Galaxy Formation: Observational Clues

Observations of the high redshift universe


Cosmological effects distort our view of high redshift galaxies:
We use imperfect clocks to judge evolution:

High redshift galaxies are intrinsically different from those around us today -- they are younger. Think of comparing a massive galaxy nearby to a massive galaxy in the early universe:

The Hubble Ultradeep Field

Field of view: 202x202 arcseconds (1/100th the area of the full moon)

    Half scale
    Full resolution

So we can look at the deep imaging, estimate redshifts (then confirm spectroscopically for better accuracy when necessary) and look at samples of galaxies at different redshifts. From Conselice, ARAA, 2014:
High redshift galaxies often look smaller, lumpier, and bluer than galaxies at intermediate and low redshift.

What about morphology? Look at massive galaxies see at a range of redshifts (again from
Conselice, ARAA, 2014). The mix of types appears to change with redshift -- Peculiar objects and disky things first, spheroidal types later. But this is almost certainly very dependent on environment and galaxy mass!

What about if we look at the stars in nearby galaxies and ask when they formed? Population synthesis studies of nearby elliptical galaxies by Thomas et al (2010) show downsizing. Plot inferred star formation rate (y-axis) against time/redshift (x-axis) for elliptical galaxies of different masses (red: most massive, blue: less massive).

So wait. Observations of the high redshift universe suggest it takes time for massive ellipticals to grow, but studies of massive ellipticals in the local universe suggest these things formed very early. What's wrong with this picture?

It actually holds together. The stars that are in massive galaxies today formed long ago when they were in smaller clumps. Over time, these smaller lumps then merged together to assemble themselves into the massive objects we see today. Lower mass galaxies we see today formed their stars later and/or over longer timescales, on average.

So what about the idea of inside out galaxy formation? Do galaxies grow their outskirts later than their inner regions?  Look at the changing sizes of massive galaxies, as measured by their effective radius (Conselice 2014 ARAA):

Also, we know that disk galaxies in the local universe show color gradients: they get bluer (younger?) as you go outwards in the disk.

The Cosmological Evolution of Star Formation

Remember that galaxy growth and star formation can be very different things. Let's forget about galaxies specifically and just ask how quickly did the universe form its stars?

(from Shapley 2011 ARAA)
Blue band shows star formation rate as observed;
red band shows the rate inferred after correcting for dust obscuration.

Star formation in the universe peaked around z ~ 2-3, when the universe was only a few billion years old. So most of the stars in the universe are ~ 10 billion years old. But...
So again, we are seeing differences between when stars formed, where stars formed, and how galaxies assemble. There are significant variations due to differences in mass and environment.