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Gas Properties of Dwarf Galaxies
Results from FIGGS: Faint (MB>-14.5) Irregular Galaxies GMRT Survey
Ayesha Begum (IOA, Cambridge)
Jayaram N. Chengalur (NCRA), Igor D. Karachentsev (SAO),
Margarita Sharina (SAO), Serafim S. Kaisin (SAO)
What is FIGGS ?


HI survey of nearby (< 10 Mpc) faint
(MB>-14.5) dwarf irregular galaxies
with the GMRT
Obtain high-quality observations of
atomic ISM at high velocity resolution
(~1.6 km/s) and variety of spatial
resolutions (~ 40” – 5”)
A systematic study of physical characteristics
of atomic ISM in faint dIrr galaxies
Giant Metrewave Radio Telescope
 GMRT
is an interferometric array
consisting of 30 antennas, each of 45 m
diameter, spread over 25 km region in a
hybrid (Y shaped configuration), 90 km off
Pune in India
 Operating
frequencies are 1420, 610, 327,
233 and 150 MHz
What is FIGGS ?


HI survey of nearby (< 10 Mpc) faint
(MB>-14.5) dwarf irregular galaxies
with the GMRT
Obtain high-quality observations of
atomic ISM at high velocity resolution
(~1.6 km/s) and variety of spatial
resolutions (~ 40” – 5”)
A systematic study of physical characteristics
of atomic ISM in faint dIrr galaxies
Why FIGGS ?

Interplay between neutral gas and star formation in faintest
gas rich galaxies
► Star formation threshold for faint galaxies

Extend the Baryonic Tully-Fisher relation to a regime of very
low mass/luminosity.

Density distribution of dark matter halos
Create archival dataset (calibrated (u,v) data, data cubes, MOMNT maps,
HI spectrum and rotation curves) for the astronomical community.
The FIGGS Sample

A sample of 65 galaxies (Catalogue of neighboring (10 Mpc) galaxies )
Karachentsev et al. 2004, AJ, 127, 2031

Selection criterion:
MB > -14.5, HI Flux > 1 Jy km/s, optical size > 1 arcmin, δ> -40 deg

HI mass typically MHI ~ 107 MO

Typical Observing time ~ 5 - 6 hours for most galaxies
 Noise per channel ~ 2 - 3 mJy/Beam

High velocity resolution ~ 1.65 km/s used for observations
 Factor of ~ 4 times better than most earlier studies
DDO 210 (MB ~ -10.6 mag)
High velocity resolution crucial for the observations of faint dwarf galaxies
Dwarfs fainter than MB ~ -14.0 have chaotic velocity fields ???
(e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)
V ~ 6.5 km/s
Lo et al. 1993 AJ, 106, 507
ΔV ~ 1.6 km/s
Begum & Chengalur 2004 A&A, 413, 525
FIGGS


GMRT HI images
HST V and I band optical images
► TRGB distance (accuracy of ~ 10%) known for most galaxies
 First large sample of faint dwarfs with accurate distances

Hα images and rotation curves from the 6m BTA

Long slit optical spectroscopic data from WHT/INT
The most detailed multi-wavelength study of faint dIrr galaxies
Survey Status
The GMRT observations over last year. Data
reduction is now complete
Time to do some science…..
Gas Fraction of FIGGS Galaxies
Gas Fraction fgas= Mgas/(Mgas+Mstar)
► Average gas fraction of FIGGS galaxies < fgas >~ 0.7
► Trend of increasing gas fraction with a decrease in luminosity/mass
 FIGGS probe the regime of faintest, very low mass, gas rich galaxies
► Extending the baseline for a comparative study of galaxy properties
Extended HI disks of FIGGS galaxies
DHI (1 X 1019 cm-2)
DHo (26.5 mag arcsec-2)
DHI/DHo
► Average HI extent of FIGGS galaxies ~ 2.7 times Holmberg radii
► Discovery of extremely extended HI disks around galaxies.
Dwarf Galaxies with Giant HI Disks
NGC 3741 (MB ~ -13.0 mag)
And IV (MB ~ -12.4 mag)
NGC 3741: Vrot derived to 38 x rd
DHI ~ 8.8 DHo
DHI ~ 6.9 DHo
NGC 3741  Galaxy with the most extended HI disk
MD/LB ~ 107  One of the “darkest irregular galaxies known
 A unique opportunity to trace the large scale mass distribution
around dwarf galaxies
Do all faint dIrr galaxies have disturbed kinematics ?
Classification of sample galaxies
NB: Number of resolution elements across the galaxy
Sample galaxies classified in the following categories
Total galaxies: 65
G: Galaxies with systematic rotation and NB > 2
D: “Chaotic” velocity field and NB > 2
GU: Systematic rotation with NB < 2
DU: “Chaotic” velocity field and NB < 2
Non detections:5
G=35 D=16 GU=3 DU=6
Most of the sample galaxies show systematic rotation
 Faintest known galaxies with such regular kinematics
Regular Vs. Disturbed Kinematics
Are external or internal processes responsible for disturbed kinematics ?
Mdyn~108 MΘ

Current star formation or external environment not correlated with observed
kinematics
 All (except one) low mass galaxies have disturbed kinematics
Star formation “threshold” in faint dwarfs

Critical threshold column density of ~ 1021 cm-2 (averaged over 500 pc) for
star formation in dwarf galaxies
(Skillman 1987 NASA conf. Pub.; Hunter et al. 1998 )
► Critical amount of dust shielding required for star formation
 Threshold column density a function of the metallicity of the galaxy

Observed column density resolution dependent

Hybrid configuration of the GMRT

Unique data set
► Require maps at angular resolution varying by a factor of ~ 8
► Produced HI maps corresponding to ~ 300 pc resolution
► Wide range in star formation rate
► Metallicities lower than for earlier studies
Hybrid configuration of GMRT
 14 antennas located in central
compact array
 Sensitivity to faint extended emission
 Rest distributed in Y shaped
configuration with a maximum
baseline of 25 km.
14 dishes in a
compact array
 High resolution
 GMRT’s hybrid configuration allows one to make both low resolution (~ 40”)
and high resolution (~ 3”) images from a single observing run
Star formation “threshold” in faint dwarfs

Critical threshold column density of ~ 1021 cm-2 (averaged over 500 pc) for
star formation in dwarf galaxies
(Skillman 1987 NASA conf. Pub., Hunter et al. 1998 )
► Critical amount of dust shielding required for star formation
 Threshold column density a function of the metallicity of the galaxy

Observed column density resolution dependent

Hybrid configuration of the GMRT

Unique data set
► Require maps at angular resolution varying by a factor of ~ 10
► Produced HI maps corresponding to ~ 300 pc resolution
► Wide range in star formation rate
► Metallicities lower than for earlier studies
HI distribution at ~ 300 pc resolution
Star formation “threshold” in faint dwarf galaxies
“Threshold” column density varies by a factor of ~4 between sample galaxies
No correlation between “threshold” column density and metallicity
FIGGS galaxies at highest resolution (~20-100 pc)
Substantial fine scale structure visible at high resolution
 Scales of energy injection into ISM through stellar winds/supernovae
► No simple, universal relationship seen between Hα emitting gas and high column density neutral gas
Conclusions

FIGGS is probing the regime of faintest, lowest mass, gas rich galaxies

GMRT Observations and HI data reduction for FIGGS complete.

Most faint dwarf Irr show systematic rotation.

Extended HI disks discovered from FIGGS


The star formation “threshold” column density varies by a factor
of ~ 4 between sample galaxies
No universal relation between the high column density HI and Halpha
emitting gas
Much more to come…
THANKS!
Typical data products from the survey
DDO 43 (~ -14.5 mag)
46” X 42”
33” X 22”
46” X 42”
32” X 22”
15” X 13”
12” X 10”
11” X 10”
6” X 5”
FIGGS
Faint (MB>-14.5) Irregular Galaxies GMRT Survey
Ayesha Begum (IOA)
Jayaram N. Chengalur (NCRA),
Igor D. Karachentsev (SAO),
Margarita Sharina (SAO),
Serafim S. Kaisin (SAO)
Robert Kennicutt (IOA)
Baryonic Tully-Fisher relation for faint galaxies
 Dwarf galaxies deviate from the TF relation defined by bright
galaxies

Baryonic correction
McGaugh et al.(2000) ApJL 533 99
 Most earlier studies using single dish observations
 Inclination corrected 50% profile widths not a good measure for
faintest dwarf galaxies  Need HI rotation curves corrected for the
pressure support
HI synthesis observations are crucial to obtain BTF relation for
extremely faint dwarf galaxies
Tully-Fisher Relation at Faint End
B Band TF
I Band TF
 FIGGS galaxies lie below the TF relation defined by bright galaxies
 Higher scatter at low luminosity end
BTF Relation
►
BTF very sensitive to IMF (Γ ) at high mass end
*
 BTF holds for low mass galaxies depending on the choice of IMF
►
Higher scatter at the low mass end
Bottemma IMF
Salpeter IMF
Properties of the FIGGS Sample
<Distance=4.6 Mpc>
<MHI/LB=1.4>
<HI mass=4 X 107 MO>
<DHI/DHo= 2.6>
Introduction

Dwarf galaxies form first in hierarchical models
► Dwarfs form building blocks of larger galaxies

Dwarf galaxies are dynamically much simpler systems
compared to bright galaxies
► Ideal to study the interplay between neutral ISM
and star formation

Study of dwarfs provide a test of various predictions of
hierarchical models
► Shape of dark matter halos, correlations between
halo parameters
Kinematics of Dwarf Irr Galaxies
 Detailed systematic HI studies limited to dwarfs brighter
than ~ -15.0 mag.
(e.g. Swaters 1999 PhD Thesis Groningen)
 Dwarfs fainter than MB ~ -14.0 have chaotic velocity fields.
(e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)
Do all faint galaxies have disturbed kinematics ?
Is NGC 3741 a unique galaxy ?
 Baryonic fraction for NGC 3741 (within the extent of the gas disk) ~ 0.18
► comparable to other galaxies with less extended HI disk
 No evidence for baryon loss (measured within the extent of gas disk) in faint
dwarf galaxies (contradiction to simulations of galaxy formation !)
 To reconcile rotation curve data with theoretical models require baryons in dwarfs
to occupy a smaller fraction of their dark matter halos compared to large galaxies
MB(mag)
MB(mag)
Begum et al. 2005, A&A Lett, 433, 1
Dissimilar morphology of high NHI and Hα emission
Star formation in dwarf galaxies

Bright spiral galaxies
► Star formation rate correlated to the gas column density
(Schmidt power-law relation)
► Suppression of star formation below critical column density
(Tomre's instability criterion)

Nature of process regulating star formation in dwarf galaxies poorly understood

Critical threshold column density of ~ 1021 cm-2 (averaged over 500 pc) for star
formation in dwarf galaxies
(Skillman 1987 NASA conf. Pub. )
► Critical amount of dust shielding required for star formation
 Threshold column density a function of the metallicity of the galaxy

Number of galaxies studied is too small to make any definite conclusions