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Dwarf LSB galaxies
in the Virgo cluster
Jonathan Davies
CDM and Dwarf galaxies
Simulation
of
Dark Matter
Observations
of
Baryons
Dwarf galaxy - 1 fluctuation of 106 M
collapsing at z=6 (13 Gyr)
Virgo cluster- 1 fluctuation of 1014 M
collapsing at z=0
Cardiff and the sub-structure problem
Does there exist a large population of dwarf
galaxies that have gone undetected ?
Optical Surveys
HI surveys
Dwarfs are ‘cursed’
1. Low luminosity (-14<MB<-10)
2. Low surface brightness (23<o<26 B )
3. Low HI mass (dE/dSph) (<107 M)
Image Detection and Selection
The algorithm we have developed is a Fourier convolution
method using matched filters to enhance faint structures
Numerical simulation of Virgo and the background
to derive the selection criteria
-14<MB<-10
23<o<26 B 
Virgo and other Environments
Roberts et al., (2004)
in prep
Sabatini et al., (2003)
MNRAS, 341, 981
Virgo
Equatorial
Strip
Roberts et al., (2004)
MNRAS, in press
The Virgo Cluster
We detect 20 gal/sq/deg in the range -14<MB<-10
23<o<26 B 
Surface density v radial
distance from M87
The selection criteria pick out the cluster population
The Luminosity function
Virgo luminosity
function
Sabatini et al., 2003
Norberg et al., 2002
The result
The dwarf to giant ratio.
N(MB<-19)/N(-14<MB<-10)
Virgo - 20:1 (Fornax - 20:1)
Field < 5:1 (Ursa Major - 5:1)
Local Group - 5:1
Field Luminosity function - 7:1
CDM – 350 (=-1.6), 8000 (=-2.0)
1. There is no LSB field dwarf galaxy
population that has been
missed by the redshift surveys.
2. There is a large dwarf galaxy
population in nearby rich clusters.
An HI survey of the Virgo Cluster
Automated galaxy detection in HIJASS data
Virgo HI
detections
Davies et al., (2004)
Survey limits
Mass 5x107 M
Column Density 6x1018 atoms cm2
The HI mass function
Davies et al., 2004
Why are the luminosity function and HI mass function
so different in different environments ?
‘Squelching’
1. Dwarfs unable to form after re-ionization, z20 (13 Gyr)!
2. A feedback mechanism.
3. Galaxy ‘Harassment’.
4. Change the initial conditions or the nature
of the dark matter particles.
Comments
Lots of DM halos around (no gas or stars) ?
Difficult physics to model
(from large scale structure to details)
Suppresses
dwarf
formation
everywhere
Ad hoc
addition
to CDM
?
Investigating the nature of dwarf
galaxies in different environments
Typical examples
1. Cluster galaxies typical of Local Group dSph.
2. Field galaxies typical of Local Group dI.
Galaxy colour and environment
Field
Galaxy colour changes
systematically with
environment
Ursa Major
Virgo
Fornax
(B-I) against number
The UKIDSS survey
Arecibo observations of
dwarf galaxy detections
1. Virgo cluster 3 detections out of 107 observed
to a mass limit of 2x107 M. Consistent with
them being mostly dE/dSph galaxies.
2. Field 4 detections out of 14 to a mass limit of
2x107 M. Consistent with them being mostly dI
galaxies.
Sabatini et al. submitted
Dark Matter dominated ?
Draco has (M/LB)=440 !
Kleyna et al., 2002, MNRAS, 330, 792
Draco
Cluster galaxy evolution
1. Ram pressure stripping
 M dyn   ICM v 2 0.4( 26.8)

 
10
 gal
 LB 
Destroyed if they enter
the cluster core ?
2. Tidal interactions (Roche limit)
1010
M galaxy 30 times more likely to
interact with another 1010 M than
with a 108 M galaxy. Dwarf galaxy
tidal stripping interactions are rare
3. Supernovae driven winds
N  nvt
 3M gal
  r 
 M dwarf
2




2/3
 M dyn  0.4 ( 26.8 )
r10
TW  10 
 LB 
M dyn 
3

TW  2 10 
 LB 
3
A viable gas stripping
mechanism ?
Pressure confinement ?
(Babul and Rees, 1992,
MNRAS, 255, 346)
TSN  105
Gas consumed by accelerate evolution? – need star formation histories
What about Harassment ?
  Galaxy 

r  RC 
  Cluster 
Moore et al., 1998,
ApJ, 495, 139
r  7 kpc much larger
than the galaxies we
detect (r  1 kpc).
Arecibo observations of
HIJASS detections with
no (obvious) optical
counterpart
VIRGOHI21
The AGES survey
VIRGOHI27
GMRT and INT observations
Faint optical source found for
VIRGOHI27
but
VIRGOHI21 ?
Summary
1. Redshift surveys have not missed a large population of LSB
field dwarf galaxies.
2. There are a large number of LSB dwarf galaxies in rich
clusters.
3. Does not appear to be a large population of HI clouds with no
optical counterparts.
4. Dwarf (mainly dE/dSph) galaxies with red colours found in
large numbers in the cluster. They are gas poor, but are they
stripped of their gas?
5. Low numbers of dwarf (dI) galaxies with blue colours and
gas rich found in the field - morphology density relation.
6. The cluster cannot have just been made up of in-falling
‘Local groups’ the dwarf to giant ratio is too large – there are
‘cluster’ (tidal?) dwarfs.
7. Cluster luminosity function steep, HI mass function shallow
compared to the field - gas more efficiently converted into
stars ?
8. The galaxies we detect are too small to have been the result
of harassment.