Sunday, October 25, 2009

A surprisingly compact early galaxy

Astronomers are beginning to learn significant details of the structure of galaxies in the early universe. And what they're learning is rather surprising: at least some early galaxies are almost as massive as otherwise similar galaxies in the present universe, yet they are much smaller in linear size, by a factor of five, thus much more compact.

What time period are we talking about here? It's not actually the time that the earliest galaxies formed, which was less than a billion years after the big bang. Instead, the time in question was around 3 billion years after the big bang.

Although that's roughly 10.7 billion years ago, many galaxies at that time were actually fairly mature, even old. This is because they had been around for more than 2 billion years, which is more than time enough for all their massive, hot, bright stars to have burned out long before. If these galaxies had depleted most of their star-forming material, not many new, hot, young stars could form. The rate of star formation might be as small as it is now in the Milky Way, only two to four solar masses worth per year, compared to thousands per year at the peak.

Young stars include proportionately more massive stars, because it's the massive ones that burn out quickly. Stars that are more than 2 billion years old have to be smaller. Smaller stars are also dimmer, cooler, and redder in color. (Recall the Hertzsprung–Russell diagram, which displays the relationship between luminosity and color.) Consequently, older galaxies that are no longer forming many new stars are also redder, and that's how astronomers estimate roughly galaxy age, or at least the length of time since rapid star formation ceased.

Certain events, such as collisions and mergers between galaxies can fire up rapid star formation again. So the correlation between color and age is not at all exact, but it's still there.

Another fact about galactic appearance is that the central part of any galaxy, even a spiral, is much brighter than the outer reaches, like the spiral arms (if any), simply because the central part of a galaxy contains most of the stars. So at the distances we're concerned with here – over 10 billion light-years – all we can really observe with existing optical telescopes is the central part of a galaxy. To a first approximation, then, very distant galaxies look ellipsoidal in shape, even if they're really spirals.

For the time period we're interested in, 3 billion years after the big bang, the redshift of light we see from objects at that time (denoted by z) is about 2.2. (See here for a fuller explanation.) The definition of redshift means that the wavelength of light emitted at z~2.2 is stretched by a factor of z+1~3.2. So visible light, with a wavelength of about 400 to 700 nanometers is shifted to 1.3 to 2.2 microns, in the near infrared. Although this kind of infrared light can be studied by ground-based spectroscopy, the best optical imagery has to be done from space-based instruments, which makes the job a lot harder.

With all that as background, the newly published result we're concerned with is really pretty simple. It has confirmed that a certain galaxy, named 1255-0, at z=2.186 is about a third as massive (~2×1011M) as the Milky Way and similar galaxies in our neighborhood at the present time, even though its central region is much smaller, with a radius of about 2500 light years. Consequently, stars in the central region of 1255-0 are packed much more closely together.

This observation raises two distinct problems: First, most existing models of galaxy formation do not predict that typical galaxies of that age will be so compact. Second, no galaxies of that sort seem to exist in our general neighborhood, so at least some of them presumably evolved from galaxies like 1255-0 – and it's not clear how that could happen.

Here's the research abstract:

A high stellar velocity dispersion for a compact massive galaxy at redshift z = 2.186
Recent studies have found that the oldest and most luminous galaxies in the early Universe are surprisingly compact, having stellar masses similar to present-day elliptical galaxies but much smaller sizes. This finding has attracted considerable attention, as it suggests that massive galaxies have grown in size by a factor of about five over the past ten billion years (10 Gyr). A key test of these results is a determination of the stellar kinematics of one of the compact galaxies: if the sizes of these objects are as extreme as has been claimed, their stars are expected to have much higher velocities than those in present-day galaxies of the same mass. Here we report a measurement of the stellar velocity dispersion of a massive compact galaxy at redshift z = 2.186, corresponding to a look-back time of 10.7 Gyr.

Note that the research isn't the first to identify very massive but compact galaxies at z~2. Rather, it's new in that it has confirmed the estimate of mass by a new method, and that's what's significant.

You see, there are basically two different ways, at present, to estimate the mass of a very distant galaxy. One method relies on a plausible assumption, that stars less than 2 billion years old, except for the very youngest, have fairly well-known distributions of mass and luminosity. And so, from the total luminosity of the galaxy that we can observe, we can form a good estimate of the total mass of stars. This is sometimes called the "photometric" mass.

This sort of measurement is what has been used to infer that a number of galaxies at z~2 may have been very massive in spite of being small in extent. As suggested above, this observation raises at least two problems, so astronomers would like to measure mass in a different way, just to be sure. Besides, perhaps the assumptions about the mass and luminosity distributions of the stars in such galaxies could be wrong.

Fortunately, there is another type of observation that can lead to good mass estimates, but it is much more difficult to make. This involves measuring the "dispersion" of velocities of stars in the galaxy. That is related to the distribution of stellar velocities. But since we can't distinguish individual stars at that distance we certainly can't measure their velocities (by very slight differences in stellar redshifts from the redshift of the galaxy as a whole).

Even though the measurement is difficult to make in practice, it's simple to describe. One simply looks at the width of a few absorption lines in the galaxy's spectrum. If the lines are wide, it means that individual stars have substantially different velocities, including a certain proportion which are quite large. This is, basically, the meaning of "dispersion".

From the relative number stars with high velocities one can infer the total mass. This yields what is called the "dynamical" mass of the galaxy. What the present research found is simply that the dynamical mass of 1255-0 is pretty close to the known photometric mass.

Why is it that lots of high-velocity stars indicates a substantial mass? Just fairly basic physics, based on two of Newton's laws. (If you're a physicist, you learned this a long time ago, so it's "obvious".) The first is Newton's law of gravitation, which is F=G×M×m/r2. This describes the gravitational force (F) between two objects having masses M and m separated by a distance r. G is a certain constant called, of course, the gravitational constant. This can be applied to a galaxy with mass M and one of its stars, with mass m, where r is the distance from the star to the center of mass of the galaxy.

The other law is Newton's second law of motion, which says F=ma. F is, again, the gravitational force of the galaxy, m is the mass of a particular star, and a is the acceleration of the star due to the force. (F and a are actually "vector" quantities, of couse, meaning they have a direction in 3-space.) You can think of the acceleration of an object as a way of measuring the force acting on it.

Putting the two laws together, we find that for any particular star its accleration will satisfy a = G×M/r2, so the star's mass doesn't matter at all, only the mass of the galaxy. Just as in our own galaxy, almost all stars are in orbit around the center of mass of the galaxy, so a star's velocity, as seen from far away, varies periodically in a predictable way, deducible from acceleration (which is the rate of change of velocity).

From calculations that are routine (at least for a physicist) one thus obtains a good estimate of the mass of a galaxy from the distribution of velocities of its stars, which in turn is deducible from the dispersion of spectral lines.

There is one additional complication: matter in any form other than what makes up stars, most especially dark matter. But the present research shows that the mass as estimated photometrically (where any nonluminous matter plays no part) and the mass as estimated dynamically (where dark matter could be important) are pretty close.

Consequently, there isn't much nonluminous matter (including nonbaryonic dark matter) in the central part of the galaxy where most of the stars are. This is as expected, since most galaxy models as well as observations have the dark matter distributed over a much larger volume than the central part of the galaxy. (Other elementary physics shows that matter outside the orbit of a star does not affect the star's motion, as long as that matter is evenly distributed.)

The net of all this is that the two problems mentioned above are real and pose questions that need to be answered.

How could massive galaxies as compact as 1255-0 have formed in the first place? It is not the case that massive compact galaxies like 1255-0 are exceptional anomalies at z~2. Instead, they seem to make up as much as 30 to 40% of galaxies whose masses have been estimated (photometrically) at that distance.

Existing models involving cold dark matter mostly do not predict such a thing. But this doesn't mean that the models can't be refined. In particular, the whole theory of cold dark matter as a driver of galaxy formation need not be discarded. It isn't necessary to invoke some exotic new physics or variations of Einstein's general relativity. The most natural approach is to find a suitable refinement of the galaxy formation model. There is research that was reported in January 2009 and offers one sort of model. It involves filaments of dark matter that conduct streams of cold gas into a central region around which a galaxy grows. Research paper: here. Additional stories: here, here, here, here.

The other problem has received less attention. The difficulty is in explaining how a galaxy (or rather, its central region) grows by a linear factor of five or so over a period of ~10 billion years, even though the mass contained in that region doesn't grow much at all. It just seems to "puff up".

Galaxies have long been presumed to grow through mergers of less massive galaxies, the most important of which are of roughly equal mass. One possibility is that few such mergers actually occur, and instead colliding galaxies mostly pass through each other without merging, but with some expansion of linear size each time. Another possibility is that there are many mergers involving mostly low-mass galaxies captured by much larger ones. That would also help explain another major puzzle: why many fewer low-mass galaxies are observed than current models predict.

Questions of this sort are not easy to resolve. They're a lot like questions about the evolution of life. All we can actually observe consists of snapshots from different points of time. Events unfold too slowly to actually see what happens. And moreover, the very small galaxies that might play a role are currently too faint to observe over most of the past 12 billion or so years of cosmic history.
van Dokkum, P., Kriek, M., & Franx, M. (2009). A high stellar velocity dispersion for a compact massive galaxy at redshift z = 2.186 Nature, 460 (7256), 717-719 DOI: 10.1038/nature08220

Further reading:

Astronomers Find Hyperactive Galaxies in the Early Universe (8/5/09) – press release

Speeding Stars Confirm Bizarre Nature of Faraway Galaxies (8/5/09) – article at

Galactic evolution: more data, no more answers (8/12/09) – article at

Galaxy formation: Too small to ignore (8/6/09) – Nature news article

Puffing up elliptical galaxies (10/3/09) – blog post (SarahAskew)

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Blogger Sarah said...

Hi Charles,
This is an interesting paper and glad to see it being discussed in so many places. Van Dokkum's result is certainly a nice one - but I think it's always a little dangerous to draw conclusions out of the observation of one object. And the data the authors used, as they themselves admit, are really pushing the limits of what we can achieve with today's instruments. I actually wrote a blog post about these issues myself -you can read it here.

10/26/2009 05:05:00 AM  
Blogger Charles Daney said...

Thanks very much for your comments, Sarah.

How did you ever find this post so fast?

You're correct, of course, that one observation isn't sufficient to base reliable conclusions on. However, the observation does seem to agree with previous results obtained by different techniques.

This paper will certainly stimulate others to study other objects as soon as the technology allows - and I gather that infrared instrumentation is just what you do...

10/26/2009 10:41:00 AM  

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