Our HST/NICMOS Pa survey of the Galactic center (GC) provides a uniform, panoramic, high-resolution map of stars and ionized diffuse gas in the central 416 arcmin^2 of the Galaxy. This survey was carried out with 144 HST orbits using two narrow-band
filters at 1.87 and 1.90 micron in NICMOS Camera 3. In this paper, we describe in detail the data reduction and mosaicking procedures followed, including background level matching and astrometric corrections. We have detected ~570,000 near-IR sources and are able to quantify photometric uncertainties of the detections. The source detection limit varies across the survey field but the typical 50% completion limit is ~17th mag (Vega System) in the 1.90 micron band. A comparison with the expected stellar magnitude distribution shows that these sources are primarily Main-Sequence massive stars (>7M) and evolved lower mass stars at the distance of the GC. In particular, the observed source magnitude distribution exhibits a prominent peak, which could represent the Red Clump stars within the GC. The observed magnitude and color of these RC stars support a steep extinction curve in the near-IR toward the GC. The flux ratios of our detected sources in the two bands also allow for an adaptive and statistical estimate of extinction across the field. With the subtraction of the extinction-corrected continuum, we construct a net Pa emission map and identify a set of Pa-emitting sources, which should mostly be evolved massive stars with strong stellar winds. The majority of the identified Pa point sources are located within the three known massive GC stellar clusters. However, a significant fraction of our Pa-emitting sources are located outside the clusters and may represent a new class of `field massive stars, many of which may have formed in isolation and/or in small groups. The maps and source catalogues presented here are available electronically.
We present preliminary results of our hst Pa$alpha$ survey of the Galactic Center (gc), which maps the central 0.65$times$0.25 degrees around Sgr A*. This survey provides us with a more complete inventory of massive stars within the gc, compared to p
revious observations. We find 157 Pa$alpha$ emitting sources, which are evolved massive stars. Half of them are located outside of three young massive star clusters near Sgr A*. The loosely spatial distribution of these field sources suggests that they are within less massive star clusters/groups, compared to the three massive ones. Our Pa$alpha$ mosaic not only resolves previously well-known large-scale filaments into fine structures, but also reveals many new extended objects, such as bow shocks and H II regions. In particular, we find two regions with large-scale Pa$alpha$ diffuse emission and tens of Pa$alpha$ emitting sources in the negative Galactic longitude suggesting recent star formation activities, which were not known previously. Furthermore, in our survey, we detect $sim$0.6 million stars, most of which are red giants or AGB stars. Comparisons of the magnitude distribution in 1.90 $mu$m and those from the stellar evolutionary tracks with different star formation histories suggest an episode of star formation process about 350 Myr ago in the gc .
We have recently carried out the first wide-field hydrogen Paschen-alpha line imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 around the central supe
rmassive black hole (Sgr A*) in the 1.87 and 1.90 Micron narrow bands with a spatial resolution of 0.01 pc at a distance of 8 kpc. Here we present an overview of the observations, data reduction, preliminary results, and potential scientific implications, as well as a description of the rationale and design of the survey. We have produced mosaic maps of the Paschen-alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. We detect a significant number of previously undetected stars with Paschen-alpha in emission. They are most likely massive stars with strong winds, as confirmed by our initial follow-up spectroscopic observations. About half of the newly detected massive stars are found outside the known clusters (Arches, Quintuplet, and Central). Many previously known diffuse thermal features are now resolved into arrays of intriguingly fine linear filaments indicating a profound role of magnetic fields in sculpting the gas. The bright spiral-like Paschen-alpha emission around Sgr A* is seen to be well confined within the known dusty torus. In the directions roughly perpendicular to it, we further detect faint, diffuse Paschen-alpha emission features, which, like earlier radio images, suggest an outflow from the structure. In addition, we detect various compact Paschen-alpha nebulae, probably tracing the accretion and/or ejection of stars at various evolutionary stages.
