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  Current Research Highlight :: January 2010 all highlights

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

An international team of astronomers lead by Tim Heckman (The Johns Hopkins University) and Roderik Overzier (Max Planck Institute for Astrophysics) is using a novel technique to gain new insights into a number of important processes related to the formation and evolution of galaxies. The new results were obtained by focusing on a rare subset of the nearby galaxy population that are the best local analogs of a type of starburst galaxy common only in the very early Universe. As part of this study, the team discovered that some of these local starbursts harbour very young, massive and dense nuclei in their centres. These nuclei are interpreted as being the possible progenitors of “cuspy” cores of typical low-mass, early-type galaxies, but seen at an early, formative stage directly after being triggered by a massive gaseous inflow event in the aftermath of a galaxy merger. These galaxy bulges-in-formation could be ideal sites for the formation or fueling of supermassive black holes.

Fig. 1: False-color HST images of the nearby starburst sample showing the (rest-frame) UV in blue/purple and optical light in yellow/red. The images measure 6" by 6". Although most objects are highly compact in both the UV and optical, a small subset consists of a very bright unresolved component in the middle of an extended, low surface brightness disk. The images demonstrate a wide range of complex morphologies often suggestive of interactions and merging of galaxies.

Fig. 2: HST optical images of six of the nearby UV-luminous starburst galaxies from Figure 1 that show a bright, unresolved object in the centre of a larger, highly disturbed disk.

Fig. 3: Mass (M*) and effective radius (Re) vs. the effective mass surface density (Σe) of the brightest clump identified in each galaxy (squares). The comparison data shown were extracted from a compilation of literature results presented in a similar fashion in Figure 45 in Hopkins et al. (2009), showing globular clusters (crosses), nuclear star clusters (triangles), and the extra-light components in early-type galaxies from the samples of Kormendy et al. (2009, stars) and Lauer et al. (2007, circles) as measured by Hopkins et al. (2009). The six unresolved nuclei from Figure 2 are indicated with filled squares. These objects are consistent with being the progenitor starbursts that formed the old central "extra-light" components seen in local, cuspy-cored elliptical galaxies. The local cusps are believed to have formed in highly dissipative merger events at high redshift, perhaps in a very similar fashion as being witnessed in the sample of local analogs of the high redshift starbursts.

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.

Roderik Overzier

Further reading

Heckman, T.M., Hoopes, C., et al., "The Properties of Ultraviolet-luminous Galaxies at the Current Epoch", 2005, ApJ, 619, L35

Hoopes, C., Heckman, T.M., et al., "The Diverse Properties of the Most Ultraviolet-Luminous Galaxies Discovered by GALEX", 2007, ApJS, 173, 441

Overzier, R.A.,Heckman, T.M., et al., "Morphologies of Local Lyman Break Galaxy Analogs. I. Evidence for Starbursts Triggered by Merging", 2008, ApJ, 677, 37

Overzier, R.A.,Heckman, T.M., et al., "Morphologies of local Lyman break galaxy analogs. II. A Comparison with galaxies at z=2-4 in ACS and WFC3 images of the Hubble Ultra Deep Field", 2009, ApJ, In Press linkPfeilExtern.gif(arXiv:0911.1279)

Overzier, R.A.,Heckman, T.M., et al., "Local Lyman Break Galaxy Analogs: The Impact of Massive Star-Forming Clumps on the Interstellar Medium and the Global Structure of Young, Forming Galaxies", 2009, ApJ, 706, 203

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