The Stellar and Gas Kinematics of the Little Things Dwarf Irregular Galaxy NGC 1569

Abstract

In order to understand the formation and evolution of Magellanic-type dwarf irregular (dIm) galaxies, one needs to understand their three-dimensional structure. We present measurements of the stellar velocity dispersion in NGC 1569, a nearby post-starburst dIm galaxy. The stellar vertical velocity dispersion, σ z , coupled with the maximum rotational velocity derived from H I observations, V max , gives a measure of how kinematically hot the galaxy is, and, therefore, indicates its structure. We conclude that the stars in NGC 1569 are in a thick disk with a V max /σ z = 2.4 ± 0.7. In addition to the structure, we analyze the ionized gas kinematics from O III observations along the morphological major axis. These data show evidence for outflow from the inner starburst region and a potential expanding shell near supermassive star cluster (SSC) A. When compared to the stellar kinematics, the velocity dispersion of the stars increase in the region of SSC A supporting the hypothesis of an expanding shell. The stellar kinematics closely follow the motion of the gas. Analysis of high resolution H I data clearly reveals the presence of an H I cloud that appears to be impacting the eastern edge of NGC 1569. Also, an ultra-dense H I cloud can be seen extending to the west of the impacting H I cloud. This dense cloud is likely the remains of a dense H I bridge that extended through what is now the central starburst area. The impacting H I cloud was the catalyst for the starburst, thus turning the dense gas into stars over a short timescale, ∼ 1 Gyr. We performed a careful study of the spectral energy distribution using infrared, optical, and ultraviolet photometry producing a state-of-the-art mass model for the stellar disk. This mass modeling shows that stars dominate the gravitational potential in the inner 1 kpc. The dynamical mass of NGC 1569, derived from V max , shows that the disk may be dark matter deficient in the inner region, although, when compared to the expected virial mass determined from halo abundance matching techniques, the dark matter profile seems to agree with the observed mass profile at a radius of 2.2 kpc.

The triggering of starbursts in low-mass galaxies

“…The datacube is strongly affected by Galactic emission, thus the total H I map and the velocity field are uncertain. Our results are in close agreement with those from Stil & Israel (1998 and Johnson et al (2012). NGC 1569 is part of the IC 432 group (Grocholski et al 2008) and has a nearby companion (UGCA 92) at a projected distance of ∼70 kpc with a similar systemic velocity (within ∼20 km s −1 ).…”

Star formation sustained by gas accretion

“…Such extreme distortion is actually common among galaxies with large specific SFR in the local universe-blue compact dwarf (BCD) galaxies often show such complex HI topology (Ekta et al 2006;Ekta and Chengalur 2010;Lelli et al 2012b;Nidever et al 2013). The survey LITTLE THINGS finds BCDs with kinematically separate components, streamers extending far beyond the optical size, kinematic HI axes offset from the optical axes, clouds associated with recent starbursts, and so on (Johnson et al 2012;Ashley et al 2013). Many other examples of HI maps showing galaxies with plumes and filaments can be found in the literature (e.g., López-Sánchez et al 2012).…”

Hierarchical Star Formation in Nearby Legus Galaxies

“…Such a formation site is suggested from the mass-metallicity relation of galaxies as a function of redshift (Mannucci et al 2009). Perhaps SSCs in small galaxies reach high pressures during dwarf-dwarf galaxy mergers (Bekki 2008), or because of the ram pressure from accreting gas streams, as appears to be the case in NGC 1569 (Johnson et al 2012) andNGC 5253 (López-Sánchez et al 2012). These perturbations would be large-scale sources of turbulence, as opposed to stellar feedback, which is a smallscale source.…”