AAS Observatory Annual Report (1993-1995)
| I. | Personnel |
|---|---|
| II. | Theoretical Research |
| III. | Observational Research |
| IV. | Other Activities |
| V. | Publications |
Professors
The major research focus of the astronomy group at U. Alabama is on extragalactic theory and observation. We summarize current results and activities by subtopic in the following two sections.
R. White and graduate student R. Dupke are trying to determine the cause of the central metal concentration observed in the intracluster gas of several galaxy clusters. They are exploring whether abundance gradients (or lack thereof) are tied to the morphological mix of cluster galaxies, are due to contamination from central dominant galaxies, or to ram pressure stripping. In a couple of galaxy clusters, the central metal enhancement is on such a small spatial scale that metals from the central dominant galaxy are likely to be the source. In other clusters, the abundance gradients track the distribution of early-type galaxies very well. White and Dupke are also working to constrain the source of intracluster metals in general, since this has a direct bearing on galaxy formation scenarios: the two competing mechanisms for the origin of the bulk of intracluster metals are galactic winds and ram-pressure stripping.
P. Godon (JPL), N. Soker (Oranim/Haifa), O. Regev (Haifa), and White found a couple of correlations between the incidence of optical filaments in cluster cooling flows and other cooling flow and cluster properties. In particular, all cooling flow clusters observed to have high Faraday rotation measures at their centers are found to have optical filaments or nuclear emission. A causal origin for this correlation was investigated, with the energy source for the optical filaments being attributed to reconnection of the magnetic field compressed in the cooling flows. In addition, the spatial extent of optical filaments was found to be correlated with the peculiar velocities of the central dominant galaxies. Several scenarios, including subcluster mergers and galaxy motion-induced turbulence, were explored to account for this latter correlation.
With T. Freeman and S. Howard (NASA), Byrd has published simulations of the one-armed spiral galaxy, NGC 4378. Previously they had found that the inner single arm of the galaxy, NGC 4622, was a leading arm (winding out opposite the disk rotation). An outer pair of arms created in the encounter trail the disk rotation. They found that a plunging orbit by a small companion opposite the disk spin could generate a single inner arm which leads the disk spin. They found that the single arm of NGC 4378 which extends over the entire disk can be created by a small companion. However, the single arm does not lead, it trails the disk rotation. As a test of this, Byrd, Buta and REU summer student D. McCormick (Tuskegee U.) have processed two color images of NGC 4378 obtained by graduate student G. Purcell. They found that the disk orientation and rotation require that the single arm does indeed trail.
Henricksen and Byrd have evaluated the effects of galaxy cluster tidal potentials on star formation in a disk galaxy falling into the cluster. Potentials most directly inferred from gravitational lensing data are evaluated, as well as classical King models and the potential calculated from X-ray gas emission. The centrally peaked lens potential is extremely effective. It is estimated that the potential will cause disk gas clouds to collide at speeds large enough to greatly enhance star formation. For galaxies in clusters, tidal perturbation can thus serve as an additional mechanism to ram pressure and individual tidal interactions for star formation and gas removal.
Byrd, with REU student S. Lester (LSU) and S. Howard and M. Carini (IUE), has carried out simulations of galaxies with two-way stellar disks. These simulations have flat rotation curves consistent with the two-way stellar disk galaxies NGC 7217 and NGC 4550. Consistent with earlier simulations by Hohl and Zang, two-way disks are found to be unstable if there is no dynamically inert halo. Byrd and his collaborators have studied a halo stabilized two-way disk that is strongly perturbed by a passing companion. The two-way tidal arm morphology is a distinctive mixture of the morphologies of companions passing opposite and in the same sense as the spin of one-way disks. Long term arms of perturbed two-way disks are smoother and more tightly wound than such arms in otherwise identical one-way disks. These characteristics make an interesting parallel with those of anemic spirals.
Byrd and REU student, J. Hallum (Mankato State), are simulating the warping effect of the Magellanic Clouds on the disk of the Galaxy. They are studying whether new values for the transverse velocity and distance of the Clouds result in a past orbit which can result in the degree of disk warp observed in HI.
Byrd and H. Salo (Oulu U.) have published a computer simulation study of the M51 system. They confirm the results of a previous study by Howard and Byrd that, in addition to the passage of the small companion galaxy away from us through the disk edge roughly 70 million years ago, there was yet another passage of this companion toward us through the disk edge about 400 million years ago. Howard and Byrd had been able to generate the shape of the bridge arm and debris near the companion as well as the long far-side HI tail. Beyond this, Byrd and Salo have reproduced the radial velocities of these features and matched the rotation curve of M51. By assuming that stars form when disk gas clouds collide, the distribution of young stars in M51 can also be reproduced. These simulations require the companion to be in a low eccentricity orbit tilted with respect to the disk.
Byrd with collaborators in Finland (M. Valtonen, J. Zheng) and Canada (M. McCall and K. Innanen) is continuing development of an alternative view of the history of the Local Group of galaxies. In this view, our galaxy and the Andromeda galaxy are the major members in terms of mass. Two other substantial galaxies (IC342 and Maffei 1), about 900 kpc beyond the Andromeda Galaxy, are flying away from us at a high speed. Because of their substantial mass, a back extrapolation of either of these two galaxies indicates that either must have strongly interacted with M31. It is surmised that, in the interaction, some galaxies merged with M31 and either of the above two galaxies were ejected. Their excess speed is thus explained. M31 slowly recoils toward our galaxy, which is a bystander to the merger/ejection. Byrd and his collaborators have published a description of consequences of this event for our galaxy. The large angular momentum of the Magellanic Clouds and the large velocity (for its distance) of Leo I can be explained if they were captured from the Andromeda galaxy as it recoiled toward us.
Hardee's research involves investigation of magnetohydrodynamic flows in astrophysical systems. His research over the past several years has been extended from a study of the dynamics of collimated outflows such as those observed in many extragalactic radio sources to the study of the dynamics of collimated outflows in Seyfert Galaxies and protostellar systems. His work involves comparison between analytical analysis of the time independent magnetohydrodynamic fluid equations and linearized time dependent magnetohydrodynamic fluid equations, and numerical simulations and numerical modeling of collimated outflows. Objectives of his research are (1) to perform fully three dimensional numerical simulations and compare results with analytically developed theory, (2) to use the results to learn how outflow morphology is influenced by the external environment, by the presence of dynamically significant magnetic fields, and by radiative cooling, and (3) to model the emission from collimated outflows and model selected objects. The numerical aspects of the research have been conducted in collaboration with M. Norman (U. Illinois), J. Stone (U. Maryland), and D. Clarke (Saint Mary's U.).
Numerical simulations were used (1) to verify predictions made by theory, (2) to study the connection between linear theory and nonlinear behavior, (3) to study the development of the nonaxisymmetric perturbations that are predicted by the linear theory to be more important dynamically than the axisymmetric perturbations, (4) to study the response of a supersonic jet to a driving frequency (precession or orbital motion of the central powersource) and (5) to make comparisons with radio source morphology. In general, the simulations confirmed predictions made by the theory. Research has shown that jet expansion associated with decreasing pressure is stabilizing, as predicted by the theory and that gradients in the external medium such as the gradients in active galaxy halos and cooling accretion flows can produce tail-like instead of lobe-like morphologies. Low power outflows are strongly influenced while high power outflows are less strongly influenced by atmospheric gradients. Qualitatively, this result agrees with the observational distinction between high power and low power radio source morphologies and suggests that the morphology of an extragalactic radio source provides a qualitative probe of the environment.
In other work, the response of a jet to a driving frequency was used to model the morphology of the radio source 3C449. The morphology of this source suggests that orbital motion of the central engine drives jet bending. The bending, along with information on the atmosphere obtained from X-ray data, made it possible to estimate the orbital period and jet velocity. Other research includes a theoretical and numerical study of the effect of strong magnetic fields on the dynamics of two dimensional adiabatic jets, a study of the differences between two dimensional and three dimensional numerical simulations of jets. In general, the simulations confirmed predictions made by the theory. The work on magnetized jets revealed the predicted destabilizing effect of strong magnetic fields on super-Alfvenic jets that are trans-magnetosonic, the predicted stability of sub-Alfvenic flows, and a rapid destabilization of magnetized flows as they make the transition from sub-Alfvenic to super-Alfvenic speeds.
Recent research has involved a detailed theoretical and numerical study of the properties of fully three dimensional jets. Comparison between two and three dimensional numerical simulations show that the additional degree of freedom in three dimensions leads to a not unexpected more rapid destabilization of the flow. The three dimensional numerical simulations of light unmagnetized jets reveal features like those seen in extragalactic jets in FR I low power radio sources. Mass entrainment and shock heating lead to relatively rapid breakup of the three dimensional jets, particularly when compared to two dimensional jets with similar properties. In more recent work performed in collaboration with D. Clarke a series of three dimensional numerical simulations of magnetized jets has been performed. Results have been compared to observed radio structures in the Cygnus A jet. The fully active magnetic fields in the numerical simulation provide simulated synchrotron intensity and polarization images which reveal features that are similar to extragalactic jets in FR II type radio sources. These images are significantly different from images generated using fluid variables and the work suggests that numerical simulations must generate emission images if simulation results are to be compared properly to observations.
Work performed in collaboration with J. Stone on the effects of radiative cooling on protostellar jet dynamics is also in progress. A series of numerical simulations has been performed and the effects of radiative cooling have been included in the theory. The effects of magnetic fields are also being studied theoretically, as it is thought that strong fields might exist in protostellar outflows.
Buta's work has centered mainly on the detection of resonances in normal galaxies and other problems of internal galaxy dynamics. The most recognizable features that we can reliably associate with dynamical orbit resonances are the inner, outer, and nuclear rings seen in many barred galaxies. These features are ubiquitous, and Buta has compiled a catalog of them for the purpose of evaluating which resonance or resonances are connected with each ring type. This Catalog of Southern Ringed Galaxies, or CSRG, was published in 1995 (ApJS, 96, 39). It is a comprehensive compilation of Hubble types, angular diameters, axis ratios, and bar/ring position angles for 3,692 ringed galaxies south of declination -17 degrees. The information was used by Buta to determine the intrinsic shapes and alignments of rings with respect to bars. Inner rings in SB galaxies have an intrinsic axis ratio of 0.81 +/- 0.06 and are aligned parallel to the bar. Outer rings and pseudorings have an intrinsic axis ratio of 0.82 +/- 0.07 and may be aligned either parallel (36%) or perpendicular (64%) to the bar. These characteristics connect inner rings to the inner 4:1 resonance close to corotation and outer rings to the outer Lindblad resonance. Nuclear rings are likely connected to the inner Lindblad resonance regions of galaxies. Identification of resonances in barred galaxies allows indirect estimation of the bar pattern speed, a fundamental unknown of galactic dynamics.
Through the CSRG, Buta has brought attention to the interesting and previously unrecognized phenomenon of intrinsic bar/inner ring misalignment. Although most inner rings in SB galaxies are aligned parallel to the bar, as indicated by CSRG statistics, definite cases of misalignment are known. The best established case is ESO 565-11, studied by Buta, Crocker, and graduate student Guy B. Purcell. In this nearly face-on galaxy, the inner pseudoring and the bar are misaligned by nearly 60 degrees. This galaxy also includes the largest known example of a starburst nuclear ring. The feature is 5 kpc in diameter compared to an average diameter of 1.5 kpc for such rings (Ho = 75), and is defined by 10 giant HII regions and super star clusters. The structure of this galaxy is not obviously connected to an interaction, and may be an example of multiple pattern speeds in a barred galaxy.
Buta, with collaborators W. van Driel (Nancay), J. Braine (Max-Planck Inst.), F. Combes (Paris Obs.). K. Wakamatsu (Gifu U.), Y. Sofue (U. Tokyo), and A. Tomita (Kyoto U.) carried out a detailed multi-wavelength study of NGC 7217, a nearby spiral galaxy with three clear rings. The galaxy is important because it is extremely axisymmetric and yet has multiple rings. The nature of rings in axisymmetric galaxies is less clear than in barred galaxies of the same Hubble type. NGC 7217 is also interesting because of the discovery of counterrotating stars by Merrifield and Kuijken. Buta and coworkers found that at large radii, the light of NGC 7217 is dominated by a nearly round stellar halo, a characteristic missed in all previous studies of this galaxy. A spheroid/disk photometric decomposition yielded a spheroid-to-disk luminosity ratio of 2.4. The three rings of NGC 7217 can be explained in terms of the same resonances as those in barred galaxies. Buta et al. suggest that a weak oval distortion, detected by Fourier analysis in an I-band image, may be driving the rings.
Crocker, Buta, and summer REU student P. Baugus carried out a study of the distribution and properties of HII regions in 30 CSRG galaxies. The study revealed that most blue-light rings are lined by HII regions, and in some galaxies a ring is the dominant feature in H-alpha. The main result from this work is that the distribution of HII regions in inner rings is sensitive to the intrinsic shape of the ring. Extremely oval inner rings have HII regions bunched up near the ring major axis, while nearly circular inner rings have HII regions more uniformly distributed in azimuth. The difference is probably connected to the fact that inner rings lie close to but inside corotation. Periodic orbits in this region are elongated along the bar axis, and material is expected to spend more time near the intrinsic major axis of such an orbit. The effect is not due to orbit-crowding. Additional findings from this study include a double nuclear ring/pseudoring in NGC 1317, distinctive HII region distributions associated with outer rings and pseudorings, and relatively normal luminosity functions of HII regions.
Buta, graduate student G. Purcell, and summer REU student M. Lewis carried out a study of the velocity fields of NGC 3081, IC 4214, IC 4290, ESO 509-98, and ESO 566-24 using data obtained with the Rutgers Fabry-Perot interferometer. These five galaxies have stronger than average inner and outer rings and are excellent examples from the CSRG. The data reveal clear evidence for noncircular motions in the inner rings of the galaxies. A model of a uniformly precessing elliptical orbit fits the run of radial velocity as a function of position angle around the rings extremely well. If the rings are driven by the bar, which is likely from their morphologies, shapes, and alignments, then the precession rate equals the bar pattern speed. In collaboration with P. Rautiainen, the velocity fields are being compared with model test-particle velocity fields of barred and ringed galaxies published by Byrd and collaborators (AJ, 108, 476, 1994).
Buta and M. L. McCall (York U.) discovered two new nearby galaxies on deep CCD images obtained with the Burrell Schmidt telescope of Kitt Peak National Observatory. The images were obtained as part of a study of the IC 342 group designed to improve magnitude and extinction estimates, and thereby distances, to the members of the group. The two new galaxies are 18'-25' south of Maffei 1, a giant elliptical galaxy and one of the most massive members of the group. Both are late-type galaxies and are likely to be physical companions of Maffei 1.
White and Keel continued a program to measure the dust extinction in spiral galaxies by using the rare cases of galaxy overlaps, involving preferably E or S0 background galaxies and symmetric foreground spirals. For the approximately 10 pairs observed so far which are symmetric enough to allow a detailed analysis, they find that the typical interarm extinction is very small at the edge of the optical disk, rising to about one magnitude at 0.3 de Vaucouleurs radii. The dust within the arms can have extinction 1-2 magnitudes at virtually all galactocentric radii. The slope of the extinction curve is flatter than seen in absorption against individual stars in the Milky Way and nearby galaxies, most likely a signature of clumping of the dust. ISO observations of several of these pairs have been scheduled, with the aim of applying a classical test for clumping by detecting the same material in emission and in absorption. A few $K$-band observations have been obtained from KPNO, which show that indeed spirals are almost transparent at 2.2$\mu$; most of the action takes place across the optical band. REU student Chris Conselice performed extensive catalog and CD-ROM sky survey searches to make sure that all bright candidates were observed, and participated in a CTIO imaging run for some of the newly uncovered overlaps.
Davis and Keel, with J. Mulchaey (CIW) and J. Condon (NRAO), obtained long $ROSAT$ HRI exposures of two spirals suspected of interacting with a surrounding hot medium, NGC 2276 and NGC 1961. In NGC 2276, the integration is long enough to detect the brightest star-forming complexes as well as the underlying disk (one of the few cases in which this has been seen for a spiral). In neither case do they see direct evidence of clumps of hot gas near the galaxies, rendering a tidal origin for their peculiar structures perhaps most plausible.
Ryder carried out a chemical abundance study of galactic disks as a follow-up to his thesis work on massive star formation in galaxies. >From a comparison of oxygen abundances in star-forming complexes with the underlying stellar surface density, he was able to reproduce an observed correlation between these two quantities by building upon the same galactic evolution model used in his thesis to account for a similar relationship between star formation rate and star formation history.
Ryder, Buta, L. Staveley-Smith (ATNF), W. Walsh (U. New South Wales), graduate student H. Shukla, and visiting student H. Toledo (UNAM) carried out a study of the distribution and kinematics of neutral hydrogen in the southern ringed barred spiral galaxies NGC~1433 and NGC~6300. This is the most detailed HI study yet carried out of the SB(r) galaxy type. NGC 1433 is the prototype "resonance ring" barred spiral because it has all of the main ring types possible in a barred galaxy and these rings can be connected to specific orbital resonances with the bar. NGC 6300 includes only a single inner pseudoring but the ring cannot easily be connected to a specific resonance. Though both galaxies are classified similarly, the HI distributions are dissimilar in the sense that the HI in NGC 1433 follows the spiral structure very closely while that in NGC 6300 extends well beyond the optical structure.
Ryder collaborated on several other projects while at the University of Alabama. With A. Zasov (Sternberg Ast. Inst.) and V. McIntyre (CFA), Ryder carried out a study of the warped extended neutral hydrogen disk of NGC~157, a bright northern Sc galaxy. With R.~Beck (Max-Planck-Institut), R.~Haynes (ATNF), and M. Ehle (Max-Planck-Institut), Ryder collaborated on mapping the distribution of radio continuum emission in the disks of the spiral galaxies M83 and NGC~1566, and with E.~Schlegel and R.~Petre (GSFC), E.~Colbert (U. Maryland), L.~Staveley-Smith (ATNF), and M.~Dopita (MSSSO), he carried out a study of supernova 1978K in NGC~1313 using the Hubble Space Telescope.
White and summer REU undergraduate K. Dendy, is re-examining evidence for environmental effects on the X-ray emission from early-type galaxies, using an X-ray galaxy data set about twice as large as in his previous study with C. Sarazin (Virginia). The correlation of X-ray properties with environment is stronger in this larger sample: {\it at a given optical luminosity, galaxies which are relatively underluminous in X-rays tend to have more bright galaxies nearby. This correlation is in the same sense as found before by White \& Sarazin, but statistically more significant. Whether the environmental correlation is causal is not yet clear. If it is causal, it is also unclear whether the environmental agent of influence is galaxy-galaxy interactions or ram pressure stripping by group gas.
Graduate student S. Gessner participated in the Space Shuttle Astro 2 mission. As dwarf galaxy team leader, she has obtained and is currently processing Ultraviolet Imaging Telescope data on the galaxies Holmberg II, Sextans A and IC 2574.
Sulentic and collaborators P. Marziani and M. Calvani (Padova), D. Dultzin-Hacyan (UNAM) and M. Moles (IAA-Granada) completed the first direct comparison of the high (HIL) and low ionization (LIL) emission line properties in a large (n=52) sample of AGN. They compared ground based spectra of H$\beta$ with HST-FOS archival spectra of CIV$\lambda$1549. They find evidence for fundamental differences in the geometry/kinematics of the high and low ionization emission in radio loud and quiet samples. Radio quiet AGN tend to show blueshifted/asymmetric HIL emission, while H$\beta$ shows only small velocity excursions relative to the local rest frame. Radio loud AGN tend to show redshifted/asymmetric H$\beta$ emission while, CIV shows only small velocity excursions. The differences between HIL and LIL are largest for radio quiet objects. The difference is interpreted as due to a predominance of radial motion (bicone or wind) for the HIL emission (which is apparently disrupted by radio jet activity). The LIL emission is suspected to arise in a more isotropic distribution above the putative accretion disk (the redshifts frequently seen in the radio loud H$\beta$ profiles cannot be easily accounted for). A comparison of UV and optical FeII emission properties is also reported and evidence is presented that photoionization by Ly $\alpha$ fluorescence plays a major role in UV FeII production.
Sulentic, Calvani, T. Zwitter (U. Ljubljana) and visiting graduate student P. Romano (Italy) have completed a study of the wings of the broad H$\alpha$ emission line in a sample of about 100 AGN. H$\alpha$ was chosen for study rather than H$\beta$ in order to avoid the contaminating effects of FeII emission. The sample is strongly weighted towards radio loud objects. The study included a comparison of the red and blue wings in the profiles. About half of the sample shows different fits to the red and blue wings. Power-law fits are favored on the blue and log-like on the red wings. This is interpreted as strong evidence that many AGN have more than one significant source of broad line emission (i.e. a composite BLR). Approximately equal numbers of log-like and power-law fits were found in the half of the sample where both wings show a similar fit. The results do not support the predictions of simple radiating disk models.
Sulentic, Calvani, Marziani and Zwitter have been comparing the predictions of radiating accretion disk and bicone models for the broad line region in AGN. They recently compared new spectra for Pictor A with data obtained by Simkin and Filippenko, respectively, five and ten years ago. This radio-loud AGN now shows very broad (FWZI$\sim$ 24,000 km s$^{-1$) and double peaked Balmer emission lines. Accretion disk and bicone models for the broad line emitting region were compared with five and ten year difference spectra. The fits uniformly favor bicone models which, admittedly have more free parameters that allow adjustment of the fits. However, the discovery that the red and blue peaks in the Balmer lines vary out of phase virtually rules out emitting disk models for this source.
Sulentic and P. Romano are studying the line profile properties of the radio superluminal AGN. Interpreted as an orientationally constrained sample, the superluminals provide the best opportunity to remove the orientation-kinematics degeneracy involved in interpreting line profile data. In this light they provide the best opportunity to distinguish between the predictions of radial and rotational models for the broad line region. They are comparing HST-FOS and ground based spectra of the high and low ionization emission lines respectively in a sample of about 20 quasars with z$\leq$1.0.
Keel, graduate student B. Irby, and G. Miley (Leiden) continued work on active nuclei selected on the basis of IRAS colors. With the completion of spectroscopy and its analysis for objects selected from the Point-Source Catalog, emphasis is now on objects selected from custom processing of the IRAS Additional Observations (AOs), a set of nearly 1000 small fields around the sky observed in a sensitive slow-scan mode. About 150 candidates were found. Redshifts are still incomplete, but several AGN at redshifts $z=0.3-0.54$ have been found.
Keel has modeled optical and IUE spectra of the nucleus of the bright Virgo-cluster spiral NGC 4569, in light of the recent HST FOC observation showing its 2200-\AA\ nucleus to be unresolved at the 0.05-arcsecond level. A velocity dispersion from an INT spectrum obtained in superb seeing shows, by mass-to-light arguments, that the nucleus cannot consist of normal stars, so a significant amount of its light (virtually all of the UV emission) must arise in an active nucleus. The combined spectra are well fitted by a $\nu^{-1}$ continuum and a starburst about 1 Gyr old (consistent with the long-observed strong Balmer absorption in this object).
Keel and REU student P. Martini (OSU) analyzed INT spectra of several candidate hot spots in radio galaxies and quasars. Optical emission from jets and hot spots is a sensitive measure of the highest-energy electrons produced in such synchrotron sources. Some candidates were rejected as projected galaxies or foreground stars (3C 319, 3C 390.3, PKS 2135-147); evidence appears for a color gradient along the 3C 273 jet, and the 3C 247 hot spot shows an apparent synchrotron cutoff. The hot spots with no spectral turnover, compared to jets with optical emission, furnish a robust argument for local particle reacceleration at these locations.
Keel, F. Owen (NRAO), and J. Eilek (NMIMT) continued analysis of a set of fiber-array spectra of M87, covering the entire H$\alpha$ emission region. The velocity structure shows no particular global pattern, with kiloparsec-sized clumps showing coherent velocity structure. The spread in velocities is consistent with the sound speed in the hot gas. Since supersonic shocks, but not subsonic shocks, should be rapidly damped in the hot atmosphere, and there are strong arguments that the emission-line filaments are velocity-coupled to the hot gas, this suggests that the X-ray gas in M87 exhibits "turbulent" motions on kiloparsec scales at speeds up to the sound speeds. This bodes ill for radially-symmetric models of cooling gas.
Keel, F. Owen (NRAO), and M. Ledlow (NMSU) analyzed nearly 200 spectra from their sample of radio galaxies in Abell clusters to test one prediction of straightforward unification of FR I radio galaxies and BL Lac objects - that lower-level BL Lac objects, not beamed directly toward us, should be rather common among FR I galaxies if observed to low levels. Using the amplitude of the 4000-\AA\ break in the galactic nuclei as a measure of possible dilution by nonthermal cores, not a single core satisfying the definition of a BL Lac object as to break amplitude and line equivalent width was found. Only a handful of candidates which showed some evidence for diluted breaks appeared. Given the many arguments in favor of BL Lac --- radio galaxy unification, some interpretations of this result include FR II objects (which avoid cluster samples) as part of the parent population, and disks obscuring AGN seen far from the jet axis (and thus not appearing as blue continuum sources).
Keel and graduate student W. Wu have re-evaluated the merger rate for nearby spiral galaxies, using the relative luminosity functions of merger remnants and a new evaluation of field spirals, plus a timescale estimated from dynamical models. They find that pair members merge at a rate $\approx 3$ per Hubble time, so the field-spiral population merges at $\approx 0.3$ per Hubble time; a two-component model based on local environment is probably most appropriate. Wu is continuing simulations of the counts and colors of faint galaxies using various extrapolations of this rate with redshift, to place constraints on the history of merging and its importance in galaxy evolution. This also ties in to counts of pairs and multiplets among faint galaxies, especially in deep fields imaged with HST.
Keel and F. Owen (NRAO) extended their earlier work, using faint B3 radio galaxies as probes of galaxy evolution. Low-power radio galaxies are especially attractive probes, retaining the radio emission and optical line emission that allow their easy identification and redshift measurements while having weak enough AGN to make the galaxies themselves much more representative than the very powerful 3C/4C "monsters". Previous results to $z=1$ showed evidence for active star-formation up to about this time and passive evolution since. Recent observations have found additional objects to $z \approx 2.5$, giving the possibility of extending this work on the star-forming history of what seem to be fairly normal elliptical systems to these high redshifts.
Keel continued collaborations with R. Windhorst (ASU) on high-redshift galaxies, concentrating on HST imaging of the field around the faint radio galaxy 53W002 at $z=2.39$. Post-servicing WFPC2 images show that this object is separated into a symmetric $r^{1/4}$ profile and a one-sided blue cloud aligned with the radio source. Galaxy counts in this field show that the faint-blue-galaxy population is dominated by irregular systems. A spectacular finding from HST cycle 5 imaging in redshifted Lyman $\alpha$ is that 53W002 is part of a cluster, with $\approx 20$ compact Lyman $\alpha$ emission objects in this field. Further studies of this phenomenon, which must relate to the early history of "typical" galaxies, have been proposed.
White, with C. Day (NASA/GSFC/USRA), I. Hatsukade (Miyazaki) and J. Hughes (CfA) jointly analyzed cluster X-ray spectra taken with two different detectors, the $Einstein$ Solid State Spectrometer (SSS) and the $Ginga$ Large Area Counters (LAC). These broad-beam detectors have very different fields of view, which allows spatial information to be extracted in spite of each instrument's inherent lack of spatial resolution. Intracluster gas temperatures were found to decrease towards the center in each of four clusters (A496, A1795, A2142 \& A2199), due to the presence of cooling flows. More surprising was the discovery of substantial enhancements in metal abundances at the centers of two of the clusters, A496 and A2142. Very few clusters were previously known to contain substantial central concentrations of metals. More recent $ASCA$ observations of clusters have revealed several more clusters with central enhancements of metals. Whether these metal enhancements are due to stripping, galaxy morphological gradients, contamination by central dominant galaxies, etc. is a matter of ongoing debate.
White, with Davis and graduate student H. Shukla, is determining the X-ray surface brightness profiles for a large number of clusters in order to compare the energetics of gas with galaxies and dark matter in clusters. The apparent fact that cool clusters tend to have shallower X-ray surface brightness slopes than hot clusters has been interpreted as a sign of energy injection into cluster gas due to galactic winds at early epochs. Galactic wind energy would have a greater impact on the gas in cool clusters, inflating the gas more than in hot clusters.
Buta, Byrd, and graduate student J. Scott have been analyzing new images and Fabry-Perot interferometry of the leading arm galaxy NGC 4622, which was discussed in detail in a previous study by Buta, Crocker, and Byrd (AJ. 103, 1526, 1992). The objective of the new analysis is to use the velocity field in conjunction with near-IR and optical images to aid in establishing which arm(s) is actually leading in this galaxy, and to obtain a kinematical and dynamical model of the system, which may owe its structure to a retrograde encounter with a low-mass companion. Determining the near side of the galaxy is very difficult owing to the low inclination ($<$ 20 degrees) and distance of nearly 60 Mpc ($H_o$ =75).
Sulentic and graduate student D. de Mello (MS: Alabama; PhD: IAG-Brazil) in collaboration with R. Rampazzo and L. Reduzzi (Brera) and R. de Souza (IAG) completed a study of morphology and star formation history in four mixed-type (E+S) pairs of galaxies. Population synthesis of the early-type components showed that they contain a significant population of young stars. Two possible origins for this young population component were considered beyond the possibility that elliptical galaxies in mixed pairs are just different than field ellipticals. 1) Mixed pairs may represent the final stage in the collapse of a compact group, where the E component is the merger remnant and the spiral is the last intact original group member. 2) Transfer of gas from the spiral component (cross-fueling) may provide the gas for a star formation event in the elliptical. Some evidence was found to support the latter hypothesis. Sulentic is continuing this work with visiting graduate student H. Toledo (Mexico). They are attempting a statistical approach by studying the FIR luminosity function for a sample of about 150 mixed pairs. They are investigating both the optical morphology and the coadded IRAS data for this sample.
Keel, REU student L. Frattare (ASU and Wesleyan U.) and E. Laurikainen (Turku and UNAM) have performed a census of H II regions in about 50 pairs of interacting galaxies from narrow-band H$\alpha$ images. Statistics of the spatial distribution and luminosity functions have been constructed, with the luminosity functions often compromised by resolution effects. No strong pattern in location, with respect to the perturber, appears. However, there is a strong enhancement pattern with radius; the strongest star formation is often concentrated in an annulus with relative width about 20\% even when the galaxy shows no particular evidence of a corresponding stellar ring.
Keel completed a kinematic study of the response of both normal spirals and Seyfert galaxies to interactions. Paired Seyfert galaxies are distinguished by (1) being systematically the brighter member of the pair, well beyond what the luminosity-dependent probability of hosting an AGN can account for; (2) having rapidly rising rotation curves like typical Sa galaxies, even for Seyfert hosts of later type; and (3) showing smaller kinematic distortions than those in a complete sample of spirals in Karachentsev pairs. Contrary to straightforward theoretical expectations, Seyferts are about equally likely to occur in direct, retrograde, and polar encounters; the mere presence of a perturbation seems much more important than its details.
The long-slit velocity slices of ordinary spirals in Karachentsev pairs were used to derive various estimators of the central redshift, based on velocity at continuum and H$\alpha$ peaks, centroid of the central H$\alpha$ peak, location of best symmetry, and synthetic profiles analogous to the ones used in 21-cm H I measurements. The dispersion among these for a given galaxy suggests that, even for long-slit spectra of arbitrary accuracy, our ability to infer "the" central redshift is limited in accuracy to about $\pm 30$ km/s by the asymmetric structures seen even in galaxies with quite normal morphologies.
Keel, Byrd, and M. Klaric (Midlands Tech. College) have analyzed $K$-band images of 110 galaxies in pairs obtained with the late KPNO 1.3-meter telescope, largely to study the incidence of bars and nuclei which are too dust-shrouded or masked by star formation to appear in the optical. Masked bars such as that in NGC 1068 are not common, with only three galaxies (NGC 5278, 5427, and 7674) showing IR bars that are not reflected in the RC3 classification. Of these, only NGC 5278 does not show the bar in $R$ images. On the kiloparsec scales that these data probe, induced bars cannot be the whole story in feeding central star formation and nuclear activity.
The Physics and Astronomy Department hosted an international meeting on "Barred Galaxies'. This meeting, which was sponsored by the International Astronomical Union as Colloquium 157 and funded in part by the National Science Foundation and the Alabama Space Grant Consortium, took place from May 30 - June 3 in the Bryant Conference Center of the University of Alabama. The meeting was attended by 116 persons from 19 countries and was the first meeting devoted specifically to the subject of barred galaxies. There were ten oral sessions spread over five meeting days, and a total of 61 poster presentations.
Hardee is making final preparations for a workshop on Energy Transport in Radio Galaxies and Quasars to be held in September 1995.
The astronomy group uses a network of 11 Sun and three Silicon Graphics workstations. Through these machines there is access to several 600 dpi B/W Postscript printers, both high and low density 8mm tape drives, 4mm DAT drives, a 1/2-inch high-density tape drive and a color Postscript printer. The computer network shares a capacity of approximately 13Gbytes of group-accessible disk space as well as another 10Gbytes of reserved disk space. All the major astronomical software reduction packages are supported and several IDL licenses are available. The new SRC Equatorial Sky Survey and Palomar II survey have been acquired and are gradually moving towards completion. An extensive collection of data has been collected on CDroms, including the digitized sky survey (north and south).
The UA Astronomy Group maintains a World-Wide Web site (URL=http://www.astr.ua.edu/). It includes information on the graduate program, Research Experiences for Undergraduates (REU) program, an extensive collection of deep-sky images obtained by group members, and instructional material including lab exercises.
The Astronomy group REU program completed its two year experimental grant and was renewed for an additional three years. Under the initial grant eight students were supported by the REU program and two were supported by the University. In the renewal grant five students will be supported each year by the REU program.
All members of the astronomy group received support from NSF/EPSCoR grant OSR-9108761. Additional funding support from NSF and NASA is acknowledged. We acknowledge observing time at KPNO, CTIO, ESO, Lowell, San Pedro Martir, Calar Alto and supercomputing time at PSC.
Publications that appeared within the report period are listed below in alphabetical order irrespective of institutional affiliation. Abstracts and popular articles are not included. Invited review papers are indicated by an asterisk.
Prepared by Dr. J Sulentic