Drilling through the M31 Halo near Mayall-II/G1 Michael Gregg University of California, Davis Michael West Lowell Observatory Brian Lemaux University of California, Davis Andreas Küpper Quantco, Inc.
Origin of the brightest Globulars Clusters Mayall-II/G1 in M31 Omega Cen in Milky Way • G1 has mass of ~10 7 M ⊙ and σ = 25 km/s • High ellipticity, ~0.2, 50% rotationally supported • Metallicity spread, ~0.45 dex • Evidence for massive blackhole in the core, ~10 4 M ⊙ • => Former dwarf galaxy nucleus, now Ultra-Compact Dwarf?
Origin of the brightest Globulars Clusters ...and connection to galaxy halo formation… HST image of nucleated Typical UCD (or bright globular) dwarf elliptical in in the same galaxy cluster: the Fornax galaxy cluster: possibly a tidally stripped nucleus nucleus + envelope of stars
Origin of the brightest Globulars Clusters... Fornax Galaxy Cluster ...and connection to galaxy halo formation… image credit: M. Hilker • Evolved tidal streams are too faint ( ∼ > 30 mag/sq. ′′ ) to detect with standard imaging (Ibata et al. 2001; Ferguson et al. 2002; etc., etc., etc.) • Need resolved star spectroscopy to identify tidal debris; characteristics can perhaps can yield insight into the precursor of G1…
G1 in Context Chemin et al. (2009) • We view G1 projected against the outer disk of M31, 2.6˚ (34 kpc) distant from the bulge. • The M31 disk has v < − 500 km s -1 at G1 (Chemin et al. 2009). • G1 systemic v = − 332 ± 3 km s -1 (Galleti et al. 2006), separation of ∼ 170 km s -1 , 5-6 times the velocity dispersion of G1 or the M31 cold disk.
Observations • Four contiguous Keck/DEIMOS slitmasks cover 16 ′ × 20 ′ area around G1. • The 1200 l mm -1 grating covers λλ 6510 − 9110, 0.33Å/pixel. • Slit width of 0.75” yields FWHM resolution of 1.7Å or 60 km s -1 at 8500Å. • More than 100 slitlets per mask. • Target stars from CFHT MegaCam r ′ and i ′ images of Kong et al. (2010) (thanks to A. Kong). • G1 tidal debris candidates selected 19.0 < i ′ < 21.5, and 0.5 < r ′ − i ′ < 2.5, with star class > 0.8, => evolved stars in M31 (RGB, AGB). • The faint limit samples to one magnitude below the tip of the RGB at the distance of G1 ( m − M = 24.42, Meylan et al. 2001)
Results: Tidal Stars from G1 • 351 reliable stellar velocities, errors typically ±5 km s -1 . • The M31 disk peak at ∼ − 500 km s -1 . • Milky Way stars from − 180 to 20 km s -1 . • Third peak in bin − 332 km s -1 , the observed systemic velocity of G1. • Clear detection of excess stars at the G1 velocity. • There are 13 stars within ±25 km s -1 of this peak (dotted lines).
Tidal Stars from G1 Uniform MW component (blue) Gradient in M31 component (red) Thirteen stars with ±25 km/s of G1 radial velocity Robust fit (dashed line) to the positions has Spearman rank coefficient of 0.59, probability significance of 0.974. Velocity correlated with position: open: negative filled: positive Tidal radius = 54” = 200pc (Meylan+ 2001)
Tidal Stars from G1 •Spearman correlation of the fourteen ∆ v points is 0.62 (0.982%), and 0.83 (0.9996%) removing the largest outlier. •There is a velocity gradient across the debris, consistent with rotation of G1 (Gebhardt et al. 2004) • The dispersion around this fit is only 8 km/s, even though selected from ±25 km/s sample •escaping stars and their velocity gradient => G1 cannot have a massive dark matter halo
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CMD of the Tidal Stars of G1 MW stars M31 stars Age 13 Gyr isochrones (Girardi+ 2010) Distribution of G1 tidal stars more consistent [Fe/H] = − 1.54, − 1.34, − 0.95, − 0.56, − 0.37 with M31 sample, except more metal poor (mean abundance of G1 and ±1 σ , ±1.5 σ ) than the disk, similar to the halo (triangles).
Mass Loss Rate from G1 ~ 10 M ⊙ in RGB tidal stars => few x 10 5 M ⊙ integrated down to 0.1 M ⊙ This is a few percent of the present mass of G1 Timescale for tidal debris to travel one Jacobi distance (r J ) is: 580pc/10 km/s = 50 x 10 6 yrs Thus even the remotest tidal stars in these fields have escaped within the last ~ 100 x 10 6 yrs At this rate, G1 will be completely gone in 3 Gyr, and of course, the mass loss rate is considerably higher at pericenter and will increase as G1 gets smaller Unfortunately, it is even less reliable to extrapolate backwards in time, but G1 has apparently contributed many times its present mass to the M31 halo.
Linking G1 to other Structures and Globulars in M31 Surface density of M31 stellar halo from Faria et al. (2007) Clump stars are known to have v<-400 km/s (Ibata et al. 2005), and [Fe/H] ~-0.4 from a CMD (Faria et al. 2007), so clump is definitely a disk population rather than halo. Our DEIMOS data show this is true even in the immediate neighborhood of G1. Perhaps the clump is a result of an interaction G1 Clump between G1 and the disk… Perhaps the clump and G1 are just a chance superposition.
Linking G1 to other Structures and Globulars in M31 NW Stream points at G1; coincidence? PA suggestively ~aligned with that of G1 and G1 tidal debris Again, the velocities are very different: G1 at -332 km/s NWS, -400 to -570 km/s G1 Clump Association 2 contains 10 GCs near G1, but except for G2, all are at much M31 stars more negative velocities, -426 to -600 km/s Veljanoski et al. (2014)
Summary • Positive identification of tidal stars from G1 • In addition to continuing to build the M31 halo, the debris contains clues to the origin of G1, and the M31 halo • For now, the precursor of G1 remains a mystery • How far can the debris be traced with spectroscopy? Future: • Analysis of spectral line strengths of tidal stars • Comparison to tidal stripping models (e.g., Pfeffer et al. 2015)
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