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Chemical Abundances, Dwarf Spheroidals and Tidal Streams. Steven Majewski (University of Virginia) Principal Collaborators:
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Chemical Abundances, Dwarf Spheroidals and Tidal Streams Steven Majewski (University of Virginia) Principal Collaborators: Mei-Yin Chou (UVa - Ph.D. thesis), Katia Cunha, Verne Smith (NOAO), David Martínez-Delgado (IAC), David Law (UCLA), Jeffrey Carlin (UVa - Ph.D. thesis), Ricardo Munoz (Yale) Image credit: David Law & SRM
Topics Discussed: • Some Motivations to Study Chemistry of Tidal Streams • Connection between dSphs and stars in the MW halo. • Reconstruct chemical distribution of original satellite galaxies. • Learn about SFHs, chemical enrichment histories, accretion histories. • Chemical fingerprinting stars to their parent source. • 2. Case Study: MDF Variation along the Sgr Stream • Find a strong metallicity gradient along the Sgr tidal tail. • Shows that Sgr originally had significant radial metallicity gradient. • 3. Case Study: Chemical Patterns in the Sgr System • Find relativechemical evolution/SFH between Sgr, MW & other satellites. • Use distinctive patterns to fingerprint other Sgr stars in Galactic halo. • 4. Case Study: Fingerprinting the Tri-And Star Cloud • Testing the connection to the Monoceros stream. • 5. The Future with New Surveys: Comments about APOGEE
Hierarchical Formation of Halos Today ~1 stream with • < 30 mag/arcsec2 attached to still-bound satellite should be visible per MW-like galaxy.(Johnston et al., in prep.) Font et al. (2006)
Prominent Tidal Streams around Disk Galaxies NGC 4013 Milky Way NGC 5907 Sgr Model (Law et al. 2005) Martinez-Delgado, Gabany et al. (2008, 2009)
Chemical HistoriesDistinctive abundance patterns-- [α/Fe], s-process (Y, La, etc.)-- reflect the unique chemical historyof the parent system, e.g., [α/Fe] (Ti, Mg, O, etc.) indicates the Type II/Type Ia SNe ratio of the parent system From McWilliam (1997)
HaloThick diskThin disk Chemical Histories:The MW Halo / dSph (Dis?)Connection dSph stars 1) dSphs appear to differ from MW halo (and even from each other) 2) Chemical fingerprinting (e.g., Freeman & Bland-Hawthorn 2002 - “tagging”) may possibly connect field stars to dSph progenitors Compilation from Venn et al. (2004)
Explaining the Halo/dSph Chemical Dichotomy Font et al. (2006), Robertson et al. (2005): Bulk of halo from massive, Magellanic Cloud-sized accreted early on, when chemistry dominated by SNII.
Explaining the Halo/dSph Chemical Dichotomy Majewski et al. (2002), Munoz et al. (2006, 2008): Satellites with prolonged chemical evolution and tidal disruption naturally leads to evolution in types of stars contributed to MW halo.
Chemical Study of the Sgr dSph + Tidal Stream • Results in Chou et al. 2007, ApJ, 670, 346, Chou et al. 2009 (~submitted), • 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.
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
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)
Sgr Leading Arms and an NGP Moving Group Brightest stars (K< 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
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
Strong Metallicity Gradient along the tidal tail! Chemical differences between the core and the tails! (Chou et al. 2007, ApJ, 670, 346) -0.4 • Time dependence in the chemistryof stars contributed to halo. • No MW dSph shows a metallicitygradient this strong -- e.g., largestis 0.5 dex variation across Sculptor (Tolstoy et al. 2004) • Either Sgr lost mass over a smallradial range with enormous gradient……or suffered a catastrophic loss withstars lost over a more normal gradient. -0.7 -1.2 -1.0 Median [Fe/H] of NGP group is similar to Sgr leading arm south
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
Chemical Distributions in Sgr Stream [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)
Chemical Distributions in Sgr Stream [Y/Fe] vs. [Fe/H] YII Sgr resembles LMC more than other dSphs
La II line affected by hyperfine splitting Chemical Distributions in Sgr Stream [La/Fe] vs. [Fe/H] Here Sgr differs a little from LMC
Chemical Distributions in Sgr Stream [La/Y] vs. [Fe/H] – metal-poor AGB produce high [hs / ls], means slower SFR than MW • Sgr resembles LMC • Sgr evolved faster than dSph, slower than MW
Similar Enrichment, Different Timescales Clear SFR difference among dSphs, LMC and Sgr Hypothetical differences in chemical history +1 dex dSphs +0.5 dex LMC
SFR differs in Galactic satellites A “universal” enrichment historyvarying only by rate?? Hypothetical differences in chemical history +1 dex dSphs SFR slow to fast: dSphs LMC Sgr MW +0.5 dex LMC
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 wrong for leading arm Proposed solution: NGP groupare Sgr trailing arm stars overlapping with Sgr leading arm north
Future Work on Sagittarius • Metallicity gradient and chemical trends along the Sgr • trailing arm • Longer, and stars stripped at specific epoch can be more cleanly isolated. • Gemini Phoenix (R~40k) H-band spectra Model (Law et al. 2005) 7+2 in these regions 10 stars in each region from Gemini South
Note that dynamically oldest of the Sgrstream stars are -enhanced -- but contributed within past few Gyr
Explaining the Halo/dSph Chemical Dichotomy Font et al. (2006): Satellites accreted >9 Gyr ago all destroyed, surviving satellites only recently accreted --> implies not major contributorsSgr exceptionary case? (e.g., only dSph presently in inner halo)
But Carina dSph is also contributing stars today… … undoubtedly some with-enhancement. Munoz et al. (2007, in prep.) Carina Koch et al. (2008)
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) APOGEE at a Glance • Bright time 2011-Q2 to 2014-Q2, co-observing with MARVELS • 300 fiber, R ~ 24,000 cryogenic spectrograph • H-band window (1.51-1.68) • Minimum S/N = 100 • Typical RV uncertainty < 0.5 km/s • 0.1 dex precision abundances for ~15 chemical elements • ~105, 2MASS-selected, giant stars probing all Galactic populations
Expected elements and S/N tests @ R=21k and 0.1 dex precision • precision will degrade for lower S/N • S/N=100 for faintest star in plugboard, higher S/N for brighter stars Element SNR/pix SNR/pix SNR/pix [Fe/H]=-2 [Fe/H]=-1 [Fe/H]=0 Na 2673.7 309.8 56.0 S 1067.2 167.2 104.8 V 1504.7 164.4 42.4 K 505.6 75.3 44.6 Mn 184.9 50.9 46.9 Ni 101.6 45.7 46.4 Ca 89.5 42.7 41.0 Al 47.2 41.8 42.1 Si 35.2 38.6 35.7 N 147.3 41.7 21.4 Ti 110.0 36.5 38.9 Mg 33.1 36.7 26.4 Fe 41.6 34.3 21.3 C 40.4 14.8 8.3 O 24.5 14.6 9.1 ”Must have” element “Important to have/very desirable” element “Nice to have” element (also not shown Cr, Co)
The Promise of Detailed Chemical Abundance Studies The Initial Mass Function [(Si+Ca) / Fe] [(Mg+Ti) / Fe] • Relative abundances of different a elements reflects mass of SN progenitors: • Probes IMF • (e.g., McWilliam & Rich 1997 differences in a elements for bulge --- on right, above)
MARVELS Coordination - APOGEE use of 30 hr fields Solar metallicity RGB tip star: int (hr) HlimAVd(kpc) 3 12.5 5 27 10 13.4 10 27 30 14.1 15 26 [Fe/H]= -1.5 RGB tip star: int (hr) Hlim AVd(kpc) 3 12.5 0 40 10 13.4 0 60 30 14.1 0 83
Summary: • Sgr Stream shows strong metallicity gradient • Sgr originally had strong to very strong radial metallicity gradient. • Recent tidal stripping released stars, producing observed gradient in tails. • Sgr core of today differs from Sgr core of “yester-Gyrs”. • Sgr recently contributed -enhanced, metal-poor stars to MW; possibly other dSphs as well (e.g., Carina). • Overall, abundance patterns along the stream are • distinct from the dSphs and MW, similar to LMC • SFR differences: dSphs LMC Sgr MW • (slower faster) • Application of chemical fingerprinting demonstrated. • Tri-And Star Cloud not chemically linked to Monoceros. • APOGEE will access ~10-15 chemical elements in streams.