Monday, August 6, 2012

What turns galaxies off?

In a previous post, we talked about the different morphological types of galaxies: spiral galaxies have a disk of stars (and often gas, mostly in the form of atomic or molecular hydrogen), whereas elliptical and lenticular galaxies have a large 'bulge' of stars. These characterizations of galaxy type key on the morphology (shape, structure) of the galaxy, and reflect the orbits of the stars in that galaxy: stars in spiral galaxies tend to rotate the center of the galaxy all in one direction, whereas stars in elliptical galaxies have a wider diversity of orbits (often termed 'random' motions, although detailed study shows that the motions are not completely random), and lenticular galaxies (oversimplifying somewhat) are supported by a mix of rotational and 'random' motions.

There is another way of characterizing galaxies: by their star formation activity. Many galaxies have a reservoir of atomic or molecular hydrogen, and gas in this reservoir collapses owing to turbulence and self-gravity, forming new stars. The majority of these star-forming galaxies have rates of star formation that correlate well with the mass of already-formed stars (the 'stellar mass' of those galaxies; some galaxies have higher star formation rates, arguably triggered by galaxy mergers). This has led many astronomers to think of the star forming galaxies as the normal or default state of galaxies.

Yet, there is a significant fraction of galaxies that does not form stars at any appreciable rate (factors of 10 or more less than a star-forming galaxy of comparable stellar mass). In terms of star formation, these galaxies are 'dead'. In this picture, these galaxies are somehow abnormal - there is a physical process that has 'turned off' star formation in those galaxies. Studying the structure of these galaxies without star formation may be important: if there is a particular structure of galaxies that correlates with a lack of star formation, the physical process that drive galaxy structure correlate (or are the same as) the processes that stop star formation on galaxy-wide scales.

A WFC3 image of the GOODS-S field
A WFC3 image of the GOODS-S field. Galaxies that are red and round are examples of the galaxies that lack star formation that we are exploring in this study. Most galaxies in this image, for example the blue star-forming disk at the bottom of the image, form stars and are more elongated (study of their shapes shows that they are consistent with being a set of disks viewed from random angles).
 NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (Arizona State University, Tempe), R. O'Connell (University of Virginia), P. McCarthy (Carnegie Observatories), N. Hathi (University of California, Riverside), R. Ryan (University of California, Davis), H. Yan (Ohio State University) and A. Koekemoer (Space Telescope Science Institute)

Intrigued by this, we used the CANDELS survey (from the UDS field) in conjunction with the Sloan Digital Sky Survey to systematically explore the relationship between a lack of star formation and the structure of galaxies. We found that there are relationships between star formation activity and galaxy structure: galaxies with big bulges (ellipticals/lenticulars) lack star formation. This is a well-known result in the nearby Universe, and the value of this study is that it shows that the relationship between the structure of galaxies holds from redshift 2 to the present day, a time span of 3/4 of the lifetime of the Universe.

There are a number of possible explanations for this behavior, and trying to identify the right ones is the subject of a number of ongoing studies, some of which are using CANDELS. The rounder shape and 'random' orbits of stars in the galaxies without star formation indicate that the gravitational field of these galaxies had to have changed quickly at some stage in the past; the merging of galaxies with each other is one way that this process happens (and can be seen on the far left of the above image). It is thought that such interactions may help massive black holes grow in the center of galaxies (see this previous post), and radiation, jets and winds from these growing black holes may heat up the gas near galaxies and stop the gas from cooling and forming stars. Further study of the properties of the galaxy population, and studies of individual galaxies caught in these short-lived but potentially crucial phases of their lives (think of short-lived but crucial phases of your own life!) will help to illuminate further why some galaxies managed to lose all their atomic and molecular gas - what turned galaxies off?

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