Effects of the radial inflow of gas and galactic fountains on the chemical evolution of M 31

¹ Department of Mathematics, University of Évora, R. Romão Ramalho 59, 7000 Évora, Portugal
e-mail: spitoni@galaxy.lca.uevora.pt
² INAF Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
³ Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, Rua Galvão Bueno 868, Liberdade, 01506-000 São Paulo, Brazil

Received: 18 September 2012
Accepted: 17 January 2013

Abstract

Context. Galactic fountains and radial gas flows are very important ingredients for modeling the chemical evolution of galactic disks.

Aims. Our aim here is to study the effects of galactic fountains and radial gas flows on the chemical evolution of the disk of Andromeda (M 31) galaxy.

Methods. We adopt a ballistic method to study the effects of galactic fountains on the chemical enrichment of the M 31 disk by analyzing the landing coordinate of the fountains and the time delay in the pollution of the interstellar gas. To understand the consequences of radial flows, we adopt a very detailed chemical evolution model. Our aim is to study the formation of abundance gradients along the M 31 disk and also compare our results with the Milky Way.

Results. We find that the landing coordinate for the fountains in M 31 is no more than 1 kpc from the starting point, thus producing a negligible effect on the chemical evolution of the disk. We find that the delay time in the enrichment process due to fountains is no longer than 100 Myr, and this timescale also produces insignificant effects on the results. Then, we compute the chemical evolution of the M 31 disk with radial gas flows produced by the infall of extragalactic material and fountains. We find that a moderate inside-out formation of the disk, coupled with radial flows of variable speed, can reproduce the observed gradient very well. We also discuss the effects of other parameters, such as a threshold in the gas density for star formation and efficiency of star formation varying with the galactic radius.

Conclusions. We conclude that galactic fountains do not affect the chemical evolution of the M 31 disk. Including radial gas flows with an inside-out formation of the disk produces a very good agreement with observations. On the other hand, if radial flows are not considered, one should assume a threshold in the star formation and variable star formation efficiency, besides the inside-out formation to reproduce the data. We conclude that the most important physical processes in creating disk gradients are the inside-out formation and the radial gas flows. More data on abundance gradients both locally and at high redshift are necessary to confirm this conclusion.