4.2. Luminous Diffuse Objects and Chain Galaxies

In Conselice et al. (2004) a new galaxy type, found abundantly between 1 < z < 2, is described and characterized. These galaxies, which have no local counterparts, were discovered based on their low light concentrations and high luminosities, and are called Luminous Diffuse Objects (LDOs). These objects are fairly common with surface densities 1.8 arcmin^-2, and co-moving number densities of 5 × 10⁵ Gpc^-3 within the GOODS South field (Conselice et al. 2004). These objects were independently discovered by Elmegreen et al. (2004a) and are likely face on counterparts of the `chain galaxies' discussed in Cowie et al. (1995) (Elmegreen et al. 2004b).

Elmegreen et al. (2004b) compared the colors of the star forming knots in their sample of LDOs with the knots found in chain galaxies, finding a very similar color distribution. This suggests that chain galaxies are the edge-on versions of LDOs. Both chain galaxies and LDOs are known to be large complexes of star forming regions (Cowie et al. 1995; Conselice et al. 2004a). Formation scenarios for these systems are discussed in Elmegreen et al. (2004b), and are consistent with large amounts of star formation occurring after gas in an initial disk fragments and produce several large clumps. In models, these clumps are predicted to form through energy dissipation and later merge together to form bulges (Immeli et al. 2004). The fact that there are no obvious bulge components in LDOs is a clue that bulge formation may occur after disk formation, not before, as is generally assumed in hierarchical models.

LDOs are likely in a phase where a large fraction of their stellar mass is being assembled. The star formation rate in LDOs is on average 4 M year^-1 before correcting for dust, and they have starburst spectral energy distributions (Figure 8). These systems account for up to 50% of all the star formation occurring between 1 < z < 2, where a large fraction of the stellar mass in galaxies formed (Dickinson et al. 2003). The effective radii of these galaxies varies from 1.5 - 10 kpc, and for these and other reasons a fraction of them are likely disks in formation (Conselice et al. 2004; Elmegreen et al. 2004b; Figure 8).

Figure 8. Left panel: The distribution of (i - z) colors for LDOs (circles) as a function of redshift with two Coleman, Wu and Weedman spectral energy distributions and a Kinney et al. starburst model plotted (see Conselice et al. 2004a). These are from bluest to reddest - starburst (solid line), Scd (dashed), Sbc (long dashed). Right panel: Absolute magnitude-effective radius relationship for LDOs. The solid line is the canonical Freeman disk relationship at z ~ 0. The dashed horizontal lines show the effective radii of different nearby galaxy types.