The Dawn Light of Blueberry Galaxies: Spectroscopic and Photometric Studies of two Starburst Dwarf Galaxies

TL;DR

This study investigates the detailed properties of two blueberry starburst dwarf galaxies using spectroscopic and photometric data, revealing their likely merging nature, high ionization, and unique emission characteristics, contributing to understanding these rare objects.

Contribution

It provides the first detailed spectroscopic and photometric analysis of two blueberry dwarf galaxies, highlighting their merger signatures and emission properties, which were previously not well understood.

Findings

RGG B and RGG 5 are likely merging dwarf galaxies.

Both galaxies exhibit high ionization parameters and nitrogen overabundances.

They are located near the maximum-starburst-line in the BPT diagram and show red mid-IR colors.

Abstract

A population of so-called "blueberry" starbursting dwarf galaxies with extremely blue colors, low-metallicities, and enormous ionization ratios, has recently been found by Yang et al. (2017). Yet we still do not know their detailed properties, such as morphologies, AGN occupations, massive star contents, infrared emission, dust properties, etc. As a pilot study of the blueberries, we investigate the spectroscopic and photometric properties of two blueberry candidates, RGG B and RGG 5, for which Hubble Space Telescope high-resolution images are available. We find that RGG B and RGG 5 perhaps are likely to be two merging dwarf galaxy systems. RGG B may have a close merging companion; yet the current evidence still cannot exclude the possibility that RGG B is just disturbed by in-situ star formation through, e.g., outflows, rather than undergoing a merger. RGG 5 presents stellar shells in the outskirt which can be the powerful evidence of galaxy merging. We also find that, all of the blueberries, including RGG B and RGG 5, are located close to the theoretical maximum-starburst-line in the BPT diagram, have very high ionization parameters, and relatively low hardness ionizing radiation fields, exhibit nitrogen overabundances, and show extremely red mid-IR colors, and reside in the so-called "ULIRGs/LINERs/Obscured AGN" region. The blueberry galaxies may not harbor AGN.