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 IHercules

Bootes II

Draco UMa II

CVn IICVn I

Willman IUMa

Segue 1

Coma

Leo IV

Leo V

“Field of streams” from V. Belokurov

We see disrupting dwarf galaxies (e.g. Sgr, Carina, Leo I), and a multitude of remnant streams, but what about the intermediate stage?

“Transition objects” – dSphs in the throes of tidal disruption

“Field of streams” from C. Grillmair

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?

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

Stellar overdensity discovered by Grillmair 2009 (ApJ 693, 1118) using matched-filter technique.

• (l,b) = (35.3, 75.4)• d = 46 kpc

Boötes III

Boo III

Boötes III

“Styx stream” passes through the same line of sight

Boötes III

“Styx stream” passes through the same line of sight

Background-subtracted CMD shows clear overdensity

Best matched by M15 ridgeline ([Fe/H] = -2.26) shifted to 46 kpc

g - i

Boötes III CMD

g - r

Prominent blue horizontal branch (BHB), also at 46 kpc.

Boötes III CMD

g - i

Boötes III spatial distribution

Boötes III spatial distribution

~ 1 sq. degree

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

R-1

BooIII surface brightness profile Background-subtracted surface density of red clump stars from Correnti et al. 2009:

R-1

Integrated magnitude: MV = -5.8, ellipticity: ~ 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:

€

Vhelio =197.5 ± 3.8km

s

VGSR = 238.8 ± 3.8km

s

σ o =14.0 ± 3.2km

s

Mean velocity, dispersion

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

€

Vhelio =197.5 ± 3.8km

s

VGSR = 238.8 ± 3.8km

s

σ o =14.0 ± 3.2km

sHigh 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:

€

Vhelio =197.5 ± 3.8km

s

VGSR = 238.8 ± 3.8km

s

σ o =14.0 ± 3.2km

s

Highest measured LOS velocity dispersion for MW dSph

Mass, M/L estimateFollowing 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)

€

M(<r1/ 2) ≈ 9.3 ×104 Rhalf100pc

σ LOSkm /s

⎛

⎝ ⎜

⎞

⎠ ⎟2

€

M(<r1/ 2) ≈1.82 ×107 Rhalf100pc

MSUN

Mass, M/L estimate

Taking MV = -5.8 (Correnti et al. 2009),

Combining with the mass estimate:

€

M

L

⎛

⎝ ⎜

⎞

⎠ ⎟V

≈1000Rhalf

100pc

M

L

⎛

⎝ ⎜

⎞

⎠ ⎟V ,SUN

€

L =1.8 ×104LSUN

[Fe/H] measurementV

helio

(km

/s)

[Fe/H] g

Metallicities based on Lick spectroscopic indices

<[Fe/H]> ≈ -2.1 ± 0.2 (but σ[Fe/H]~0.6 dex)

Spatial distribution of members/targetsLarge, filled symbols: BooIII RV membersSmall 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)

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 sizeAssociated tidal stream? (Styx stream)Low surface brightnessPower-law component of surface

brightness profileHigh 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 studyDeep 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