Characterizing Ultraviolet Excesses in the Outskirts of a Local Early-Type Sample

TL;DR

This study investigates ultraviolet excesses in the outskirts of early-type galaxies, revealing faint NUV emissions linked to low-level star formation, with differences based on galaxy luminosity and spatial distribution.

Contribution

It provides the first detailed analysis of UV excesses in early-type galaxy outskirts, highlighting star formation activity and its dependence on galaxy brightness.

Findings

Faint NUV excesses are present in early-type galaxy outskirts.

Faint NUV excesses increase with galaxy luminosity.

Bright sources are more numerous around fainter early types.

Abstract

We present an analysis of ultraviolet (UV) emission in the outer regions of a local, volume-limited sample of 56 early-type galaxies, where H{\alpha} emission from massive star formation is typically absent. We find excess faint NUV emission in the environments of our early-type galaxies compared to blank sky measured in the same tiles, indicating that the excesses are not due to background contamination. We do not observe corresponding faint FUV excesses. Faint NUV excesses increase with galaxy luminosity and are not correlated with the presence or absence of HI in the environments of these galaxies. The faint NUV excesses in the outskirts of early-type galaxies can be interpreted as being due to star formation at or above a few \times 10-5 M\odot yr-1 kpc-2; star formation at this rate can create a few percent of the mass of an early-type galaxy in a Gyr. Faint early types (with MB > -21.3) have on average four times as many bright UV sources within 30 kpc compared to bright early types (with MB < -21.3). The peak of the source distribution detected around faint early types is less luminous and slightly bluer than the peak of the sources detected around bright early types, indicating that early types with MB > -21.3 are more actively building up their mass with young stars. The spatial distribution of bright sources around all early types increases approximately linearly out to 20 kpc and subsequently flattens.