Overview of the Project:
At a distance of only 8.0+/-0.5 kpc (24,000 ly), the center of our Galaxy provides a unique opportunity to study the environment around a supermassive black hole in detail.  Perhaps not surprisingly, the central 10 parsecs of the Galaxy is a place of much activity!  Based on stellar orbits, it has been determined that there is a 3.6 million solar mass black hole at the center of the Galaxy.  Although extremely dim at optical & IR wavelengths, the accretion disk around this black hole can be seen as a bright radio source, called Sgr A*.  (Sgr * has also been detected at x-ray and near-IR wavelengths.)

Sgr A* is surrounded by a number of streams of dust and ionized gas are falling towards the black hole. Theses arcs are, in turn, surrounded by the circumnuclear disk, which is a lumpy, warped, ring that appears to orbit the nucleus at a radius of ~6 ly. There are also a number of supernova remnants (SNR) expanding in the region.  One of the most important SNR is Sgr A East, which is expanding outwards and pushing much of the molecular gas away from Sgr A*, thereby depriving the galaxy of its molecular "meal".

There are currently many unanswered questions about the Galactic Center. In particular, large scale (from >5 pc (15 ly)) accretion of material towards the black hole is not well understood (though we're finding that Sgr A* may be significantly hindering accretion at the current time). In addition, recent IR observations have found very young stars <0.5 pc (1.5 ly) from Sgr A*. These stars must have recently formed in dense molecular clouds, but they are too young to have migrated there and the local conditions are too tidally disrupted to permit in situ formation.

Over the past 5 years, we have observed emission from dense molecular gas in the central 10 pc (30 ly) of the Galaxy in order to better understand the environment around Sgr A*. We have observed emission from ammonia (NH 3) molecules in the central 10 pc (33 light years) of the Milky Way using the VLA in order to better understand the environment in the region.  These spectral lines are ideal for studying the Galactic center because they trace warm, dense gas.   By observing different lines (with different energies) we can compare the locations of cool (low E) and hot (high E) gas (see above)  The Doppler shift of the emission to calculate how the gas is moving towards or away from us!  But that's not all... we can also directly measure many of the physical parameters of the clouds (see below), including temperatures, opacities, and masses.

From these detailed physical parameters, we have been able to improve the understanding of the relative locations and interactions of the many features at the Galactic Center (see below).


Emission from Hot Gas near Sgr A*

In 2002, we observed the Galactic Center in yet another NH3 tracer, NH3(6,6).  This transition traces gas that is very warm (it has 4 times the energy of our next most energetic tracer).  We detect many of the same features in NH3(6,6) as in the other transitions, with one important difference.  NH3(6,6) is dominated by a bright cloud of emission less than 1.5 pc (5 ly) from Sgr A* (Herrnstein & Ho 2002).  This material appears to be located interior to the ring of emission called the "circumnuclear disk" (CND) making it some of the closest molecular material to Sgr A* that has ever been detected!  You can see our NH3(6,6) image in panel (b) above.

Upon closer inspection, we find taht this new cloud has many interesting features. It isn't seen in the lower Energy transitions because a cool intervening layer of the cloud absorbs that emission away. In addition, if you look at the kinematics of this feature, it shows evidence for rotation and expansion/contraction (Herrnstein & Ho 2005). Upcoming high-resolution maps should allow us to determine the morphology and kinematics of this cloud in more detail.

Physical Conditions in Clouds

NH3 has many "nice extras" when compared to other molecular tracers.  The main line is bracketed on each side by 2 hyperfine lines that allow us to calculate the opacity (thickness) of the gas.  In addition, we can use the opacity and the ratio in brightness of (2,2) to (1,1) to calculate the temperature of the gas.  This sounds easy, but the linewidths at the Galactic Center are so broad that the main and hyperfine lines blend making estimation of opacity and temperature difficult.  I have developed a method which uses the observed line width of (1,1) and (2,2) to simultaneously solve for the opacity and real linewidth of NH3 (McGary & Ho 2002 ). This information is then combined with the (2,2)/(1,1) line ratio to get the temperature!

Our latest temperature plot is shown below.  Temperature information is really helpful in understanding which clouds may have been impacted by shocks or may be being heated by the nucleus!

A 3-D Model of the Central 10 pc

Cartoon showing the face-on arrangment of features at the GC.  The southern streamer, western streamer, northern ridge, and CND as well as the edge of the 50 km/s GMC and the molecular ridge are all detected in NH3 and are labeled in the NH3(3,3) figure above.

Two different views (left: "above", right: "side") of  the line-of-sight arrangement of the main features in the central 10 parsecs of the Galaxy (Herrnstein & Ho 2004).  For more information, please see this mini-poster.

Work in Progress

To date, our conclusions have been limited by the low resolution of our data (~14''). The low resolution is not the result of the resolution of the VLA. Rather, we had to degrade the resolution (by throwing away long baselines) in order to detect extended emission. Part of the problem is that the VLA is an interferometer, and is not sensitive to emission on the largest scales. Recently, we took data at the Green Bank Telescope to fill in these "missing short spacings." The combined images should have a resolution of ~2''. Below, a preliminary velocity integrated map of NH3(3,3) shows the result of shows the immediate improvement from adding the GBT data.

With the addition of the GBT data, you now see that the emission is dominated by the nearby 20 and 50 km/s GMCs (see the cartoons above for camparison). The exciting features such as the northern ridge, western arm, and CND are thin filaments on top of this underlying structure.

Publications & Posters

Herrnstein, R.M. & Ho, P.T.P. "The Nature of the Molecular Environment within 5 pc of the Galactic Center", 2005, ApJ, 620, 287

Herrnstein, R.M. & Ho, P.T.P.  "The Forest from the Trees:  A 3-D View of the Inner 10 pc of the Galaxy from High-Resolution Spectral Line Images," 2004, poster presented at "Sgr A* at 30" conference, Green Bank, WV (PDF mini-poster)

Herrnstein, R.M. & Ho, P.T.P. "Hot Molecular Gas in the Central 10 Parsecs of the Galaxy,"  2003, Astron. Nachr. Special Supplement "The central 300 parsecs of the Milky Way", Eds. A. Cotera, H. Falcke, T. R. Geballe, S. Markoff, 324, No. S1, 583 

Herrnstein, R.M. "Ammonia at the Galactic Center:  A Detailed Study of the Molecular Environment in the Central 10 Parsecs of the Galaxy," 2003, PhD Thesis

Herrnstein, R.M. & Ho, P.T.P. "Hot Molecular Gas in the Galactic Center," 2002, ApJL, 579, 83

McGary, R.S., Ho, P.T.P., & Coil, A.L.  "Temperature, Opacity, and Kinematics of NH3 at the GalacticCenter," 2002, poster presented at American Astronomical Society meeting, Washington, DC (PDF mini-poster)

McGary, R. S. & Ho, P. T. P.  ``NH3 in the Central 10 pc of the Galaxy. II. Determination of Opacity for Gas with Large Linewidths, '' 2002, ApJ, 577, 757

McGary, R. S., Ho, P. T. P., & Coil, A. L.  ``NH3 in the Central 10 pc of the Galaxy. I. General Morphology and Kinematic Connections Between the CND and GMCs,'' 2001,  ApJ, 559, 326

GBT Proposal