Download New ultra faint dwarf galaxy candidates discovered with the Dark

New ultra faint dwarf galaxy candidates
discovered with the Dark Energy Survey
Brian Yanny
DES
Fermilab Community Advisory Board
Meeting
March 26, 2015
DES, the Dark Energy Survey, is a currently
active experiment (E-939), led by Fermilab,
with several hundred collaborators from
around the world, and aims to survey a large
area of sky in the southern hemisphere over
5 years using a FNAL-assembled mosaic CCD
camera. The survey has recently completed
its 2nd year of observations.
The primary science goal of DES is to constrain
what we know about the nature of the phenomenon
of Dark Energy, which is apparently responsible
for the accelerating expansion of the universe.
Tonight, I'd like to tell you about a recent discovery
that was made with the survey, and how it fits
into DES's science goals.
http://arxiv.org/abs/1503.02584 (March 9, 2015)
The DES result involves the discovery of what
are known as '(candidate) dwarf galaxies' in the
neighborhood of our own Milky Way Galaxy.
This paper discovered 8 candidates, of which
at least 3 are certainly dwarfs and the remaining
may be dwarfs or faint globular star clusters.
What's the difference between a dwarf galaxy and
a globular cluster?
What's the difference between a dwarf galaxy and
a globular cluster?
A dwarf galaxy is a collection of stars, dust and
other matter, containing between
10,000 and 10,000,000 stars, all held together by
the gravity of the dwarf galaxy's own contents.
A globular cluster is a collection of stars, containing
between 1,000 and 1,000,000 stars all held together
by the gravity of the cluster's stars.
So....
What's the difference between a dwarf galaxy and
a globular cluster?
Globulars
Dwarfs
While they may seem similar, there are some
visible differences: globulars are tight balls, dwarfs
are extended blobs. The key difference, however, is
invisible....
globular cluster M3
Fornax dwarf galaxy
Dynamical mass can be estimated by
The 'virial theorem':
2
Mass(dynamical) = v R/2G
Where v is the velocity dispersion (spread)
of the stars in the object,
and R is the radius of the object, G= Newton's
const.
If the object is bound by gravity, then this
formula gives the Mass of the cluster/dwarf.
One can also estimate the mass of an object
simply by counting stars, and assuming each
star weighs about the same as the sun.
One measures the movements of stars in
globulars and dwarfs by taking spectra of their
stars and measuring the 'radial velocity', the speed
at which individual stars are moving toward and
away from us (like a radar gun doppler shift
measurement). When we do this, the
two types of objects separate very cleanly:
Globulars: tight velocity dispersions ~ 1 km/s
(implied mass matches star count)
Dwarfs: broad dispersions:
~ 10 km/s
(implied mass falls short of star count by factor 100!)
globular cluster M3:
Visible Mass = 100,000 solar masses
Dynamical Mass ~ 100,000 M_sun
Fornax dwarf galaxy:
Visible mass: 1 Million M_sun
Dynamical mass ~ 100 Million M_sun.
For globulars, these two methods of estimating
Mass generally agree very well (within 30%).
For dwarfs, they disagree by factors of 100x or
more.
Typically, for a dwarf galaxy such as Fornax,
there will be 1,000,000 stars seen, but an
implied dynamical mass of 100,000,000 solar
masses, 100x more.
What's going on?????
The extra mass is assumed to be in the form
of a substance called 'Dark Matter' (distinct from
Dark Energy). This substance has been
puzzling astronomers and physicists for 80 years
now, and its nature is still unknown.
**The ultimate nature of Dark Matter is a
KEY UNSOLVED PROBLEM in basic physics today**,
Just the sort of problem a fundamental science lab
like Fermilab is interested in exploring.
The LHC/CMS experiment is looking for it in
the form of SUSY particles.
DES is looking for how it affects the motions
of stars and galaxies, as part of its main Dark Energy Mission.
The factor of 100 dark/ordinary matter ratio
makes dwarf galaxies ideal laboratories to try
and learn about dark matter. The more, the closer the better..
Why do we need to find more,
and why 'the closer the better?'
There is an unexplained excess of gamma-ray (high
energy photons, energies of 1-10GeV), coming from the
Galactic center, as seen by the Fermi-GLAST gamma
ray telescope (in space now).
These gamma-rays may be due to dark matter particles,
however, they may also be due to other things (spinning
neutron stars for example).
If they are due dark matter, then dwarf galaxies should
also show gamma ray emmission, since dwarf galaxies
have lots of dark matter. But.... dwarf galaxies are
further away from us than the galactic center (20-200 kpc vs 8 kpc)
And the signal drops as distance squared and the galactic center
Has much more dark matter than these dwarfs. Still....
dwarf galaxies are our best know place to confirm or deny the
hypothesis that Dark Matter is a type of basic physics particle that annihilates
to gamma rays.
So the plan is:
Find as many dwarfs as one can.
See if any/all of them are emitting gamma rays.
1. If so: **Major discovery regarding the nature
of Dark Matter** :-)
2. If not: add together the faint noisy gamma ray
signal from all the dwarf you can (the closer ones
carry more weight because of the inverse square
law effect). [We are here]
3. If signal shows up, go to 1! :-)
4. If still no signal is present at limits which are in conflict
With the galactic center signal: gamma rays in GC
Not caused by dark matter – but important constraint set!
http://arxiv.org/abs/1503.02584 (March 9, 2015)
q
Must isolate the blue stars
from the foreground stars.
An ultra-faint dwarf galaxy
is only 1% of the
length of a full size spiral
(80 pc vs. 8 kpc in radius)
and contains fewer than 100,000 stars (vs. 100 billion for
the full size galaxy).
Figure 4 from Bechtol et al. Stars in the dwarf are separated from the forground galaxy
stars by looking at their colors and looking for a 'blue turnoff' sequence of stars all at
the same distance.
Another dwarf/globular candidate found.
Colored dots on the right indicate stars with
high probabilities of being candidate members.
Status:
Eight new candidates found, led by UC and Fermi
researchers (Keith Bechtol, Alex Drlica-Wagner).
Includes one close dwarf (30 kpc distant, high weight)!
Good gamma-ray followup candidate.
Spectroscopic follow-up underway in order to obtain
velocities and 'Dynamical Mass' of this candidate: J0335'
Once dynamical mass is known, then can be added to
the sum of all known dwarfs, and we can see if we can
answer confirm (or deny) that dark matter is a possible
source for gamma rays in the galactic center.
Stay Tuned!