The Least Luminous Galaxies
recent discoveries and what they mean
in a cosmological context
Beth Willman, Ohio State, April 10 2007
What is “least luminous”?
• “Dwarf galaxy” = less
luminous than MV ~ -17
• Very few galaxies with
MV > -11 are known
outside LG
• More than a dozen
galaxies with MV > -8
now known inside LG
Luminosity function from Blanton et al 2005
The least luminous
galaxies known are
all nearby
Image of KKH98 courtesy of Anil
Seth and the ANGST team
12 kpc
Palomar 5
M5
M13
M15
Images of globular clusters from SDSS data
Dwarf galaxies
250 kpc
250 kpc
Accreted stellar halo picture from
Kathryn Johnston & James Bullock
Numerous streams in
MW’s and M31’s stellar halo
Image from Belokurov et al 2006
Why Not To pursue the
least luminous galaxies
(in the Local Group)
 First structures to collapse
 The building blocks of galaxies
 Cold Dark Matter models predict that the
Local Group should be teeming with dwarf
galaxies
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Simulation presented in Governato, Willman et al 2007
Why To pursue the
least luminous galaxies
(in the Local Group)
 Complex
 The leftovers from galaxy formation
 They are our most direct tracers of dark
matter on small scales
 Small potential wells = very affected by
reionization and stellar feedback
Pre-SDSS dwarf galaxy
population of the Milky Way
Willman 1 object
Willman et al 2004
Sloan Digital Sky Survey
Image from Belokurov et al. 2007
SDSS Search for Dwarf Galaxies
• Uniform and well-defined (see also - Irwin et al 1990,
Kleyna et al 1997, Whiting et al.)
• Put upper limit on stellar content of High
Velocity Clouds (Willman et al 2004b, Willman & Dalcanton
2007, in prep; See also - Simon and Blitz 2001, Siegel et al. 2004)
• Compare with theoretical predictions
• Find new objects to study (see also - Newberg et al
2002, Yanny et al 2003, Ibata et al 2003, Rocha-Pinto et al 2003, Majewski
et al 2003, Martin et al 2004, Zucker et al 2004, Huxor et al 2005)
Sloan Digital Sky Survey
key ingredients:
Uniform, precision photometry
Multi-color
Excellent star-galaxy separation
Excellent astrometry
In the near
field,
we can resolve
stars
How To Find
Resolved Dwarf Galaxies
Red Color Cut, Magnitude Cuts, Spatial Smoothing
Isolate subset of stellar population with strongest signal
bright
faint
blue
red
blue
red
Two New Objects:
Ursa Major and Willman 1
Ursa Major dwarf:
D ~ 100 kpc
Size ~ 250 pc
MV ~ -6.5
age = old (10 Gyr?)
[Fe/H] = low (-1.7?)
Willman et al. 2005a,b, 2006
Two New Objects:
Ursa Major and Willman 1
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Willman 1 object:
D ~ 40 kpc
Size ~ 20 pc
MV ~ -2.5
age = old (10 Gyr?)
[Fe/H] = low (-1.7?)
Willman et al. 2005a,b, 2006
Detection Limits
faint
bright
Willman et al. 2003, dissertation; Walsh, Jerjen & Willman in preparation
Figure from Walsh, Jerjen, & Willman, ApJL submitted and ApJ in prep
New Satellites: Milky Way
Ursa Major I
Willman et al. 2005
Willman I
Willman et al. 2005
Bootes
Belokurov et al. 2006
Canes Venatici I
Zucker et al. 2006a
Ursa Major II
Zucker et al. 2006b
Coma Berenices
Belokurov et al. 2007
Canes Venatici II
ibid
SEGUE 1
ibid
Hercules
ibid
Leo IV
ibid
Leo T
Irwin et al. 2007
Slide and data compilation from Steve Majewski
New Satellites: M31
And IX
And X
And XI
And XII
And XIII
And XIV
And XV and XVI
Zucker et al. 2004
Zucker et al. 2006c
Martin et al. 2006
ibid
ibid
Majewski et al. 2007
Martin et al, in prep
Slide and data compilation from Steve Majewski
New Dwarfs: Milky Way
L (Lsun)
Ursa Major I
Bootes
Canes Venatici I
Ursa Major II
Coma Berenices
Canes Venatici II
Hercules
Leo IV
Leo T (200 Myr pop)
HI
1x104
2x104
1x105
3x103
2x103
7x103
2x104
1x104
6x104
2x105
D (kpc)
Rcore(pc)
100
60
220
30
45
150
140
160
420
290
230
550
125
70
135
310
150
170
Slide and data compilation from Steve Majewski
New Dwarfs: M31
L (Lsun)
And IX
And X
And XI
And XII
And XIII
And XIV
1.7x105
1.4x105
6.9x104
3.0x104
4.8x104
2.0x105
D (kpc)
Rcore(pc)
805
~700
~780
~780
~780
~740
>500
~270
~115
~125
~115
~623
Slide and data compilation from Steve Majewski
Willman 1 object
Willman et al 2004
What are
these things?
Dwarfs or clusters?
Dwarf - dark matter halo
Cluster - no dark matter halo
• traditionally classified by their
size and luminosity
• these are insufficient metrics,
particularly at low luminosity
• difference is important b/c we
are trying to trace dark matter
distribution
• new discriminants: kinematic
and metallicity distributions
Ursa Major dwarf
Willman 1 object
Willman et al 2005a,b, Zucker et al 2006 a,b, Belokurov et al 2006,2007
Velocity dispersions of 7 of the new dwarfs
Figure from J. Simon, Simon & Geha, in prep
What are
these things?
Dwarfs or clusters?
Dwarf - dark matter halo
Cluster - no dark matter halo
• traditionally classified by their
size and luminosity
• these are insufficient metrics,
particularly at low luminosity
• difference is important b/c we
are trying to trace dark matter
distribution
• new discriminants: kinematic
and metallicity distributions
Ursa Major dwarf
Willman 1 object
Willman et al 2005a,b, Zucker et al 2006 a,b, Belokurov et al 2006,2007
What are these things?
An in-depth investigation of Willman 1
Geha, Willman, Strader & Rockosi 2007, in prep
What are these things?
An in-depth investigation of Willman 1
Willman et al 2006, Geha, Willman, Strader & Rockosi 2007, in prep 29
What are these things?
An in-depth investigation of Willman 1
True metallicity spread?
Geha, Willman, Strader & Rockosi 2007, in prep
So what’s the deal with
the missing satellite problem?
Image courtesy J. Diemand
How can we use nearby dwarfs to learn about the density,
mass spectrum, abundance, and/or spatial distribution of
dark matter on small scales?
Image courtesy J. Diemand
What do we know, and does it
make sense with CDM?
• Models need to allow for 50+ dwarf satellites
• “central” velocity dispersions - masses within
0.6 kpc (Strigari et al 2007, in prep):
• Mass spectrum of observed dwarfs and predicted
sub-halos diverges
• Dwarf satellites with vastly different luminosities
may have similar total masses
• Radial distribution
What do we know, and does it
make sense with CDM?
• Models need to allow for 50+ dwarf satellites
• “central” velocity dispersions - masses within
Compare
this variety of metrics with some simple
0.6 kpc:
models
to simultaneously
learn
about
dark matter
• Mass
spectrum of observed
dwarfs
and predicted
on
smalldiverges
scales and galaxy formation
sub-halos
• Dwarf satellites with vastly different luminosities
may have similar total masses
• Radial distribution
Number of predicted CDM sub-halos
All sub-halos within r200,
in the highest resolution
simulation
All sub-halos within r200,
in a 1/27th resolution
simulation
Same, but within 0.1r200
Figure from: Diemand, Kuhlen, & Madau 2007
Number of predicted CDM sub-halos
Plenty of physics to render low mass
halos dark:
•
•
Reionization
Stellar feedback
Atomic cooling efficient to only 104 K
•
Toomre-stable (Verde et al 02, Jimenez et al 97, Taylor &
•
Webster 05)
Too many dwarfs for Warm Dark Matter models??
Lots of people have explored these with both semi-analytic
and numerical models: Benson et al, Bullock, Kravtsov & Weinberg,
Somerville (2002), Stoehr et al, Hayashi et al (2003), Kravtsov, Gnedin, & Klypin
(2004), Moore et al (2006), Governato, Willman et al (2006)
Divergent mass spectrum of
CDM sub-halos and Milky Way dwarfs
Figure from: Diemand, Kuhlen, & Madau 2007
Strigari et al, 2007 in prep
Divergent mass spectrum of
CDM sub-halos and Milky Way dwarfs
Plenty of physics to render
low mass halos dark:
• Reionization
• Stellar feedback
• Atomic cooling
efficient to only 104 K
• Toomre-stable (Verde et al 02,
Jimenez et al 97, Taylor & Webster 05)
Figure from: Diemand, Kuhlen, & Madau 2007
Strigari et al, 2007 in prep
A pile-up or rapid drop off in the mass
spectrum of observed dwarfs?
Figure from J. Simon, Simon & Geha, in prep
Tassis, Kravtsov & Gnedin 2007
Willman et al, in prep
Read, Pontzen & Viel (2006), Tassis, Kravtsov, & Gnedin (2007),
Governato, Willman et al (in prep) all predict a sharp decrease in
the stellar content of dark matter halos near some transition mass
Update of Willman et al 04; simulation data from Diemand, Kuhlen & Madau. See also Taylor
& Babul 2004, Moore et al 2006, Kravtsov, Klypin & Gnedin 2004 for similar arguments
Update of Willman et al 04;
Update
simulation
of Willman
data from
et al 04;
Diemand,
simulation
Kuhlen
data&from
Madau.
Diemand,
See also
Kuhlen
Taylor&
2005 also
investigated
r.d.s
& Babul 2004, Moore et al
Madau;
2006, Moore
Kravtsov,
et alKlypin
2005,
&
Kravtsov,
Gnedin
2004
Klypin
for
&similar
Gnedinarguments
2004
Update of Willman et al 04; simulation data from Diemand, Kuhlen & Madau
Update of Willman et al 04; simulation data from Diemand, Kuhlen & Madau
MW1 simulation described in Governato, Willman et al 2007
Take home messages
1.
2.
3.
4.
5.
6.
7.
Our view of the Local Group is being revolutionized!
New class of dwarf galaxies discovered?
Models may need to allow for existence of more than 50
dwarfs, most fainter than MV = -7; could be curtains for models
with reduced small-scale power
Central mass distributions of nearby dwarfs are well
characterized and can be directly compared with dark matter
only simulations (new Strigari et al result)
Dwarf satellites are certainly a biased tracer of CDM sub-halos
Spatial distribution (and # and mass spectrum) of dwarfs agrees
very well with that of vmax > 30 km/sec sub-halos
www.thelocalgroup.info
The End
Dwarf satellites as dark matter
tracers the good, the bad, the ugly, and the reality
All sub-halos within r200,
in the highest resolution
simulation
All sub-halos within r200,
in a 1/27th resolution
simulation
Same, but within 0.1r200
Figure from: Diemand, Kuhlen, & Madau 2007
Dwarf satellites as dark matter
tracers the good, the bad, the ugly, and the reality
Plenty of physics to render
low mass halos dark:
• Reionization
• Stellar feedback
• Cooling timescales
• Toomre-stable (Verde et al 02,
Jimenez et al 97, Taylor & Webster 05)
Figure from: Diemand, Kuhlen, & Madau 2007
Dwarf satellites as dark matter
tracers the good, the bad, the ugly, and the reality
•
Cosmological model
+
baryonic physics =>
observations
Lots of people have done this with
both semi-analytic and numerical
models: Benson et al, Bullock et al, Somerville
(2002), Stoehr et al, Hayashi et al (2003), Kravtsov,
Gnedin, & Klypin (2004), Kravtsov, Klypin and
Gnedin (2004), Governato, Willman et al (2007)
•
What’s the problem?
Lots of models yield ~ 12 luminous Milky
Way satellites
•
Why is this a problem?
a) Not a problem for CDM models, but we
aren’t yet learning a whole lot about CDM
or galaxy formation
b) There are more than 12 dwarfs
Dwarf satellites as dark matter
tracers the good, the bad, the ugly, and the reality
• Observers disagree with observers
observations
+
detailed observations
=>
properties of the dark
matter halos that
they trace
• Theorists disagree with theorists
• 6-ish least luminous dwarfs all
consistent with 5d7 Msun within their
luminous extent
• Total masses of Milky Way’s lowest
luminosity dwarfs are uncertain by
up to 2 orders of magnitude
• … but we are making progress!
Step 1 - select a subfield
Step 2 - get cataloged stars
Step 3 - red color cut (or not)
Step 4 - one magnitude bin
Step 5 - compute number density
Step 5 - compute number density
Step 6 - smooth with a 3 ´ or 7´ spatial filter
Detection Limits
Globular Cluster vs dSph
globular clusters
 -10 < MV < 0.2
 Few < d < 120 kpc
 Single stellar
population
 Spherical
 Not dark matter
dominated now, or in
the past





dwarf spheroidals
-16 < MV < -8
23 < d < 250 kpc
Extended star
formation
Elliptical
Dark matter
dominated
CDM on Small Scales
 Cusp/Core problem
(NFW, de Blok et al 2003, Swaters et al
2003, Reed et al 2003, Valenzuela et
al 2005, Kleyna et al 2003, Goerdt et
al 2006)
 Missing Satellite
Problem (Kaufmann et al 93,
Moore et al 99, Klypin et al 99, Font
et al 01)
Dark matter only sim of LG formation, Ben Moore’s website