Figure caption. Example distributions of the first four
velocity “moments” (called v, σ, h3 and h4 ) measured from the GMOS-N
IFS data for two of the MASSIVE survey galaxies. For each galaxy, the
top row shows two-dimensional maps, while the bottom row shows two-sided
radial profiles from Gemini/GMOS-N (magenta circles) and McDonald
Observatory (green squares) data. For more information, see the study by
Berkeley graduate student Irina Ene, in the June issue of The Astrophysical Journal. Hi-res image
Every galaxy has a story, and every galaxy has been many others in
the past (unlike for humans, this is not purely metaphorical, as
galaxies grow via hierarchical assembly). Generally speaking, the most
massive galaxies have led the most interesting lives, often within
teeming galactic metropolises where they are subject to frequent
interactions with assorted neighbors. These interactions influence the
structure and motions of the stars, gas, and dark matter that make up
the galaxies. They also affect the growth of the supermassive black
holes at the galaxies’ centers.
Although the detailed life stories of most galaxies will remain
forever uncertain, the key thematic elements may be surmised in various
ways. A particularly powerful probe of a galaxy’s dynamical structure is
called integral field spectroscopy (IFS), which dissects a galaxy’s
light at each point within the spectrograph’s field of view. In this
way, it is possible to construct a map of the motions of the stars
within the galaxy and infer the distribution of the mass, both visible
and invisible. IFS observations of the outskirts of a galaxy can provide
insight into its global dynamics and past interactions, while IFS data
on the innermost region can measure the mass of the supermassive black
hole and the motions of the stars in its vicinity.
The MASSIVE Galaxy Survey, led by Chung-Pei Ma of the University of
California, Berkeley, is a major effort to uncover the internal
structures and formation histories of the most massive galaxies within
350 million light years of our Milky Way. A recent study by the MASSIVE
team presents high angular resolution IFS observations of 20 high-mass
galaxies obtained with GMOS at Gemini North, combined with wide-field
IFS data on the same galaxies from the 2.7-meter telescope at McDonald
Observatory in Texas. The study, led by Berkeley graduate student Irina
Ene, appears in the June issue of The Astrophysical Journal.
The accompanying figure shows example maps of four indicators, or
“moments” (called v, σ, h3 , and h4), of the stellar motions within two
galaxies in the MASSIVE survey. The maps, based on the GMOS IFS data,
cover the central regions of the galaxies. The figure also shows graphs
of how these indicators vary with distance from the centers of these
galaxies. Although both galaxies exhibit ordered central rotation, they
are strikingly different in how the motions of the stars vary within the
galaxy. Interestingly, for galaxies in the MASSIVE Survey, the
directions of the motions of the stars in the central regions are often
unaligned with the motions at large radius. This indicates complex and
diverse merger histories.
As a proof of concept, the new study performs detailed dynamical
modeling of the IFS data for NGC 1453, the galaxy in the sample with the
fastest rotation rate. The team’s analysis reveals the amount of dark
matter in this galaxy and shows how the shapes of the stars’ orbits
change with radius. In addition, the team found an impressively large
mass for the central black hole, more than three billion times the mass
of our Sun. The MASSIVE Survey team is currently performing detailed
modeling for all the rest of the galaxies in the sample. The results
will provide further insight into the assembly histories of the largest
galaxies in the local Universe and refine our understanding of the
coevolution of galaxies and their central black holes up to the most
extreme masses.
Source: Gemini Observatory
