NGC 1705. I. Stellar populations and mass loss via a galactic wind

Abstract

We present a detailed multi-waveband study of the galaxy NGC 1705. The data include optical and ultraviolet spectroscopy, optical and infrared photoelectric aperture photometry, narrowband and broadband images, H I spectra, and data from the IRAS point source catalog. NGC 1705 is shown to be a nearby (D≈5 Mpc for H0 = 75 km s-1 Mpc-1) nucleated blue compact dwarf (BCD) galaxy. It contains two stellar populations, distinct in structure and color: an inner high surface-brightness (HSB) population with a radius of 500 pc, and a relatively low surface-brightness (LSB) population extending out to a radius of at least 1.5 kpc. Both are star forming populations whose colors suggest either (i) ages of about 1 and 3 Gyr, respectively, or (ii) a difference in their initial mass functions, with the LSB population being depleted in high mass stars. The HSB population defines NGC 1705's BCD morphology. Numerous embedded objects can be seen within its boundaries. These objects include star clusters, some of which are ionizing sources, and perhaps individual supergiant stars. The brightest object is the unresolved off-center nucleus which is a young (13 Myr old) globular-like cluster with a mass of ∼ 1.5 × 106 Script M sign⊙. In the light of Hα, NGC 1705 has a bipolar morphology, suggesting a flow with its axis roughly aligned along the continuum minor axis. The expulsive nature of this flow is confirmed by emission and absorption line kinematics. A comparison of the integrated Ha and H I velocity profiles suggests that neutral material is entrained in this flow. A simple expansion model is constructed which yields an expansion timescale on the order of the age of the nucleus for any likely set of its free parameters. This and the nucleocentric flow geometry are strong evidence that the nucleus powers the outflow. The outflow has reached escape velocity for total mass to light ratios Script N signT/LB < 3.6 using the most conservative acceptable set of free parameters. This limit is high for BCDs, indicating that NGC 1705 is almost certainly losing mass in a galactic wind. The evolutionary outcome of this mass loss depends on the mass of gas entrained in the flow, and the dark matter content, both ill-constrained quantities. Possibilities include (i) complete destruction of the galaxy, (ii) rapid evolution into a nucleated dwarf elliptical galaxy, or (iii) retention of some gas for future star formation. The implications of our results are discussed and we list four other BCD and amorphous galaxies with a minor axis flow morphology. Show more