Dissecting star formation in the “Atoms-for-Peace” galaxy

UVIT observations of the post-merger galaxy NGC7252^★

¹ Indian Institute of Astrophysics, Koramangala II Block, Bangalore, India
e-mail: koshy@iiap.res.in
² Department of Physics, Christ University, Bangalore, India
³ National Research Council of Canada, Herzberg Astronomy and Astrophysics Research Centre, Victoria, Canada
⁴ National Centre for Radio Astrophysics, Pune, India
⁵ Tata Institute of Fundamental Research, Mumbai, India
⁶ University of Calgary, Calgary, Alberta, Canada
⁷ ISRO Satellite Centre, HAL Airport Road, Bangalore, India
⁸ Inter-University Center for Astronomy and Astrophysics, Pune, India

Received: 25 January 2018
Accepted: 23 February 2018

Abstract

Context. The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. Aims. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations.

Methods. The post merger galaxy NGC7252 (“Atoms-for-Peace” galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star-forming regions in the tidal tails and study the spatial variation in star formation rates.

Results. Based on ultraviolet imaging observations, we discuss star-forming regions of ages <200 Myr in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate (SFR) of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 ± 0.01 M_⊙ yr⁻¹. We show that the integrated SFR can change by an order of magnitude if the extinction correction used in SFR derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR = 0.02 M_⊙ yr⁻¹ and NGC7252NW, SFR = 0.03 M_⊙ yr⁻¹) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252.