RM 18apr07

Dwarf Galaxies in Groups and Clusters: 

Dwarf Galaxies in Groups and Clusters

Luminosity Function: 

Luminosity Function The number of galaxies in each bin of luminosity. Normally parameterised as a Schechter Function: Or, in magnitude form:

Luminosity Function: 

Luminosity Function

Why the LF is important: 

Why the LF is important The LF provides a statistic that can be compared to cosmological models. The LF of rich clusters like Virgo appears to match predictions from DM The LF of less dense environments is much shallower than predicted Understanding why is at the heart of the missing satellites problem.

Determining the LF: 

Determining the LF The slope (α) of the LF is given by the faint galaxies – the dwarfs. These are the hard to find due to their low luminosity and (often) low surface-brightness. Field galaxies: α = -1.2 (2dF: Cross et al. 2000; SDSS: Blanton et al. 2001) Assigning membership is an added complication in groups and clusters.

Assigning membership: 

Assigning membership Measuring redshifts: Pro: rules out most background sources. Con: difficult for faint galaxies, ambiguous in large clusters and very time-consuming. Statistical subtraction of background: Pro: relatively easy to do, particularly with automated finders. Con: you are subtracting one large number from another to get a small answer.

Assigning Membership: 

Assigning Membership Measuring Distances (TRGB, SBF, etc.) Pros: Unambiguous. Cons: Only generally possible for a subset of nearby galaxies. Using Morphology Pros: Fairly simple. Cons: Will tend to either include BG galaxies or exclude true group members.

Nearby Groups: 

Nearby Groups Jerjen et al. (2000) looked at dwarfs in Scl, Cen A, M81 andamp; LG. Scl: poor group, few dwarfs. LG andamp; M81 - slightly richer, more dwarfs. Cen A - richest, most dwarfs. Combined LF has α = -1.29 ± 0.09, M*B = -21.8 ± 1.8.

Virgo Cluster: 

Virgo Cluster α = … -1.25 (Sandage et al. 1985) - VCC. -1.7 (Impey et al. 1988) - stacked plates. -2.0 (Phillipps et al. 1998) - bg subtraction. -1.6 (Trentham andamp; Hodgkin 2002) - CCD survey -1.6 (Sabatini et al. 2003a) - CCD survey Not just scatter - slope has steepened as LSB galaxies have been included. May not be well described by a single Schechter Function.

Other Clusters: 

Other Clusters UMaj: α = -1.1 ± 0.2 (Trentham et al. 2005) Coma: α ~ -1.7 at faint end (Smith et al. 1997; Trentham 1998a) Fornax: α ~ -2.0 at faint end (Kambas et al. 2000) Abell 2554: α ~ -1.7 at faint end (Smith et al. 1997) Abell 963: α ~ -1.7/-1.8/-1.15 at faint end (Driver et al. 1994; Smith et al. 1997; Trentham 1998b) Abell 665: α ~ -2/-1.18 at faint end (Wilson et al. 1997; Trentham 1998b) Abell 1689: α ~ -2 at faint end (Wilson et al. 1997)

Groups & Clusters: 

Groups andamp; Clusters Muriel et al. (1998) made a statistical analysis of LFs in the literature and found: Field: α -1.0 ± 0.15, M* = -19.5 ± 0.1 Groups: α -1.0 ± 0.2, M* = -19.6 ± 0.2 Poor Clusters: α -1.2 ± 0.1, M* = -19.6 ± 0.1 Rich Clusters: α -1.5 ± 0.1, M* = -19.7 ± 0.1 Groups look very like the field, but rich clusters, in particular, are different.

The dwarf-giant ratio: 

The dwarf-giant ratio Sabatini et al. (2003b) surveyed Virgo, UMaj, 4 field spirals and a void with the results: Virgo: 20 dwarfs/giant (40 in centre, 4 at the edge) UMaj: 5 dwarfs/giant Field spirals: 3 dwarfs/giant Void: 0 dwarfs, 0 giants LG (viewed from 15 Mpc): 4 dwarfs/giant

Dwarf morphology-density relation: 

Dwarf morphology-density relation Proposed by Ferguson andamp; Sandage (1990) that: 'Compared to dwarf ellipticals, dwarf irregulars form a more extended population in nearby clusters and may in fact be entirely absent from the cluster cores' Also that non-nucleated dEs with MB andlt; -14.5 cluster like dIrrs and giant spirals while nucleated dEs and non-nucleated dEs with MB andgt; -13.5 cluster like giant ellipticals and S0s.

Dwarf morphology-density relation: 

Dwarf morphology-density relation Kambas et al. (2000) different distributions for high and low SB galaxies in Fornax Also found dwarfs less strongly clustered than giants

A New Type of Galaxy: 

A New Type of Galaxy Ultra-Compact galaxies - galaxies that appear star-like due to their small size - were predicted by Disney (1976) and Disney andamp; Phillipps (1983) as the HSB analogues to LSB galaxies Discovered in Fornax (Drinkwater et al. 1999; Phillipps et al. 2001), also seen in Abell 1689 (Mieske et al. 2004) and in Virgo (Drinkwater et al. 2004; Jones et al. 2006).

Possible Causes: 

Possible Causes Theories of galaxy evolution must address the difference between field, group and cluster LFs. Two schools of though: nature (initial conditions in which galaxies formed) or nurture (interactions undergone by galaxies since forming). Must also address the morphological differences and the existence of UCDs.

References: 

References Disney, 1976, Nature, 263, 573 Disney andamp; Phillipps, 1983, MNRAS, 205, 1253 Drinkwater et al., 2004, PASA, 21, 375 Drinkwater et al., 1999, ApJ, 511L, 97 Driver et al., 1994, MNRAS, 268, 393 Ferguson andamp; Sandage, 1990, in 'The Interstellar Medium in External Galaxies', 281 Impey et al., 1988, ApJ, 330, 634 Jerjen et al., 2000, AJ, 119, 593 Jones et al., 2006, AJ, 131, 312 Kambas et al., 2000, AJ, 120, 1316 Mieske et al., 2004, AJ, 128, 1529 Muriel et al., 1998, ApJ, 506, 540 Phillipps et al., 2001,ApJ, 560, 201 Phillipps et al. 1998,ApJ, 498L, 119 Sabatini et al., 2003a, MNRAS, 341, 981 Sabatini et al., 2003b, Apandamp;SS, 285, 97 Sandage et al., 1985, AJ, 90, 1759 Smith et al., 1997, MNRAS, 287, 415 Trentham et al., 2005, MNRAS, 357, 783 Trentham andamp; Hodgkin, 2002, MNRAS, 333, 423 Trentham, 1998a, MNRAS, 293, 71 Trentham, 1998b, MNRAS, 295, 360 Wilson et al.,, 1997, MNRAS, 284, 915