Implications for the origin of early-type dwarf galaxies – the discovery of rotation in isolated, low-mass early-type galaxies

Abstract

We present the discovery of rotation in quenched, low-mass early-type galaxies that are isolated. This finding challenges the claim that (all) rotating dwarf early-type galaxies in clusters were once spiral galaxies that have since been harassed and transformed into early-type galaxies. Our search of the Sloan Digital Sky Survey data within the Local volume (z < 0.02) has yielded a sample of 46 galaxies with a stellar mass M ⋆ 5 × 10 9 M ⊙ (median M ⋆ ∼ 9.29 × 10 8 M ⊙ ), a low Hα equivalent width EW Hα < 2Å, and no massive neighbour (M ⋆ 3 × 10 10 M ⊙ ) within a velocity interval of ∆V = 500 km s −1 and a projected distance of ∼ 1 Mpc. Nine of these galaxies were subsequently observed with Keck ESI and their radial kinematics are presented here. These extend out to the half-light radius R e in the best cases, and beyond R e /2 for all. They reveal a variety of behaviours similar to those of a comparison sample of early-type dwarf galaxies in the Virgo cluster observed by Toloba et al. Both samples have similar frequencies of slow and fast rotators, as well as kinematically decoupled cores. This, and especially the finding of rotating quenched low-mass galaxies in isolation, reveals that the early-type dwarfs in galaxy clusters need not be harassed or tidally stirred spiral galaxies.

Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane’s Kinematic Scaling, and the Spin–Ellipticity Diagram

“…To achieve our desired spectral resolution of ∼20 km s −1 for the velocity dispersion, we used the 0 .75 wide (and 20 long) slit which gives an F W HM spectral resolution of 55.9 km s −1 . Therefore, with a good ratio of signal-to-noise (S/N ) we are able to measure velocity dispersion σ down to F W HM/2.35 ≈ 24 km s −1 (see Janz et al 2017, and their Figure 5, for further details).…”

“…has recently found 46 isolated dwarf ETGs with masses less than 5×10 9 M and with redshifts z < 0.02. These have been tabulated in Janz et al (2017). Rotation curves, obtained by one of us (J.J.), are presented there for nine of those galaxies with M B −18 mag.…”

“…We measure a central stellar velocity dispersion σ cen = 37±2 km s −1 (see Janz et al 2017 for further information). This value is greater than the spectral resolution limit of ∼24 km s −1 noted in the previous section, and is therefore a robust measurement rather than a lower limit.…”

“…Although CG 611, and the other isolated dwarf ETGs presented in Janz et al (2017), undermine the "Galaxy harassment" and "tidal stirring" models for the creation of all dwarf ETGs, those two mechanisms might preferentially act on lower mass, late-type spiral, and dwarf irregular galaxies, turning them into dwarf spheroidal (dSph) galaxies (Klimentowski et al 2009;Kazantzidis et al 2011Kazantzidis et al , 2013Paudel & Ree 2014) by removing much of their disk mass. These dSph galaxies (with M B −13 to −14 mag) may then be somewhat different from the dwarf/ordinary ETG sequence at M B −13, with many of the most luminous ETGs (M B −20.5 mag) additionally displaying a departure in the luminosity-(central surface brightness) diagram due to their partially depleted cores .…”

“…Nurture, as the (sole) formation path for dwarf ETGs. We have undertaken such an observing campaign, and the results are presented in Janz et al (2017). Here we provide a more detailed presentation for one of these galaxies -specifically, for the galaxy CG 611 -than can be afforded, or is necessary, for all of the survey galaxies.…”