36 AUSTRALIAN SKY & TELESCOPE May | June 2017
R. JAY GABANY (BLACKBIRD OBS.) IN COLLABORATION WITH D. MARTÍNEZ-DELGADO (MPIA AND IAC) ET AL.universe’s history, so they’re not the most obvious places
to look for middle-aged AGB stars. Still, the gravitational
forcesthatshredadwarfduringitsaccretionintoalarger
halomightpresslatentgasintonewstars.Oryoungstars
mightinsteadhaveformedin—andthenbeencastout
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,observationsofindividualstarsinthehalo
hint at a single long-ago encounter with one massive dwarf
galaxy,orperhapsseveralsmallerdwarfs,indicatingan
otherwise quiet past. But while our halo’s sedate nature putsit on the opposite end of the spectrum from the dynamic
Andromeda Galaxy, the variety of halos seen around other
nearbygalaxiesmakesithardtotellwhethertheMilkyWay’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
promisesadeeperview,andperhapssomeanswers.
TheEuropeanSpaceAgency’sGaiasatellite,aswellasthe
ground-based Large Synoptic Survey Telescope (LSST) that’s
set to begin observing in 2022, will enable astronomers to
maphalostarsfromveryneartheSuntofarbeyondthe
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 —
andwe’rereadytolisten.◗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, givenannually to a US 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 spin-off project that
devotes amateur time to searching
for whole dwarf satellites around
nearby spirals. Both projects presentunique 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 attidalstreamsurvey@
gmail.comAmateur astrophotographers wanted!— David Martínez-Delgado
STHE 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.Visit https://is.gd/milkywayhalo to view additional
illustrations of the Milky Way’s stellar streams.You can also keep up with new results
on Twitter: @astro_delgadoMILKY WAY HALO