Brackett-γ Line Emission Maps of Four Galaxies

Abstract

Observations of the Br-γ hydrogen recombination line have been made in a sample of four galaxies. The data were acquired with the new Palomar Prime-Focus Infrared Camera (PFIRCAM) mounted at the prime focus (f/3.3) of the Hale 5-m telescope. The galaxies were imaged with narrow band filters centered on the Br-γ (2.166 μm) line and nearby K-band continuum. This project was undertaken with two goals in mind. First, to demonstrate the feasibility of extragalactic narrow-band imaging with near-infrared arrays, and second, to measure the Br-γ to Hα flux ratio corresponding to regions undergoing massive star formation. The galaxies are all at low redshift, but of a variety of morphologies: late-type galaxies, NGC 3683 (Sbc, 1656 km/s) and NGC 4713 (Sbd, 653 km/s), early-type galaxy, NGC 5866 (S0-f , 770 km/s), and irregular-type galaxy, NGC 2537 (IRR, 447 km/s). In three of the galaxies, NGC 2537, NGC 3683 and NGC 4713, we detect Br-γ emission at the ≥3σ level, and in NGC 5866 we obtain an upper limit. The spatial resolution of the raw 2.2μm data was about 1″, with the final maps smoothed to a resolution of 1 – 1.5″. Our Br-γ detections resolve individual H II region complexes in the galaxies. By comparing the Br-γ emission with Hα emission, we may derive an effective extinction measure, and infer a Lyman continuum flux from massive stars. This Lyman luminosity can then be converted into a total mass estimate for young stars, assuming an initial mass function and an upper mass cut-off. To date we have acquired Ha images of NGC 3683, NGC 4713, and NGC 5866. The preliminary results of NGC 4713 are presented in this paper.