Ancient Galaxies Packed with Stars

by Paul Gilster on April 30, 2008

Just how different were things in the early universe? One answer comes from a study of galaxies whose light has taken eleven billion years to reach us. In this early era — the universe would have been less than three billion years old — researchers have found galaxies so unusually compact that they compress a galaxy’s worth of stars into a space only five thousand light years across. Such objects would be able to fit into the central hub of the Milky Way. What’s more, these ultra-dense galaxies may account for as much as half the number of all galaxies of their mass that existed at this time.

“In the Hubble Deep Field, astronomers found that star-forming galaxies are small,” said Marijn Franx of Leiden University, The Netherlands. “However, these galaxies were also very low in mass. They weigh much less than our Milky Way. Our study, which surveyed a much larger area than in the Hubble Deep Field, surprisingly shows that galaxies with the same weight as our Milky Way were also very small in the past. All galaxies look really different in early times, even massive ones that formed their stars early.”

View within an ultradense galaxy

I sometimes try to imagine the scene from within a globular cluster, a sky awash with tightly packed stars. A compact galaxy must have offered any denizens of eleven billion years ago quite a view as well, as in the artist’s concept above, which shows the view from a hypothetical planet in a distant ultradense galaxy, its sky packed with thousands of stars. We would see 200 times more stars here than we see in Earth’s night-time sky. Image credit: NASA, ESA, G. Bacon (STScI), and P. van Dokkum (Yale University).

Today’s far more slowly rotating galaxies have correspondingly slower moving stars. But these early, compact galaxies show stars moving around their cores at 500 kilometers a second. The speculation now turns to just how such galaxies wound up becoming enlarged over the ensuing aeons. Galactic collisions are an obvious possibility, but there is much to learn about other processes that may have been at work, and the role of dark matter in all this may be consequential. The paper on this work goes out of its way to note that the main mechanism for moving from these early galaxies to the size and density of a more mature galaxy like the Milky Way may not have been identified.

Another wild card: The observed densities may be overestimated because of uncertainties in the analysis, about which the authors list several possibilities. Hubble’s Wide Field Camera 3, set for fall of 2008 installation via Servicing Mission 4, may be able to tighten things up. The paper is van Dokkum et al., “Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at Z ~ 1.3: Early-Type Galaxies Did Not form in a Simple Monolithic Collapse,” Astrophysical Journal Letters 677, pp. L5–L8 (April 10, 2008). Abstract available.

{ 8 comments }

dad2059 April 30, 2008 at 18:19

Quite a mystery Paul, how these early galaxies could have attained mass as they matured. Dark matter? Seems reasonable for the time being because there isn’t any observable visible means for these galaxies to grow.

The view from one of the early planets is great, imagine an intelligent being looking out from there!

James M. Essig April 30, 2008 at 20:37

Hi Paul and dad2059;

There are still vantage points in particular locations where the night sky might appear increadably and beautifully dense with night time stars. In particular, dense large globular clusters come to mind. I can imagine that for dense globular clusters where a variety of stellar spectral types exist with an average seperation of say 1/4 to 1/3 LY, one could probably make out the colors of the stars with the unaided eye from the surface of a terrestrial planet.

When one imagines the incredable variety of nebula, supernova remnents, planets with mutliple moons etc., then one can imagne the incredable views that are available throughout the depths of our universe. Lots of beautiful vistas for our decendants to behold. All of this is obvious, but when I think of the beauty, I wish I could book a flight to such a planetary system already.

I would happily volunteer for a 1/3 C optimized Isp fusion rocket flight to Alpha Centuari in a heartbeat. Too bad I will be a very old man before any such hardware could be ready for flight.

Thanks;

Jim

Adam May 1, 2008 at 3:05

Hi Guys

On a planet with an atmosphere the sky-glow from the brightest stars would wash out the stars further away, so the sky wouldn’t be so crowded. The Moon similarly rather jealously limits the stars around her when she’s full. A star 10,000 times brighter than the Sun within a light-year would be as bright as the Moon. A few within a few parsecs would dominate the sky.

Such heavy, compact little galaxies is a real puzzler. I do wonder what errors might have crept in or what exotic gravitational effects might make a little knot of stars look like a massive galaxy.

Alex May 1, 2008 at 9:59

Hmm, I wonder if it would be possible for an advanced civilization to rearrange its home galaxy into a dense, compact form to cut down travel and communication lag time.

Instead of taking hundreds or tens of thousand of years, a compact galaxy only requires a few thousands to cross from one side to the other.

I’m not saying that the discovered compact galaxies are actually megaengineering artifacts, they’ve just given me an idea.

dad2059 May 1, 2008 at 14:13

Great idea Alex, why not?

I bet we’ve seen many mega-engineering projects through our observations and just assumed they’re natural occurrances.

ljk May 2, 2008 at 12:13

Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse

Authors: Pieter van Dokkum, Marijn Franx, Mariska Kriek, Bradford Holden, Garth Illingworth, Daniel Magee, Rychard Bouwens, Danilo Marchesini, Ryan Quadri, Greg Rudnick, Edward Taylor, Sune Toft

(Submitted on 27 Feb 2008)

Abstract: Using deep near-infrared spectroscopy Kriek et al. (2006) found that ~45% of massive galaxies at z~2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star-assisted Keck/AO.

Considering that their median stellar mass is 1.7×10^11 Solar masses the galaxies are remarkably small, with a median effective radius of 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5 kpc and average stellar densities which are two orders of magnitude lower than the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm previous studies at z>2 which lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies.

Our findings demonstrate that fully assembled early-type galaxies make up at most ~10% of the population of K-selected quiescent galaxies at z~2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z~2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.

Comments: Accepted for publication in ApJ Letters

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.4094v1 [astro-ph]

Submission history

From: Pieter van Dokkum [view email]

[v1] Wed, 27 Feb 2008 21:34:01 GMT (278kb)

http://arxiv.org/abs/0802.4094

ljk May 2, 2008 at 12:20

Cosmic Variance brings up some issues on this find:

http://cosmicvariance.com/2008/05/02/teeny-weenie-massive-galaxies/

ljk December 5, 2008 at 11:00

How long has the Milky Way got? About 8 trillion years:

http://oklo.org/?p=302

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