In the first four lectures of this series, we reviewed star formation in local galaxies. Recall that the star formation rate is proportional to the CO emission, from which we concluded that star formation occurs only in molecular gas and that the consumption rate from molecules to gas is constant. This derivation assumed a fixed CO to H2 conversion rate to get the molecular gas mass, a fixed IMF, uniform grain properties, and certain extinction corrections to get the star formation rate. In addition, the molecular fraction scales almost linearly with pressure, and the pressure depends on the mass column densities, gas and stars, and the velocity dispersions, gas and stars. We also saw that spiral waves promote star formation in the arms, or organize the star formation, but do not affect the average rate much. The same is apparently true for star formation in shells, bright rims and pillars, which trigger star formation in these regions, i.e., organize where it happens, without changing the global average rate much. Star formation seems saturated in inner disks, so the detailed mechanisms of cloud formation do not appear to matter.
In addition, we saw that stars form in hierarchical patterns with star complexes, OB associations, clusters, and so on, because of turbulence compression and self-gravity. As a result, there are power-law mass functions for clouds, clusters, and stars, and there are space-time correlations for clusters. There are probably similar space-time correlations for young stars which are not observed yet.
We would like to discuss here what changes for young galaxies at high redshift. At first, we expect high redshift galaxies to look like normal galaxies viewed in the restframe ultraviolet. They would look dimmer because of cosmological surface brightness dimming, and the star formation regions would be blurred out because of poor spatial resolution. But still, we might expect to see the uv restframe versions of normal galactic features, i.e., exponential disks, spiral arms, bars, lots of small star-forming regions, and a general diversity in the relative prominence of disks and spheroids (i.e., the Hubble types).
Barden, Jahnke & Häussler (2008) made model images of redshifted SDSS galaxies to z = 0.15, 0.5, and 1, and even increased the intrinsic brightness for the z = 1 images. The result was a significant loss of faint structures, including the outer disk and the faint star-forming regions. Overzier et al. (2010) redshifted "Lyman Break Analogs" to z = 2,3, and 4. They found that small clumps blend together and faint peripheral tidal features disappear. Petty et al. (2009) looked at the standard structural measures: the Gini coefficient, M20 (central concentration), and the Sersic index for redshifted local galaxies. They found that the model galaxies were smoother (lower Gini) and more centrally concentrated (lower M20) than their local counterparts. These studies reinforce our notion that high redshift galaxies should look somewhat smooth and centrally concentrated if they are at all like local galaxies.
In fact, when deep high resolution images of the sky were taken, particularly by the Hubble Space Telescope (HST), disk galaxies did not look anything like these expectations from local galaxies. Beyond z ~ 2, galaxies are mostly irregular, asymmetric, and clumpy (van den Bergh et al. 1996, Abraham et al. 1996, Conselice et al. 2005). In particular, there is a class of galaxies that is almost entirely clumpy, with nearly half of the light in several big star-forming clumps and no obvious underlying exponential disk (Elmegreen & Elmegreen 2005). Figure 1 shows two examples of clumpy disks, with UDF catalog numbers indicated. On the left are SkyWalker 1 images using the ACS camera and on the right are NICMOS images in the near-infrared with 3× lower resolution. The galaxies contain several large star-forming clumps with no central concentration from a bulge or exponential disk.
Figure 1. Two examples of clump cluster galaxies from the HST UDF with color ACS camera images on the left and NICMOS images on the right (from Elmegreen et al. 2009a). These galaxies are characteristic of this class, having several large clumps of star formation and no obvious interclump disk.
In a catalog of galaxy morphologies in the HST Ultra Deep Field, considering only galaxies larger than 10 pixels so their internal structure can be observed, Elmegreen et al. (2005a) recognized 6 basic types: Chain galaxies (121 examples), Clump Clusters (192), Double (134), Tadpole (114), Spiral (313), and Elliptical (129). Only the spirals and ellipticals resemble local galaxies, and even then the spirals tend to have bigger star-forming regions than local spirals and the ellipticals are clumpy as well (Elmegreen et al. 2005b). Photometric redshifts of these galaxies (Elmegreen et al. 2007a) suggested that all of the clumpy types extend out to at least z ~ 5 with extreme starburst spectral energy distributions. The spirals and ellipticals end at about z ~ 1.5, which could be because either their number density drops, or they are too faint in the restframe uv to see at higher redshifts.
1 designed by K. Jahnke and S.F. Sánchez, AIP 2004 Back.