Spatially resolved star formation relation in two HI-rich galaxies with central post-starburst signature

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: aklitsch@eso.org
² Ludwig-Maximilians-Universität München, Faculty of Physics, Schellingstraße 4, 80799 München, Germany
³ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
⁴ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
⁵ Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia
⁶ The Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia

Received: 8 December 2015
Accepted: 18 November 2016

Abstract

Context. E+A galaxies are post-starburst systems that are identified based on their optical spectra. These galaxies contain a substantial young A-type stellar component but display no emission lines, which indicates only little ongoing star formation (SF). HI 21 cm line emission is found in approximately half of the nearby E+A galaxies, indicating that they contain a reservoir of gas that could fuel active SF.

Aims. We study the distribution and kinematics of atomic and molecular gas in two HI-rich galaxies, which show a typical E+A spectrum at the centre and SF at larger radii. From our results we aim to infer whether the SF activity of these galaxies is consistent with the activity seen in disc galaxies, or if it indicates a transition towards another evolutionary phase.

Methods. We present newly obtained high spatial resolution radio interferometric observations of the HI 21 cm emission line using the Karl Jansky Very Large Array (VLA) and of the CO(1–0) emission line using the Atacama Large Millimeter/submillimeter Array (ALMA). We combine these data sets to predict the star formation rate (SFR) using a pressure-based SF relation and show that it does not correlate well with the SFR derived from Hα on sub-kpc scales. We apply a recently developed statistical model for the small-scale behaviour of the SF relation to predict and interpret the observed scatter.

Results. We find regularly rotating HI gas that is smoothly distributed across the entire disc. The CO(1–0) emission line is not detected for either of the two galaxies. The derived upper limit on the CO mass implies a molecular gas depletion time of t_depl ≲ 20 Myr. However, because of the low metallicity, the CO-to-H2 conversion factor is highly uncertain. In the relations between the Hα-based SFR and the HI mass, we observe a substantial scatter we demonstrate results from small-number statistics of independent SF regions on sub-kpc scales.

Conclusions. We confirm the HI-richness of ESO534-G001 and 2dFRS S833Z022, and find that the scale dependence of the atomic SF relation in these galaxies is consistent with the predicted increase in the scatter towards small size scales. This is due to the incomplete sampling of independent HI clouds and SF regions. This finding adds to the existing literature, which has reported a scale dependence of the molecular SF relation, showing that the atomic and molecular phases are both susceptible to the evolutionary cycling of individual regions. This suggests that the atomic gas reservoirs host substantial substructure, which should be observable with future high-resolution observations.