Galactic mergers are some of the most dynamic events in the universe. Tidal (just gravitational) forces strip stars and gas out of a galaxy into elegant tidal features while other material is compressed causing copious star formatio n. The fact that all these effects are the result of simple gravity is astonishing.

For many years, scientists did not believe that the features seen in peculiar galaxies could possibly be the result of Newton's simple gravitational law, Fg = G M m / r^2. (Newton, 1726, Royal Academic Society) There were various theories that related these features to the jets seen in distant radio objects. Looking at Figure 1 (left), a picture of NGC 4676, or "The Mice", one can understand where they would get such an impression. The northern tail extends straight out from the top, why shouldn't that be a jet?

Astrophysicists were led back to simple gravity by Alar and Juri Toomre in 1972 (ApJ 178, p. 623). The Toomres ran 3-body code to simulate galactic mergers. The three bodies are two galactic potentials and one test particle that moves at the behe st of these combining potentials. In one simulation, it is possible to have multiple test particles. Each particle tests a different starting position. On the order of 100 test particles were thrown into these potential wells.

Figure 2: Evolution of interacting galaxies using 3-body method, note development of "tails" and "bridges" (Toomre & Toomre 1972)

The features that formed by running the model were arcs streaming out of the interacting galaxy as shown in figure 2. The Toomres called these arcs "tails". They also noticed similar features coming out the other side of galaxies which they called "anti-tails", but which are commonly called "bridges" because they appear to be material connecting two galaxies. The Toomres’ found that they could manipulate the size of these tidal features by altering the interaction geometry. There are four main parameters, the size of the orbit (or distance between the two galaxies), the ellipticity of the orbit, the inclination angle of each galaxy in relation to the orbital plane, and the amount which you rotate each galaxy about the axis perpendicular to the orbital plane, or argument of periapse. This geometry is shown in figure 3.

In general, the tail will be ejected in the plane of the galaxy, so if the galaxy is inclined to the orbital plane, the tail will come out at that inclination angle. As the inclination angle becomes greater (that is, as the galaxy’s plane become s more perpendicular to the orbital plane, assuming one starts with the galaxy rotating in the same direction as it is orbiting the other galaxy), the tail decreases in size. And then as that inclination angle becomes greater than perpendicular (the gala xy is rotating the direction opposite to the orbital direction), no tail is formed.

The other angular parameter with which one can fiddle, the argument of periapse, does not have as large an effect on the creation of the tidal features. The main effect is in viewing angle or the angle at which the tail appears to be in relationship t o the other galaxy’s tail. This is useful in the case where tails are coming out of both galaxies in an interacting system and one needs to have the two tails relate to one another at a certain angle. A much slighter effect is a widening of the tai l when the argument of periapse is increased.

It should be noted that the Toomres’ were not the first to use computers to simulate the gravitation effects on interacting galaxies. Nine years earlier, Pfleiderer (1963, Zs.f.Ap, 58, 12), a single grad student was unsuccessful in the formation o f tidal features because his galaxies moved past one another faster than they could have interacted.

Using all these tools, the Toomres’ were able to match the morphology (visual shape) of galaxies. They matched four systems: two with spirals and companion, Arp 295 and M51 (the "Whirlpool Galaxy"), and more typical early stage mergers, NGC 4038/ 9, "the Antennae", and NGC 4676, "the Mice".

At the time these matches were done, there was only good morphological data. Techniques for obtaining kinematics (velocity) data were confined to one-dimensional long slit spectra. These could only give kinematics in especially bright regions and the re only along a line. Today, Fabry-Perot interferometers can be used for bright optical sources while spectral line data collected by radio interferometers can be used for regions rich in neutral Hydrogen gas. Both of these techniques provide velocity i nformation for each point on an image. Thus, one can know the velocities at which points in the tail are moving relative to one another.

This information has made it possible to match more than morphology. My project stemmed from this desire to match n-body simulations to detailed observations. Several systems have already been matched using kinematics data including a rematch of NGC 4676 and NGC 7252. One might wonder at the need to rematch what the Toomres’ have already matched. It turned out that this additional exploration lead to the discovery that material in tails that is closer to the galaxy has a smaller apocenter than material farther out in the tail. This return of tail material to the merging body may cause fine gas structure in the resulting elliptical galaxy.

After matching such relatively simple interactions, there was a desire for something more challenging. In a survey of 14 interacting systems using the VLA, three particularly odd systems were found. Arp 299 was one of these. Using optical images, this galaxy looks like a relatively normal interacting system, with one galaxy producing a tail. But, when the VLA gas data was laid over the optical, the gas tail appeared separate from the optical. Reasons for this were guessed at: A) A superwind, created by the outflow from the numerous supernovae ignited by the galactic interaction could have created a hot, gaseous medium that would ram pressure strip the gas out of the tail, leaving the stars as an optical remnant. While this seems reasonable, the two tails seem like fully formed tidal features, each having been created on it’s own. This leads to a second possibility, B) The two tails are actually one very wide tail, that appears to be two because of projection effects. In this case, an explanation for the separation of materials needs to be found. In the Toomres’ model, outer rings of test particles begin the formation of the tail, followed by inner rings of test particles. Thus, the outside parts of tails are composed of the material from the outside of a galaxy while the inside of tails is from further in the galaxy. In the disk of spiral galaxies, optical surface brightness falls off exponentially, while the gas density falls off at a much less precipitous rate. Combining these two observations, it seems plausible that the outer part of a tail would be composed of primarily gas while the inner part would have a higher optical content. Then, if the galaxy were oriented towards us just right, we would see this disparity between tails.

It was this second possibility that led to my investigation of n-body simulations. What needed to be determined was whether or not just gravity could create a wide tail with different makeups at different locations. To do this, we first needed to run gravitational n-body simulations. The code that we used was created by Josh Barnes and uses a tree as a data structure to hold the particles that make up the galaxy. With this method, it is possible to quickly run a true n-body simulation. What separates an n-body simulation from a three-body simulation is the changing of potentials. In a three-body simulation, the potentials of the two massive bodies remain constant, while the third particle is effected by the changing summation of these 2 constant potentials. But, in an n-body, the potential energy contours are recalculated with each iteration based on the location of all n particles. The n-body method is better able to model the intricate gravitational effects than 3-body simulations.

The first step in running any simulation is to narrow parameter space by making assumptions based on observations. In our case, we knew that one of the galaxies did not create a tail. From the Toomres’, we know that this galaxy must be spinning with a retrograde orientation or be perpendicular to the orbital plane. A couple of initial attempts showed that the perpendicular orientation produced the proper morphology. The separation of the galaxies at periapse also needed to be determined. At too great a distance the galaxies would barely interact, too close and the interaction would happen too quickly producing an undersized tail. One-half of a scale unit was determined to be the proper distance. The remaining parameter to be manipulated was the angle at which the tail-forming galaxy should be to the orbital plane. This parameter could not be so easily determined. The tails formed by almost every inclination angle, except for extreme values (i > 50š), formed a tail with approximately the same morphology. fairly intense investigation of various inclination angles paired with various arguments of periapse did not produce significant variation. The most important piece of information found in this trade study was that none of the orbit al parameters created a wide tail. This finding is not yet a conclusion, though. The galaxy models that we were using contained hollow centers. It may be that these centers are what creates the tail width. The piece of evidence supporting this is Figu re 4 which is a much larger simulation that includes the central disk particles which were left out of the simulations I ran this summer. The large particles represent those from the outer half (?) of the disk, while the small particles are those from th e center of the disk. The greater tail width and separation of inner and outer disk particles is noticeable. Thus, a larger simulation needs to be run.

Figure 4: 64K particle Simulation. Small particles are from inside of disk, large from outer portion

If a larger simulation is not successful, there is another possibility for modeling Arp 299. Many disk galaxies that have been observed have a warp, meaning that a given ring of particles has a different inclination than more inner rings, and as a result is projected onto a different plane. While doing these simulations, we came to realize that a number of the 14 tailed systems had much less drastic bifurcated or parallel features, and that this might be a common phenomena. It seems plausible tha t a warp would cause a split tail, each component having a slightly different observable make up.

The viewing of these 6 dimensional simulations (3 spatial and 3 velocity components) is not a trivial matter. A program to do this viewing had been created prior to my arrival at NRAO, but its controls were cumbersome. To alleviate this problem, I a dded a graphical user interface to control the rotation, scaling, and other properties of the model. The experience in graphical user interface programming was invaluable.

The modeling of interacting galaxies has advanced since the days of Toomre and Toomre. Now we can examine kinematics data rather than just morphological and make more accurate models. We have also found out details about the process of galactic inter action which are critical to our understanding of the evolution of all types of galaxies.

• Arp 299 HI data
• Model match to outer filament
• Model match to inner filament

3) Tidal Dwarf Formation in Tidal Tails in collaboration with J. Hibbard (NRAO) It has long been known (eg Schweizer 1978) that clumps of star formation with properties similar to dwarf galaxies exist within tidal features. Some of these features show enhancements also in the underlying stellar light and in the neutral gas column density and line-profile width, leading to the suggesting that they will evolve into independent dwarf galaxy satellites. This suggestion is supported by detailed numerical simulations which demonstrate that self-gravitating structures can indeed develop in tidal material torn of during galaxy interactions. However, tidal features which simultaneously exhibit enhancements in the HI column densities, HI line widths, optical surface brightness, and H-alpha emission are rare. This has not deterred many investigators from applying a Jeans-mass criteria and some reasonable range of M/L to any and all HI or optical enhancements, and claim such features are Tidal Dwarfs. There is never any mention made whether other non-enhanced regions of the tail satisfy the same requirements. The goal of this study is to see if the Jeans criteria is generally satisfied within tidal features, or if it only occurs at regions with an underlying optical condensation. In this project, the student would use my existing data on 14 tailed systems and use the HI momO, moml maps and calibrated B+R images to evaluate the Jeans mass and MHI/L of various features, to see if their are indeed any special regions within the tails. These data include 4 well studied tidal dwarfs as well as plenty of other knots and such which can be intercompared. http://deneb.physics.carleton.edu:8080/people/greg/summer.html

4) The Local Environment of Star Forming and Active Galaxies in collaboration with J. Hibbard (NRAO) and perhaps J. Condon (NRAO) In this project, the student would correlate the IRAS and NVSS data bases with the Tully Atlas of nearby bright galaxies - a volume limited catalog of galaxies brighter than Mb<-16 within 40 Mpc. In particular, we are interested in whether IR or Radio bright galaxies prefer certain types of environments, as measured by the local density parameter listed by Tully. Other density measures should also be examined. We would also examine morphologically peculiar galaxies in the Tully Atlas and examine if these are distributed differently than the radio or IR bright galaxies.

Andrew West , of Haverford College

works with Ken Kellerman
on

Application of the Angular Size Redshift Relation

The NRAO Very Long Baseline Array (VLBA) has been used over a period of 4 years to obtain repeated images of a large sample of quasars and active galactic nuclei (AGN) at 15 GHz. The resolution is better than 1 milliarcsecond (mas) and we are able to located the relative position of small features with an accuracy better than 0.01 mas. This corresponds to about 0.1 parsecs at cosmological distances (H = 65 km/sec/Mpc). Thus we are able to determine component motions even for sources with apparent subluminal velocities.

In most quasars and AGN, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c. The median observed apparent velocity is about 5c, corresponding to a typical intrinsic velocity of about 98 percent of the speed of light oriented about 10 degrees to the line of sight. There is some evidence for accelerations along the jet, but no evidence of infall into the central engine As indicated by previous studies, (Vermeulen & Cohen 1994, ApJ, 430, 467; Vermeulen 1995, PNAS, 92, 11385) consideration of the effects of Doppler boosting suggests that the distribution of apparent velocities is not consistent with simple ballistic models or that there is a significant spread in intrinsic velocity.

We have examined the apparent angular velocity-redshift relation which offers a potentially powerful test of world models. Our data are consistent with standard Friedmann models and somewhat favor a deceleration constant between 0 and 1/2 for a zero cosmological constant. Further observations of a bigger source sample, especially at large redshift, along with an increased time base, may lead to more precise constraints on world models.

This work was presented at the 193rd AAS Meeting Session 107

NRAO/Socorro 1998 Research Experience for
Undergraduates (REU) Program

The summer REU program at NRAO/Socorro in 1998 consists of 4 main categories of activity:

1 - student research projects, in collaboration with an NRAO advisor
2 - lectures to the students by NRAO staff members
3 - a joint student project, involving observations with the Very Large Array (VLA)
4 - guided tours of the VLA, given by the students on weekends

The 1998 REU program at NRAO/Socorro is under the direction of James Herrnstein , and Tim Bastian. Dr. Herrnstein is a Jansky Postdoctoral Researcher at NRAO/Socorro, and Dr. Bastian is a member of the scientific research staff.

Student Research Projects

Each of the REU students will work with one or more advisors on one or more projects throughout the summer. This is the main focus of the program, and the majority of the students' time will be spent on these research endeavors. These projects involved observing, data reduction and analysis, equipment development, and theoretical studies. At the end of the program, each student gave a lunch talk explaining the main project(s) he or she worked on during the summer. The possibility exists for the students to present their original research at scientific meetings deemed appropriate by their advisor(s). Following is a detailed list of the specific projects carried out by the NRAO/Socorro REU students:

Melissa Nysewander of Agnes Scott College
works with Miller Goss
on

The Distant HII complexes NGC 3576 and NGC 3603

NGC 3576 (l = 291.3^o, b = -0.7^o) and NGC 3603 (l = 291.6^o, b = -0.5^o) are optically visible, luminous HII regions located at distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australian Telescope Compact Array (ATCA) observations of these two sources in the continuum and the H90alpha, He90alpha, C90alpha and H113beta recombination lines with an angular resolution of 7" and a velocity resolution of 2.6 km s^-1. All four recombination lines are detected in the two sources. Broad radio recombination lines are detected in both NGC 3576 (dV_FWHM > 50 km s^-1) and NGC 3603 (dV_FWHM > 70 km s^-1). In NGC 3576 a prominent N-S velocity gradient (~30 km s^-1 pc^-1) is observed, and a clear temperature gradient (6000 K to 8000 K) is found from east to west, consistent with a known IR color gradient in the source. In NGC 3603, the H90alpha, He90alpha and the H113beta lines are detected from 13 individual sources. The Y+ (He/H) ratios in the two sources range from 0.07+/-0.01 to 0.21+/-0.05. The H113beta/H90alpha ratio in NGC 3576 is close to the theoretical value, suggesting that local thermodynamic equilibrium (LTE) exists. This ratio is enhanced for most regions in NGC 3603, which may be the result of high optical depth or stimulated emission. We compare the morphology and kinematics of the ionized gas at 3.4 cm with the distribution of stars, 10micron emission and H₂O, OH, and CH₃OH maser emission. These comparisons suggest that both NGC 3576 and NGC 3603 have undergone sequential star formation.

This work was presented at the 193rd AAS Meeting Session 16

Joshua Eisner of Harvard University
works with J. Herrnstein
on

The High Mass Star Forming Region W51-IRS2 in W51

This past summer, I worked with James Herrnstein, Lincoln Greenhill and Karl Menten on the W51IRS2 star formation region. The region is particularly interesting in that it is only one of three star formation regions where an SiO maser source has been detected (in comparison, SiO masers have been detected around tens of late-type stars). The summer research consisted of the reduction of VLA data from March 1998, and some data from July 1998. We had data at 22 and 43 GHz, and we has both continuum and spectral line data at each of these wavelengths. I spent the first 6 weeks or so of my time reading the current literature, and reducing the data using AIPS, IDL and IslandDraw. The remaining 6 weeks were spent in analyzing the results of the data reduction. This analysis was very fruitful, and there is much more to be done, because of the richness of the data set: the final images contained H2O masers, SiO masers, a compact HII region, and an ultracompact HII region. We found that the dominant region of water maser activity, which also corresponded to the location of the SiO maser source, was not coincident with any of the radio continuum sources, nor was it coincident with any IR point sources. We attempted to come up with models to explain this result, and the best model we have come up with thus far involves an outflow from a protostar (or possibly multiple protostars).

This work was presented at the 193rd AAS Meeting Session 71

Scott Schnee , of Columbia University
works with Rick Perley
on

Array Studies

Last summer I worked with Rick Perley on the flux density scale. I learned to edit and calibrate continuum data in AIPS. I worked with about 15 sources, mostly quasars and Seyferts, but also four planets. The data was taken in all bands from 90 cm to 7 mm. From the calibrated data I made images of the sources to determine the measured flux density of these calibrators. By determining the ratios of the flux densitiies of these secondary calibrators to 3C295 and using the results from a paper by Baars et al to calculate the absolute flux density of 3C295 we were able to determine the true flux densities of the secondary calibrators. By comparing these values to those determined in previous years by Rick Perley we checked for variability in these calibrators.

Charlie Silver , of Columbia University
works with Greg Taylor
on

The Nucleus of NGC 1275

During the summer of 1997 Charlie Silver worked with Taylor on a low-frequency study of 3C84 using the VLBA. The surprising result from those observations was the discovery of a steep-spectrum "milli-halo" surrounding the parsec-scale core and jets (Silver, Taylor \& Vermeulen 1998, ApJL, submitted). The relativistic particles that produce the milli-halo may diffuse out from the core and jets, or may originate in Supernovae. Charlie and I plan to search for "milli-halos" in starbursting systems like Arp220 \& Mrk231 (where a structure that might be a milli-halo has been reported by Carilli, Wrobel \& Ulvestad 1998), and in a few other nearby AGN. We hope to work on this project this summer, although the observing time for it has not yet been granted.

Our primary research project for summer 1998 will be the study of 5 intra-day variable (IDV) sources. These compact extragalactic radio sources are known to exhibit variability on time scales as short as hours (Heeschen \etal 1987; Wagner \etal 1990). The fundamental nature of these variations is uncertain, including the question of whether they are intrinsic to the source or caused by extrinsic propagation effects. If intrinsic, these variations challenge the conventional beaming models that explain the origin of the radio emission. Existing observations have been mostly confined to single-dish monitoring of flux density and polarization at centimeter wavelengths, often combined with simultaneous optical photometry. In the fall of 1996 we observed 5 IDV sources over 8 days with the VLBA (project BK32), the 60" optical telescope at Palomar, and the ROSAT X-ray telescope. The VLBA data, which includes polarimetry, should allow us to discriminate between intrinsic and extrinsic mechanisms. Charlie will lead the reduction of the VLBA data. The goal of this project is a publication in the ApJ.

Jane Rigby, of Penn State University,
works with K. Anantharamiah
on

Recombination Lines in Active Galactic Nuclei

Jane R. Rigby, of Penn State University worked with K. Anantharamaiah Recombination Lines in External Galaxies The student learned about Seyfert and starburst galaxies, synthesis imaging, and radio recombination lines (RRLs). She then searched two galaxies for radio recombination lines, which probe ionized gas in nuclear regions. Radio recombination lines are produced in ionized gas. In a few percent of collisional recombinations, the electron is captured into a very excited state. The electron then cascades down, quantum level by quantum level; these small energy changes resul t in emission of radio photons. Though these lines are weak, they are largely unaffected by dust; therefore, they can be used to probe the conditions and kinematics of ionized gas, particularly in the nuclear regions of starburst and Seyfert galaxies, re gions which are optically obscured. Observing extragalactic recombination lines is an observation challenge because it requires high dynamic range spectral line observations. The student reduced, calibrated, and analyzed spectra from two galaxies: ultraluminous IR galaxy Markarian 251 ( a Seyfert 1); and elliptical galaxy NGC 1052. Both have bright nuclei, which could produce stimulated emission of recombination lines. In Mrk 231, a "hole" in HI absorption against the central core, combined with a turnover in the nuclear spectrum, suggested the existence of radio recombination lines. However, at X and L bands, detection is uncertain (sensitivity of ~0.5 mJy.) For the observed emission measure of EM = 10^7, if the observed spectral turnover were due to free-free absorption, for any temperature, we would have seen RRLs from the central VLBI core component. Since we saw no strong lines, we conclude that the observed sp ectral turnover must be due to syncrotron self-absorption, not free-free absorption. For the southern VLBI core component, we conclude that the gas density is above 10^3 cm^-3. We also looked for recombination and OH lines in elliptical galaxy NGC 1052. No elliptical galaxy has been searched for RRLs, because elliptical galaxies have little gas. However, NGC 1052 has good evidence for ionized gas: it is known to have an inne r, ionized gas disk; it has strong, extended Halpha, OII, and NII emission; and it has water megamasers. We searched for recombination lines at X band in NGC 1052, but found none down to a sensitivity of ~2 mJy. We also searched for OH emission in NGC 1052 at L band and found a possible detection at v=1600 km/s. (NGC 1052 is receeding at 1500 km/s.) VLA observations to confirm this observation were recently made. If this line is real, we have detected molecul ar gas falling into the center of NGC 1052 at 100 km/s, presumably feeding the AGN.

John W. Weiss , of Carleton College
works with Tim Bastian and Ketan Desai
on

The Nature of Microturbulence in the Solar Wind

My time at NRAO was spent researching scattering inthe ISM and solar wind. Usually, scattering would be an annoyance to astronomers, since it degrades images. However, if one is interested in the scattering medium (in our cases, the ISM and solar wind), scattering can give us a great deal of information on the medium. Among other things, once can learn about how the turbulent pockest are distubted in terms of their sizes, the alignment of anisotropies and the density of the medium. This is how I spent my summer. With Ketan Desai, I examined scattering in the ISM by looking at quasar 2050+365, a double source. In this case, we are taking images taken a few years apart an measuring how the seperations change. This differential wander measurement may allow us to reject or support certain models of the turbulence in the ISM. With Tim Batian, I looked at the scattering of quasar (point) sources as they passed through the solar wind. By measuring the amount of scattering, we were able to get a measurement of the power of the scatter, as well as picking up some density and anisotropy information. My summer was spent, then, learning about synthesis imaging, learning the specifics needed for scattering (by no means an easy field, but that's what made it so exciting!), and doing the anove research. It was an enlightening, fun, and well-spent summer.

Michael Crawford, of New Mexico Tech
works with Bob Hjellming
on

The Appearance of Highly Relativistic Radio Sources

Under the supervision of Robert Hjellming, Michael Crawford worked on a project to compute, and display as images, the appearence of sources moving at relativistic speeds. It was part of an ongoing effort the compare models of Galactic jet sources assocatiated with X-ray binaries and transients with VLA, VLBA, Merlin, and ATCA radio light curves and images. The source models combined precessing jet kinematics to compute the apparent location of jet material, Doppler boosting of the intensities to predict the apparent surface brightness of the jet; and convolution with elliptical beams to smear the predicted image to the resolution of instruments like the VLA or the VLBA. All the computations were done on a PC using MathCAD. Once the computations were being done correctly, Michael was able to predict the correct general appearance of the jets in SS433 and the Feb. 1997 one-sided jet ejection during an 11 Jy flare in Cyg X-3. The resulting programs allow one to predict the appearence of adiabatically expanding relativistic jets for any values of the kinematic parameters. The next stage of the project was to compute the radio light curves of the sources modeled in this way, but that was just begun when the 12 weeks ended. The light curve computation has since been completed by Robert Hjellming, and is being successfully applied to transient events in a large number of Galactic X-ray binaries and transients.

,Josh Bloom of Caltech
works with Dale Frail
on

Radio Afterglows from Gamma-ray Bursters

A student might be expected to work on several things including (1) the monitoring of known radio afterglows (2) searching for radio emission from new bursts, (3) modeling the flux evolution of the synchrotron-emitting fireball, (4) developing observing/reduction code to simplify the process of obtaining radio data on GRBs. Depending on the student's talents I can see this as a pure observing project, a theory project or a partial software project. The detection of an afterglow would almost certainly involve the publication of paper.

Andreea Petric, of Columbia University and NMIMT
works with Michael Rupen
on

Line Widths and the Universal HI Profile in NGC 1058

We use HI observations to trace the vertical motions of the neutral gas in the face-on spiral galaxy NGC 1058. The combination of high sensitivity and low inclination permits the accurate measurement of line profiles at high spatial and spectral resolution, virtually uncontaminated by planar motions associated with disk rotation or spiral density waves. Although the width of the line profile varies from one beam to the next (FWHM= 14 -- 30 km/s), some global trends are evident. The lines are broader in the central 1.5arcmin, where the star formation is most vigorous. However, outside this region the linewidths are uniformly smaller in the spiral arms than in the interarm regions; and there is a general anticorrelation between H$\alpha$ emission and broad profiles. The line profiles are not Gaussian, and hence cannot solely be determined by single-temperature thermal broadening. The surprise is that the observed shape does seem to be universal, in the sense that the line profiles at every pixel, when scaled by their FWHMs, appear identical. This implies that whatever mechanism determines the relative distribution of kinetic energies operates in the same fashion throughout the galaxy -- both within and beyond the stellar disk, in quiescent regions and in regions with active star formation.

This work was presented at the 193rd AAS Meeting Session 8

Lectures, etc...

Several lectures about radio astronomy and interferometry will be presented, allowing the students to obtain a good understanding of the technique. Talks were also given on general topics in astronomy, presented by members of the scientific staff. The astronomy talks were designed to give the students an understanding of what sort of research goes on at NRAO, and in radio astronomy in general. The detailed list of lectures and events for the summer is in the following table.

1998 Calendar of Events -- New Mexico

June 1998

June 17, 8:30 am: Sixth Summer School in Synthesis Imaging begins at Workman Center.
June 23, 8:30 pm: Sixth Summer School in Synthesis Imaging ends at Workman Center.

July 1998

July 10, 1000 Radiative Transfer Bryan Butler
July 14, 1000 Scattering Ketan Desai
July 16, 1000 Radio Galaxies Frazer Owen
July 21, 0900 HI in Galaxies Michael Rupen
July 24, tbd Gamma Ray Bursters Dale Frail
July 28, tbd Recombination Lines Anantha
July 30, tbd Masers Jim Herrnstein

August 1998

Aug 4, tbd Galactic Sources Bob Hjellming

Green Bank, West Virginia (NRAO 43m and 100m Telescopes)

Students conducting their research at the NRAO Green Bank Site in West Virginia included the students in the list below, along with others. The program at Green Bank is under the direction of Dr. Ron Maddalena.

1998 Summer and co-op Students (L-R): Marc Apgar, Jennifer Lockman, Steve Hicks, Naomi Bates, Lisa Wray, and Nicole Wiersgalla

1998 Summer and co-op Students and NRAO staff members after a caving trip (L-R): Frank Ghigo, Jennifer Lockman, Pat Matheny, Ron Maddalena, Nicole Wiersgalla, and Steve Hicks.

1998 Calendar of Events -- West Virginia

June 1998

June 1 - 5 -- Future VLBI Missions from Space (workshop)
June 14 - 27 -- Teacher's Institute
June 12: Mark Clark: M&C architecture
June 17: Toney Minter: Turbulence
June 26: Lisa Wray: Radio Astronomy Receivers
June 29: Frank Ghigo: Earth Wobble

July 1998

July 1: Dan Pedtke: OVLBI
July 7: Jim Braatz: Active Galactic Nuclei
July 9: Rick Fisher: Antennae
July 14: Dana Balser: Radiative Processes
July 16: Joe McMullin: Spectroscopy
July 21: Ron Maddalena: Holography
July 22: Jay Lockman: HI in the Milky Way
July 23: Gary Anderson: History of Radio Technology
July 27 - 29 -- Workshop on Science with the Green Bank Telescope (workshop)

Naomi S. Bates, of Princeton University
works with Ron Maddalena
on

Extragalactic Hydrogen Wings

The project concentrates on the analysis and interpretation of observations of the wings of HI profiles of galaxies. The project attempts to characterize the component of gas that has a higher velocity dispersion than the gas typically attributed to galaxies. That is, the project attempts to study what in our galaxy would be called high-velocity clouds or a warm/hot component to the neutral ISM.

Nicole Wiersgalla, of the University of Minnesota
works with Dana Balser and Tony Minter
on

Turbulence in HII regions and the Diffuse Ionized Gas (DIG)

The student will use both observations and models to explore the role of turbulence in classical HII regions which are usually confined to the galactic plane and the diffuse ionized gas (DIG) which pervades the galaxy. The observations will consist of both single-dish and interferometric radio recombination line data. Additional observations have been proposed using the Virginia Tech Spectral Line Imaging Camera (SLIC) which the student may use to measure the [SII]/H alpha abundance ratio; these measurements can constrain possible heating mechanisms. A computer program called CLOUDY will be used to simulate the physical conditions of the ionized gas. Depending on the student's background a specific aspect of the project could be emphasized.

Laura Woodney, of The University of Maryland
works with Joe McMullin
on

Chemistry in Comets

Laura Woodney worked on two separate projects while at Green Bank. The first was finishing a paper for submission to Icarus on flashes off of Jovian satellites during the D/Shoemaker-Levy 9 impact. It was predicted that these flashes would be quite bright, however, nothing but tentative detections was reported from various observers around the world. In this paper Woodney examines her own possible detections, and discusses the physical implications the weak or non-existent reflections of light from the impacts have on the impact conditions.

Woodney's main project was to examine the morphology of the relationship of HCN and CN in Comet Hale-Bopp. Hale-Bopp is the first comet for which there has been simultaneous high resolution imaging of this parent and daughter pair, so that this is the first time the morphology of their relationship can be examined. A greater understanding of the HCN/CN relationship will place constraints on the much disputed nature of other proposed CN parents. Of particular interest is what these HCN images can reveal about the nature of the association of the nuclear HCN source with the CN jets since there is enough spatial resolution to detect HCN jets.

The HCN maps of the J=1-0 transition at 3mm were made with the Berkeley-Illinois-Maryland Association (BIMA) Array in Northern California, with a beam size of ~ 9''. The maps have been created so that they are phased with the rotational period of the comet to reduce the smearing of any spatial features seen in the maps. At each phase examined, data spanning several days has been summed both to increase the signal to noise ratio and give more complete uv coverage than a 1 to 2 hour observation would provide on its own. When these maps are compared to similarly phased narrowband CN images obtained at Lowell Observatory taken near or at the same time as the HCN images it is clear that at least some of the CN jets are a product of HCN jets, but that there is probably a secondary source. A likely candidate for this secondary source is dust, however more exotic parent molecules such as C₂N₂ as suggested by Festou et al. (1998) are also possible.

Keri Eberhardt, of The University of Nebraska
works with Glen Langston
on

Highly Redshifted Molecular Lines

Marc Apgar, of West Virginia University
works with Wes Grammer
on

Green Bank Telescope Diagnostics

Jennifer Lockman, of College of Charleston
works with Jay Lockman
on

Hydrogen Emission in the Milky Way

Jessica Golub, of Vassar
works with Jim Braatz
on

Water Masers in Nearby Galaxies

1. Galaxies which have been searched for H2O masers, but not detected, tend to get lost in people's work and go unpublished. As there are many groups now searching for masers, it would be useful to have a list of all undetected galaxies. The student could solicit such lists from each research group, organize it and distribute it back to them. I have actually started this already, and I think it would be a good short-term project to complete it.

2. Conduct a study of nearby, edge-on galaxies. I have two interests in such sources. One, they may be candidates for new maser searches, but it would be ideal to identify any signs of activity (compact radio cores, IR excess, ...) in them before targeting them for observation. The second, I have found an unusual discrepancy in the inclination angle distribution of AGNs, and it could be compared to other galaxy populations.

3. To calibrate, reduce, and organize several rounds of 140-ft observations done in the past year. I've done preliminary reduction already, but I'd like to have some more detailed spectra and studies done on several H2O maser observations. This would involve some unipops work.

This work was presented at the 193rd AAS Meeting Session 6

Tucson, Arizona (NRAO 12m and VLBA Telescopes)

Students conduct their research at the NRAO Tucson Site in Arizona. The program in Tucson is under the direction of Jeff Mangum. As the NRAO offices are across the street from KPNO/NOAO offices, the REU group shares in the activities of the NOAO REU program there.

In addition to the general activities carried out at the KPNO/NOAO offices, the NRAO and KPNO/NOAO REU students participated in two group activities organized by the NRAO staff. The first was a night at the 12 Meter Telescope, where the REU students spent a day at the 12 Meter Telescope. Following a tour of the telescope and lab facilities, where the students were introduced to the instrumentation used in millimeter wavelength astronomy, the students were given the opportunity to participate in some actual millimeter wave astronomical observations. With this experience the students got an introduction to the observing techniques used in millimeter wavelength astronomy.

The second general activity was a lecture series on millimeter wavelenth astronomy given by members of the NRAO scientific staff. Three lectures were given. Jeff Mangum gave a presentation on millimeter wavelength research into the properties of objects in the Solar System and molecular clouds. Darrel Emerson gave a lecture on millimeter wavelength observing techniques. Finally, to complete the survey of millimeter astronomy, Simon Radford gave a lecture on extragalactic astronomy at millimeter wavelengths.

Three REU students conducted research at the NRAO Tucson site in Arizona during the summer of 1998.

The following are detailed reports describing the work done by each REU student at NRAO Tucson.

Beth Biller, of Swarthmore College
works with Tamara Helfer
on

The Physical Conditions and Structure of HCN and CS Emission in the Milky Way Plane

In a recent study of the large-scale HCN and CS distribution and emission properties as a function of location in the Milky Way, Helfer \& Blitz (1997) used the NRAO 12m to observe the 3 mm emission from HCN, CS, and CO along 30 positions in the first quadrant of the Milky Way. Emission from HCN and CS is surprisinly common; however, the lines are relatively weak, which suggests that either the transitions are subthermally excited or that the filling fraction of dense clumps is small. This REU project is divided into two parts, either of which could satisfy a completed project should time pressures prevent combining the two: (1) An analysis of NRAO 12m CS J=3-2 observations towards selected lines of sight; these observations are to be combined with the CS J=2-1 data in order to model quantitatively the physical conditions in the emitting gas. (2) The reduction and analysis of BIMA observations of HCN, CS, and CO toward one line of sight, in order to determine whether the sprectral feature comes from compact, dense subclumps that have relatively low filling fractions on pc scales, or whether it comes from a smooth distribution of gas at relatively low density.

For the main part of this REU project, Beth compared NRAO 12m observations of the 2 mm CS J=3-2 emission along 9 lines of sight in the Milky Way plane to their 3 mm counterparts from Helfer \& Blitz (1997). This involved retrieving the raw CS J=3-2 spectra, fitting baselines to emission-free regions of the spectra, and averaging together 4-point cross observations to compare to the existing 3 mm data. Beth then learned how to run a Large Velocity Gradient radiative transfer model to interpret the CS J=3-2 to CS J=2-1 line ratios over the pc-scale features. We derived molecular hydrogen densities of $<$ 10$^5$ cm$^{-3}$ for features at all lines of sight, with most of the derived densities in the range 2.5 $\times$ 10$^4$ to 1 $\times $ 10$^5$ cm$^{-3}$. These densities are about an order of magnitude lower than those derived from CS observations of cloud cores in M17, S140, and NGC2024 by Snell et al. (1984), and they are one to two orders of magnitude lower than the densites derived using HC$_3$N observations of cores in Orion, M17 and Cepheus A (Bergin et al. 1996). In addition to the radiative transfer study, Beth learned some basic techniques of millimeter interferometry by reducing CO, CS, and HCN BIMA observations along one line of sight. While the CO emission was extremely complex, the CS and HCN emission was concentrated on size scales of about half a parsec, a scale comparable to the size of starforming cores in nearby GMCs. Beth has written a report to summarize many of the details of her work this summer, and she plans to attend the January 1999 AAS meeting in Austin to present our results.

This work was presented at the 193rd AAS Meeting Session 71

Alexis Johnson, of The University of Virginia
works with Jeff Mangum
on

Molecular Spectral Line Images of Comet Hale-Bopp

Images of the CO, HCN, HCO+, and CH3OH emission from Comet Hale-Bopp were obtained near perihelion passage of this famous celestial visitor in spring 1997. These images were obtained using the On-The-Fly (OTF) observing technique at the 12m telescope. Alexis concentrated on the analysis of these data to (1) derive the spatial and spectral correspondence between the molecular emission distributions; (2) measure the spatial and temporal variations in the abundances of these molecules within the comet; (3) study the kinematic structure of the comet as traced by each species. Alexis spent most of her time developing a set of analysis tools using the IDL analysis package. The results of this work will be presented at the AAS meeting in Austin, Texas in January 1999.

This work was presented at the 193rd AAS Meeting Session 11

Laura A. Snyder, of Iowa State University
works with Simon Radford
on

Study of Chajnantor and Other High Astronomical Sites

The NRAO has proposed to build the Millimeter Array (MMA), which will be substantially more powerful than any existing telescope for astronomical observations at millimeter and submillimeter wavelengths. At these wavelengths, atmospheric water vapor can limit the observational sensitivity and resolution. As part of MMA development, therefore, NRAO has conducted an extensive site test campaign. For the last three years, we have studied a high altitude (5000 m) site near Cerro Chajnantor in the Andean altiplano on the eastern edge of the Atacama desert in northern Chile. Autonomous instruments record the atmospheric transparency at 225 GHz and 350 um, the atmospheric stability at 12 GHz, and meteorological parameters. The NRAO and other groups have also operated similar instruments at other sites, notably Mauna Kea and the South Pole, where existing (NSF supported) telescopes are located.

These data clearly demonstrate Chajnantor is a world class site for astronomy at millimeter and submillimeter wavelengths. This conclusion is based, however, on a relatively simple statisical analysis. The data are rich enough to support more detailed investigations such as correlations with meteorolgy databases.

Ms. Snyder completed the following studies of the site:

Cloud cover: Ms. Snyder analyzed surveillance images of the Chajnantor site's southwestern horizon for the nine 9 months, 1997 June to 1998 February. From these images, she determined the average cloud cover and its diurnal and monthly variations. An MMA memo is in preparation.

Subsurface temperature: Ms. Snyder analyzed data from a subsurface temperature probe for the six months, 1997 June to October. She determined the thermal diffusivity of the soil, the diurnal and monthly temperature fluctuations in the subsurface temperature, and the properties of subsurface freezing and melting episodes, including the effective soil salinity. An MMA memo is in preparation.

Digitized maps: Ms. Snyder measured published contour maps to extend a digital elevation model of the site. This will be used as input for hydrodynamic modeling of wind flow over the site.

El nino correlations: Ms. Snyder investigated possible correlations between sea surface temperature indexes and conditions measured at the site. The data hint at an influence of the El Nino/Southern Oscillation, but no definite conclusions were reached.