Spectroscopic and Photometric Landscape of Andromedas Stellar Halo Dark Matter on Dwarf Galaxy to Ga

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Spectroscopic and Photometric Landscape of
Andromedas Stellar HaloDark Matter on Dwarf
Galaxy to Galaxy Group Scales
Raja Guhathakurta UCO/Lick Observatory University
of California Santa Cruz
Friday March 23
Astrophysical Probes of Dark Matter
Beckman Ctr, Irvine
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Collaborators
Karrie Gilbert, Jason Kalirai, Kirsten Howley,
Evan Kirby, Ryan Montgomery, Greg Laughlin, Jürg
Diemand (UCSC) Mark Fardal (U Mass), Marla Geha
(HIA/NRC), Jedidah Isler (Fisk / UCSC) Steve
Majewski, Rachael Beaton, Jamie Ostheimer, Ricky
Patterson, Ricardo Muñoz, Mike Skrutskie (U
Virginia) David Reitzel, Mike Rich (UCLA),
Michael Cooper (UC Berkeley) Arif Babul, Alan
McConnachie (U Victoria), James Bullock, Marc
Seigar (UC Irvine) Andreea Font (Durham), Kathryn
Johnston (Columbia U) Lia Athanassoula
(Marseille), Martin Bureau (Oxford) Tom Brown
(STScI), Mikito Tanaka (U, Tokyo), Masashi
Chiba (Tohoku U) Phil Choi (Pomona),Chris Sneden
(UT Austin)
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Outline

• Stellar halo (and inner spheroid)
• Discovery
• Global structure
• Chemical enrichment star formation history
• Tidal debris from past accretion events
• Other features bar, boxy bulge, star-forming
  ring
• Global dynamics

• Dwarf satellites
• Tidal disruption
• Tracers of M31s gravitational potential
• Properties of these building blocks

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Conclusion from Previous Studies M31s halo
(R 10 30 kpc) looksnothing 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 Ways
  bulge than its halo
• M31s spheroid has also been likened to
  elliptical galaxies
• The age and star-formation history of M31s
  spheroid are unusual intermediate-age / young
  population found in Brown et al.s (2003)
  ultra-deep HST / ACS photometry

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M31s Extended Stellar Haloand Inner Spheroid
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M31 Data Sets
Star-Count Map(Ferguson et al. 2002)
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Remote Outer Halo of M31
Majewski/Ostheimer KPNO 4-m/MOSAIC DDO51 filter
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Isolating M31 Red Giant Stars

• Possible contaminants
• (1) Background galaxies
• (2) Foreground Milky Way dwarf stars
• ? similar brightnesses/colors
• ? similar line-of-sight velocities
• We expect to find very few (if any) Andromeda red
  giant branch stars in the remote outer fields
  in order to isolate them we need to do a careful
  job of rejecting the above contaminants

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165 kpc
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Photometry in the DDO51 BandPre-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
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Isolating a clean sample of M31 RGB stars Use
probability distribution functions based on 5
photometric/spectroscopic diagnostics to
eliminate foreground Milky Way dwarfs. Will use
5 more in the future. (1) Radial Velocity
(2) DDO51 photometry (3) Na I
equivalent width (4) Position in the CMD
(5) Fe/Hphot vs Fe/Hspec (67) KI
line strengths (810) TiO band strengths
Gilbert et al. (2006, ApJ, 652, 1188)
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Overall Likelihood Distributions

• Weighted average of the first 5 individual
  likelihoods
• In general
• Li gt 0 M31 RGB
• Li lt 0 MW dwarf
• where
• Li log(Pgiant/Pdwarf)i

Gilbert, et al. 2006, AJ, in press
(astro-ph/0605171)
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Surface Brightness Star-counts in outer fields
(R gt 60 kpc) well above extrapolation of Sersic
inner spheroid. Best fit power law R-2.5 halo.
Guhathakurta et al. (2005, astro-ph/0502366)
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Radial Gradient in Metallicity
Kalirai, Gilbert, PG, et al. 2006b, ApJ
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Photometric vs. Spectroscopic Fe/H Estimates
Kalirai, Gilbert, PG, et al. 2006b, ApJ
It is reassuring to see that there is a
reasonably good correlation between the two
Fe/H estimates
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Detailed Elemental Abundances from Coadded Spectra
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Dissecting a Recent Collision
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Giant Stream and Young Shell System in M31
NE Shelf
W Shelf
Giant S Stream
Star-Count Map(Irwin et al. 2005)
Fardal, PG, Babul, McConnachie 2006, MNRAS,
submitted (astro-ph/0609050)
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The Merger of a Dwarf Galaxy with Andromeda
Karoline Gilbert Andromeda Press
Conference January 7, 2007
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The Remnants of a Cannibalized Galaxy

• Andromeda shows many signs of disturbance

• Mark Fardals simulations many of these features
  can be traced to a single parent galaxy

• We have discovered tidal debris in the location
  predicted by the Fardal simulations

• Will help astronomers to model the mass
  distribution of the Andromeda Galaxy, including
  dark matter

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Survey of Andromedas Halo

• Photometry from the MegaCam instrument on the
  3.6-m Canada-France-Hawaii Telescope
• Spectroscopy from the DEIMOS spectrograph on the
  10-m Keck II Telescope

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Metal-Rich Substructure on SE Minor Axis
Gilbert et al. 2007, astro-ph
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Substructure in Our Fields

• Bulge and Halo stars move in all directions,
  wide spread in line of sight velocities
• Tidal Streams stars move together, small spread
  in line of sight velocities

• 3 Fields show signs of both populations
• Spread in velocities of the tidal debris
  decreases with increasing radial distance from
  the center of Andromeda

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Measuring of the Mass of Andromeda

• Observed feature is in the same location and has
  the same distribution of velocities as the
  feature predicted by the Fardal simulations
• Observed feature, along with velocities from the
  other debris features, will allow a precise
  measurement of the amount of dark matter in the
  Andromeda galaxy

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Substructure
Fardal et al. (2006, MNRAS, submitted,
astro-ph/0609050)
February 23rd, 2007


UC Santa Cruz
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Substructure
Gilbert et al. (2007, ApJ, submitted)
February 23rd, 2007


UC Santa Cruz
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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 galaxys center than previous studies
• We detect M31 red giant stars in all our fields
  the star counts in the outer fields are well
  above the extrapolation of the r1/4 or Sersic law
  that fits the inner spheroid

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M31s Surface Brightness ProfileBulge, Disk,
and halo
PG et al. 2005, astro-ph/0502366
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M31s Boxy Bulge and Central BarAn Unobstructed
Wide-field View in the Near Infrared
BVRZ
Beaton et al. 2006, ApJL, submitted
(astro-ph/0605239)
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M31s Boxy Bulge and Central BarDetailed
Comparison to Dynamical Models
Effect of changing inclination
Effect of changing bar angle
Athanassoula Beaton 2006, MNRAS, in press
(astro-ph/0605090)
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Substructure
Bullock Johnston (2005, ApJ, 635, 931)
February 23rd, 2007


UC Santa Cruz
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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.

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M31s Dwarf Satellites
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NGC 205 ObservationsKeck / 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
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Keck / DEIMOS Targets
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NGC 205 Major-axis Velocity Profile
Inner rotation speed 10 km/s Radial velocity
curve turns over beyond 2.5 reff (
rtidal) Velocity turnover is coincident with
radius at which isophotal twisting starts to
occur
Radial velocity (km/s)
Radius (arcmin)
These data indicate that NGC 205 is in a prograde
orbit around M31
Geha, PG, Rich Cooper 2006, AJ
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Modeling the NGC 205-M31 Encounter Using a
Genetic Algorithm
Howley, Geha, PG, Montgomery Laughlin 2006, in
preparation
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M31 dE Rotation Curves
NGC 205
NGC 147
velocity (km/s)
NGC 185
radius (arcmin)
Simien Prugniel (2000)
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M31 dE Rotation Curves
NGC 205
NGC 147
velocity (km/s)
NGC 185
radius (arcmin)
Simien Prugniel (2000)
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M31 dE Rotation Curves
NGC 205
NGC 147
velocity (km/s)
NGC 185
radius (arcmin)
Simien Prugniel (2000)
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M31 dE Rotation Curves
NGC 205
NGC 147
velocity (km/s)
NGC 185
radius (arcmin)
Simien Prugniel (2000)
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Andromeda Dwarf Galaxies
Currently, 12 dSphs (And I-III, V-XIII) and 4
dEs (M32, M110, NGC 147, NGC 185).
Martin et al. 2007 (2006, MNRAS, 671, 1983)
February 23rd, 2007


UC Santa Cruz
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Andromeda Dwarf Galaxies
The discovery of And XIV !
Beaton et al. (2007, ApJL, submitted)
February 23rd, 2007


UC Santa Cruz
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Andromeda Dwarf Galaxies
Keck/DEIMOS studies of And I, II, and III
underway.
Kalirai et al. (2007, ApJ, in prep.)
February 23rd, 2007


UC Santa Cruz
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Summary

• Stellar halo (and inner spheroid)
• Discovery
• Global structure
• Chemical enrichment star formation history
• Tidal debris from past accretion events
• Other features bar, boxy bulge, star-forming
  ring
• Global dynamics

• Dwarf satellites
• Tidal disruption
• Tracers of M31s gravitational potential
• Properties of these building blocks

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Andromeda Dwarf Galaxies
Keck/DEIMOS studies of NGC 147, NGC 205, NGC 185
underway.
Geha et al. (2006, AJ, 131, 332)
February 23rd, 2007


UC Santa Cruz
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Mauna Kea Sunset and Moonrise