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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.
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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.
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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.
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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
(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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