Near-Infrared Flux Limits for Sgr A* Based on NICMOS Data

Images of the central arcsec of the Galaxy at near-infrared(IR) wavelengths reveal a tight stellar cluster, however, no point source exactly coincident with the radio source Sgr A* has been clearly detected in the near-IR. "Contaminating" emission due to overlapping point spread functions (PSFs) from this stellar cluster as well as from the bright IRS16 sources 1-3" away from Sgr A* makes it difficult to assess directly the near-IR characteristics of Sgr A*. Taking advantage of the stability of the PSF achieved by HST/NICMOS observations, we employ PSF subtraction techniques in order to investigate the true nature of the near-IR emission from Sgr A*. New limits on the maximum possible flux emanating from a point source coincident with Sgr A* are presented at 1.1, 1.45, 1.6, 1.9, and 2.2 mu m based on NICMOS data taken at several epochs. These are the faintest reported near-IR flux limits for wavelengths shorter than 2.2 mu m. The resulting extinction-corrected fluxes are compared to theoretical models (including both quiescent and flare models) for the spectrum of Sgr A*. We also present NICMOS narrow-band emission line images in Pa alpha where significant stellar PSF residuals have been removed to reveal new diffuse structures in the ionized gas near Sgr A* at two epochs.