Download “Missing” Local Group Satellites

“Missing” Local Group Satellites
Daniel Zucker
Macquarie University / Australian Astronomical Observatory
How Do Galaxies Form?
Theory:
Hierarchical
Cold Dark
Matter
Paradigm
Steinmetz/Potsdam
Cold Dark Matter (CDM)
•  ~Works for largescale structure
•  However, too much
small-scale structure,
compared to what is
observed
Gravitational Scales for Dark Matter
What Do We Expect to See Today?
In the hierarchical CDM
scenario, we should see (around
the Milky Way, M31 and other
galaxies):
•  Many surviving satellites
•  Many stellar streams
•  But do we see them?
Johnston & Bullock
The “Missing Satellite” Problem
Dark Matter
Diemand+ 2006
Luminous Matter
Bullock & Johnston 2005
•  CDM models predict far more lowmass dark subhalos and
substructure than dwarfs and
streams observed -- “missing
satellitesӠ
•  Some theoretical solutions:
inhibited star formation; observed
satellites much more massive;
observed satellites originally more
massive but tidally stripped‡
† Klypin+
1999, Moore+ 1999, Benson+ 2002 ‡ Somerville 2002, Benson+ 2002; Stoehr+ 2002; Kravtsov+ 2004
The Ultra-Low-Luminosity Explosion
•  Wide-area surveys (SDSS:
MW and INT/CFHT: M31) 
~20 new low-luminosity LG
dwarfs since 2004, almost all
dSphs
•  Many of the new dwarfs
have very low measured
masses (< 108 M)  most
sensitive to reionisation and
feedback processes
Walsh+ 2008
The Milky Way’s Growing Entourage
Relative Distance of Stellar Structures: Closest
Intermediate Farthest
Spying on the Neighbours
•  CFHT Survey of
the halo of M31
(including M33)
•  A wealth of
stellar structures
and faint
satellites
•  M33 may be
tidally disrupting
from close
passage to M31
Pan-Andromeda Archaeological
Survey (PAndAS), McConnachie
et al.
Properties of the New Milky Way Dwarfs
•  Low luminosities (-8 <~ MV <~
-3), low metallicities
•  Distances: ~30 to 420 kpc
•  Comparable sizes to
“classical” dwarf spheroidals
•  High velocity dispersions for
luminosity  potentially very
high M/L
•  But: some appear to have
irregular/distorted shapes
(e.g., UMa II, Her)  tidal
disruption?
CVn I
CVn II
Boo I
Com
Something Different: Leo T, The Smallest StarForming Galaxy
•  Leo T not dead yet: stars
formed within few x 108 yr
•  HIPASS, GMRT, WSRT: ~3
x 105 M H I, σV ~ 7 km/s,
RV ~ 35 km/s •  MH I/M✸ ~ 2, (Virialized)
Mdyn~ 7x106 M (<
“minimum” 107 M*) -- how
did it keep/accrete gas? Are
there many more like Leo T?
Irwin+ 2007, Ryan-Weber+ 2008
* Read+ 2006
[Fe/H] = -1.7, 14 Gyr
GMRT
[Fe/H] = -0.7,WSRT
200Myr
INT
INTData
g,r
Star Formation in Extreme Dwarfs
•  Leo T: a unique (?) laboratory –
a very low mass, relatively
isolated system with gas and
recent star formation •  Hα (Gemini) and UV (pointed
GALEX + Swift/UVOT)
observations to study recent
star formation: no detected H II
regions  no ongoing SF… or
no O stars?
•  LG dwarfs may still have gas at
≥ ~270 kpc from MW, M31
(e.g., Grcevich & Putman 2009)
Leo
Leo T
T
Leo T: A Holy Grail of Rosetta Stones?
+
•  Can we understand a “simple” galaxy like Leo T?
•  Are there (many) more such objects in the Local Group?
What’s WALLABY Got to Do With It?
•  WALLABY: Southern all-sky HI
survey, ~4 km/s velocity
resolution, 5σ HI detection
limit at 1 Mpc ~ 5×104 M •  Skymapper: Southern all-sky
optical survey in 6 bandpasses
•  The combination of WALLABY
and Skymapper will be uniquely
well-suited for finding new Leo
T-like dwarfs in the Local
Group…and perhaps beyond
Summary
•  CDM predicts many more satellites than have been
observed  “missing satellite” problem
•  Recent surveys have yielded a wealth of new dwarf
galaxies in the Local Group, somewhat alleviating
the “problem”; among these dwarfs was Leo T, a
low-luminosity star-forming galaxy with H I gas
•  WALLABY + Skymapper will make the discovery of
more Leo T-like objects possible