Ages of Elliptical Galaxies - McMaster Physics and Astronomy

Ages of Elliptical Galaxies
NGC 5128
(Centaurus group)
W.E.H.
R.Gendler
M32 (Local Group)
SDSS
NGC 3377
(Leo group)
NGC 3379
(Leo group)
In these galaxies …. the color and
magnitude distributions of the halo stars
directly constrain age and metallicity
distribution
Old halo and bulge red-giant stars are
readily accessible with HST imaging; for
Local Group and NGC 5128, can also
reach the helium-burning HB
(V,I) photometry works well (high
metallicity sensitivity and takes full
advantage of the optical cameras)
Interpolate within RGB tracks (calibrated
onto the Milky Way globular cluster grid)
Fast, efficient way to derive first-order
Metallicity Distribution Function
Metallicity
Age
NGC 5128
Unique chance to study a halo
population in an E/S0 giant at
close range
d = 3.8 Mpc from several
standard candles:
TRGB, PNLF, SBF, Cepheids
Previous work:
“20 kpc field”
Allocated in Cycle 5,
mid-1994
Exposures taken
August 1997!
Harris, Harris, & Poole 1999, AJ 117, 855
Harris, Poole, & Harris 1998, AJ 116, 2866
But are these really mostly former
disk stars from the merged
progenitors, IF the galaxy formed
by major merger?
More fields ….
7’
8.0 kpc = 1.4 Reff
18’ 20 kpc = 3.7 Reff
27’ 30 kpc = 5.4 Reff
33’ 37 kpc = 6.7 Reff
deepest, with ACS
More fields ….
7’
8.0 kpc = 1.4 Reff
18’ 20 kpc = 3.7 Reff
27’ 30 kpc = 5.4 Reff
33’ 37 kpc = 6.7 Reff
Peng, Ford & Freeman 2002
Malin 1983
Harris & Harris 2000, 2002
Rejkuba, Greggio, Harris, Harris &
Peng 2005
Metal-rich
all the way
out!
Mean age = 8.5 Gy
Employ age-sensitive features of CMD: at the moment,
only the “40 kpc field” has enough leverage for both age
and metallicity distributions simultaneously
AGB bump
RHB
(red
clump)
-Luminosity function in both I and V
Rejkuba, Harris, Harris &
Greggio 2008
-Full distribution across CMD
Generate simulated CMDs from evolutionary model tracks:
“Teramo” models (Pietrinferni et al. 2004 + later papers)
-full evolutionary phases through AGB++
- 11 distinct metallicity values x α-enhanced or scaled Solar
Simulation with uniform 13 Gy age + Z-distribution, convolved
with observational measurement scatter and incompleteness
Compare two CMDs divided into grid elements: numbers of stars
n1(i,j), n2(i,j) in each grid element and Nbox total grid elements
Form the total
1
(n1 − n2 ) 2
χ =
Σ
N box (n1 + n2 )
2
and find simulation that
minimizes it by varying
over input MDF and ADF
Consistency tests with both luminosity functions,
complete CMD, and Z-distribution
Single-age “burst” formation
models with Z-mixture
Simulation versus simulation:
tests of internal age sensitivity of
method Æ +/- 1 Gy at best
Simulation versus actual CMD:
“best” mean age ~ 11 Gy? within
the context of these models
Single-age (i.e. “starburst”) simulations aren’t adequate. Need
formation period lasting > 1 Gy and mean age > 10 Gy
Mixture including age/metallicity relation: [preliminary!]
8 Gy
13 Gy
Local Group compact elliptical M32:
12 Gy isochrones
not sufficient by
themselves. Wide
age mixture?
Leo Group ellipticals: d = 10.5 Mpc
NGC 3377
NGC 3379
MV = -19.9
MV = -20.9
38.5 ksec V, 22.3 ksec I
38.5 ksec V, 22.3 ksec I
field center at 12 kpc
field center at 33 kpc
(1.5 Æ 5.2 Reff )
(10.3 Æ 13.6 Reff )
Harris, Harris, Layden & Stetson 2007, AJ 134, 43
Harris, Harris, Layden & Wehner 2007, ApJ 666, 903
NGC 3377
NGC 3377
No trace of metallicity gradient!
Entirely old, simple formation
history, intermediate metallicity
NGC 3379
ACS
NICMOS fields
Gregg et al. (2004)
– old and high-Z
DeVaucouleurs & Capaccioli 1979
(“note close agreement with r1/4 law”)
Textbook standard giant
elliptical! “A walking
advertisement for the
deVaucouleurs law” (Statler
& Smecker-Hane 1999)
NGC 3379
ACS
V filter cutoff
NGC 3379
2-stage chemical evolution
Are we seeing the “true”
metal-poor halo for Z < 0.2 ?
-2.4
-5.6
Are we seeing the region of transition to
the classic metal-poor halo?
Are we seeing the region of transition to the classic metal-poor halo?
Why didn’t we see it in the others?
Are we looking at two distinct components ?? (bulge + halo)
NGC 3377
1.5 Æ 5.2 Reff
NGC 3379
10.3 Æ 13.6 Reff
NGC 5128
1.4 Æ 6.7 Reff
Should we expect to find the transition
starting routinely around 12 Reff ?
Kalirai et al. 2006 &&
M31 halo Z-gradient
Metal-poor past R > 10 Reff
Increasing
luminosity of
host galaxy
Higher
effective yield
in chemical
evolution
Further simulations and analysis in progress ….