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

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

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

Direct Look Back to Early Epochs eep xtragalactic volutionary robe

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/0410721

Panchromatic Studies of Galaxies at z ~ 1

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

Previous Studies of M31

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

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. 2001 — 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. 2003 — Age constraints from ultra-deep HST / ACS images: significant intermediate-age population

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

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

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

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. 2000 Palma et al. 2003 Ostheimer 2002, PhD thesis, U Virginia

Sample Keck / DEIMOS Spectra PG et al. 2006a, AJ, in press (astro-ph/0406145)

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/0406145

Grouping stars for coaddition of spectra

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

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

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

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. 8190 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. 7700 Å Strength of three surface-gravity-sensitive TiO absorption-bands at 7100, 7600, and 8500 Å Gilbert, PG, et al. 2006, AJ, in preparation

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

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

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

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

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

M31’s Surface Brightness Profile

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

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

M31: Halo versus Bulge

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

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).

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

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

M31: Bulge to Disk Ratio

Stellar Kinematics in Major-axis Fields G1 H13d Stellar Kinematics in Major-axis Fields Keck DEIMOS/LRIS Ultra Deep HST ACS

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

Debris Trails in the M31 Halo

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

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

Progenitor / Orbit of the Giant Southern Stream Test-particle orbits in a static disk galaxy potential (Ibata et al. 2004; 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

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.

Tidal Disruption of M31’s Dwarf Satellites

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

Keck / DEIMOS Targets

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

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

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

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

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

Mauna Kea Sunset and Moonrise