The Globular Clusters Dwarf Galaxies Connection Michigan Aug
“The Globular Clusters - Dwarf Galaxies Connection” Michigan, Aug. 2007 Structural Parameters for confirmed Globular Clusters in NGC 5128 Matias Gomez Universidad de Concepcion - Chile Collaboration with: K. Woodley, W. E. Harris (Mc. Master), D. Geisler (Concepcion), G. L. H. Harris (Waterloo)
Why Structural Parameters? (SPs) 1) Properties of Globular Clusters (GCs) are correlated with properties of their host galaxies (Brodie & Strader 2006) fundamental key to understand galaxy assembly and evolution 2) GCs seem to occupy a well-defined region in the parameter space ( 0¸c¸ ) (Djorgovski 1995; Mc. Laughlin 2000; Mc. Laughlin & van der Marel 2005) 3) GC formation scenarios! Need to understand the physical conditions that produce outliers high ellipticity of LMC GCs (Geisler & Hodge 1980), N 5128 (Rejkuba et al. 2007) Population of bright/extended/compact/etc GCs in some environments and not in others
4) Effective (half-light) radius remains constant along the entire GC life (Spitzer & Thuan 1972; Spitzer 1987; Meylan & Heggie 1997; Aarseth & Heggie 1998) learn from GC formation by using sizes (Jordan 2004; Brodie & Strader 2006) 5) Sizes seem to indicate that a large variety of objects could have a common origin: GCs/UCDs/FFs/DSCs (but not d. Sph, Gilmore et al 2007) 6) distances for free! (Kundu & Whitmore 2001; Jordan et al. 2005)
A puzzling difference: blue clusters are, on average, 20% larger than red ones (Kundu & Whitmore 1998; Larsen et al 2001; Kundu & Whimore 2001) What causes this? 1) Mass segregation + metallicity dependence of stellar lifetime under the assumption of metallicity-independent half-mass radii (Jordan 2004; Jordan et al. 2005) little change predicted in relative sizes with Rgc 2) Projection effects (Larsen & Brodie 2003; Spitler et al. 2006) size differences should be largest at small Rgc and disappear at large Rgc Need to study sizes at large Rgc
1) In our Galaxy: re increases with galactocentric distance re ~ √Rgc (van den Bergh et al. 1991) 2) In nearly all GC Systems red clusters are more centrally concentrated than blue ones within any given radius, red (metal-rich) GCs will tend to lie at smaller Rgc If GCs follow the same correlation found in the MW, red cluster will appear smaller on average than blue ones.
Why NGC 5128? 1) d = 3. 8 Mpc, nearest giant elliptical (albeit merger product). GCs can be resolved 2) N ~ 2000 (Harris et al. 2007) (reasonable sample in contrast with MW) 3) Since merger product potentially rich GC Age-Distribution-Function, spanning many Gyr (cepheid distance! Ferrarese et al. 2007) 4) 3 mag brighter than Fornax & Virgo, 2 mag brighter than Leo ideal target for addressing many questions ELT science case
However, nearby is good and bad. . . 1) GCS very spread out (individual GCs found out to Rgc = 40', Peng et al. 2004 b) 2) Low galactic latitude (b~19°) and contamination is a serious problem a) blue compact, faint background galaxies b) a rich population of foreground objects (Halley and NGC 5128 - ©A. Gomez, CTIO)
Plus. . . NGC 5128 has its reputation!
Previous work: (just a few examples) - HST structural parameters (Holland et al. 1999, Harris et al. 2002, Harris et al. 2007 ACS) reduced FOV - Peng et al. 2004, 4 m CTIO, BVI - Harris et al. 2004, 4 m CTIO, CMT 1 (most extensive study) less than ideal seeing (Dirsch et al. 2003) Needed: a clean sample of GCs from the 105 sources in the field (2%)
This work: IMACS Observations 25 fields observed at Las Campanas Observatory (Chile) with the Magellan 6. 5 m and the IMACS camera (0. 11“/pix). Seeing between 0. 4" and 0. 7". Short and long exposures in B, R. 1. 2° Image taken from DSS (North up, East to the left)
Confirmed GCs taken from: Convolution techniques Peng et al. (2004) Woodley et al. (2005) Beasley et al. (2007) Harris (2007, ACS imaging) (ishape, Larsen 1999, 2001) + + new Magellan/LDSS-2 data We were able to derive structural parameters for ~370 GCs (no contamination, homogeneous sample, large FOV, very good seeing) combining with existing Washington photometry from Harris et al. (2004) colours, magnitudes and metallicities
Some results: 1) good 1: 1 comparison with entirely independent HST/ACS sizes (different approach too) 2) Red/blue subpopulations: usual behaviour (but note higher dispersion for metal-rich GCs)
relative sizes of blue and red cluster depends on Rgc (flattening of red GCs at ~1. 5 times reff of NGC 5128)
Dividing into inner/outer metal-poor/metal-rich subpopulations: size difference in agreement with usually observed trend at small Rgc No difference at large Rgc (sample extending up to ~8 times reff of NGC 5128) Note also the lack of large blue clusters at large Rgc ! Results in favour of projection effects as the origin of size difference (Spitler et al. for NGC 4594)
Conclusions: 1) 0. 5” high resolution imaging across a 1. 2°x 1. 2° field 2) sizes and SPs for ~370 confirmed GCs out to Rgc = 40' (~45 kpc) 3) size difference between metal-rich and metal-poor disappears at large Rgc Projection effects are enough to explain difference Coming attractions: 1) GMOS ages and metallicities of a large sample of GCs (Ph. D thesis of K. Woodley) 2) VIMOS: homogeneous PA coverage (e. g. along minor axis)
thanks !!
and. . . South !!
- Slides: 17