Star formation and gas in the minor merger UGC 10214^⋆

¹ Departamento de Física Teórica y del Cosmos, Facultad de Ciencias, University of Granada, Av. Fuentenueva s/n, 18071 Granada
e-mail: darobel@correo.ugr.es
² Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Spain
³ Departamento de Astrofísica, University of La Laguna, 38206 La Laguna, Spain
⁴ Instituto Carlos I, Facultad de Ciencias, University of Granada, Av. Fuentenueva s/n, 18071 Granada, Spain
⁵ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
⁶ School of Earth & Space Exploration, Arizona State University, 550 E. Tyler Mall, Room PSF-686, PO Box 876004, Tempe, AZ 85287-6004, USA
⁷ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
⁸ Unidad de Astronomía, Fac. Cs. Básicas, Universidad de Antofagasta, Avda. U. de Antofagasta, 02800 Antofagasta, Chile
⁹ Astrophysics, Cosmology and Gravity Centre, Astronomy Department, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 1000012, PR China
¹¹ South American centre for Astronomy, CAS, Camino El Observatorio 1515, Las Condes, Santiago, Chile

Received: 18 July 2018
Accepted: 20 November 2018

Abstract

Minor mergers play a crucial role in galaxy evolution. UGC 10214 (the Tadpole galaxy) is a prime example of this process in which a dwarf galaxy has interacted with a large spiral galaxy ∼250 Myr ago and produced a perturbed disc and a giant tidal tail. We used a multi-wavelength dataset that partly consists of new observations (Hα, HI, and CO) and partly of archival data to study the present and past star formation rate (SFR) and its relation to the gas and stellar mass at a spatial resolution down to 4 kpc. UGC 10214 is a high-mass (stellar mass M_⋆ = 1.28 × 10¹¹ M_⊙) galaxy with a low gas fraction (M_gas/M_⋆ = 0.24), a high molecular gas fraction (M_H2/M_HI = 0.4), and a modest SFR (2–5 M_⊙ yr⁻¹). The global SFR compared to its stellar mass places UGC 10214 on the galaxy main sequence (MS). The comparison of the molecular gas mass and current SFR gives a molecular gas depletion time of about ∼2 Gyr (based on Hα), comparable to those of normal spiral galaxies. Both from a comparison of the Hα emission, tracing the current SFR, and far-ultraviolet (FUV) emission, tracing the recent SFR during the past tens of Myr, and also from spectral energy distribution fitting with CIGALE, we find that the SFR has increased by a factor of about 2–3 during the recent past. This increase is particularly noticeable in the centre of the galaxy where a pronounced peak of the Hα emission is visible. A pixel-to-pixel comparison of the SFR, molecular gas mass, and stellar mass shows that the central region has had a depressed FUV-traced SFR compared to the molecular gas and the stellar mass, whereas the Hα-traced SFR shows a normal level. The atomic and molecular gas distribution is asymmetric, but the position-velocity diagram along the major axis shows a pattern of regular rotation. We conclude that the minor merger has most likely caused variations in the SFR in the past that resulted in a moderate increase of the SFR, but it has not perturbed the gas significantly so that the molecular depletion time remains normal.