MPA :: Current Research Highlight

The "Lyman break galaxy analogs" project is a Key Project carried out with the Galaxy Evolution Explorer (GALEX). GALEX is a UV satellite that has imaged a large fraction of the sky in the far- and near-ultraviolet at an unprecedented combination of depth, resolution and sky coverage. By combining the GALEX all-sky survey with the large spectroscopic database of galaxies provided by the Sloan Digital Sky Survey, the team was able to select and study a sample of galaxies in the nearby Universe (z<0.3) that are both the most luminous and the smallest galaxies detected in the far-UV. This selection was specifically chosen in order to search for starburst galaxies having high star formation rates, low dust attenuations, and compact sizes that are most typical for the population of the UV-selected Lyman Break Galaxies (LBGs) at high redshift (z~3). Such galaxies may serve as a local training set, useful for studying typical processes occurring in starburst galaxies and for comparing with LBGs for which only limited information can be derived as their high redshifts render them small and faint.

The technique proved to be very successful, showing that there exists a rare but detectable population of nearby galaxies that is similar to LBGs in most of its physical properties, such as stellar mass, age, metallicity, extinction, star formation rate, size, morphology, (gas) kinematics and the conditions of the interstellar medium. A collage of images obtained with the Hubble Space Telescope (HST) is shown in Figure 1. The sample of nearby LBG analogs consists of a wide variety of disturbed and “clumpy” galaxies. In the UV, the galaxies are characterized by massive complexes of relatively unobscured starburst regions, while in the optical they most often show the faint tidal features that are typically associated with mergers and interactions between galaxies.

As part of an HST follow-up study, the team uncovered a small sample of galaxies (Figure 2) in which the UV and optical morphology is dominated by a single, unresolved component that is the source of a large fraction of the continuum and line emission. On the basis of their optical spectra it has been ruled out that these are unobscured (Type I) active galactic nuclei. Instead, the peculiar nuclei are interpreted as being due to the intense light from young stars. Modeling of the intensity and colour of the light shows that the nuclei must house between a few hundred million and a few billion solar masses in stars, distributed in a very compact region of only a few hundred parsecs in diameter. The very high stellar mass densities implied are similar to those found in the most massive globular clusters and nuclear star clusters seen in late-type galaxies, but the total masses and sizes of the new class of objects are up to two orders of magnitude larger. However, the dense nuclei resemble, at least structurally, the central “cusps” that are observed in typical low-mass early-type galaxies (Figure 3). In early types, these so-called cusps are due to an excess central stellar mass relative to that expected from extrapolating the galaxy's outer Sersic profile inwards. These cusps are believed to be the remnants of (hypothesized) dissipative galaxy mergers at high redshift. These mergers fueled massive central starbursts in the progenitor population of present-day low mass early-type galaxies. The discovery of young, dense nuclei in the analogs of high redshift starburst galaxies suggests that exactly this process may still be occurring in a small fraction of galaxies observed in the nearby Universe. Further evidence for this is given by the fact that the host galaxies of the dense nuclei have total stellar masses comparable to those of typical cuspy early-types (a few times 10¹⁰ solar masses) and that they have disturbed optical morphologies (Figure 3) indicating that their central starburst was triggered by a major inflow event.

The new finding also provides an attractive scenario for the formation of supermassive black holes. The centres of local massive galaxies have been shown to host (dormant) supermassive black holes that contain about one thousandth of the total mass of the galaxy. It is not exactly known when and how these black holes formed, but the strong correlation between their masses and those of their hosts suggests a common origin. The massive reservoirs of cold gas in the dense nuclei of the LBG analogs may provide ideal nurseries for the formation of these massive black holes. At present, the nuclei are likely too young (a few tens of Myr) to be efficiently growing a supermassive black hole, as they are still in a supernova-dominated outflow phase which would prevent gas from settling onto an accretion disk surrounding a central black hole. However, 50-100 Myr after the most recent starburst, the strong mechanical feedback from the winds of the most massive stars and core-collapse supernovae will have subsided and the low-velocity winds of evolved stars will dominate the local gas kinematics. This may offer a steady flow of material that can be retained within the potential well of the nucleus where it can cool and be (partially) accreted by a black hole. In order to investigate whether any weakly accreting black holes are already present in the dense nuclei, the team has recently started a follow-up campaign using X-ray imaging spectroscopy, infrared spectroscopy and radio interferometry. The results are expected to shed new light on the formation of intermediate mass black hole “seeds” forming deep inside the forming bulges of young galaxies.

Massive, Dense Nuclei in Young Starbursts: Progenitors of Early Type Galaxy Cusps?

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