boötes iii: a disrupted dwarf galaxy?
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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