The spatial distribution and origin of the FUV excess in early-type galaxies

TL;DR

This study investigates the spatial distribution and origins of the FUV excess in early-type galaxies, revealing correlations with galaxy properties and proposing a helium abundance gradient as a key factor.

Contribution

It provides the first detailed analysis of the FUV excess distribution and its relation to stellar populations and galaxy dynamics in a diverse galaxy sample.

Findings

FUV excess shows a positive gradient in most early-type galaxies.

The gradient correlates with central velocity dispersion but not with stellar population parameters.

The strength of the FUV excess correlates more strongly with [alpha/Fe] than with [Z/H].

Abstract

We present surface photometry of a sample of 52 galaxies from the GALEX and 2MASS data archives, these include 32 normal elliptical galaxies, 10 ellipticals with weak Liner or other nuclear activity, and 10 star forming ellipticals or early-type spirals. We examine the spatial distribution of the Far Ultra-Violet excess in these galaxies, and its correlation with dynamical and stellar population properties of the galaxies. From aperture photometry we find that all galaxies except for recent major remnants and galaxies with ongoing star formation show a positive gradient in the (FUV-NUV) colour determined from the GALEX images. The logarithmic gradient does not correlate with any stellar population parameter, but it does correlate with the central velocity dispersion. The strength of the excess on the other hand, correlates with both [alpha/Fe] and [Z/H], but more strongly with the former. We derive models of the underlying stellar population from the 2MASS H-band images, and the residual of the image from this model reveals a map of the centrally concentrated FUV excess. We examine a possible hypothesis for generating the FUV excess and the radial gradient in its strength, involving a helium abundance gradient set up early in the formation process of the galaxies. If this hypothesis is correct, the persistence of the gradients to the present day places a strong limit on the importance of dry mergers in the formation of ellipticals.