Faint Low Surface Brightness Galaxies in the Virgo Cluster

Jonathan Davies, Sabina Sabatini and Sarah Roberts

Faint Low Surface Brightness Galaxies in the Virgo Cluster

Jonathan Davies, Sabina Sabatini and Sarah Roberts

SAO September 2009

CDM and Dwarf galaxies

Simulationof Dark Matter Observations

ofBaryons

1 fluctuation of 106 M (dwarf Galaxy) collapses at z=6 (13 Gyr)

1 fluctuation of 1014-15 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.

Importantphysicalprocesses?

What is the relation between the Dark Matter Mass Function and the

Luminosity Function?

Is there a large populationof dwarf galaxies that has gone undetected?

Number of galaxies fainterthan in the VCC?Number of galaxies beyondthe last point in the SDSS/2dFluminosity function?

Low luminosityLow Surface Brightness

Many no gasLow 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

EquatorialStrip

30 sq deg

M101

Image Detection and Selection

The algorithm we 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

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 centreDetect about 4 sq deg at cluster edge andin the Equatorial strip.

Basic resultLooking through the Virgo cluster isvery different to looking through thegeneral 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 M101 - 5:1)

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 (MB≈-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<MB<-10)/ N(MB<-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 reacha noise level of about 0.7 mJy. Full proposal is for 2000 hours or 200 sq degof 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 massof about 107 M0. Some of the detections have extremely blue colours.

?

Part of a typical data cube in thiscase the NGC7448 group.

Evolutionary clues - galaxy colours

Galaxy colour changessystematically with

environment

(B-I) against number

Field

Ursa Major

Virgo

Fornax

Measuring and understandingthe stellar populations in galaxiesof different mass and in differentenvironments is the next greatchallenge.

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?