Figure 13.
The slope η of the
velocity dispersion
profile is plotted
against the central
velocity dispersion
σ 0 for individual
galaxies in multiple
different samples
of galaxies
(indicated in the
legend). The blue
points represent
brightest galaxies
in groups (BGGs)
of high (square)
and low (circles)
density, while the
green, red, and
yellow points
represent brightest
cluster galaxies (BCGs)
in various samples of
galaxy clusters. The grey
points indicate generic
“early-type galaxies”
(ETGs). The central
dispersion σ 0 is larger
for massive galaxies
because their stars move
more rapidly in the
stronger gravitational
fields of such galaxies.
The dispersion profile η
is negative if the stars
farther from the galaxy
center move more slowly,
and it is positive if the
stars move rapidly at
larger distances. Thus,
massive BCGs tend to
have rising profiles, with
the stars increasing
their velocities at larger
distances from the galaxy
center.
36
lipticals that are not
BCGs, including many
brightest galaxies in
smaller groups.
mation about the mass of a galaxy and how
that mass is distributed.
In a recent study, astronomers used the
GMOS instruments at both Gemini North
and South to measure the stellar velocity
dispersions (a measure of the mean random
stellar speed) and velocity dispersion pro-
files (its variation with galactocentric radius)
in a sample of 32 massive elliptical galaxies,
each of which is the brightest member with-
in a large cluster of galaxies. Such brightest
cluster galaxies (BCGs) tend to reside near
the centers of their respective clusters, and
therefore they are generally embedded
within very extended distributions of both
light and dark matter. The sample of BCGs in
this study included some of the most mas-
sive galaxies in the Universe out to a dis-
tance of five billion light years.
The researchers found a surprising variety
in the shapes of the velocity dispersion pro-
files for the BCGs, with a large fraction of
them showing rising dispersion profiles. This
means that the stars within these galaxies
are moving faster at larger galactocentric
distances in response to an increasing gravi-
tational force. In comparison, rising velocity
profiles are much rarer in other massive el-
GeminiFocus
“You would naively
think that massive el-
liptical galaxies are a
homogeneous, well-
behaved class of ob-
jects, but the most
massive beasts, those
in the centers of
groups and clusters,
continue to surprise
us,” said Ilani Loub-
ser, an astronomer at
North-West Universi-
ty in South Africa and the lead author of the
study. She also noted, “The quality, and the
wealth of information we can measure from
the GMOS spectra (even in poor weather!), is
remarkable.”
The study also found that the slopes of the
velocity dispersion profiles correlate with the
galaxy luminosity (Figure 13), in the sense
that the increase in the speed of the stars
was greater in brighter BCGs, as well as in
brightest group galaxies. Whether the full
diversity in the observed velocity dispersion
profiles is consistent with standard models
for the growth of massive galaxies is not yet
clear. The researchers present their results as
a challenge for detailed cosmological simula-
tions.
The work has been accepted for publication
in Monthly Notices of the Royal Astronomical
Society, and a preprint is available online.
An Ultra-diffuse Galaxy Devoid
of Dark Matter
In most galaxies, the stars are like a luminous
frosting on the predominant mass of dark
matter whose gravity holds the whole con-
January 2019 / 2018 Year in Review