Boötes III: a Disrupted Dwarf Galaxy? Jeff Carlin (University of Virginia)

Collaborators Ricardo Muñoz (Yale) Carl Grillmair (Spitzer Science Center) Steve Majewski (UVa) David Nidever (UVa) For more details, see Carlin et al. 2009 (astro-ph 0907.3738) – ApJL accepted.

“Transition objects” – dSphs in the throes of tidal disruption Bootes I Hercules Bootes II Draco UMa II CVn II CVn I Willman I UMa Segue 1 Coma Leo IV Leo V “Field of streams” from V. Belokurov Intermediate: between bound dwarf spheroidal and completely unbound tidal stream We see disrupting dwarf galaxies (e.g. Sgr, Carina, Leo I), and a multitude of remnant streams, but what about the intermediate stage? 3

“Transition objects” – dSphs in the throes of tidal disruption “Field of streams” from C. Grillmair Transition objects are important tools for understanding action of tides on dwarf galaxies We see disrupting dwarf galaxies (e.g. Sgr, Carina, Leo I), and a multitude of remnant streams, but what about the intermediate stage?

Some expected properties of transition objects Distorted morphology, large size Associated tidal stream? Low surface brightness Power-law component of surface brightness profile High velocity dispersion Rotation / velocity gradient Radial (i.e. destructive) orbit Metal poor Metallicity gradient? None of these alone is sufficient to characterize an overdensity. See, e.g., Muñoz, Majewski, & Johnston 2008; Peñarrubia et al. 2009; Oh, Lin, & Aarseth 1995; Piatek & Pryor 1995 for modeling of tidal effects on dSphs

Boötes III Stellar overdensity discovered by Grillmair 2009 (ApJ 693, 1118) using matched-filter technique. (l,b) = (35.3, 75.4) d = 46 kpc Boo III

Boötes III “Styx stream” passes through the same line of sight

Boötes III “Styx stream” passes through the same line of sight Unclear yet whether BooIII associated with Styx stream, but is plausible b/c of distance, spatial coincidence “Styx stream” passes through the same line of sight

Boötes III CMD g Background-subtracted CMD shows clear overdensity Best matched by M15 ridgeline ([Fe/H] = -2.26) shifted to 46 kpc g g - i g - r

Boötes III CMD Prominent blue horizontal branch (BHB), also at 46 kpc. g g - i

Boötes III spatial distribution Extends over large area East-West extension, distorted morphology

Boötes III spatial distribution ~ 1 sq. degree Munoz, Majewski, & Johnston 2008 – size increases near complete destruction (4x in the case they discuss)

BooIII surface brightness profile Unfiltered surface density from Grillmair 2009: (-1 < (g-i) < 1) r a R-1

BooIII surface brightness profile Background-subtracted surface density of red clump stars from Correnti et al. 2009: r a R-1 Integrated magnitude: MV = -5.8, ellipticity: e ~ 0.5

Tidal disruption and surface brightness With each pericentric passage, outer SBP exhibits a break, with power-law “break” population. As tidal disruption proceeds, the power-law portion moves inward, until in final stages, complete SBP approaches a power-law. Peñarrubia et al. 2009

Spectroscopic Observations, Feb. 2009 MMT 6.5m + Hectospec multiobject spectrograph 227 targets, 18.5 < g < 22.5, along the turnoff and lower RGB Includes 6 BHB candidates 4550-7050 Angstroms, R~3000 9 x 1800 sec. exposure RV uncertainties: 3 – 15 km/s

BooIII Radial Velocities 193 stars with reliable RVs (i.e. S/N > 10) Central peak matches predicted MW halo distribution (from Besançon model) Two peaks in RVs: ~ 200 km/s ~ -200 km/s Vhelio (km/s)

BooIII Radial Velocities All 6 BHB candidates in our sample are in 200 km/s peak  Boötes III RV signature (red arrow on figure) Vhelio (km/s)

CMD w/ RV membership candidates RV candidates follow isochrone for [Fe/H]=-2.3, 10.2 Gyr population at 46 kpc (which also fits BHB and turnoff) We excluded as likely foreground stars those more than 0.25 mags from this isochrone  20 candidates in final sample (large filled symbols)

Mean velocity, dispersion Using maximum likelihood method for all 20 candidates, we obtain the systemic velocity and velocity dispersion of Boötes III:

Mean velocity, dispersion Using maximum likelihood method for all 20 candidates, we obtain the systemic velocity and velocity dispersion of Boötes III: High Galactocentric RV for an object at b=75.4, dist=46 kpc  radial (and thus potentially destructive) orbit.

Mean velocity, dispersion Using maximum likelihood method for all 20 candidates, we obtain the systemic velocity and velocity dispersion of Boötes III: Highest measured LOS velocity dispersion for MW dSph

Mass, M/L estimate Following Wolf et al. 2009 (see talk on Tuesday), we estimate mass based on: No reliable measurement yet for half-light radius. Substitute σo = 14.0 km/s: (similar to common mass scale found by Strigari et al. 2008, Mateo 1998) Depends on assumption of dynamical equilibrium, which is probably not true, but we do the exercise anyway. Relies solely on observables. 100 pc is typical size for ultra-faints (see Martin et al. 2008) At 46 kpc, 100 pc  7.5 arcmin = 0.125 deg.

Mass, M/L estimate Taking MV = -5.8 (Correnti et al. 2009), Combining with the mass estimate: Similar to some other UFDs at this luminosity (but remember the assumption of dynamical equilibrium!)

[Fe/H] measurement Metallicities based on Lick spectroscopic indices Vhelio (km/s) Stacked member spectra  [Fe/H] = -2.0 g [Fe/H] Metallicities based on Lick spectroscopic indices <[Fe/H]> ≈ -2.1 ± 0.2 (but σ[Fe/H]~0.6 dex)

Spatial distribution of members/targets Large, filled symbols: BooIII RV members Small diamonds: all observed stars Contours from Grillmair 2009 data

Radial metallicity gradient? [Fe/H] r (arcmin)

No sign of velocity gradient or rotation Vhelio (km/s) Mean, median RV of members on either side both agree to within 1 km/s position angle (degrees)

No sign of velocity gradient or rotation Vhelio (km/s) RA (degrees) – roughly along major axis

Expected properties of transition objects – comparison to Boötes III properties  Distorted morphology, large size  Associated tidal stream? (Styx stream)  Low surface brightness  Power-law component of surface brightness profile  High velocity dispersion (σo = 14.0 km/s) Rotation / velocity gradient ???  Radial (i.e. destructive) orbit (VGSR=239 km/s)  Metal poor ([Fe/H] ~ -2.1) Metallicity gradient ???

Further study Deep photometry to derive structural properties Identify more RV members, both in the core and over a larger area Velocity dispersion profile Rotation or velocity gradient? High-resolution spectra for detailed abundances Detailed comparison with models of tidally disrupting satellites For more details, see Carlin et al. 2009 (astro-ph 0907.3738).

Absolute magnitude vs. half-light radius Martin et al. 2008

[Fe/H] vs. abs. magnitude for dSphs, globular clusters Simon & Geha 2007

Simon & Geha 2007

Boötes III