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The Least Luminous Galaxies:
Faint But Not Dull
Daniel Zucker
Macquarie University/ Anglo-Australian
Observatory
I. Introduction
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
The “Missing Satellite” Problem, circa
2005
Dark Matter
Diemand+ 2006
Luminous Matter
• 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+
Bullock & Johnston 2005
27 July 2009
1999, Moore+ 1999, Benson+ 2002
2002, Benson+ 2002; Stoehr+ 2002; Kravtsov+ 2004
‡ Somerville
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
The Observational Perspective, circa
2005
• 2003: only 9 Milky Way
(MW) dwarf spheroidal
galaxies (dSphs), still
fewer around M31
• 2004: Andromeda IX
Cetus And
dSph
IX
(Zucker+)
• 2005: Ursa Major
(Willman+), Andromeda X
(Zucker+); runts or tip of an
iceberg?
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Whiting+
1999
Subaru
g,r,i
Ann Arbor, Michigan
The Sloan Digital Sky Survey
• SDSS-I: Largearea imaging and
spectroscopic
survey, covered
~25% of the sky
• SDSS-II: just
finished, SDSS-III
now underway
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Stellar Density in SDSS Data
• SDSS data for the North Galactic Cap cover over 8000 sq. deg.
• ~56 million stellar objects identified by SDSS pipeline in DR5
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
The SDSS “Field of Streams”
Density composite from magnitude slices:
probing distances in upper main sequence and
turn-off stars
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
A Field of Streams...and Dots
Canes Venatici I
NGC 5466
NGC 5272
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
The Ultra-Low-Luminosity Explosion
• Wide-area surveys
(SDSS,CFHT)  ~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
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Walsh+ 2008
Ann Arbor, Michigan
And Now, for Something Different: Leo T, The
Smallest Star-Forming Galaxy
• Leo T not dead yet: stars
formed within few x 108 yr
• HIPASS, GMRT, WSRT:
~3 x 105 M H I, RV ~ 35
km s-1
• MH I/M ~ 2, (Virialized)
Mdyn~ 7x106 M -- how did it
keep/accrete gas? Are
there many more like Leo
T?
Irwin+ 2007, Ryan-Weber+
2008
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
GMRT
WSRT
INT g,r
Ann Arbor, Michigan
Luminosity 
MV vs. rh for the New Dwarfs
No DM
 Gap? 
DM
Size 
• Few objects with rh
between ~40 pc and
~100 pc 
characteristic size
scale for objects with
dark matter?
Implications+?
• What is the nature of
the objects in the
“gap” --star clusters
or dwarfs?
+
27 July 2009
e.g. Gilmore+ 2008, Strigari+2008
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
A Common (Minimum) Mass for
Galaxies?
Matteo+ 1993
Strigari+ 2008
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Some Big Questions about Little
Galaxies
• How many low-luminosity dwarfs are there? What are
their properties, and how are they distributed?
• Is there a minimum galaxy mass? (Is there a
common galaxy mass?) If so, what does this tell us
about dark matter/ star formation/ both?
• Is there a minimum galaxy size? What are the objects
in the “gap”?
• What are their stellar populations, and what can they
tell us about the conditions of star formation in such
extreme systems over a cosmic time?
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
II. Stellar Populations of LowLuminosity Dwarfs*
“The stellar populations of ultra-faint dwarfs are still a mystery” – O. Gnedin (2
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Stellar Populations with HST
• 101-orbit HST program
to observe 4 new Milky
Way satellites, 3 M31
satellites and Leo T,
using F606W (broad ~V)
and F814W (~ I) filters
• WFPC2 (RIP): 4-chip
batwing, 160” on a side
And XIII F606W
Zucker+, in prep; Martin+, in
prep
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Hercules
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
MV = -6.6
Ann Arbor, Michigan
CVn I
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
MV = -8.6
Ann Arbor, Michigan
Leo T
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
MV = -8.0
Ann Arbor, Michigan
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Star Formation/Chemical
Enrichment Histories
• Hercules: purely old (~14
Gyr), metal poor
• CVn I: extended star
formation (14 - 8+ Gyr), no
clear sign of younger
population (cf. Martin et al.
2008), increasing metallicity
• Leo T: extended, multiepisodic star formation (14 6 Gyr, 4 Gyr - 1 Gyr, 250
Myr)
27 July 2009
Leo T
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Her
CVn I
Ann Arbor, Michigan
Current Star Formation in Extreme Dwarfs
• Leo T: a unique (?)
laboratory
• H (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?
Zucker+, in prep.
27 July 2009
Leo
Leo TT
Gemini
Swift NUV,
UVW1,
UVW2
INT g+r,Hα,
GALEX
FUV
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Star Formation History vs. DG
5-8 Gyr
>10 Gyr
<~1 Gyr
(0.5<z<1
(z>2)
(z<0.1)
)
DG(kpc)
MV
And XI
≥100
-7.3
Old
And XIII
≥100
-6.9
Old+
Her
140
-6.6
Old
CVn II
150
-4.9
Old+
Leo IV
160
-5.0
Old+
CVn I
220
-8.6
Old+
Inter+
420
-8.0
Old+
Inter+
Young!
Ann Arbor, Michigan
Leo T
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Summary
• The “ultra-faint” dwarfs appear to be part of the same continuum
of properties with the previously-known dwarfs – e.g., derived star
formation histories are consistent with the apparent proximity
effect: ultrafaint  luminosity-challenged
• None of the dwarfs studied (Hercules? And XI?) look like purely
ancient, single stellar populations  what impact did reionization
have on such objects?
• Leo T has had multiple epochs of star formation, as recently as
200 Myr ago – and still has neutral gas – but no H II regions (or
no O stars). Is Leo T a “Rosetta Stone” for understanding this
extreme regime of star formation?
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
Epilogue: Some Things Coming Up
Down Under…
• Skymapper: southern allsky 6-filter photometric
survey
• HERMES: 400 fiber highres (R~30000) spectrograph
with 2° FOV for AAT
• WALLABY: southern all-sky
H I survey proposed for
ASKAP
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan
27 July 2009
Daniel Zucker • Extreme Star Formation in Dwarf Galaxies •
Ann Arbor, Michigan