Faint Low Surface Brightness Galaxies in the Virgo Cluster Jonathan Davies, Sabina Sabatini and Sarah Roberts SAO September 2009

 CDM and Dwarf galaxies Simulation of Dark Matter Observations of Baryons 1  fluctuation of 10 6 M  (dwarf Galaxy) collapses at z=6 (13 Gyr) 1  fluctuation of M  (Virgo Cluster) collapses at z=0 Dark Matter Halo mass function is steep (-2.0<  <-1.6) Relatively more small Dark halos in low density environments. Important physical processes?

What is the relation between the Dark Matter Mass Function and the Luminosity Function? Is there a large population of dwarf galaxies that has gone undetected? Number of galaxies fainter than in the VCC? Number of galaxies beyond the last point in the SDSS/2dF luminosity function? Low luminosity Low Surface Brightness Many no gas Low metalicity so no dust

Virgo and other Environments an INT WFC survey Sabatini et al., (2003) MNRAS, 341, 981, and (2005) MNRAS, 357, 819 Roberts et al., (2004) MNRAS, 352, 478 Roberts et al., (2007) MNRAS, 379, 1053 Virgo 15 sq deg 7 sq deg Equatorial Strip 30 sq deg M101

Image Detection and Selection The algorithm we 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<M B <-10 23<  o <26 B  Same data and same objective detection method for all fields

What did we find? 30 arc sec E-W strip Detect about 40 sq deg at cluster centre Detect about 4 sq deg at cluster edge and in the Equatorial strip. Basic result Looking through the Virgo cluster is very different to looking through the general field with regard to the numbers of faint smudges detected.

The result (involving some modelling) Virgo - 20 (E-W) and 31:1 (N-S) (  =-1.3, VCC  =-1.4) Field < 5:1 (Ursa Major and M :1) Local Group - 5:1? Sloan/2dF 20:1 (  =-1.2)  CDM – 350 (  =-1.6), 8000 (  =-2.0) 1. There is no LSB field dwarf galaxy population that would lead to a sharp rise in the luminosity function beyond the last measured point (M B ≈-16) of the redshift surveys (SLOAN, 2dF). 2. There is a relatively large LSB dwarf galaxy population in a nearby cluster compared to the field and the Local Group. The dwarf to giant ratio. N(-14<M B <-10)/ N(M B <-19)

The Arecibo Galaxy Environment Survey (AGES) Are dwarf galaxies detected in surveys for atomic hydrogen? AGES is a survey of nearby isolated galaxies, groups and the Virgo cluster. It consists of multi-beam scans of large areas of sky (5-20 sq deg) to reach a noise level of about 0.7 mJy. Full proposal is for 2000 hours or 200 sq deg of sky. It is currently about 40% complete.

Result is that we find very few new dwarf galaxies in all of the environments. There are one or two new HI detections for each giant galaxy down to an HI mass of about 10 7 M 0. Some of the detections have extremely blue colours. ? Part of a typical data cube in this case the NGC7448 group.

Evolutionary clues - galaxy colours Galaxy colour changes systematically with environment (B-I) against number Field Ursa Major Virgo Fornax Measuring and understanding the stellar populations in galaxies of different mass and in different environments is the next great challenge.

Main issues 1.If clusters are assembled from small groups of galaxies where do the excess numbers of dwarfs in clusters come from? 2.The numbers detected are still way short of the numbers predicted using the dark matter mass function. Feedback or what? 3.What are the important evolutionary effects that operate on dwarf galaxies in clusters, but not in the field? Is this the origin of the large numbers of dE/dSph galaxies (harassment, tidal) ? 4.There is no evidence for a large population of field dwarf galaxies. 5.Neither optical or atomic gas surveys reveal the predicted population, Dark galaxies or an incorrect theory?