LOCAL LYMAN BREAK GALAXY ANALOGS: THE IMPACT OF MASSIVE STAR-FORMING CLUMPS ON THE INTERSTELLAR MEDIUM AND THE GLOBAL STRUCTURE OF YOUNG, FORMING GALAXIES

We report on the results of Hubble Space Telescope optical and UV imaging, Spitzer mid-IR photometry, and optical spectroscopy of a sample of 30 low-redshift (z ∼ 0.1 to 0.3) galaxies chosen from the Sloan Digital Sky Survey and Galaxy Evolution Explorer surveys to be accurate local analogs of the high-redshift Lyman break galaxies. The Lyman break analogs (LBAs) are similar in stellar mass, metallicity, dust extinction, star formation rate (SFR), physical size, and gas velocity dispersion, thus enabling a detailed investigation of many processes that are important in star-forming galaxies at high redshift. The main optical emission-line properties of LBAs, including evidence for outflows, are also similar to those typically found at high redshift. This indicates that the conditions in their interstellar medium are comparable. In the UV, LBAs are characterized by complexes of massive clumps of star formation, while in the optical they most often show evidence for (post-)mergers and interactions. In six cases, we find a single extremely massive (up to several ×109 M☉) compact (radius ∼102 pc) dominant central object (DCO). The DCOs are preferentially found in LBAs with the highest mid-IR luminosities (L24 μm = 1010.3–1011.2 L☉) and correspondingly high SFRs (15–100 M☉ yr−1). We show that the massive star-forming clumps (including the DCOs) have masses much larger than the nuclear super star clusters seen in normal late-type galaxies. However, the DCOs do have masses, sizes, and densities similar to the excess light/central cusps seen in typical elliptical galaxies with masses similar to the LBA galaxies. We suggest that the DCOs form in the present-day examples of the dissipative mergers at high redshift that are believed to have produced the central cusps in local ellipticals (consistent with the disturbed optical morphologies of the LBAs). More generally, the properties of the LBAs are consistent with the idea that instabilities in a gas-rich disk lead to very massive star-forming clumps that eventually coalesce to form a spheroid. Finally, we comment on the apparent lack of energetically significant active galactic nuclei in the DCOs. We speculate that the DCOs are too young at present to grow a supermassive black hole because they are still in a supernova-dominated outflow phase (age less than 50 Myr).