Local Volume dwarf KK242: radial velocity, SF region, and metallicity

TL;DR

KK242, a transition-type dwarf galaxy in a void, has a significantly different radial velocity from its nearby galaxy NGC6503, and new spectroscopic data reveal details about its star-forming regions and metallicity, informing its origin.

Contribution

This study provides new spectroscopic measurements of KK242's star-forming regions, clarifies its radial velocity discrepancy, and estimates its metallicity, offering insights into its evolutionary history.

Findings

Radial velocity of KK242's SF complex is ~100 km/s lower than NGC6503.

Detected H-alpha and H-beta emission in two compact regions within KK242.

Estimated metallicity of KK242 is 12+log(O/H) ~7.35, typical for a stripped void dwarf galaxy.

Abstract

KK242 is a LV dwarf of transition type residing in the void environment. Koda et al. present clear indications on its connection with Scd galaxy NGC6503. This implies the distance to KK242 of ~6.3 Mpc and its M_B = -10.5 mag. Its radial velocity, known from the Effelsberg radio telescope \HI\ observations, reveals, however, the difference with that of NGC6503, dV ~ 400 km/s. If real, this fact implies the substantial constraints on its origin. To clear-up the issue of KK242 radial velocity, we obtained with the SAO 6-m telescope spectra of its faint star-forming (SF) complex. H-alpha and H-beta emission is detected in two adjacent compact regions, the southern and northern, separated by ~2" (~60 pc). Their mean radial velocity is V_hel = -66 km/s, ~100 km/s lower than that of NGC6503. We use the HST Legacy Archive images and photometry of individual stars from the Extragalactic Distance Database, available for KK242, to identify in the SF complex the exciting hot stars, the probable BHeB and RHeB stars and a supernova remnant. We address, based on the possible range of its gas metallicity, the probable evolutionary paths of KK242. Using package Cloudy and parameters of the exciting B0V stars, we conclude that the observed flux ratio of [Sii] doublet to H-alpha is consistent with the value of 12+log(O/H) ~7.35+/-0.18 dex, expected for a stripped void dIrr galaxy.