Sunday, February 28, 2010

Spiral galaxies are taking over

Everybody knows what a spiral galaxy looks like. Here's a typical nearby example (M74):

Among the largest and brightest galaxies close to our own, about 72% are of this spiral type, like M74. There is a classification system for galaxy shapes, and the remaining 28% of large, bright, nearby galaxies fall into classes called "elliptical", "lenticular", or simply "peculiar". (For simplicity and for other reasons that will become apparent, we're ignoring smaller, dimmer galaxies – such as the Magellanic Clouds – which may be very small and difficult to detect. Such galaxies tend to have more irregular shapes.)

Actually, two types of spiral galaxies are normally recognized: "ordinary" spirals, such as M74, above. In these the spiral arms reach all the way in to the center of the galaxy. However, in the variant known as a "barred" spiral the arms originate at either end of a "bar" through the center of the galaxy. The figure to the right shows how the Milky Way is thought to appear if viewed from outside.

Our own galaxy, the Milky Way, has been placed in this category. Although we can't see it as it looks from a distance, the classification is based on detailed surveys of star frequencies in varying directions and distances. The Milky Way's bar is fairly small. Here's another, more dramatic, barred spiral, NGC 1300:

The classification system for galaxy shapes was developed by Edwin Hubble, and so it's called the Hubble sequence. In addition to the two types of spirals, it includes elliptical galaxies and lenticular galaxies. A final type that Hubble recognized is the irregular galaxy, which is anything that doesn't fit in another classification.

Elliptical galaxies are classified as such based on their 2-dimensional appearance being elliptical. The actual 3-dimensional shape is generally ellipsoidal, in which the three principal axes may be of different lengths. A purely spherical galaxy would be a special case. Elliptical galaxies lack any distinct internal features besides smoothly increasing brightness towards the center. A lenticular galaxy is a transition type between elliptical and spiral. It has a central bulge that is ellipsoidal in shape, but also a flattened disk outside the bulge, like the disk of a spiral galaxy, except that there are no distinct spiral arms.

The Hubble classification scheme recognizes distinctions within each type, based on the degree of elongation (for an ellipse) or how tightly wound the spiral arms are. For our purposes here, it's enough to consider just the main types: ellipticals, lenticulars, spirals, and everything else (irregulars).

Hubble constructed his classification based on nearby galaxies, whose structure was easily observable at the time (1936). There is no compelling reason to think that a very similar sequence could be used to classify galaxies in existence at some very different age of the universe, whose present age is ~13.7 billion years. So, now that our instruments are capable of studying galaxies at much greater distances, and hence at earlier times, a very interesting question is how much different would a useful classification system be for a very different time period. Would there be entirely different types? Or would the types be much the same, but perhaps in different proportions relative to each other?

Astronomers have been wondering about this for some time, and reaching out to greater distances as their equipment allows. Now there is some recent research that covers a substantial number of selected galaxies (143) at redshifts around z~.65, which are about 6 billion light-years distant, and hence observed as they were 6 billion years ago, about 7.7 billion years after the big bang. We have no way to know how old any particular galaxy is, but most must have been little more than 7 billion years old.

For comparison, an appropriate sample of 116 local galaxies was selected, to determine the relative proportion of each type at the present time. All galaxies sampled, both local and distant, were required to have at least a certain minimum intrinsic brightness.

In a nutshell, what the research found is that the types observed are just the same as today. There are no obviously distinct new types such as (say) circular rings with little in the center. However, the percentages of spiral and irregular galaxies are dramatically different, though the percentages of ellipticals and lenticulars are almost the same as in the present-day universe.

The research deliberately included only intrinsically bright, massive galaxies in both samples – because, in the distant group, dimmer, less massive galaxies would mostly not be sufficiently bright to reveal useful details of their shape, if they were even detectable at all. But on top of that, even today the less massive galaxies tend to have more irregular shapes that don't fit in the Hubble classification.

In the resulting sample, fully 72% of local galaxies were spirals, but only 31% of distant ones were. On the other hand, irregular galaxies made up only 10% of the local sample but 52% of the distant one. Yet there was hardly any difference in the percentages for ellipticals and lenticulars, being 3% and 15% (respectively) in the local case and 4% and 12% in the distant case.

Stated differently, on a percentage basis spirals are 2.3 times as abundant now as they were 6 billion years ago. Yet irregulars are now 5.2 times less abundant.

Here's the research abstract:

How was the Hubble sequence 6 Gyr ago?
The way galaxies assemble their mass to form the well-defined Hubble sequence is amongst the most debated topic in modern cosmology. One difficulty is to link distant galaxies, which emitted their light several Gyr ago, to those at present epoch. Such a link is affected by the evolution or the transformation of galaxies, as well as by numerous selection and observational biases. We aim to describe the galaxies of the Hubble sequence, 6 Gyr ago. We intend to derive a past Hubble sequence that can be causally linked to the present-day one.... We found that our single criterion is particularly appropriate to relating distant and nearby galaxies, either if gas is transformed to stars in relatively isolated galaxies or, alternatively, if they accrete significant amounts of gas from the intergalactic medium. Subsequent mergers during the elapsed 6 Gyr, as well as evolution of the stellar populations, are found to marginally affect the link between the past and the present Hubble sequence.... We do find an absence of number evolution for elliptical and lenticular galaxies, which strikingly contrasts with the strong evolution of spiral and peculiar galaxies. Spiral galaxies were 2.3 times less abundant in the past, which is compensated exactly by the strong decrease by a factor 5 of peculiar galaxies. It strongly suggests that more than half of the present-day spirals had peculiar morphologies, 6 Gyr ago, and this has to be taken into account by any scenario of galactic disk evolution and formation. The past Hubble sequence can be used to test these scenarios and to test evolution of fundamental planes for spirals and bulges.

One obvious concern with this kind of analysis is to eliminate as much as possible the chance that the results might be biased by selection effects. This is why the primary criterion from including galaxies in either the local or distant sample is that they all have more than a minimum intrinsic brightness. It goes a long way to ensure that enough detail is present in images of the remote galaxies that accurate classification is possible. There is additional analysis to show that effects such as very recent mergers of galaxies and overall aging of stellar populations should not make substantial differences.

Further, the criteria used to classify galaxies are the same for both samples, and simple enough that they can be applied just as accurately to the distant galaxies as the nearby ones. Consequently, it should not be the case that so many irregular (or "peculiar", in the terminology of the paper) types are found in the distant sample simply because the level of detail is inadequate.

Here are some of the characteristics necessary for a galaxy to be classified as irregular: (1) If more than half the object's light comes from a central region less than 1 kiloparsec (3.26 thousand light-years) in radius, the galaxy is classified as "compact". (2) The object appears to have more than one component – a possible merger of two or more galaxies. (3) The object is asymmetrical, with a central bright area and "tail" on only one side. (4) The light in the central area is bluer than the surrounding part (in which young, blue stars of spiral galaxies are usually found) – in the spiral galaxy pictures above you can see that the arms are bluer than the center.

Among objects that are not classified as irregular, the further distinction between elliptical, lenticular, and spiral is based on the percentage of light coming from the central region, with the highest percentage leading to an elliptical classification and the lowest (<50%) to spiral.

The results of this study are actually rather surprising. It has long been assumed that galaxies at present are largely the product of multiple mergers over time between smaller galaxies, and further, that this should lead to more rather than fewer irregularly shaped galaxies.

Instead, just the opposite seems to the case. The percentage of galaxies in both samples that are considered elliptical or lenticular is small and very similar, and therefore probably not representative of the typical evolutionary progression. This is so even if some galaxies in either class transitioned from (or to) the classes of spirals and irregulars.

The most natural conclusion would seem to be that a large percentage of galaxies 6 billion years ago either never had a regular shape or else were midway in the process of merging with others, whereas this is uncommon now. And further, the result at present of all those mergers consists mostly of symmetrical spiral galaxies, rather than ellipticals, lenticulars, or asymmetrical "peculiar" galaxies.

So it seems that the normal course of galaxy evolution is to produce galaxies with a spiral shape much like that of the Milky Way. This is surprising and unexpected.

The obvious question, then, is how so much symmetry and regularity evolved. The same research group responsible for the paper just described has made some specific hypotheses about how this process worked. But that is reported in another paper (here or here) – which we'll look at another time.
Delgado-Serrano, R., Hammer, F., Yang, Y., Puech, M., Flores, H., & Rodrigues, M. (2010). How was the Hubble sequence 6 Gyr ago? Astronomy and Astrophysics, 509 DOI: 10.1051/0004-6361/200912704

Further reading:

Forming the present-day spiral galaxies (2/4/10)

Today's Spiral Galaxies Were Once the Ugly Ducklings (2/8/10)

Spiral Galaxies Exist — But Why? (2/14/10)

Older galaxies more peculiar, census shows (2/8/10)

How was the Hubble sequence 6 Gyrs ago? – open access arXiv version of the paper

The Hubble sequence: just a vestige of merger events? – companion paper (arXiv version)

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