Figure 2.
A comparison of
ellipticity and Sersic
index distributions
from local early-type
galaxies (contours)
with those of the
high-redshift massive
compact galaxies
published in van der
Wel et al. (2011, black
filled triangles) and
Damjanov et al. (2011,
black stars). Contours
are normalized and
smoothed, increasing
in number density
from yellow to red
in logarithmic steps.
Different panels show
changing mass ranges
for the local earlytype galaxy sample,
denoted in the upper
right corner or each
panel. The last panel
(lower right) shows
the distribution for
local massive diskdominated galaxies.
Figure taken from
Chevance et al., 2012.
9
Until recently, we assumed that these ultracompact objects are early-type galaxies —
elliptical systems, shrunken in size by a factor
of three, while retaining their overall mass.
This seemed like a fairly safe assumption, and
in some ways likely, because elliptical galaxies are not only pretty massive, as the objects
seen seem, but also appear structureless, as
do ellipticals.
However, there are growing indications that
this may actually be a bad assumption — after all, the galaxies are so compact that even
if they harbored lots of interesting substructure, we would not resolve it with the Hubble
Space Telescope (HST). This is a nice argument, by the way, for imaging these objects
with Gemini’s new Multi-Conjugate Adaptive
Optics system.
Furtheremore, recent deep, high-resolution images taken with the HST’s Wide Field
Camera 3 and Near-Infrared Camera 2 have
shown, based on observed ellipticities and
Sersic profile fits, that some of these compact,
high-redshift galaxies may contain disks. Indeed, based on a sample of 14 quiescent, ultra-compact galaxies, van der Wel et al. (2011)
claim that disks dominate the majority of
these systems.
GeminiFocus
To investigate this claim, we compared the
ellipticity distribution of 31 carefully selected
quiescent, ultra-compact galaxies to a set of
mass-selected ellipticity and Sersic index distributions (obtained from 2D structural fits to
~ 40,000 nearby galaxies from the Sloan Digital Sky Survey). A Kolmogorov-Smirnov test
shows that the distribution of ellipticities for
the high-redshift galaxies is consistent with
the ellipticity distribution of a similarly chosen sample of massive early-type galaxies.
However the distribution of Sersic indices for
the high-redshift sample is inconsistent with
that of local early-type galaxies, and instead
resembles that of local disk-dominated populations. In other words, while the ellipticities
argue for these compact galaxies being earlytype systems, their profiles argue for them being disks (Figure 2).
The correct conclusion, then, seems to be that
nothing works. The mismatch between the
properties of high-redshift compact galaxies
and those of putative local analogs leads us to
conclude that the basic structures of ultra-compact galaxies probably do not closely resemble
those of any single local galaxy population. Any
galaxy population analog to the high-redshift
compact galaxies that exists at the current ep-
June2012