1. The Stellar Halo of the Andromeda Spiral Galaxy
Raja Guhathakurta
University of California Observatories (Lick, Keck, TMT)
University of California at Santa Cruz
Fossil record of galaxy formation / evolution
Discovery of extended metal-poor stellar halo: (sub)structure, dynamics, metallicity distribution
Relation of halo to: extended “bulge”, boxy bulge/bar, outer disk, debris trails, satellite disruption
STScI Wed, Jan 25, 2006

2. Collaborators Many thanks to……
Halo: Observations
Karoline Gilbert, Jason Kalirai,& Carynn Luine (UCSC)
Steve Majewski & Jamie Ostheimer (U Virginia)
David Reitzel & Mike Rich (UCLA)
Michael Cooper (UC Berkeley)
Halo: Dynamical Modeling
Andreea Font & Kathryn Johnston (Wesleyan U)
Arif Babul & Jonathan Geehan (U Victoria)
Mark Fardal (U Massachusetts)
Bulge
Rachael Beaton, Mike Skrutskie (U Virginia)
Martin Bureau (Oxford), Lia Athanassoula (Marseilles)
Dwarf Satellites
Marla Geha (OCIW), Phil Choi (Spitzer Sci Ctr)
MOTIVATION / BACKGROUND
Direct look back (e.g. DEEP) vs fossil record
r ~ 200 kpc is halo radius of typical large galaxy
Many thanks to……
Sandy Faber, Drew Phillips (UCSC) & the DEIMOS instrument team

3. Galaxy Formation and Evolution
Direct look back to distant galaxies
versus
Fossil record studies

4. Direct Look Back to Early Epochs
eep
xtragalactic
volutionary
robe

5. Sample of Preliminary DEEP2 Results
Redshift survey of 50,000 galaxies half-way to the Big Bang:
Clustering of galaxies vs. redshift and galaxy type/mass
Samples of galaxy groups, including Local Group analogs
Gerke et al. 2005, astro-ph/

6. Panchromatic Studies of Galaxies at z ~ 1

7. Present-day Stellar Halos of Galaxies “Fossil Records” of Galaxy Formation
ΛCDM vs WDM
Bode, Ostriker, & Turok 2004
The amount of structure on sub-galactic scales is different for different flavors of dark matter

8. Previous Studies of M31

9. Brief History of M31 Spheroid Stellar Population Studies (A highly incomplete review)
Baade 1944 — Resolved Population II stars in inner regions
van den Bergh 1968; Hodge 1981 — Reviews: ~ 50 years of studies of luminous Population I stars and integrated light
Mould & Kristian 1986 — First detailed star-count study: metal rich population with large [Fe/H] spread at R = 7 kpc
Pritchet & van den Bergh 1988, 1994; Christian & Heasley 1991 — New star-counts & data compilation: steep radial brightness profile at R = 20 kpc; spheroid follows single de Vaucouleurs r1/4 law for R < 20 kpc; metal-rich population

10. More Recent Work on M31’s Spheroid
Rich et al. 1996
More Recent Work on M31’s Spheroid
Brown et al. 2003
Freeman 1996 — Contrasts M31 vs. Milky Way spheroid
Reitzel, PG & Gould 1998 — Cleaner sample: metal-rich population at R = 20 kpc; high stellar density
Durrell et al. 2001, 2004 — Star-counts: metal-rich in the mean, with metal-poor tail; obeys r1/4 law out to R < 30 kpc
Holland et al. 1996; Rich et al. 1996; Belazzini et al — HST study of field stars: widespread metal-rich population
Reitzel & PG 2002 — Spectroscopy of RGB stars at R = 20 kpc: presence of definite metal-poor population
Brown et al — Age constraints from ultra-deep HST / ACS images: significant intermediate-age population

11. Conclusion from Previous Studies: M31’s “outer” spheroid (r ~ 20 – 30 kpc) is nothing like the Milky Way halo
The combination of the r1/4 law surface brightness profile and high metallicity makes the M31 spheroid look much more like the Milky Way’s bulge than its halo
M31’s spheroid has also been likened to elliptical galaxies
The age and star-formation history of M31’s spheroid are unusual —intermediate-age / young population found in Brown et al.’s (2003) ultra-deep HST / ACS photometry

12. Discovery of an extended metal-poor stellar halo
Our Survey of M31
Discovery of an extended metal-poor stellar halo

13. M31 Data Sets Deep, wide-field imaging: Ultra-deep, hi-res imaging:
KPNO/Mosaic DDO51;
CFHT/MegaCam;
Subaru/S-Cam (WASSAHBI)
Faint red giant branch stars, and even
horizontal branch stars
Metallicity (photometric estimates)
Global structural parameters (radial
extent, flattening) and sub-structure
Clean sample of bright M31 red giants
Kinematics: rotation and sub-structure
Chemical abundance patterns
Spectroscopy:
Keck/DEIMOS (+LRIS/ESI/HIRES);
MMT/Hectospec
Ultra-deep, hi-res imaging:
HST/ACS (+WFPC2)
Reaches below main-sequence turnoff!
Direct age determination
Metallicity (photometric estimates)
Horizontal branch morphology

14. Photometry in the DDO51 Band Pre-selection of M31 RGB candidates for spectroscopy
M31 red giant vs. Milky Way dwarf star spectra
DDO51 color-color diagram
Majewski et al Palma et al. 2003
Ostheimer 2002, PhD thesis, U Virginia

15. Sample Keck / DEIMOS Spectra
PG et al. 2006a,
AJ, in press
(astro-ph/ )

16. Co-added Keck / DEIMOS Spectra
Many weaker lines; should be able to constrain [α/Fe]
Ca II triplet
Na I doublet: surface gravity sensitive; strong in MW dwarfs – weak in M31 giants
PG et al. 2005a, astro-ph/

17. Grouping stars for coaddition of spectra

18. M31 Data Sets Star-Count Map (Ferguson et al. 2002)
NGC 205
“Normal” M31 Image (Choi et al. 2002)
CFHT MegaCam
Subaru SuprimeCam
Keck DEIMOS
Keck LRIS
HST ACS
Ultra Deep HST ACS

19. Remote Outer Halo of M31 R = 160 kpc R = 85 kpc
Majewski/Ostheimer KPNO 4-m/MOSAIC DDO51 filter

20. Our Study of the M31 Halo
The spectroscopic sample combined with our method for isolating a clean sample of M31 RGB stars gives us an unprecedented ability to detect sparse groups of M31 stars
Explores the halo of M31 3 to 5 times further out from the galaxy’s center than previous studies
We detect M31 red giant stars in all our fields; the star counts in the three outermost fields are well above the extrapolation of the r1/4 law that fits the inner spheroid

21. Isolating M31 Red Giants Ten criteria used to reject foreground MW dwarf stars
Radial velocity
DDO51 parameter based on the (M – DDO51)
versus (M – T2) color-color diagram
Strength of surface-gravity-sensitive Na I
absorption-line doublet at approx Å
Position within color-magnitude diagram
[Fe/H]spec (based on strength of Ca II absorption-
line triplet) versus [Fe/H]phot (CMD based)
Strength of two surface-gravity-sensitive K I
absorption-lines at approx Å
Strength of three surface-gravity-sensitive TiO
absorption-bands at 7100, 7600, and 8500 Å
Gilbert, PG, et al. 2006, AJ, in preparation

22. Overall Likelihood Distributions
Sum of upto 10 individual likelihoods
In general:
High ΣLi: M31 RGB
Low ΣLi: MW dwarf
where:
Li = log(Pgiant/Pdwarf)

23. M31’s Surface Brightness Profile
PG et al. 2006b, Nature, submitted

24. M31’s Surface Brightness Profile: Bulge, Disk, and halo
PG et al. 2006b, Nature, submitted

25. Are the Stars in the Remote Halo Fields Really M31 Red Giants?
The stars in the three outermost halo fields follow the expected locus of red giant stars, not that of dwarf stars, in each of the five diagnostics:
— radial velocity
— DDO51 criterion
— Na I doublet
— color-magnitude diagram
— [Fe/H]spec vs. [Fe/H]phot

26. PG et al. 2006b, Nature, submitted
Outer Halo Fields
PG et al. 2006b, Nature, submitted

27. M31’s Surface Brightness Profile

28. Metallicity Distribution Inner Spheroid is more Metal-rich than Outer Halo
Kalirai et al. 2006b, ApJ, in preparation

29. Radial Gradient in Metallicity?
Spectroscopic metallicities tell the SAME story!
The inner spheroid (bulge) is significantly more metal-rich than the outer halo; need larger outer halo samples to characterize the shape of the outer [Fe/H] distribution
Kalirai et al. 2005b, ApJ, in preparation

30. M31: Halo versus Bulge

31. Kinematics M31’s Extended Bulge vs Halo
The dynamically cold peak is only marginally significant; most of it comes from the R=60 kpc field; there is a hot component in the outer halo
Kalirai et al. 2006b, ApJ, in preparation

32. Kinematics of the Extended Bulge
The fields dominated by M31’s extended bulge display a relative large spread in line-of-sight velocities (Gaussian sigma ~ 100 km/s).

33. K-band image minus ellipse fit
M31’s Boxy Bulge and Central Bar An Unobstructed Wide-field View in the Near Infrared
K-band image minus ellipse fit J
BVRZ
JHK
Model disk galaxy with central bar and boxy bulge (Bureau & Athanassoula 2005) K
Model A
Model disk galaxy with central bar, boxy bulge, and classical bulge (Beaton & Athanassoula 2006)
BVRZ
Beaton, Majewski, PG et al. 2006, in preparation

34. Beaton, Majewski, PG et al. 2005, in preparation
M31’s Boxy Bulge and Central Bar Detailed Comparison to Dynamical Models
Effect of changing inclination
Effect of changing bar angle
Beaton, Majewski, PG et al. 2005, in preparation

35. M31: Bulge to Disk Ratio

36. Stellar Kinematics in Major-axis FieldsG1
H13d
Stellar Kinematics in Major-axis Fields
Keck DEIMOS/LRIS
Ultra Deep HST ACS

37. Major-axis Disk Fields (H13d and G1)HI HI
Emphasize: BULGE, DISK, and HALO
Never see only a dynamically cold component
INCONSISTENT with Worthey et al claim of a dominant disk
Note the prominent cold peak due to the giant southern stream and the second, smaller cold peak at v ~ –425 km/s (a different debris trail?).
Reitzel et al. 2006, in preparation

38. Debris Trails in the M31 Halo

39. Stellar Kinematics in M31’s Giant Southern Stream
H13s
Keck DEIMOS
Ultra Deep HST ACS

40. Giant Southern Stream Field (H13s)
Note the prominent cold peak due to the giant southern stream and the second, smaller cold peak at v ~ –425 km/s (a different debris trail?).
Kalirai et al. 2006a, ApJ, in press

41. Progenitor / Orbit of the Giant Southern Stream
Test-particle orbits in a static disk galaxy potential (Ibata et al ; Font et al. 2006)
Refinement of disk galaxy potential to better match M31 (Geehan et al. 2006; Widrow et al. 2006)
N-body satellite in this new M31 potential (Fardal et al. 2006)
Could the stream, PNe concentration, “eastern shelf”, and “western shelf” be associated with a single accretion event?
Could M32 being the surviving bulge of the progenitor disk galaxy?
Fardal, Babul, Geehan & PG 2006, in preparation

42. Simulated Galaxy Halos
The most prominent debris trails in the simulations are expected to be the most metal-rich. This trend is seen in our M31 halo data.

43. Tidal Disruption of M31’s Dwarf Satellites

44. NGC 205 Observations Keck / DEIMOS multislit spectroscopy
Integrated light spectra cannont probe beyond effective radius
We have targeted individual red giant branch stars
Accurate radial velocities for 723 red giant stars in NGC 205
Geha, PG, Rich & Cooper 2006, AJ

45. Keck / DEIMOS Targets

46. NGC 205: Major-axis Velocity Profile
Inner rotation speed: 15 km/s
Radial velocity curve turns over beyond 2.5 reff
These data indicate that NGC 205 is in a prograde orbit

47. Fundamental Plane Measure: reff , SBeff , so Kappa-space:dE E
dSph GC
Measure: reff , SBeff , so
Kappa-space:
k1 ~ log ( Mass )
k2 ~ log ( SBeff x M/L )
k3 ~ log ( M/L )
dEs lie in distinct region of FP in relation to other “hot” stellar systems
Geha, PG & van der Marel 2002, AJ

48. Rotation vs. Anisotropy in Virgo Cluster dEs
Rotational flattening
6 out of 19 dEs are roughly consistent with rotational flattening
Vrot / s
Ellipticity: e
Geha, PG & van der Marel 2003, AJ

49. Rotating Versus Non-rotating Dwarfs
Virgo Cluster dEs:
Rotating Versus Non-rotating Dwarfs
Despite very different internal kinematics,
rotating and non-rotating dEs:
share the same region of FP space
cannot be distinguished via morphology
contain similar stellar populations
have similar local environments
It is difficult for a single formation scenario to create
the apparent dichotomy in rotational properties
Courtesy of J. S. B.
It is difficult for a two separate scenarios to create
dEs with similar properties

50. M31’s Stellar Halo: Summary / Future Work
Discovery of an extended halo of M31 red giants: r > 150 kpc
Spatial structure: extent, profile shape, flattening
Global dynamics
Sub-structure: streams from past accretion events
Origin of various “features” in the halo
> Disk-halo transition region
> Ongoing tidal disruption of existing satellites
> Detailed modeling of M31’s disk
Chemical abundance: [α/Fe] vs. [Fe/H] for structural subcomponents
Statistical comparison to numerical simulations

51. Mauna Kea Sunset and Moonrise