2019-06-01+Sky+and+Telescope

NEWS NOTES

12 JUNE 2019 • SKY & TELESCOPE

GALAXIES

Gaia Peeks at Local Group’s

Past and Future

ASTRONOMERS HAVE USED the

European Space Agency’s Gaia satellite

to measure stellar motions in Androm-

eda (M31) and its satellite Triangulum

(M33) — the largest galaxies in the

Local Group outside of the Milky Way.

The results answer some key questions

about the galaxies’ interactions.

Roeland van der Marel (Space

Telescope Science Institute) and col-

leagues averaged the motions of 1,

and 1,518 stars in the Andromeda and

Triangulum galaxies, respectively. After

measuring the galaxies’ rotation and

their motion across the sky, the team

then simulated their past and future

movement, looking billions of years

backward and forward in time.

The projections enabled the astrono-

mers to investigate the relationship

between the two galaxies. Hubble Space

Telescope images and radio data from

the Very Long Baseline Array had previ-

pThe European Space Agency’s Gaia satellite

monitored stars (blue dots) in the Andromeda

Galaxy, shown here at ultraviolet wavelengths.

Astronomers averaged the stellar motions (ar-

rows) to understand the galaxy’s rotation and

its movement on the sky.

ASTRONOMERS MAPPING out lumi-

nous stars across the Milky Way are

exploring what bent our galaxy’s disk.

We’ve known since 1957 that the

Milky Way is warped like thrown pot-

tery gone wrong. It’s not alone — at

least half of all galaxies are warped.

But the cause remains unclear: Are we

seeing the collective motions of billions

of stars that pull on each other, or is

the cause external, such as gravitational

interactions with satellite galaxies?

Using 1,339 Cepheid variable stars

from the Wide-fi eld Infrared Survey

ously suggested that they might have

passed close by each other 6 billion

years ago and are swinging by each

other again. But the new Gaia-based

simulations reveal that Triangulum is

only now making its fi rst approach to

Andromeda. This result suggests that

the galaxies in the Local Group have

come together relatively recently.

The team’s analysis also recalcu-

lated the Milky Way’s meet-up with

Andromeda. While astronomers have

long known that Andromeda is barrel-

ing toward us at about 110 km/s, it has

been hard to gauge how fast our sister

galaxy is moving sideways. Hubble data

had indicated a small sideways motion

of about 17 km/s, indicating a head-on

collision. But now, after averaging the

Hubble results with the new Gaia mea-

surements, van der Marel’s team fi nds

that Andromeda is sliding sideways

more quickly, at around 60 km/s.

The two galaxies are still due for a

merger, but their fi rst encounter will be

more of a glancing blow than a head-

on collision, says van der Marel. The

revised velocity also means the expected

encounter will occur 600 million years

later than originally thought, in 4.5 bil-

lion years’ time.

■ BEN SKUSE

Explorer catalog and a number of

visible-light surveys, Xiaodian Chen

(Chinese Academy of Sciences) and his

colleagues have mapped out two-thirds

of our galaxy’s stellar disk to better

understand its shape.

The Cepheids’ pulsations relate

directly to their luminosity, making

them reliable markers of distance. The

scientists thus use the stars to trace the

warped disk out 65,000 light-years from

our galaxy’s center. Chen and his col-

leagues replicated the disk’s shape with

a mathematical model, which they then

used to probe the source of the warp-

ing. Their results appear February 4th in

Nature Astronomy.

The team found that the warp within

the central Milky Way arises naturally,

as the inner stellar disk pulls on and

distorts the outer part of the disk. But

in the outer reaches of the galaxy, more

than 50,000 light-years from the center,

the warp’s shape changes. The research-

ers speculate that infalling dwarf

galaxies might have sculpted the outer

Milky Way, or the warp might refl ect

a misalignment between the galactic

disk and the larger halo of dark matter

around it.

As infrared surveys continue to fi nd

more Cepheids on the farside of the

Milky Way, astronomers will eventually

be able to map out the whole disk for

better insight into its structure.

■ MONICA YOUNG

MILKY WAY

What Has Our Galaxy in a Twist?

pLike the Milky Way, the spiral galaxy ESO

510-G13 has a pronounced warp in its gaseous

disk, as well as a less pronounced warp in its

disk of stars.

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