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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, z20 (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 nvt 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 TSN 105 3 A viable gas stripping mechanism ? Pressure confinement ? (Babul and Rees, 1992, MNRAS, 255, 346) 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.