Galaxies playing possum

Title: Investigating early-type galaxy evolution with a multiwavelength
approach II: the UV structure of 11 galaxies with Swift-UVOT

Authors: R. Rampazzo, P. Mazzei, A. Marino, M. Uslenghi, G. Trinchieri, and A. Wolter

First Author’s Institution: INAF Osservatorio Astronomico di Padova

Status: Accepted for publication in Astronomy & Astrophysics, open access

Introduction

Early-type galaxies: much-maligned, misnamed, and misunderstood. Maligned for being boring (including by me), misnamed by tradition (the old idea was that formless elliptical blobs settle down into ‘late-type’ spirals: more-or-less the opposite of the truth), but misunderstood? I’ll explain.

You see, early-type galaxies (ETGs) encompass a wide range of galaxy types, from elliptical to lenticular, which all appear to have one thing in common: they’re dead as a doornail. At least, that is, in the sense that they are no longer forming any stars. A galaxy is essentially a really big collection of stars held together by gravity: thus, a galaxy that isn’t forming stars is ultimately doomed. Those stars will eventually die, and then what you have isn’t really a galaxy anymore, but rather just a collection of burnt-out cinders. But are all ETGs really like this? Or are some of them not quite as dead as they might first appear?

The Paper

For this paper, eleven ETGs were observed at ultraviolet (UV) wavelengths using the Swift space-telescope. UV observations are particularly useful for studying low-level, ongoing star formation because the brightest stars at these wavelengths are high mass and short-lived. Thus, UV observations are most sensitive to these stars, which must have formed recently.

The authors were principally interested in the structure of these galaxies. In particular, while the galaxies are pretty featureless in visible light (as is typical for ETGs) they were seeking interesting sub-structures in the UV images, such as rings or spiral arms. These stick out like a sore thumb in certain of their images (see the figure), revealing that these galaxies still play host to at least some ongoing star formation. Moreover, such structures ought to appear in galaxies that have recently suffered the indignity of a collision with another, lesser galaxy. They are akin to ripples spreading out from the impact site. All in all, such features show up in six of the galaxies they look at. These galaxies might look dead to the casual viewer, but the reality is they are still stirring faintly.

NGC 1533 is a particularly good example. The left panel shows an image in visible light, in which the galaxy appears to be pretty much a boring, featureless blob (the bright points around it are stars in our own galaxy and thus unrelated). However, in the right panel (an ultraviolet image) a wealth of subtle substructure is revealed! These structures, somewhat reminiscent of spiral arms, are regions where some star formation is still ongoing (Fig.7 in the paper).

Conclusions

The evidence is mounting: far from being rare, this study of a handful of galaxies suggests that many ETGs have recently experienced a minor star formation event of some sort. This rejuvenation of apparently quiescent galaxies seems to be due to the arrival of fresh fuel for star formation, delivered by collisions with minor galaxies. This takes us beyond a simple picture in which galaxies fall neatly into two categories (‘alive’ and ‘dead’): it seems that some linger on, living off a drip-feed of fresh gas.

About Paddy Alton

I am a fourth year PhD student at Durham University's Centre for Extragalactic Astronomy, where I work with Dr John Lucey and Dr Russell Smith. My research is on the stellar populations of other galaxies - with a specific focus on those of the largest elliptical galaxies, whose stars formed under radically different conditions to those in our own Milky Way. I graduated in 2013 from the University of Cambridge with an MSci in Natural Sciences, having specialised in Astrophysics. Out of the office I enjoy a variety of sports, but particularly rowing (whenever Durham's fickle river Wear allows it).

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