Sky and Telescope - April 2017 - 29

the gravitational forces that shred a dwarf during its accretion into a larger halo might press latent gas into new stars.
Or young stars might instead have formed in - and then
been cast out of - the larger galaxy's disk. Regardless, the
stars' presence complicates our ideas about stellar halos and
the dark matter around them.

Laying Ghosts to Rest
So let's sum up what we know - or rather what we don't
- about the Milky Way's halo. We know the halo contains
about a trillion Suns' worth of mass, but we only know that
to within a gaping factor of two. Moreover, we still don't
really know how clumpy this dark mass is, or even what general shape it has.
Meanwhile, observations of individual stars in the halo
hint at a single long-ago encounter with one massive dwarf
galaxy, or perhaps several smaller dwarfs, indicating an
otherwise quiet past. But while our halo's sedate nature puts
it on the opposite end of the spectrum from the dynamic
Andromeda Galaxy, the variety of halos seen around other

nearby galaxies makes it hard to tell whether the Milky Way's
history is typical or far from it.
Since the SDSS and its iconic Field of Streams image ﬁrst let
us take a close look at the Milky Way's outermost stars, questions about our halo and its past have been haunting astronomers. Now, a second wave of astronomical data (S&T: Sept.
2016, p. 14) promises a deeper view, and perhaps some answers.
The European Space Agency's Gaia satellite, as well as the
ground-based Large Synoptic Survey Telescope (LSST) that's
set to begin observing in 2022, will enable astronomers to
map halo stars from very near the Sun to far beyond the
Milky Way's own halo. As computer simulations continue to
advance at a rapid pace, these new surveys will rise to their
predictive challenge. Finally, the ghosts are ready to speak -
and we're ready to listen.
¢ MONICA YOUNG is the web editor of Sky & Telescope.

* Visit https://is.gd/milkywayhalo to view additional
illustrations of the Milky Way's stellar streams.

Amateur Astrophotographers Wanted
- David Martínez-Delgado

R. JAY G A BA N Y (BL ACK BIRD OBS.) IN COLL A BOR ATION WITH D. M A RTÍNE ZDELG A DO (MPIA A ND IAC) E T A L.

◗ Is the Milky Way's history "typical"?
We have to look at other galaxies to answer that question. Simulations predict
that deep (long-exposure) observations
should detect stellar streams in the
outskirts of almost all nearby galaxies.
But such features are extremely faint
and difficult to find. Few were spotted
until 1997, when astrophotographer
David Malin used enhancement techniques on photographic plates to image
structures around the spiral galaxies
M83 and M104. I was inspired by his
pioneering work and began observing
other nearby spirals using professional
telescopes - but without success.
By chance, years later, I stumbled
upon some deep galaxy images that
astrophotographer R. Jay GaBany had
posted on his website. I invited him
to join me. Within a couple of years,
we had detected a wide assortment
of giant tidal structures in the halos of
several classic spiral galaxies. In 2010
the American Astronomical Society
honored GaBany with the Chambliss
Amateur Achievement Award, given annually to an American amateur who has

helped advance scientific research.
Today, our "stream team" includes
a group of high-class astrophotographers interested in ultra-deep galaxy
imaging, including Ken Crawford, Karel
Teuwen, Adam Block, and Johannes
Schedler. I also recently started a spinoff project that devotes amateur time
to searching for whole dwarf satellites
around nearby spirals. Both projects

present unique opportunities for
amateurs to contribute to the field of
galaxy formation. The nearby universe
abounds with faint galaxies ripe for exploration, so I am continuing to recruit
talented astrophotographers to my
team. Interested amateurs can contact
me at tidalstreamsurvey@gmail.com.
* You can also keep up with new
results on Twitter: @astro_delgado.

p THE UMBRELLA NGC 4641, a galaxy 35 million light-years away, shows an umbrella-like
structure, the remains of a smaller dwarf companion that once orbited the larger galaxy.

s k y a n d t e l e s c o p e .c o m

* APRIL 2017

https://www.is.gd/milkywayhalo http://www.skyandtelescope.com

Sky and Telescope - April 2017

Table of Contents for the Digital Edition of Sky and Telescope - April 2017