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Clumpy young galaxies, whether somewhat face-on and called "clump clusters," or edge-on and called chains, have properties that are consistent with their youth, and also show variations that are consistent with their gradual evolution into modern disks.

Their youthful appearance is reinforced by the observation that they are highly molecular (Tacconi et al. 2010, Daddi et al. 2008, 2010a) and highly turbulent (Förster Schreiber et al. 2009, Law et al. 2009, see below). Presumably the turbulence is a result of energy gained from intergalactic accretion (Elmegreen & Burkert 2010) and gravitational instabilities in the disk (Bournaud, Elmegreen & Martig 2009). Many young galaxies have a ratio of rotation speed to twice the dispersion speed that is less than unity. Förster Schreiber et al. (2009) consider that when this ratio is less than 0.4, the disks are dispersion-dominated. Their galaxies have stellar masses in the range from 1010 Modot to 1011 Modot and dynamical masses that are 3 to 5 times larger, on average.

Tacconi et al. (2010) made CO (3-2) maps of several clump clusters and found that typical clumps in these galaxies have 5 × 109 Modot of H2 with radii < 1-2 kpc, SigmaH2 = 300-700 Modot pc-2, and sigma ~ 19 km s-1. They also derived a high gas fraction in the disks. Daddi et al. (2010a) observed CO in 6 galaxies at z ~ 1.5, finding rotation in some cases and a generally high gas fraction. The timescales for gas consumption, stellar build-up, and galactic dynamics were all comparable in the Daddi et al. study, which implies that the galaxies are very young. Daddi et al. also found that the efficiency of star formation is about the same as in normal galaxies today.

The clump stellar masses in clump clusters are ~ 100 times larger than star complex masses in modern spiral galaxies of similar luminosities (Elmegreen et al. 2009b). Bulges or bulge-like objects in clump clusters and chains are sometimes observed, and they are more like the clumps in terms of mass and age than are the bulges in spiral galaxies (Elmegreen et al. 2009a). The interclump surface density and age relative to the clump surface density and age also show variations among different clump clusters (Elmegreen et al. 2009b). All of these variations suggest an evolution from highly clumpy, bulge-free galaxies to smooth spiral galaxies with bulges.

There are essentially no barred clump cluster galaxies. Even if bars were present, they could hardly be recognized in such irregular disks. Bars appear only when the galaxies calm down and develop exponential disks and central concentrations or bulges. Still, there are elongated clumps in some clump clusters, suggesting protobars (Elmegreen et al. 2004b). If these objects really turn into bars, then this suggests bar formation can be a gas-rich process, including significant energy dissipation, and not a pure stellar process as in standard numerical models.

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