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

Abstract

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 μm based on NICMOS data taken at several epochs. These are the faintest reported near‐IR flux limits for wavelengths shorter than 2.2 μm. The resulting extinction‐corrected .uxes 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 Paα where signi.cant stellar PSF residuals have been removed to reveal new diffuse structures in the ionized gas near Sgr A* at two epochs.