The Second Case of a Major Merger Triggering a Starburst in a Green Pea Galaxy

TL;DR

This study presents the second H I 21cm emission map of a Green Pea galaxy, revealing a merger-triggered starburst with detailed gas distribution, dynamics, and absorption features, highlighting the galaxy's interaction-driven evolution.

Contribution

First detailed H I spatial mapping of a Green Pea galaxy showing merger evidence and starburst triggering mechanisms.

Findings

H I emission extends in an inverted 'C' shape around the galaxy and companions.

Starburst likely triggered by ongoing merger with neighboring galaxies.

H I depletion timescale is approximately 0.69 Gyr, shorter than typical galaxies at similar redshift.

Abstract

We have used the Karl G. Jansky Very Large Array (VLA) to map H{\sc i} 21\,cm emission from the Green Pea galaxy GP~J1148+2546 at $z\approx0.0451$, only the second measurement of the H{\sc i} spatial distribution of a Green Pea. The VLA H{\sc i} 21\,cm image, the DECaLS optical image, and Sloan Digital Sky Survey spectroscopy show that GP~J1148+2546 has two neighbours, the nearer of which is only $\approx 17.5$~kpc away, and that the H{\sc i} 21\,cm emission extends in an inverted ``C'' shape around the Green Pea and its companions, with the highest H{\sc i} column density between the two neighbouring galaxies. The starburst in GP~J1148+2546 is likely to have been triggered by the ongoing merger with its neighbours, although the velocity field and velocity dispersion images do not show clear merger signatures at the Green Pea location. The H{\sc i} mass of the Green Pea and its immediate surroundings is $(3.58 \pm 0.37) \times 10^9 \, M_\odot$, a factor of $\approx 7.4$ lower than the total H{\sc i} mass of the system of three interacting galaxies, while the H{\sc i} depletion timescale of GP~J1148+2546 is $\approx 0.69$~Gyr, much shorter than that of typical galaxies at $z \approx 0$. We detect damped Ly$\alpha$ absorption and Ly$\alpha$ emission from the Green Pea in a Hubble Space Telescope Cosmic Origins Spectrograph spectrum, obtaining a high H{\sc i} column density, $\approx 2.0 \times 10^{21}$~cm$^{-2}$, and a low Ly$\alpha$ escape fraction, $\approx 0.8$\%, consistent with the relatively low value ($\approx 5.4$) of the ratio O32~$\equiv$~[O{\sc iii}]$\lambda 5007 + \lambda 4959$/[O{\sc ii}]$\lambda$3727,3729.