Modelling the Ultra- Faint Dwarf Galaxies and Tidal Streams of the Milky Way M. Fellhauer Universidad de Concepcion in collaboration with N.W. Evans 1 , V. Belokurov 1 , D.B. Zucker 1 , M.I. Wilkinson 2 , G. Gilmore 1 , M. Irwin 1 1 Institute of Astronomy; 2 Univ. Leicester
63
Embed
Modelling the Ultra-Faint Dwarf Galaxies and Tidal Streams of the Milky Way M. Fellhauer Universidad de Concepcion in collaboration with N.W. Evans 1,
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Modelling the Ultra-Faint Dwarf Galaxies and Tidal Streams of the Milky Way
M. FellhauerUniversidad de Concepcion
in collaboration with
N.W. Evans1, V. Belokurov1, D.B. Zucker1,
M.I. Wilkinson2, G. Gilmore1, M. Irwin1
1Institute of Astronomy; 2Univ. Leicester
Ladies and gentleman
SDSS Proudly presents:
The ‘Field of Streams’
The SDSS survey60 million stars are catalogued in SDSS in 5 colours
All stars of the Milky Way in SDSS:
And then we apply a simple colour-cutAnd are left with only the halo stars…
“Field of Streams”Belokurov et al. 2006
A gallery of SDSS dwarfs
D = 220 kpcrh = 550 pcMV = -7.9
D = 60 kpcrh = 220 pcMV = -5.8
D = 150 kpcrh = 140 pcMV = -4.8
D = 44 kpcrh = 70 pcMV = -3.7
CVn I Boo CVn II Com
Some Implications• Numbers: 10 new MW dwarfs (including UMa I, Leo V &
Boo II) have been found to date, in SDSS data covering ~20% of the sky tens more likely remain undiscovered
• Properties: Ultra-low luminosities (-3.8 ≥ MV ≥ -7.9) and surface brightnesses (µV < 27 mag arcsec-2), odd morphologies are these truly dwarf galaxies or fuzzy star clusters? Are these a distinct class of object?
Hobbit Galaxies?
MV vs. Log(rh) Mind the Gap?
But there is even more:
Leo T: A New Type of Dwarf?
• MV ~ -7.1, • µV~ 26.9 mag
arcsec-2
• (m - M)0 ~ 23.1, ~420 kpc
• Recent < 1 Gyr star formation --blue loop/MS stars
SDSS data
INT DataIrwin et al. 2007
The Smallest Star-Forming Galaxy?
• Not dead yet: stars formed within past few x 108 yr
• HIPASS: Coincident H I • RV ~ 35 km/s• if @ 450 kpc, ~ 2 105
M in H I (MH I/M ~ 1, cf. Local Group dIrrs)
• Is Leo T the tip of a Local Group “free floating” iceberg? HIPASS
Constraining the progenitorof UMa II and the Orphan Stream
Initial model for UMa II:
use simple Plummer spheres to constrain parameter space in initial mass & scale-length
Constraining the Progenitor: I. Length of the Tails
Tails as function of progenitor mass and simulation timeProgenitor must be>105 Msun & <107 Msun
Simulation time must be longer than 7.5 Gyr
Constraining the Progenitor:II. Morphology of UMa II
• Progenitors with more than 105 Msun
must be almost destroyed to account for the patchy structure, the low mass of the remnant and the high velocity dispersion of UMa II
• Progenitors with more than 106 Msun do not get sufficiently disrupted to account for the substructure
Comparing 2 UMa II models:
One component model• Plummer sphere:
– Rpl = 80 pc
– Mpl = 4 x 105 Msun
Two component model• Hernquist sphere:
– Rh = 200 pc
– Mh = 5 x 105 Msun
• NFW halo:
– RNFW = 200 pc
– MNFW = 5 x 106 Msun
inserted at the position of UMa II 10 Gyr ago
Orphan stream UMa II
1-comp. 1-comp.2-comp. 2-comp.
Comparison of the 2 models - Reproduction of Orphan Stream &
UMa II
Comparing the appearance & the kinematics of the two models:
One component (B)
Before(A), while (B)& after dissolution [c]
Two component (D)
A
B
C
D
Patchy structure (B) vs. round, bound, sound & massive (D)
Both models show high velocity dispersion
Mean vrad is patchy with gradient (B) vs. constant within object (D)
A: before dissolution is low and vrad constant
B: patchy structure, high , patchy vrad
with gradient
C: no density enhancement, low ,gradient in vrad
Conclusions:
• It is possible that UMa II is the progenitor of the Orphan Stream
• If UMa II is a massive star cluster or a dark matter dominated dwarf galaxy ?Decide for yourself…
or wait for better data.
But then we have some predictions:
If better data will be available:• Predictions from our models:
– At the Orphan Stream: if the progenitor was more massive than 106 Msolar than we should see the wrap around of the leading arm at the same position but at different distances & velocities
– At UMa II: if the satellite is DM dominated the contours should become smoother; if UMa II is the progenitor of the Orphan Stream the satellite is not well embedded in its DM halo anymore (otherwise there would be no tidal tails)
– A disrupting star cluster will show a patchy structure in the mean line-of-sight velocities with a gradient through the object; a DM dominated bound satellite will have a constant vrad within the object
Latest News:
Simon & Geha (2007):
Seem to confirm gradient in radial velocity
New unpublished data searching for tidal tails around UMa IIshow no sign of tidal tails -
Formation of Dwarf Galaxies:(PhD project of P. Assmann (Concepcion))
• Consider star formation in a DM halo• Stars form in star clusters, which suffer from gas-
expulsion• Star clusters inside the DM halo merge and form a
dwarf galaxyAim:• Constrain the parameter space of successful
progenitors (halo shapes, SFEs, profile of star cluster distribution)
• Look for fossil records of the formation in velocity space
The Sagittarius Tidal Stream
Some words aboutTidal Tails…
How does the ‘Field of Streams’ connect with the tidal tails of the Sagittarius dwarf galaxy ?
The Bifurcation (overlap of at least two branches of the tails)
Upper Stream (B)
Lower Stream (A)
Stream (A) and (B) havealmost the same distanceStream (C) is locatedbehind stream (A)
“Houston - we have a Problem”:
• How can the two streams be so close in position and distance– Is there no peri-centre shift ?– Is there almost no shift of the plane of the orbit ?– Is it caused by two objects orbiting each other ?
• No, see LMC & SMC
– Did Sagittarius collide with another object ?• Maybe, but that’s not causing a bifurcated stream