Transcript Document 7285714

FIGGS: The Faint Irregular Galaxy
GMRT Survey
Jayaram N Chengalur
NCRA/TIFR
Ayesha Begum
I. D. Karachentsev
Sambit Roychowdhury
S. Kaisin, M. Sharina
Near field cosmology from Dwarf
Galaxies
 Extremely faint dwarfs are particularly
interesting in the context of hierarchical
galaxy formation models
 The smallest objects collapse first, larger
galaxies from by merger of smaller ones
 The process of galaxy merger is highly
inefficient
 Every large galaxy should be surrounded
by dozens of left over “dwarf galaxies”
 Detailed study of these galaxies could
lead to insights into the formation and
evolution of galaxies in the early universe.
Numerical simulation: Each large
Dark Matter halo is surrounded by
several, as yet unmerged, smaller
halos.
The Faint Irregular Galaxy GMRT
Survey: FIGGS
GMRT observations of the neutral
hydrogen (HI) in a well defined
sample of nearby dwarf galaxies.
Selected from KK catalog
65 galaxies identified using the
selection criteria
MB > -14.5, HI Flux > 1 Jykm/s,
Dopt > 1',
δ > -40o
Accurate distances are known for a
large fraction of the sample
complimentary multi-wavelength data
(HST, Ground based optical broad
band, Ha, GALEX UV…)
By far the largest interferometric
HI study of dwarf galaxies
The Giant Meterwave Radio
Telescope (GMRT)
• Aperture Synthesis Radio
Telescope (interferometer)
•30 Antennas each 45m in
diameter
•About 70 km N of Pune, 160 km
E of Mumbai.
1 km x 1
km
• Hybrid configuration
• 12 dishes in central
compact array
• Remaining along 3 “Y”
arms
• Allows one to
simultaneously make
low and high resolution
images
14
km
Properties of the FIGGS sample
<D>~ 4Mpc
<MB> ~ -13
<MHI> ~ 3 x 107 Msun
FIGGS vs. other large interferometric HI surveys
Average gas fraction of FIGGS galaxies < fgas >~ 0.7
 FIGGS probes the regime of faint, very low mass, gas rich galaxies
extending the baseline for a comparative study of galaxy properties
FIGGS Data Products
HI Spectrum
Velocity
HI
(11”)
Field
HI
HI
(45”)
(5”)
HI (28”)
++++
GALEX
UV
CCD
H
R+B
a
CCD V + B
FIGGS: Expected Outcomes
• Study the relationship between dark and
baryonic matter in the smallest gas rich
galaxies
• Dark Matter Halos, Tully-Fisher relation, Baryonic
Tully Fisher relation, baryon fraction….
• Study the modes of conversion of gas into
stars in the smallest units of star formation
• Star formation thresholds, recipes, effective yield…
Scaling relations for gas disks
Begum et al. MNRAS,386,1667 (2008)
•
HI mass correlates tightly with DHI (1 Msun/pc2)
•
Relationship matches within error bars with that found for large spirals
(Broeils & Rhee 1997)
– Over 3 orders of magnitude of HI mass, disks of galaxies can be described as
being drawn from a family with fixed average SHI
•
Correlation of HI mass with optical diameter weaker than for spirals
– Coupling between gas and star formation in dwarfs not as tight as in spirals?
Dark Matter
DDO 210 (MB -10.6 mag)
Is there an ordered component to the gas motions in the
faintest dwarf galaxies?
DV ~ 6.5 km/s (VLA Darray)
Lo et al. 1993 AJ, 106, 507
DV ~ 1.6 km/s (GMRT)
Begum & Chengalur 2004 A&A, 413, 525
Large Scale Kinematics of the HI
All FIGGS galaxies
show large scale
velocity gradients
- Not always consistent
with that expected
from a rotating disk
HI in Cam B (MB ~ -12.0)
Begum, Chengalur & Hopp New Ast, 2003, 8, 267
Cam B : Rotation Curve
Begum et al. New Ast, 2003, 8, 267
Peak rotation velocity
comparable to velocity
dispersion
Important to correct for
pressure support before
determining the dynamical
mass (Model Dependent!)
Mass model for CamB
Begum et al. New Ast, 2003, 8, 267
Constant density core
provides a good fit, but
“NFW” type halo does not
» NWF halos in general do not
provide a good fit to our sample
galaxies.
Rotation curve derived at
resolutions ranging from
300pc to 750 pc
» beam smearing does not seem
to be important
TF relations
Baryonic Mass
I band Magnitude
Begum et al., MNRAS,386,138, (2008)
Vrot
Vrot
Compare FIGGS sample with HST large spiral Sample (both samples
have accurate distances)
FIGGS galaxies are slightly under luminous for their velocity width, but
have about the right amount of baryons (Model dependent
conclusion!)
Dwarf galaxies have been less efficient at converting gas into stars
Scatter about the TF
Begum et al., MNRAS,386,138, (2008)
FIGGS
HST
The scatter about the BTF/TF (normalized by the
measurement error) is much larger for the FIGGS
sample as compared to the HST sample
Star formation efficiency is lower and shows more
scatter in dwarf galaxies as compared to large spirals
Star
formation
in extremely
faint
dwarfs
Star formation recipes for spirals
and starburst galaxies
(Kennicutt ApJ 498, 541, 1998)
•
Star formation occurs only above a fixed
threshold gas column density
– Star formation rates determined from Ha
observations
– Related to instabilities in thin disks?
(Safronov AnAp 23, 979, 1960;Toomre ApJ
139, 1217, 1964)
Above this threshold column density, the star
formation rate has a power law dependence on the
gas column density
SSFR = (2.5 ± 0.7)10−4(Sgas/1Msunpc−2)1.4±0.15Msunyr−1kpc−2
(Kennicutt ApJ 498, 541, 1998)
Unclear if recipes derived from large spirals
apply to dwarf (“primordial”) galaxies
Star formation in Dwarfs
(As traced by Ha)
(Begum et al. 2006 MNRAS 365 1220)
• Star formation recipes have traditionally used Ha as the
tracer of star formation rate Ssfr
• Since observed gas density SHI varies with resolution, all
HI images have to be constructed at the same linear
resolution
• Linear resolution used here is 300pc
• There is no one to one correspondence between HI
column density and star formation (as traced by Ha)
• The observed “threshold” column density is much lower
than that predicted from disk instability model
Ha vs. HI on fine (~ 20pc scales)
(Begum et al. 2006 MNRAS 365 1220)
On very small scales Ha emission is
sometimes co-incident with HI peaks, and
sometimes with HI “holes”
Measuring SFR in faint Galaxies
• At low SFR, Ha measurements of SFR
are susceptible to stochastic errors
• Ha is produced by massive OB stars, i.e.
traces the instantaneous SFR
• The number of OB stars in small clusters is
susceptible to large Poisson fluctuations
even if the IMF is universal
• UV may be a better indicator of SFR
Roychowdhury et al. MNRAS,397,1435, (2009)
Pixel by Pixel Correlation with FUV
• Several galaxies show “Schmidt Law” type behaviour but the slope is
• Generally much steeper than 1.4
• Highly variable from galaxy to galaxy
• Power law continues until one reaches the sensitivity limit of the GALEX
data
• At the highest Sgas, the SSFR approaches the value predicted by the
“Kennicutt-Schmidt” law
Fine scale structure of the ISM in dwarf
galaxies
Fine Structure in the HI gas
The HI emission is
clumpy and shows
considerable structure
even on scales of 20100 pc.
Expected in turbulent
ISM models – BUT Can
this be quantified?
Power spectrum of HI fluctuations
in the faintest dwarf galaxies
• HI signal is very weak – estimators used earlier won’t
work
• Suggest a new estimator, based on correlation between
visibilities on adjacent baselines which is well suited to
dragging the signal out of the noise
• Similar estimator is used in interferometric CMBR work
e.g. Hobson et al. MNRAS 275, 863 (1995)
HI fluctuation PowerSpectrum in
DDO 210 (MB ~ -10.9)
• The power spectrum has a slope
a ~ -2.75 ± 0.45
Begum et al. MNRAS,372,L33 (2006)
• over scales from 80 – 500 pc
• the intensity fluctuations appear to
come from modulation of the
density
• similar slope to that seen in the
Milkyway
• In thin disks expect a ~ -1.6
• Since fluctuations happen in 2D
• DDO210 must have a relatively
fat disk
See also Dutta et al. MNRAS,398,887 (2009)
Intrinsic axial ratio of dwarf HI disks
b
a
Edge on
q=b/a
b
a
a
Observed Distribution
Intrinsic Distribution
Intermediate
Face on
p
 ( p)  p 
 (q)dq
2 1/ 2
2
2 1/ 2
(
1

q
)
(
p

q
)
0
a
c
a
a
p=c/a
Invert directly,
or using Lucy
deconvolution
Intrinsic axial ratio of HI disks
Observed axial ratios
Intrinsic axial ratios
Dwarfs have quite fat HI disks <q> ~ 0.57
--Consistent with their higher s/Vrot ratios
Thank you
Caveat’s
Roychowdhury et al. MNRAS,397,1435, (2009)
Correcting the SFR
calibration for low metalicity
will increase the deviation
from the “Kennicutt-Schmidt”
SFR law
Correcting Sgas for possible
molecular gas content will
have the same effect
To bring agreement with
the “Kennicutt-Schmidt” law
one will need truncation of
the IMF at the upper end.
Star formation recipes and galaxy
formation simulations
• Numerical simulations accurately
follow the gravity driven merger
of the dark matter halos
• Physics of the baryonic material
is complex and poorly
understood
– heating/cooling, collapse to form
stars, feed back from star
formation
• Most simulations of galaxy
formation use “recipes” for
following the star formation
process
– Recipes are generally derived
from observations of nearby
galaxies.
Numerical simulations of galaxy formation
in LCDM cosmology are able to produce
realistic disk galaxies. While the gravity
driven infall and merger are accurately
computed, star formation and feedback
are incorporated largely via “recipes” (e.g.
Fabio et al. MNRAS 2007, 374,1479)