Using Radial Metallicity Gradients in Dwarf Galaxies to Study
Environmental Processing
Ryan Leaman - IAC
8th July, 2013 EWASS, Turku
K. Venn, A. Brooks, G. Battaglia, A. Cole, R. Ibata, M. Irwin, A. McConnachie, T. Mendel, E. Tolstoy, E. Starkenburg
WLM Dwarf Irregular Galaxy
• (Relatively!) Isolated:
– 1 Mpc from MW/M31, 250 kpc from Cetus dSph – With current position and velocities has had at most
one MW pericentre passage which would have been at least 11 Gyr ago (z > 2.5)
• Chance to study effects of internal evolution in absence of external factors (tides, ram pressure)
• Possibly constrain relative strength of stellar feedback, secular instabilities in dictating
chemistry/structure/dynamics of isolated dwarf
• Compare gaseous/stellar dynamics (i.e., V/σ ),
test tidal transformation models (e.g., Mayer et al.)
Medium Resolution CaT Spectra
• Possible to compare the
resolved velocity fields of the gas and stars over the whole body of the galaxy in the
absence of tides (i.e., LMC/SMC)
Gas and Stellar
Kinematics
Velocity Dispersion Time Evolution
•Stars born cold, dispersion grows via some internal heating mechanism
•GMC encounters can only produce a maximum of α=0.25
•Others? DM substructure encounters right magnitude?
• OR... ISM pressure floor variation, bar/disk
instabilities (Sotnikova et al.
2003)
• Tidal effects may actually decrease dispersion.
Comparative Metallicity of WLM
• Have [Fe/H] measurements (+/- 0.25 dex) for 126 stars where S/N was high enough.
• Metallicities computed using empirically calibrated CaT method, places stars on [Fe/H] scale of galactic GCs.
• Recent Studies (Starkenburg et al. 2010) have updated the CaT-[Fe/H] calibration to correct for non-linearities at low [Fe/H]
and faint magnitudes
• Used here to assure method of deriving metallicity for each star is not biased, and updates to [Fe/H] measurements of comparison galaxies have been applied to create a more homogeneous sample.
• Would like to see if a more isolated rotating, gas rich dwarf has global metallicity properties different than dispersion supported, tidally influenced gas poor dSphs - or tidally influenced gas rich Magellanic Clouds
Can Look at
Differential MDFs…
• WLM shows similar mean, spread to other high
luminosity galaxies (Leo I, Fornax)
• Expectations in line with luminosity-metallicity
relation
• Average chemical
enrichment still dictated by halo mass despite
isolation??
Radial [Fe/H]
Gradients for dIrrs and dSphs
•Here spatial coverage is absolutely crucial.
•dSphs seem to show steeper gradients than the rotating dIrrs
•Suggested by Schroyen et al. (2011) that a “centrifugal barrier mechanism” in rotating dwarfs may prevent the intense
central SF episodes that lead to radial gradients
Radial [Fe/H]
Gradients for dIrrs and dSphs
•dSphs show chaotic behavior in inner 1.5 core radii before falling at larger radii. Angular Momentum? SFH?
• LG dEs NGC147, NGC185 (V/σ ~ 1) trace the dIrrs also
• Uncertainties clouding interpretation: Age of tracer population, structural parameter, V/σ estimates
0 2 4 6 8 10
r/rc
−0.7
−0.6
−0.5
−0.4
−0.3
−0.2
−0.1 0.0 0.1
∆[Fe/H]
dIrrs: LMC SMCWLM
dSphs: Sculptor Fornax Sextans Carina Leo I Leo II
25
References
24 Leaman et al.
TABLE 1
Local Group Dwarf Galaxy Sample
Galaxy Nstars rmax/rt a Reference
WLM 180 0.79 Leaman et al. (2009, 2012)
LMC 373, 59, 383 0.78 Cole et al. (2005); Pomp´eia et al. (2008); Carrera et al. (2008b)
SMC 349, 364 1.14 Carrera et al. (2008a); Parisi et al. (2010)
Fornax 870 1.20 Battaglia et al. (2006)b
Sculptor 629 1.34 Tolstoy et al. (2004)b
Sextans 180 0.75 Battaglia et al. (2011)b
Leo I 825 1.18 Kirby et al. (2010)
Leo II 256 1.10 Kirby et al. (2010)
Carina 327 1.06 Koch et al. (2006)b
aColumn shows what fraction of the tidal radius the outer most star in the spectroscopic sample extends to.
b“DART sample” - original data from these papers updated with additional observations and the [Fe/H] calibration from Starkenburg et al. (2010).
TABLE 2 MDF Properties
Galaxy 10th % 50th % 90th % p [Z!] [Fe/H]0
LMC −1.06 −0.45 −0.18 0.430 −∞
0.363 −1.30
SMC −1.53 −1.05 −0.64 0.100 −∞
0.085 −1.91
WLM −1.74 −1.24 −0.75 0.070 −∞
0.064 −2.34
Fornax −2.04 −1.17 −0.74 0.093 −∞
0.090 −2.66
Leo I −1.84 −1.42 −1.11 0.090 −∞
0.037 −2.18
Sculptor −2.45 −1.96 −1.41 0.014 −∞
0.013 −3.14
Leo II −2.29 −1.59 −1.28 0.036 −∞
0.033 −2.68
Sextans −2.89 −2.26 −1.66 0.007 −∞
0.007 < −5.0
Carina −2.47 −1.87 −1.51 0.019 −∞
0.017 −3.02
Note. — Effective yields in the first row for each galaxy represent the best fitting value from a Leaky box model, second row the effective yield in the pre-enriched model and initial [Fe/H].
26
Summary of Chemodynamics
• Thickening/heating increases with time, likely due to internal processes given WLM’s isolation. Could be a possible baseline amount of secular evolution in absence of tides, ram pressure.
•Global [Fe/H] properties and age-metallicity relation are in good agreement with more luminous dSphs/dEs, and SMC despite likely differences in dynamics and/or isolation... passive evolution to dE?
• Intrinsic spread in [Fe/H] constant over 4 orders of magnitude for dwarfs with L > 5x105
• WLM along with the SMC/LMC show radial [Fe/H] profiles that are statistically flatter than the dSphs.
• More semi-isolated systems will help further quantify interplay between environment, angular momentum in dictating radial metallicity and velocity gradients.
• Many signatures (gradients, dispersions) would be hard to detect with samples that were not so spatially extended. Large spatial
coverage crucial in accurately comparing L-Z, AMRs, [Fe/H](r), V(r)
