvainis 1993 for a review), (b) optically thick thermal emission from the innermost regions of an accretion disk in both Schwarzschild and Kerr geometries (e.g., Shields 1978, Malkan \& Sargent 1982, Czerny \& Elvis 1987, Sun \& Malkan 1989, Laor 1990, Fiore, $et al.$ 1994). We present the locus of each model in optical/UV/soft X-ray color-color diagrams. The colors were optimized to be indicators of particular spectral components and they provide stronger discriminants than the standard UBV diagrams. These diagrams clarify the limitations of both models in their pure form. Addition of extra components is needed and we show how the simplest possible addition (an underlying power law) brings disk models into agreement with the data. We compare our results with the sample of 47 low z quasars for which we have collected observations over a wide range of frequencies (Elvis $et al.$ 1994). d, we propose that the polarization arises in an asymmetric distribution of scatterers which are coincident with and/or interior to the BAL and BEL regions, and the latter two need not be distinct. The bluer \newcommand{\MgII}{Mg\,{\small II}}

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ectrum of polarized vs. totaS1109.tex 644 34 12 3144 5555512725 5333 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.09] Associated Mg\,II Absorption in Red Quasars

In a spectroscopic survey of 56 intrinsically faint, steep-spectrum quasars, we detected an excess of quasars with associated \MgII\ absorption line systems, in which $z_{abs} \approx z_{em}$. This is in contrast to other surveys using apparent magnitude selected samples which have found no such excess above the background expected from distributed absorbers. In our sample there is a strong correlation between very red rest-frame B-V colors of the quasar and the presence of associated \MgII\ absorption. In order to further understand this result, we have obtained deep R-band images of two of the red quasars (3C\,68.1 and 3C\,212) and begun a spectroscopic survey of quasars selected for very red B-V color. In 3C\,68.1 ($z=1.24$), we observe a prominent extension outside the quasar point source, and what is most likely a foreground cluster. Spectroscopy of three faint red quasars revealed no associated \MgII\ absorption lines. The implications of these results are discussed. results with the sample of 47 low z quasars for which we have collected observations over a wide range of frequencies (Elvis $et al.$ 1994). d, we propose that the polarization arises in an asymmetric distribution of scatterers which are coincident with and/or interior to the BAL and BEL regions, and the latter two need not be distinct. The bluer

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ectrum of polarized vs. totaS1110.tex 644 34 12 3321 5555512700 5311 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.10] HST and Ground-Based Spectroscopy of High Luminosity AGNs

We are carrying out quasi-simultaneous multiwavelength spectrophotometry from below Ly $\alpha$ to beyond H $beta$ for a sample of $\sim 50$\ radio-loud quasars with $z \sim 0.3$\ to 1.3. The aim is to understand the structure and physics of the nuclear environment -- through investigations of the Baldwin effect, comparison of line profiles and ratios, continuum shape, and X-ray and radio properties. In particular the sample is selected with constrained radio lobe luminosity, but with a wide range in the ratio of radio core to lobe flux density. We use the latter ratio as a prime indicator of orientation of the central engine, thus enabling us to investigate angle dependence of kinematics, optical depth, and anisotropy of the broad-band continuum. We will present some striking results derived from our sample, which is so far about 60\% complete.

Manchester} P.S. Coppi (University of Chicago), F.A. Aharonian, H. V\"olk (Max-Planck-Institut f\"ur Kernphysik), M. Sommer (MPE), K. Hurley, P. Li (UC Berkeley SSL), C. Kouveliotou, G.J. Fishman (MSFC), M. Boer, M. Niel (CESR), J. Laros (U. Arizona), T. Cline (NASA/GSFC), S.Sampson, A.Borione, C.E.Covault, J.W.Cronin, B.E.Fick, L.F.Fortson, K.G.Gibbs,B.J.Newport, T.A.McKay, R.A.Ong, L.J Rosenberg (EFI,U of Chicago), M.Catanese, K.D.Green, A.Kennedy, J.Matthews, D.Nitz, D.Sinclair, J.C. van der Velde (U of Michigan), D.B.Kieda (U of Utah), V. C. Vo (Mankato State U.), J. Hakkila (Mankato State U.), L. M. Pearson (Mankato State U.), C. A. Meegan (NASA/MSFC), D. H. Hartmann (Clemson U.), J. M. Horack (NASA/MSFC), G. J. Fishman (NASA/MSFC), G. N. Pendleton (UAH), R. B. Wilson (NASA/MSFC), M. N. Brock (NASA/MSFC), W. S. Paciesas (UAH), M. S. Briggs (UAH), (), B. A. Thompson (Mankato State U.), J. Hakkila (Mankato State U.), V. C. Vo (Mankato State U.), C. A. Meegan (NASA/MSFC), J. M. Horack (NASA/MSFC), G. J. Fishman (NASA/MSFC), G. N. Pendleton (UAH), R. B. Wilson (NASA/MSFC), M. N. Brock (NASA/MSFC), W. S. Paciesas (UAH), M. S. Briggs (UAH), Alar Toomre (MIT), H. D. Aller, P. A. Hughes, M. F. Aller (U. Michigan), C.-Y.Hwang, S.Bowyer (UCB Astron/CEA), S.Kahn (UCB Phys \& Astron), A.Konigl (U Chicago Astron), Paolo Marziani, Jack W. Sulentic, William C. Keel (U. Alabama), Toma\v{z} Zwitter (U. Ljubljana/U. Padova), Massimo Calvani (Padova Obs.), S. M. Simkin (Mich State), E. M. Sadler (Sydney U), R. Sault (ATNF ), S. M. Simkin (Mich State), E. M. Sadler (Sydney U), R. M. Wagner, H. D. Aller, P. A. Hughes (U. Michigan), J.Glenn, G.D.Schmidt (U.Ariz.), C.B.Foltz (MMTO), F.W. Hamann, T.A. Barlow, E.A. Beaver, E.M. Burbidge, R.D. Cohen, V.T. Junkkarinen and R.W. Lyons (UCSD), A. Siemiginowska, O. Kuhn, F. Fiore, M. Elvis, J. McDowell, B. Wilkes and S. Mathur (SAO), T. Aldcroft (SAO), J. Bechtold (U. Arizona/Steward), M. Elvis (SAO), B.J.Wills (Univ.TX Austin), J.A.Baldwin (CTIO), M.S.Brotherton (Univ.TX Austin), H.Netzer (Tel Aviv Univ.), G.J.Ferland (Univ.KY Lexington), D.Wills (Univ.TX Austin), I.W.A.Browne (Jodrell Bank, Univ. Manchester), R.F.Carswell (IOA, Cambridge, U.K.), M.Han (Univ.Wisconsin, Madison), R.F.Carswell (IOA, Cambridge, U.K.), M.Han (Univ.Wisconsin, Madison)

We are carrying out quasi-simultaneous multiwavelength spectrophotometry from below Ly $\alpha$ to beyond H $beta$ for a sample of $\sim 50$\ radio-loud

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asars with $z \sim S1111.tex 644 34 12 2642 5555512674 5331 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.11] Compact Double Radio Sources in the MIT-Green Bank-VLA (MGV) Gravitational Lens Search Sample

We have made radio and optical observations of a selection of compact double radio sources discovered in the course of the the MIT-Green Bank-VLA gravitational lens search. These sources were chosen for followup by virtue of having two or more unresolved components separated by less than two arcseconds in VLA A-array maps at X Band (3.6 cm). Statistical studies have shown that the class of ``compact double radio sources'' are not, in general, larger objects seen end-on. This class is not well-studied at the present time, but they appear to be radio galaxies at large redshifts. We will present results from our 6 cm MERLIN observations of a sample of these sources. We will also present preliminary results of our optical followup program using the 1.3m McGraw-Hill telescope.

a foreground cluster. Spectroscopy of three faint red quasars

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vealed no associated \MgII\ S1112.tex 644 34 12 3027 5555514227 5324 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.12] A New Technique for Identifying High Redshift Quasars

This report presents preliminary results of a search for high redshift quasars based on the Palomar Transit Grism Survey of Schmidt, Schneider \& Gunn. The original survey covers $\sim$ 60 square degrees of the sky with low-resolution slitless spectra of approximately 600,000 objects in the wavelength range from 4400~{\AA} to 7500~{\AA} and was designed to find emission line candidates of a given signal--to--noise ratio and equivalent width.

Our goal is to establish whether there is a significant number of high redshift quasars with weak or narrow emission lines by searching for objects with a continuum depression blueward of the Lyman break. In addition to providing an interesting sample of quasars with different emission line properties, these objects could explain the steep decline in the comoving space density of strong emission line quasars at redshifts higher than $\sim$ 2.5. The follow--up observations will be carried out with the new spectrograph at the 3.5m ARC Telescope, Apache Point, NM.

implications of these results are discussed. results with the sample of 47 low z quasars for which we have collected observations over a wide range of frequencies (Elvis $et al.$ 1994). d, we propose that the polarization arises in an asymmetric distribution of scatterers which are coincident with and/or interior to the BAL and BEL regions, and the latter two need not be distinct. The bluer

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ectrum of polarized vs. totaS1113.tex 644 34 12 3514 5555512522 5322 % Session 11 -- Luminous AGN and QSOs

Display presentation, Monday, 30, 1994, 9:20-6:30

[11.13] Very High Energy Gamma-Ray Observations of Markarian 421

Markarian 421 is the closest (z=0.031), yet dimmest identified AGN observed by EGRET. As yet, it is also the only AGN to be observed at TeV energies by the Whipple Observatory imaging gamma-ray telescope. The energy spectrum of Mrk421 between 0.3 GeV and 4 TeV was previously reported to be 1.02x$10^{-11} E^{-2.06}$ photons/cm$^{2}$/s/TeV. At TeV energies, this represents a 6$\sigma$ excess above background. However, the EGRET and Whipple observations used to determine the spectrum were not taken contemporaneously. Previous EGRET and Whipple observations showed no strong evidence of variablility. In April 1994, Markarian 421 was observed with both the Whipple telescope and EGRET. The preliminary results of the Whipple observations in the energy range between 0.25 TeV and 10 TeV will be reported. Of special interest are energies above 3 TeV where the effects of intergalactic absorption from the IR background should become noticeable. distribution of scatterers which are coincident with and/or interior to the BAL and BEL regions, and the latter two need not be distinct. The bluer

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ectrum of polarized vs. totaS1201.tex 644 34 12 4765 5555514256 5337 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.01] The Effects of Dust on the Ultraviolet Spectral Energy Distribution of Starburst Galaxies

The effects of dust on the spectral energy distribution (SED) of starburst regions of galaxies was investigated using Monte Carlo techniques to model the transport of radiation in systems where the dust and stars are mixed. In a recent paper, Calzetti, Kinney, \& Storchi-Bergmann (ApJ, 10 July 1994) derive an extinction curve from observations of starburst galaxies assuming the dust is in a screen geometry. This gives an extinction curve where the geometrical effects of mixing of the dust and stars are convolved with the extinguishing effects of the dust. The resulting extinction curve is greyer than the Galactic extinction curve and featureless in the ultraviolet, i.e. lacking both the 2200 \AA\ bump and far-UV rise.

In an attempt to explain this ``effective'' starburst extinction curve we have modeled the effects of dust on the SED of starbursts. A simple starburst model was used to determine the different populations of stars as a function of the starburst age. The flux at 23 wavelengths, ranging between 1000 \AA\ to 5500 \AA\, was computed using Monte Carlo techniques assuming the dust and stars were spherically distributed. The dust was assumed to have similar properties as dust in our Galaxy. The distribution of different star types ranged from mostly centrally located for O stars to constant density for A and later stars. In addition, the fraction of stars lying outside the dust ranged from very few for O stars to a majority for A and later stars. Combining the two models, it was found that the SED was strongly dependent on the distribution of the different types of stars relative to the dust, the age of the starburst, and the amount of dust. The ``effective'' UV extinction curve became greyer and featureless as the amount of dust was increased. For example, the 2200 \AA\ bump was almost non-existent for large amounts of dust.

This work was supported by NASA LTSA Grant NAGW-3168.

o orders

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S1202.tex 644 34 12 2531 5555513367 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.02] Spectroscopic Observations of Hot Spot Galaxies

Long-slit spectroscopic observations of hot spot galaxies NGC 4321 and NGC 2903 will be presented. These galaxies display knots of radio and optical emission in their nuclear regions which suggest regions of intense star formation. Spectroscopic observations taken with the Kitt Peak 2.1m will be compared with radio and H-alpha images of these galaxies. Spectra of the optical hot spots reveal strong variations from spot to spot in the nature of the underlying continuum. Emission-line ratios in the nuclear regions also display a wide range of physical properties among the hot spots. These nuclear line ratios are significantly different from line ratios in comparison HII regions in the disks of these spriral galaxies. Optical spectra of knots of non-thermal radio emission will also be presented.

extinction curve we have modeled the effects of dust on the SED of starbursts. A simple starburst model was used to determine the different populations

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stars as a S1203.tex 644 34 12 2035 5555513672 5326 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.03] Off-Center Star Formation in The Nuclear Starburst Galaxy NGC 2903

NGC 2903 is a strongly barred galaxy harboring a nuclear starburst. We have mapped the 1.28$\mu$m Pa-$\beta$ recombination line using CRSP, the KPNO long-slit near IR spectrograph, on the KPNO 1.3m telescope. The map shows that star formation is strongly concentrated about 3" or 150pc to the north of the nucleus. We discuss the advantages and disadvantages of our observational technique compared to conventional narrow band and Fabry-Perot imaging.

display a wide range of physical properties among the hot spots. These nuclear line ratios are significantly different from line ratios in comparison HII regions in the disks of these spriral galaxies. Optical spectra of knots of non-thermal radio emission will also be presented. extinction curve we have modeled the effects of dust on the SED of starbursts. A simple starburst model was used to determine the different populations

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stars as a S1204.tex 644 34 12 2271 5555513643 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.04] Multi-wavelength Observations of Galaxy Pairs in the Early Stages of Merger

Multi-wavelength observations of interacting galaxy pairs provide a view the early stages of the merger process. VLA 21cm HI observations and H$\alpha$\ imaging provide evidence of minor-axis gas outflows, as well as the funneling of gas into the nuclei of some galaxies (Arp 270 and Arp 283). Observations are ongoing for other systems that appear to be at about the same stage of interaction as these pairs. Comparisons are made between the near-infrared and HI, as well as optical and ROSAT PSPC observations in order to understand further this type of star formation in colliding systems. ing CRSP, the KPNO long-slit near IR spectrograph, on the KPNO 1.3m telescope. The map shows that star formation is strongly concentrated about 3" or 150pc to the north of the nucleus. We discuss the advantages and disadvantages of our observational technique compared to conventional narrow band and Fabry-Perot imaging.

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nS1205.tex 644 34 12 3517 5555513177 5336 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.05] A Radio Continuum Survey of Ring Galaxies

As part of a multi-wavelength study of interacting galaxies with ring-like structure, we observed 13 systems with the VLA in D-array at 8.4 GHz. Four objects were also observed at 1.4 GHz and 4.8 GHz. Snapshots of 15-20 minutes were done to identify candidates for future deeper and higher resolution observations. Nine of the 13 objects were detected at X-band above a threshhold of about 0.2 mJy. Results will be compared with H-alpha and infrared observations of these objects to study star formation processes in the wake of a density wave resulting from a collision. The regions of recently formed O/B stars are expected to be bright in H-alpha and radio thermal bremstrahlung radiation. The internal optical extinction in the star formation regions can in principle be estimated by comparisons of H-alpha and radio thermal flux, thus allowing the stellar composition of the star clusters in these regions to be deduced. We expect that diffusion of relatavistic electrons from supernova sites results in non-thermal radiation that lags behind the sites of young star formation, the latter being richer in thermal radiation. Our preliminary results for ARP 10 are consistent with this scenario in that we find the flattest spectrum part of the radio map coincides with the region of brightest H-alpha flux. ntrally located for O stars to constant density for A and later stars. In addition, the fraction of stars lying outside the

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st ranged from very few for O stars to a majority S1206.tex 644 34 12 2142 5555513716 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.06] A Spectrophotometric Study of Ring Galaxies

We present our analysis of longslit spectrophotometric data for a sample of ring galaxies. A detailed calculation of the radial variation of chemical abundances is used to examine the star formation history and possible collisional nature of these systems. We focus our attention on Arp 10, a peculiar ring galaxy, which exhibits an azimuthal variation of the star formation rate along the ring. Such a behavior is consistent with a collisionally induced density wave. We estimate the kinematics inferred from the spectra and present a plausible scenario of the evolution of the galaxy. ormed O/B stars are expected to be bright in H-alpha and radio thermal bremstrahlung radiation. The internal optical extinction in the star formation regions can in principle be estimated by comparisons of H-alpha and radio thermal flux, thus allowing the stellar composition of the star clusters in these regions to be deduced. We expect that diffusion of relatavistic electrons from supernova sites results in S12071207.tex 644 34 12 7173 5555513613 5647

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st ranged from very few for O stars to a majority S1206.tex 644 34 12 2142 5555513716 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.07] Where Does Star Formation End in Disk Galaxies?

Much work has focused on understanding the key processes which govern star formation in galactic disks and a major goal has been to establish the link between star formation activity and the physical and dynamical state of the interstellar medium. An important issue is how star formation ends in disks. Does it end abruptly, or is there a smooth decline in star formation activity? Is the location of the outermost HII region determined solely by galaxian properties, or is there a dependence on the local environment? Are there sites of recent star formation beyond the ``edge" of the old stellar disk?

HII regions are the clearest tracers of present day massive star formation. In order to address the above issues, we have undertaken a deep H$\alpha$ imaging study of the outer regions of a sample of spiral galaxies. Our sample includes both field and Virgo cluster galaxies which have been mapped in HI. We will present some preliminary data from which we have identified a number of galaxies with HII regions located at large galactocentric radii. These regions often lie beyond the critical radius for star formation predicted by simple gravitational instability theories. We will discuss these data and investigate the possible implications for disk evolution.

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st ranged from very few for O stars to a majority S1206.tex 644 34 12 2142 5555513716 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.07] Where Does Star Formation End in Disk Galaxies?

Much work has focused on understanding the key processes which govern star formation in galactic disks and a major goal has been to establish the link between star formation activity and the physical and dynamical state of the interstellar medium. An important issue is how star formation ends in disks. Does it end abruptly, or is there a smooth decline in star formation activity? Is the location of the outermost HII region determined solely by galaxian properties, or is there a dependence on the local environment? Are there sites of recent star formation beyond the ``edge" of the old stellar disk?

HII regions are the clearest tracers of present day massive star formation. In order to address the above issues, we have undertaken a deep H$\alpha$ imaging study of the outer regions of a sample of spiral galaxies. Our sample includes both field and Virgo cluster galaxies which have been mapped in HI. We will present some preliminary data from which we have identified a number of galaxies with HII regions located at large galactocentric radii. These regions often lie beyond the critical radius for star formation predicted by simple gravitational instability theories. We will discuss these data and investigate the possible implications for disk evolution. star formation predicted by simple gravitational instability theories. We will discuss these data and investigate the possible implications for disk evolution.

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st ranged from very few for O stars to a majority S1206.tex 644 34 12 2142 5555513716 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.08] Star Formation Rates in Low Surface Brightness Galaxies

The low surface brightness (LSB) disk galaxies found in recent surveys (e.g.,\ Schombert et al.\ 1992, AJ, 103, 1107) tend to be blue and gas rich. These properties along with their low mean surface luminosity and {\sc H i} densities imply an inefficient mode of star formation. The H$\alpha$ images that we present of a sample of these galaxies show the weak star formation in these galaxies relative to high surface brightness, Hubble sequence spirals. Most LSB disks harbor only a handful of widely scattered {\sc H ii} regions and generally have low total H$\alpha$ fluxes. However, in some cases these scattered areas of star formation can be quite vigorous. For example, the giant LSB disk galaxy F568-6 contains a complex that is comparable in size, mass, and star formation rate to a luminous irregular galaxy, but contributes only about 5\% of the galaxy's total blue luminosity (Bothun et al.\ 1990, ApJ, 360, 427). We present neutral hydrogen images of some of the largest known LSB galaxies. The gas in these galaxies is distributed in regularly rotating, giant, and also very low {\sc H i} surface brightness disks. A preliminary analysis shows that if most of the gas is in atomic hydrogen, large parts of the disks have total surface densities below the threshold for star formation to set in.

s Hopkins University,\\ Dept. of Physics and Astronomy\\

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st ranged from very few for O stars to a majority S1206.tex 644 34 12 2142 5555513716 5327 % Session 12 -- Starburst Galaxies

Display presentation, Monday, 30, 1994, 9:20-6:30

[12.09] Early Results from the HST Medium Deep Survey with WFPC2

We present some of the initial results from the HST Medium Deep Survey (Key Project) using WFPC2 in cycle 4. WFPC2 has been used in parallel mode for the survey at the rate of a few fields every week since late January 1994, where each field is typically covered by several 30-40 minute "orbits", with exposures split between "wide V" (F606W) and "wide I" (F814W). The extragalactic part of ther Medium Deep Survey is directed towards an understanding of galaxy morphology at intermediate redshifts (0.1 to 0.7), with systematic ground-based spectroscopic and photometric follow-up.

Some of the initial WFPC2 images show evidence for compact, superluminous starburst regions within galaxies. We also detect a high fraction of disturbed and interacting systems, and estimates are made of these fractions relative to local galaxies.

We are performing a search for serendipitous objects, and present images of candidate lensing systems and merging/interacting galaxies.

ing the two models, it was found that the SED was strongly dependent on the distribution of the different types of stars relative to the dust, the age of the starburst, and the amount of dust. The ``effective'' UV extinction curve became greyer and featureless as the amount of dust was increased. For example, the 2200 \AA\ bump was almost non-existent for large amounts of dust. This work was supported by NASA LTSA Grant NAGW-3168. o orders

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S1301.tex 644 34 12 5441 5555514066 5327 % Session 13 -- Models or Gas Flows and Spectral Formation

Display presentation, Monday, 30, 1994, 9:20-6:30

[13.01] Global Oscillations in Near-Critical Accretion Flows

Bright accretion-powered systems exhibit many varieties of time-variabilty, including flaring and quasiperiodic oscillations. These may reflect variations in the source itself intrinsic to rapid accretion; to assess this idea we examine the time-dependent response of radiation-dominated spherical accretion flows to radial and nonradial perturbations. Such flows may serve as useful first approximations to the actual accretion flows in low-mass X-ray binaries (LMXBs) and other systems. Feedback between the production of radiation and the flow of gas to the radiation producing regions promotes the existence of global modes, each of which is associated with a characteristic oscillation about steady flow. The feedback mechanism is very general, implying that radiation hydrodynamic oscillations may be a universal feature of rapidly accreting systems. These oscillations are accompanied by characteristic variations in the radiation output of the accretion flow. Because heavily damped modes are unlikely to be observable, we examine how the frequencies and the damping rates of global modes depend on parameters such as the mass accretion rate. Under conditions thought to be typical of LMXBs, the radial modes are weakly damped and some nonradial modes are even linearly unstable. Our results for the frequencies and structure of radial modes confirm similar findings by Fortner, Lamb, and Miller~(1989). When nonradial modes are active, the flow develops regions through which material preferentially accretes, and regions through which radiation preferentially escapes. Our linear stability analysis suggests that these modes dominate the accretion flow behavior when the system luminosity approaches the Eddington limit, the critical point beyond which steady spherical accretion becomes impossible. Nonradial mode frequencies behave very differently from those of the radial modes, as they appear to depend only weakly on the system luminosity. A connection may exist between nonradial radiation hydrodynamic modes and the NB/FB quasi-periodic intensity oscillations found in LMXBs, which also display striking frequency stability.

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S1302.tex 644 34 12 2322 5555513344 5321 % Session 13 -- Models or Gas Flows and Spectral Formation

Display presentation, Monday, 30, 1994, 9:20-6:30

[13.02] Subsonic Accretion through Stellar Boundary Layers