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Dwarf LSB galaxiesDwarf LSB galaxiesin the Virgo clusterin the Virgo cluster
Jonathan Davies
CDM and Dwarf galaxiesCDM and Dwarf galaxies
Simulationof Dark Matter
ObservationsofBaryons
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 problemCardiff 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 SelectionImage Detection and Selection
The algorithm we have developed is a Fourier convolutionmethod using matched filters to enhance faint structures
Numerical simulation of Virgo and the backgroundto derive the selection criteria
-14<MB<-1023<o<26 B
Virgo and other EnvironmentsVirgo and other Environments
Virgo
EquatorialStrip
Sabatini et al., (2003)MNRAS, 341, 981
Roberts et al., (2004)MNRAS, in press
Roberts et al., (2004)in prep
The Virgo ClusterThe Virgo ClusterWe detect 20 gal/sq/deg in the range -14<MB<-1023<o<26 B
Surface density v radialdistance from M87
The selection criteria pick out the cluster population
The Luminosity functionThe Luminosity function
Virgo luminosityfunction
Norberg et al., 2002Sabatini et al., 2003
The resultThe result
Virgo - 20:1 (Fornax - 20:1)Field < 5:1 (Ursa Major - 5:1)
Local Group - 5:1Field 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.
The dwarf to giant ratio.N(MB<-19)/N(-14<MB<-10)
An HI survey of the Virgo ClusterAn HI survey of the Virgo ClusterAutomated galaxy detection in HIJASS dataAutomated galaxy detection in HIJASS data
Survey limitsMass 5x107 M
Column Density 6x1018 atoms cm2
Virgo HIdetections
Davies et al., (2004)
The HI mass functionThe HI mass function
Davies et al., 2004
Why are the luminosity function and HI mass functionWhy are the luminosity function and HI mass functionso different in different environments ?so different in different environments ?
1. Dwarfs unable to form after re-ionization, z20 (13 Gyr)!
2. A feedback mechanism.
3. Galaxy ‘Harassment’.
4. Change the initial conditions or the nature of the dark matter particles.
‘Squelching’
Suppressesdwarf
formation everywhere
Ad hocadditionto CDM
?
CommentsCommentsLots of DM halos around (no gas or stars) ?Difficult physics to model (from large scale structure to details)
Investigating the nature of dwarf Investigating the nature of dwarf galaxies in different environmentsgalaxies in different environments
Typical examples
1. Cluster galaxies typical of Local Group dSph.2. Field galaxies typical of Local Group dI.
Galaxy colour and environmentGalaxy colour and environment
Galaxy colour changessystematically with
environment
(B-I) against number
Field
Ursa Major
Virgo
Fornax The UKIDSS survey
Arecibo observations ofArecibo observations ofdwarf galaxy detectionsdwarf 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 ?Dark Matter dominated ?
Draco has (M/LB)=440 !
Kleyna et al., 2002, MNRAS, 330, 792
Draco
Cluster galaxy evolutionCluster galaxy evolution
1. Ram pressure stripping )8.26(4.02
10
gal
ICM
B
dyn v
L
M
Destroyed if they enterthe cluster core ?
2. Tidal interactions (Roche limit)
3. Supernovae driven winds
3/2
2 3
dwarf
gal
M
Mr
vtnN
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
A viable gas strippingmechanism ?
Pressure confinement ?(Babul and Rees, 1992,
MNRAS, 255, 346) 5
3
)8.26(4.03
10
102
1010
SN
B
dynW
B
dynW
T
L
MT
rL
MT
Gas consumed by accelerate evolution? – need star formation histories
What about Harassment ?What about Harassment ?
Cluster
GalaxyCRr
r 7 kpc much largerthan the galaxies wedetect (r 1 kpc).
Moore et al., 1998, ApJ, 495, 139
Arecibo observations ofArecibo observations ofHIJASS detections withHIJASS detections withno (obvious) opticalno (obvious) opticalcounterpartcounterpart
VIRGOHI21 VIRGOHI27
The AGES survey
GMRT and INT observationsGMRT and INT observations
Faint optical source found forVIRGOHI27
butVIRGOHI21 ?
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.
Summary
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