Transcript HI in Local Group Dwarf Galaxies

HI in Local Group
Dwarf Galaxies
Jana Grcevich
Advisor: Mary Putman
HI in Local Group Dwarfs
• Limits on HI content of the newly
discovered dwarfs
• HI in Leo T
• HI in other low-mass local group dwarfs
• Galactocentric Distance vs. HI content
• Halo Density Estimation
• Gas Accretion
Data
HIPASS
Declination -90 to +25
Range
Spatial
15.5’
Resolution
Velocity
26.4 km/s
Resolution
LAB
GALFA
-90 to +90
-1 to +38
35.7’
3.4’
1.3 km/s
0.2 km/s
Mass Limit Relations
HIPASS
(26.4 km/s)
LAB
(10 km/s)
M=2.38 x 10-2 D2kpc M0
M=6.24 x 10-1 D2kpc M0
HI Mass Upper Limits
Object
Distance
HI Mass
(kpc)
(solar masses)
Bootes I
60
< 86
Bootes II
60
< 86
Coma
44
Berenices
< 46
Hercules
< 466
140
Ursa
Major I
100
< 6240
Ursa
Major II
30
< 562
Willman I
38
< 901
220
< 3.0 x 104
< 1.4 x 104
Leo IV
160
< 609
Canis
Venetici I
Segue
23
< 13
Canis
150
Venetici II
HIPASS
LAB
Leo T in GALFA
• Lowest luminosity
galaxy discovered
which has current
star formation (Irwin
et al. 2007
• T - “transition”
• Optical Vel. = 38.1
km/s
• HI Vel. = 35 km/s
Leo T
(Ryan-Weber et al. 2007 in prep)
Non-detections &
Confident Detections
• Additional galaxies not detected:
Cetus, Sextans, Leo I, And III, And V,
And VI, Leo II, Leo IV, Ursa Minor,
Draco, and Sagittarius.
• Confident Detections: Antlia,
Phoenix, Pegasus, Aquarius, and
LGS3.
Fornax
• Moment Map 32.87 to 59.26 km/s
• Unclear if cloud is
part of typical MW
emission, an HVC
of separate origin,
or the Fornax Dwarf
• Optical Vel. = 53
km/s
• HI Cloud Vel. = ~40
km/s
Contours at 3, 7, 11, and 15 sigma
Sculptor
• Two clouds
discovered by
Carignan et al.
1998 with
Parkes/ATCA
• Optical Vel. = 102
km/s
• HI Vel. = 105 km/s
Contours at 3, 5, 7, and 9 sigma
Sculptor
• Sculptor Dwarf is in
the same direction
as the Magellanic
Stream and Sculptor
Group
• Sky is crowded at
this velocity
Contours at 3, 5, 7, and 9 sigma
• Cloud could be a
filament extending
toward the sculptor
group or a chance
superposition
Sculptor
(Putman 2003)
Sculptor Dwarf l = 287.5 b = -83.2
Tucana
• HI cloud first
detected by
Oosterloo et al.
(1996) who claimed
it was associated
with the Magellanic
Stream
• Optical Vel. = 184
km/s
• HI Vel. = 130 km/s
• ~54 km/s Velocity
Difference
Contours at 3, 5, 7, and 9 sigma
Tucana
(Putman 2003)
Tucana l = 322.9 b = -47.4
HI vs GC Distance
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
HI vs GC Distance
NonDetections
And
All NonAmbiguous
Detections Detections
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TIFF (LZW) decompressor
are needed to see this picture.
Majority
Confident
Detections
At > 105
Solar Masses
Mass Loss Mechanism
• Simulations suggest that ram
pressure is the primary mass loss
mechanism, assisted by tidal and
possibly internal effects (Mayer et al
06; Mori & Burkert 01; Quilis & Moore
2001)
Diffuse Halo Medium
• Assume dwarfs in the transition
region are being actively stripped of
gas
• Density of hot halo medium is given
by (Gunn & Gott 1972):
rIGM v2 > s2 rgas/3
Diffuse Halo Medium
rIGM ~ s2 rgas/(3 v2) = 2.2 x 10-4 cm-3
Typical values for a Leo T-like progenitor
skm s-1
rgas ~ NHI,core/R ~ 1 x 1020 cm-2/600 pc = 5.4
x10-2 cm-3
v ~ 60 km s-1 (1D velocity dispersion for Local
Group dwarf galaxies from Van den
Bergh 1999a)
Diffuse Halo Medium
rIGM ~ 2.2 x 10-4 cm-3
Observations suggest a hot gaseous corona with a
mean density of 2 x 10-5 cm-3 within 150 kpc
(Sembach et al. 2003)
Explanations:
Orbits take them further in than they are now seen
The diffuse halo medium is or was “clumpy”
Leo T doesn’t represent the progenitor
Other mass lowering mechanisms - reionization?
Gas Accretion
• Average HI mass of galaxies 300 kpc
out or more: 4 x 106 M0
• Galaxies within 300 kpc would
contribute about 8 x 107 M0 to the
MW
Conclusions
• All of the SDSS dwarfs except Leo T are devoid of gas
to our detection limits, and these upper limits are
lower than the HI mass of any known dwarf which has
HI.
• Dwarf galaxies at smaller galactocentric distances
have less HI on average than those at larger
distances.
• The HI -distance trend supports data from simulations
which suggest ram-pressure stripping/tidal effects are
responsible for the low HI content of dSphs
• The diffuse halo density can be estimated, but yields
densities higher than expected
• Accretion from the dwarfs provides insufficient fuel to
support long term star formation in the MW at the
observed rate
Future Work
Rel. Velocity
= 150 km s-1
Cloud:
• Analysis of the star
R = 25 pc
formation histories of the T = 1 x 104 K
dwarfs and how this
n = 0.5 cm-3
correlates with HI content
• Galfa observations of HI
in the vicinity of local
group dwarfs
• Simulations of gas clouds
being stripped and study
of head tail clouds
Ambient Gas:
T = 5 x 106 K
n = 1 x 10-3 cm-3
HVC simulation
by Fabian
Heitsch
Quic kT ime™ and a
YUV420 codec decompres sor
are needed to s ee this pi cture.