Chemical Abundances Along the Sagittarius Stream

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Chemical Abundances Along the Sagittarius Stream

• Mei-Yin Chou
• (University of Virginia)
• Principal Collaborators
• Steve Majewski (UVa), Katia Cunha, Verne Smith
  (NOAO), David Martínez-Delgado (IAC)
• Results in Chou et al. 2007, ApJ, 670, 346 Chou
  et al. 2008 (in prep.)

Image credit David Law
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• Outline
• Motivation
• Find connection between Sgr and stars it
  contributed to MW halo
• Reconstruct chemistry of original satellite
  galaxy
• 2. Metallicity Distribution Function (MDF) in Sgr
  Stream
• Found strong metallicity gradient along the Sgr
  tidal tail
• Shows that Sgr originally had strong radial
  metallicity gradient
• 3. Chemical Patterns in Sgr Stream
• Find relative chemical evolution between Sgr, MW
  and other satellites
• Use distinctive patterns to fingerprint other Sgr
  stars in Galactic halo

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Prominent Tidal Streams around Disk Galaxies
NGC 4013
Milky Way
NGC 5907
Sgr Model (Law et al. 2005)
(Gabany et al.)
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Chemical HistoryDistinctive abundance
patterns-- a/Fe, s-process (Y, La, etc.)--
reflect the unique chemical history of the parent
system, e.g., a/Fe (Ti, Mg, O, etc.)
indicates the Type II/Type Ia SNe ratio of the
parent system
From McWilliam 1997
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Chemical History
Halo Thick disk Thin disk
dSph stars
dSphs clearly differ from MW (and even from each
other) Chemical fingerprinting (e.g. Freeman
Bland-Hawthorn 2002) may be possible
Compilation from Venn et al.2004
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• My work
• High resolution, high S/N (50-200)
• spectroscopy of 2MASS-selected
• M giants in Sgr and its stream
• 31 stars from KPNO 4-m (R 35000)
• 12 stars from TNG 3.5-m (R 45000)
• 16 stars from Magellan 6.5-m (R 19000)
• Use of predominantly northern telescopes leads
• to focus on the leading arm

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R 35000
Derivation of Abundances MOOG (Sneden 1973) An
LTE Stellar Line Analysis Program
Ti
Ti
Teff from J-K (Houdashelt et al. 2000) log g from
isochrone (Girardi et al. 2000) Initial
metallicity guess
EW measurements
Model Atmosphere Line List
log g
MOOG
log Teff
Fe/H and x/Fe
If the output Fe/H not consistent
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The expected dynamical age of debris along the
tidal stream Stars lost from Sgr 1 orbit ago
0.5 Gyr 2 orbits ago 1.4 Gyr 3 orbits ago
2.2 Gyr 4 orbits ago 3.1 Gyr 1 radial
period 0.85 Gyr
Model (Law et al. 2005)
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Sgr Leading Arms and an NGP Moving
Group Brightest stars (Klt 10) in Sgr
core Leading arm north (lost 2 Gyrs
ago) Leading arm south (lost 3 Gyrs ago) Also,
peculiar group of NGP M giant stars having
radial velocities different from the main
leading arm trend
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• Iron Abundance Analysis
• 11 Fe I lines in a narrow spectral window
  7440-7590 Å
• (Smith Lambert 1985, 1986, 1990)
• LTE code MOOG
• combined with a
• Kurucz ATLAS9 (1994)
• solar model
• Solar gf-values of
• Fe I lines

R 35000
R 45000
R 19000
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Strong Metallicity Gradient along the tidal
tail! Chemical differences between the core and
the tails!
(Chou et al. 2007, ApJ, 670, 346)
-0.4
-0.7
-1.2
P
-1.0
Median Fe/H of NGP group is similar to Sgr
leading arm south
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• Reconstructed MDF of Sgr core several Gyrs ago
• Relatively flat, more
• metal-poor than
• presently in the
• Sgr core
• The observed
• chemical properties
• of the presently
• surviving satellites
• may depend on
• their tidal stripping
• history

MDF of Sgr core
MDF of Sgr core
MDF of Sgr tails
MDF of Sgr tails
Sum
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Chemical Trends in Sgr Streams Ti/Fe vs. Fe/H
Fe/H
Crosses are MW stars from Gratton, R. G.
Sneden, C. (1994), Fulbright, J. P. (2002),
Johnson, J. (2002), and Reddy, B. E. et al.
(2003) Triangles are dSph stars from Shetrone et
al. (2001 2003), Geisler et al.
(2005), Sadakane et al. (2004)
Sgr resembles LMC more than other dSphs LMC
stars from Pompéia et al. (2008)
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Chemical Trends in Sgr Streams Y/Fe vs. Fe/H
YII
Sgr resembles LMC more than other dSphs
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La II line affected by hyperfine splitting
Chemical Trends in Sgr Streams La/Fe vs. Fe/H
Here Sgr differs a little from LMC
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Chemical Trends in Sgr Streams La/Y vs. Fe/H
metal-poor AGB, high hs / ls means slower
SFR than MW

• Sgr resembles LMC
• Sgr evolved slower than MW

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Similar Enrichment, Different Timescales
Clear SFR difference among dSphs, LMC and Sgr
Hypothetical differences in chemical history
1 dex dSphs
0.5 dex LMC
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SFR differs in Galactic satellites
Hypothetical differences in chemical history
1 dex dSphs
SFR slow to fast dSphs ? LMC ? Sgr ?MW
0.5 dex LMC
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• Chemical Fingerprinting
• What is the peculiar NGP
• group?
• Fe/H -1, similar to
• Sgr leading arm south
• (dynamical age 3 Gyrs)
• Ti/Fe, Y/Fe, La/Fe
• and La/Y resemble
• Sgr leading arm south

Suggests NGP stars are Sgr stars of same
dynamical age as leading arm south, but
dynamics are wrong
Proposed solution NGP group are Sgr trailing
arm stars overlapping with Sgr leading arm north
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• Future Work
• Metallicity gradient and chemical trends along
  the Sgr
• trailing arm
• Longer, and stars stripped at specific epoch
  can be more cleanly isolated.
• Apply the same
• method to other
• possible halo streams,
• e.g. Monoceros stream

Model (Law et al. 2005)
72 in these regions
10 stars in each region from Gemini South
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• Summary
• Sgr Stream shows strong metallicity gradient
• Sgr originally had strong radial metallicity
  gradient
• Recent tidal stripping released stars, producing
  observed gradient in tails
• Sgr core of today differs from Sgr core of
  yester-
• gigayears
• Chemical abundance patterns along the stream are
• distinct from the MW, and more like the LMC
• ? Suggests SFR differences
• dSphs ? LMC ? Sgr ? MW
• (slower faster)