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

Omega Cen in Milky WayMayall-II/G1 in M31

•Evidence for massive blackhole in the core, ~104 M⊙

•High ellipticity, ~0.2, 50% rotationally supported•G1 has mass of ~107 M⊙ and σ = 25 km/s

•Metallicity spread, ~0.45 dex

•=> Former dwarf galaxy nucleus, now Ultra-Compact Dwarf?

Origin of the brightest Globulars Clusters

HST image of nucleated dwarf elliptical in the Fornax galaxy cluster: nucleus + envelope of stars

Typical UCD (or bright globular) in the same galaxy cluster: possibly a tidally stripped nucleus

...and connection to galaxy halo formation…

Origin of the brightest Globulars Clusters...

•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.)

image credit: M. Hilker

...and connection to galaxy halo formation…

•Need resolved star spectroscopy to identify tidal debris; characteristics can perhaps can yield insight into the precursor of G1…

Fornax Galaxy Cluster

• 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.

G1 in ContextChemin et al. (2009)

Observations

• Four contiguous Keck/DEIMOS slitmasks cover 16′ × 20′ area around G1.

• The 1200 l mm-1 grating covers λλ6510−9110, 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)

• 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).

Results: Tidal Stars from G1

Tidal Stars from G1

Uniform MW component (blue) Gradient in M31 component (red)

Tidal radius = 54” = 200pc (Meylan+ 2001)

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

Thirteen stars with ±25 km/s of G1 radial velocity

• Spearman correlation of the fourteen ∆v points is 0.62 (0.982%), and 0.83 (0.9996%) removing the largest outlier.

Tidal Stars from G1

• There is a velocity gradient across the debris, consistent with rotation of G1 (Gebhardt et al. 2004)

•escaping stars and their velocity gradient => G1 cannot have a massive dark matter halo

• The dispersion around this fit is only 8 km/s, even though selected from ±25 km/s sample

Uniform MW component (blue) Gradient in M31 component (red)

Tidal radius = 54” = 200pc (Meylan+ 2001)

Instantaneous tidal radius or Jacobi radius (von Hoerner 1957):

Tidal tails can be distributed over even wider area, 1.5-3x the tidal radius (Just et al. 2009)

Tidal Stars from G1

rt = R

✓mG1

2 MM31

◆ 13

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CMD of the Tidal Stars of G1

Age 13 Gyr isochrones (Girardi+ 2010) [Fe/H] = −1.54, −1.34, −0.95, −0.56, −0.37 (mean abundance of G1 and ±1σ, ±1.5σ)

MW stars M31 stars

Distribution of G1 tidal stars more consistent with M31 sample, except more metal poor than the disk, similar to the halo (triangles).

~ 10 M⊙ in RGB tidal stars

Thus even the remotest tidal stars in these fields have escaped within the last ~ 100 x 106 yrs

Mass Loss Rate from G1

=> few x 105 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 (rJ) is: 580pc/10 km/s = 50 x 106 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.

Surface density of M31 stellar halo from Faria et al. (2007)

G1 Clump

Linking G1 to other Structures and Globulars in M31

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 between G1 and the disk…

Perhaps the clump and G1 are just a chance superposition.

Linking G1 to other Structures and Globulars in M31

M31 stars

G1 Clump

NW Stream points at G1; coincidence? PA suggestively ~aligned with that of G1 and G1 tidal debris

Association 2 contains 10 GCs near G1, but except for G2, all are at much more negative velocities, -426 to -600 km/s

Veljanoski et al. (2014)

Again, the velocities are very different: G1 at -332 km/s NWS, -400 to -570 km/s

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? • Analysis of spectral line strengths of tidal stars • Comparison to tidal stripping models (e.g., Pfeffer et al. 2015)

Future: