Near-infrared spectroscopy of the Blue Compact Dwarf galaxy Markarian 59

TL;DR

This study provides detailed near-infrared spectroscopic observations of the blue compact dwarf galaxy Mrk 59, revealing its low extinction, the nature of molecular hydrogen excitation, and confirming stellar radiation as the primary ionization source.

Contribution

First comprehensive NIR spectrum of Mrk 59 combined with optical data, demonstrating the excitation mechanisms and ionization sources in a low-metallicity BCD galaxy.

Findings

Low extinction (A(V)=0.24 mag) consistent across optical and NIR.

H2 emission from dense clumps excited by photons, no shocks needed.

Photoionization model matches observed emission lines without additional ionization sources.

Abstract

We present near-infrared (NIR) spectroscopic observations of the blue compact dwarf (BCD) galaxy Mrk 59, obtained with the TripleSpec spectrograph mounted on the 3.5m APO telescope. The NIR spectrum of Mrk 59, which covers the 0.90 - 2.40 micron wavelength range, shows atomic hydrogen, molecular hydrogen, helium, sulfur and iron emission lines. The NIR data have been supplemented by a SDSS optical spectrum. We found extinction in the BCD to be low [A(V)=0.24 mag] and to be the same in both the optical and NIR ranges. The NIR light does not reveal hidden star formation. The H2 emission comes from dense clumps and the H2 vibrational emission line intensities can be accounted for by photon excitation. No shock excitation is needed. A CLOUDY photoinization model of Mrk 59 reproduces well the observed optical and NIR emission line fluxes. There is no need to invoke sources of ionization other than stellar radiation.The [FeII] 1.257 and 1.643 micron emission lines, often used as supernova shock indicators in low-excitation high-metallicity starburst galaxies, cannot play such a role in high-excitation low-metallicity HII regions such as Mrk 59.