Sky and Telescope - May 2015 - 14

News Notes

GALACTIC I Charting the Andromeda Galaxy

This composite image of M31 spans 48,000
light-years. At full resolution, you can see
individual stars, even though the galaxy is 2.5
million light-years away. Explore the full image
at http://is.gd/m31zoom.
NASA / ESA / J. DALCANTON / B. F. WILLIAMS / L. C. JOHNSON / PHAT / R. GENDLER

Astronomers working with the Panchromatic Hubble Andromeda Treasury
(PHAT) project released the biggest,
sharpest image yet of M31 on January 5th
at the winter American Astronomical
Society meeting in Seattle.
The ﬁnal image includes 12,834 shots
from more than 400 pointings taken
through ultraviolet, visible, and nearinfrared ﬁlters. Astrophotographer Robert
Gendler stitched the images together to
create the seamless mosaic, a stitching so
careful that it's aligned at the level of individual stars - 117 million of them - or
to better than one-tenth of an arcsecond.
Estimating the amount of dust needed
to redden more distant stars, Julianne
Dalcanton (University of Washington) and
colleagues mapped dust across the galaxy,
creating a 3D map of M31 with more than
four times better resolution than previous
dust-mapping methods. Surprisingly, the
team found that other widely used maps
predict twice as much dust as is really
there, potentially from a calibration issue.
Nearby galaxies might therefore have
much less dust than previously thought.
Another surprise lies in M31's
structure. Astronomers know that starforming regions riddled with young, massive stars trace out spiral galaxies' iconic
arms. Computer simulations show that
such arms should move and evolve over
time. Because stars' colors and luminosities reveal their ages, the PHAT images
enable astronomers to look back in time
and determine M31's star-forming history
in various locations.
14

May 2015 sky & telescope

What the team found is that the arms
aren't all transitory: a ring present today
was also forming stars between 500 and
630 million years ago, a time scale much
longer than astronomers predicted for the
structure's survival. (The inner and outer
rings vary as expected.)
"This was really a surprise," Dalcanton
says. The density of stars in this ring is

about 40% higher than in other regions in
Andromeda, and it contains both old and
young stars - it's not just the young stars
tracing it out, as is common with spiral
structure. "So it's this long-lived dynamical thing that's just kind of sitting there,
for reasons we don't understand," she says.
■ CAMILLE M. CARLISLE
& MONICA YOUNG

BLACK HOLES I Binary En Route to Merger?
Two decades of observations reveal
what looks like a pair of supermassive black
holes closing in for a merger.
When galaxies merge, astronomers
expect the supermassive black holes lurking in the galaxies' cores to form their
own dancing duos, inspiraling and ﬁnally
coalescing. Yet black hole binaries have
proven diﬃcult to ﬁnd, and astronomers
only have circumstantial evidence for them.
Now astronomers have found the closest-hugging black hole binary candidate yet,
Matthew Graham (Caltech) and colleagues
announced January 7th in Nature and at the
American Astronomical Society.
The team's source, PG 1302-102, is a
"vanilla" quasar that mysteriously pulsates
with a period of about 5 years. Quasars are
notoriously variable at all wavelengths, but
randomly so - usually there's no regularity
to the changes in brightness. Yet when on
a whim the team ran algorithms to check a
sample of 247,000 quasars for regular pulsations in brightness, 20 sources popped

up, including PG 1302-102, which is the
"best-looking" of the bunch. That discovery
rate (20 out of 247,000) is close to what's
expected by theorists for binaries separated
by less than a tenth of a light-year, as this
one seems to be.
The exact distance between the two
black holes depends on what their masses
are. The team estimates a combined mass
for PG 1302-102's pair of a few hundred
million solar masses; the individual masses
are unknown but are likely comparable.
Astronomers still don't have a good
handle on what happens in the ﬁnal few
light-years of a black hole merger, but PG
1302-102's (purported) black holes will
likely merge in a few hundred thousand to
a couple million years. The elliptical galaxy
they sit in also shows signs of being the
product of a merger; it's hard to say exactly
how long ago it formed, but probably a few
hundred million years ago.
■ CAMILLE M. CARLISLE
& MONICA YOUNG

http://www.is.gd/m31zoom

Sky and Telescope - May 2015

Table of Contents for the Digital Edition of Sky and Telescope - May 2015