NGC 3377

The Nearest Giant Ellipticals:
Rosetta Stones or Fool’s Gold?
A hypothesis or theory is clear, decisive, and
positive, but it is believed by no one but the man
who created it.
Experimental findings, on the other hand, are
messy, inexact things which are believed by
everyone except the man who did that work.
Harlow Shapley
NGC 5128
(Centaurus group)
W.E.H.
SDSS
NGC 3377
(Leo group)
NGC 3379
(Leo group)
In these three galaxies ….
Old halo and bulge RGB stars are
readily accessible with HST imaging
(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
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
Metallicity
Age
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?
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
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 in press
Harris, Harris, Layden & Wehner 2007, submitted
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
Williams et al. 2007
Virgo ICM stars
Kalirai et al. 2006 &&
Metal-poor past R > 10 Reff
Increasing
luminosity of
host galaxy
NGC 5128 Cycle 15 – ACS target fields at R = 70, 110, 140 kpc
(13, 20, 25 Reff)
What about the globular
cluster population? N5128
permits detailed comparison
VERY DIFFICULT
TO WORK WITH:
- Finding them in the first place is hard.
Field is at intermediate latitude (b =
19o), thus field contamination by both
foreground stars and faint background
galaxies is very significant
- Low SN = 2 (1500 GC’s total), thus “signal” is low
- Nearby (D = 4 Mpc), thus GCS is very dilute against the field
- Observing runs are always plagued by bad weather,
bad seeing, or instrument failure [this includes HST]
~450 clusters now known, 340
with radial velocities
Woodley et al. 2007, AJ, in
press: kinematics & dynamics
Radial velocity measurements
Hi-res imaging (HST, Gemini,
VLT, Magellan)
G.Harris et al. 2004 CMT1 database
Metallicity distribution function
(C-T1)0 Æ metallicity
Metallicity distribution function
S N ( MP)
≥ 5
S N ( MR)
The new “specific
frequency problem” !
SN = 4.2 +- 0.6
SN = 0.85 +- 0.12
Probably a common feature
of gE galaxies.
Assuming the low-metallicity
clusters formed in massive
pregalactic dwarfs, did they
form –
-preferentially early relative
to the field stars, followed by
truncation of star formation?
-at ~5 x higher efficiency?
Harris & Harris 2002
Beasley et al. 2002
Jordan et al. 2004
Rhode et al. 2005
SN = 4.2 +- 0.6
Major star formation phase of the gE
SN = 0.85 +- 0.12
Age estimates from Lick indices:
Beasley et al. 2007
courtesy Thomas Puzia
Highest-metallicity GCs appear
6-8 Gy old. cf. Rejkuba &&
halo-star mean age of 8.5 Gy
But -- Great majority > 10 Gy
Gemini GMOS study of
inner GC population
Preliminary !!
Woodley, Harris, Harris, Geisler,
Gomez, & Puzia 2007
N5128: GC kinematics
Woodley et al. 2007,
astro-ph/0704.1189
Blue, MP
Red, MR
NGC 3379
NGC 5128
GCS
PNe
Woodley et al. 2007
Romanowsky et al. 2003
Bergond et al. 2006
Dynamical connection of a giant
central galaxy with its surroundings
NGC 5128: Woodley 2006, AJ 132, 2424
M87: Cote et al. 2001
There appears to be good reason to think that the metal-rich
globular clusters and the main population of stars in E
galaxies formed together
and
We may now have reason to think that the metal-poor
globular clusters and the (more elusive) metal-poor halo
stars formed together as well (but in a different ratio).
But are these generalizations risky? Prospects for continuing the halo-star
studies are great if we can get HST back with WFC3 (or ACS) … Æmore
extended coverage of N5128, 3379, plus Virgo, Fornax E’s and many others.
Stellar populations are what happen
when galaxy populations run out !
There appears to be good reason to think that the metal-rich
globular clusters and the main population of stars in E
galaxies formed together
and
We may now have reason to think that the metal-poor
globular clusters and the (more elusive) metal-poor halo
stars formed together as well (but in a different ratio).
All depends on keeping the eye steadily fixed on the facts of
nature, and so receiving their images as they are. For God
forbid that we should give out a dream of our own imagination
for a pattern of the world.
Francis Bacon (1623)