Gas Properties of Dwarf Galaxies Gas Properties of Dwarf Galaxies Ayesha Begum (IOA, Cambridge) Faint (M B >-14.5) Irregular Galaxies GMRT Survey Results from FIGGS: Faint (M B >-14.5) Irregular Galaxies GMRT Survey Jayaram N. Chengalur (NCRA), Igor D. Karachentsev (SAO), Jayaram N. Chengalur (NCRA), Igor D. Karachentsev (SAO), Margarita Sharina (SAO), Serafim S. Kaisin (SAO) Margarita Sharina (SAO), Serafim S. Kaisin (SAO)

What is FIGGS ? HI survey of nearby (< 10 Mpc) faint HI survey of nearby (< 10 Mpc) faint (M B >-14.5) dwarf irregular galaxies (M B >-14.5) dwarf irregular galaxies with the GMRT with the GMRT Obtain high-quality observations of Obtain high-quality observations of atomic ISM at high velocity resolution atomic ISM at high velocity resolution (~1.6 km/s) and variety of spatial resolutions (~ 40” – 5”) (~1.6 km/s) and variety of spatial resolutions (~ 40” – 5”) systematic study of physical characteristics A systematic study of physical characteristics of atomic ISM in faint dIrr galaxies of atomic ISM in faint dIrr galaxies

Giant Metrewave Radio Telescope 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 HI survey of nearby (< 10 Mpc) faint (M B >-14.5) dwarf irregular galaxies (M B >-14.5) dwarf irregular galaxies with the GMRT with the GMRT Obtain high-quality observations of Obtain high-quality observations of atomic ISM at high velocity resolution atomic ISM at high velocity resolution (~1.6 km/s) and variety of spatial resolutions (~ 40” – 5”) (~1.6 km/s) and variety of spatial resolutions (~ 40” – 5”) systematic study of physical characteristics A systematic study of physical characteristics of atomic ISM in faint dIrr galaxies of atomic ISM in faint dIrr galaxies

Why FIGGS ?  Interplay between neutral gas and star formation in faintest gas rich galaxies gas rich galaxies ► S tar formation threshold for faint galaxies ► S tar formation threshold for faint galaxies  Extend the Baryonic Tully-Fisher relation to a regime of very low mass/luminosity. low mass/luminosity.  Density distribution of dark matter halos Create archival dataset (calibrated (u,v) data, data cubes, MOMNT maps, Create archival dataset (calibrated (u,v) data, data cubes, MOMNT maps, HI spectrum and rotation curves) for the astronomical community. 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: M B > -14.5, HI Flux > 1 Jy km/s, optical size > 1 arcmin, δ> -40 deg   HI mass typically M HI ~ 10 7 M O  Typical Observing time ~ hours for most galaxies Noise per channel ~ mJy/Beam  Noise per channel ~ mJy/Beam  High velocity resolution ~ 1.65 km/s used for observations Factor of ~ 4 times better than most earlier studies  Factor of ~ 4 times better than most earlier studies

DDO 210 (M B ~ mag) Lo et al AJ, 106, 507 High velocity resolution crucial for the observations of faint dwarf galaxies  V ~ 6.5 km/s ΔV ~ 1.6 km/s Begum & Chengalur 2004 A&A, 413, 525 Dwarfs fainter than M B ~ have chaotic velocity fields ??? Dwarfs fainter than M B ~ have chaotic velocity fields ??? (e.g. Cote et al AJ , Lo et al AJ ) (e.g. Cote et al AJ , Lo et al AJ )

FIGGS  GMRT HI images  HST V and I band optical images ► TRGB distance (accuracy of ~ 10%) known for most galaxies ► TRGB distance (accuracy of ~ 10%) known for most galaxies First large sample of faint dwarfs with accurate distances  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 The GMRT observations over last year. Data reduction is now complete reduction is now complete Time to do some science….. Time to do some science…..

Gas Fraction of FIGGS Galaxies ► f gas ► Average gas fraction of FIGGS galaxies ~ 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 ► baseline for a comparative study of galaxy properties ► Extending the baseline for a comparative study of galaxy properties Gas Fraction f gas = M gas /(M gas +M star )

Extended HI disks of FIGGS galaxies ► ► Average HI extent of FIGGS galaxies ~ 2.7 times Holmberg radii ► ► Discovery of extremely extended HI disks around galaxies. D HI (1 X cm -2 ) D Ho (26.5 mag arcsec -2 ) D HI /D Ho

NGC 3741 (M B ~ mag) D ~ 8.8 D D HI ~ 6.9 D Ho D HI ~ 8.8 D Ho D HI ~ 6.9 D Ho  A unique opportunity to trace the large scale mass distribution around dwarf galaxies around dwarf galaxies  A unique opportunity to trace the large scale mass distribution around dwarf galaxies around dwarf galaxies And IV (M B ~ mag) Dwarf Galaxies with Giant HI Disks  Galaxy with the most extended HI disk NGC 3741  Galaxy with the most extended HI disk M D /L B ~ 107  One of the “darkest irregular galaxies known NGC 3741: Vrot derived to 38 x r d

Classification of sample galaxies Sample galaxies classified in the following categories G: Galaxies with systematic rotation and N B > 2 D: “Chaotic” velocity field and N B > 2 GU: Systematic rotation with N B < 2 DU: “Chaotic” velocity field and N B < 2 N B Number of resolution elements across the galaxy N B : Number of resolution elements across the galaxy Do all faint dIrr galaxies have disturbed kinematics ? Total galaxies: 65 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

Are external or internal processes responsible for disturbed kinematics ? Cur Current star formation or external environment not correlated with observed kinematics M dyn ~10 8 M Θ Regular Vs. Disturbed Kinematics   All (except one) low mass galaxies have disturbed kinematics

Star formation “threshold” in faint dwarfs  Critical threshold column density of ~ cm -2 (averaged over 500 pc) for star formation in dwarf galaxies star formation in dwarf galaxies (Skillman 1987 NASA conf. Pub.; Hunter et al ) (Skillman 1987 NASA conf. Pub.; Hunter et al ) ► Critical amount of dust shielding required for star formation ► Critical amount of dust shielding required for star formation  Threshold column density a function of the metallicity of the galaxy  Threshold column density a function of the metallicity of the galaxy  Observed column density resolution dependent ► Require maps at angular resolution varying by a factor of ~ 8 ► Require maps at angular resolution varying by a factor of ~ 8  Hybrid configuration of the GMRT ► Produced HI maps corresponding to ~ 300 pc resolution ► Produced HI maps corresponding to ~ 300 pc resolution  Unique data set ► Wide range in star formation rate ► Wide range in star formation rate ► Metallicities lower than for earlier studies ► Metallicities lower than for earlier studies

Hybrid configuration of GMRT  GMRT’s hybrid configuration allows one to make both low resolution (~ 40”) and high resolution (~ 3”) images from a single observing run   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.  High resolution 14 dishes in a compact array

Star formation “threshold” in faint dwarfs  Critical threshold column density of ~ cm -2 (averaged over 500 pc) for star formation in dwarf galaxies star formation in dwarf galaxies (Skillman 1987 NASA conf. Pub., Hunter et al ) (Skillman 1987 NASA conf. Pub., Hunter et al ) ► Critical amount of dust shielding required for star formation ► Critical amount of dust shielding required for star formation  Threshold column density a function of the metallicity of the galaxy  Threshold column density a function of the metallicity of the galaxy  Observed column density resolution dependent ► Require maps at angular resolution varying by a factor of ~ 10 ► Require maps at angular resolution varying by a factor of ~ 10  Hybrid configuration of the GMRT ► Produced HI maps corresponding to ~ 300 pc resolution ► Produced HI maps corresponding to ~ 300 pc resolution  Unique data set ► Wide range in star formation rate ► Wide range in star formation rate ► Metallicities lower than for earlier studies ► 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  “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 (~ 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 FIGGS is probing the regime of faintest, lowest mass, gas rich galaxies GMRT Observations and HI data reduction for FIGGS complete. GMRT Observations and HI data reduction for FIGGS complete. Most faint dwarf Irr show systematic rotation. Most faint dwarf Irr show systematic rotation. Extended HI disks discovered from FIGGS Extended HI disks discovered from FIGGS The star formation “threshold” column density varies by a factor The star formation “threshold” column density varies by a factor of ~ 4 between sample galaxies of ~ 4 between sample galaxies No universal relation between the high column density HI and Halpha emitting gas No universal relation between the high column density HI and Halpha emitting gas Much more to come… Much more to come…

THANKS!THANKS!

DDO 43 (~ mag) 46” X 42” 33” X 22”15” X 13”11” X 10” 46” X 42”32” X 22”12” X 10”6” X 5” Typical data products from the survey

Ayesha Begum (IOA) Ayesha Begum (IOA) Jayaram N. Chengalur (NCRA), Jayaram N. Chengalur (NCRA), Igor D. Karachentsev (SAO), Igor D. Karachentsev (SAO), Margarita Sharina (SAO), Margarita Sharina (SAO), Serafim S. Kaisin (SAO) Serafim S. Kaisin (SAO) Robert Kennicutt (IOA) Robert Kennicutt (IOA) Faint (M B >-14.5) Irregular Galaxies GMRT Survey FIGGS

Baryonic Tully-Fisher relation for faint galaxies  Dwarf galaxies deviate from the TF relation defined by bright galaxies galaxies  Baryonic correction  Baryonic correction McGaugh et al.(2000) ApJL  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 faintest dwarf galaxies  Need HI rotation curves corrected for the pressure support pressure support HI synthesis observations are crucial to obtain BTF relation for HI synthesis observations are crucial to obtain BTF relation for extremely faint dwarf galaxies extremely faint dwarf galaxies

Tully-Fisher Relation at Faint End 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 Bottemma IMFSalpeter IMF ► ► Higher scatter at the low mass end

Properties of the FIGGS Sample

Introduction  Dwarf galaxies form first in hierarchical models ► Dwarfs form building blocks of larger galaxies ► Dwarfs form building blocks of larger galaxies  Dwarf galaxies are dynamically much simpler systems compared to bright galaxies compared to bright galaxies ► Ideal to study the interplay between neutral ISM ► Ideal to study the interplay between neutral ISM and star formation and star formation  Study of dwarfs provide a test of various predictions of hierarchical models hierarchical models ► Shape of dark matter halos, correlations between ► Shape of dark matter halos, correlations between halo parameters halo parameters

Kinematics of Dwarf Irr Galaxies  Detailed systematic HI studies limited to dwarfs brighter than ~ mag. than ~ mag. (e.g. Swaters 1999 PhD Thesis Groningen) (e.g. Swaters 1999 PhD Thesis Groningen)  Dwarfs fainter than M B ~ have chaotic velocity fields. (e.g. Cote et al AJ , Lo et al AJ ) (e.g. Cote et al AJ , Lo et al AJ ) 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 ► 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 !) dwarf galaxies (contradiction to simulations of galaxy formation !) M B (mag) Begum et al. 2005, A&A Lett, 433, 1  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

Dissimilar morphology of high N HI and Hα emission

Star formation in dwarf galaxies  Bright spiral galaxies ► Star formation rate correlated to the gas column density ► Star formation rate correlated to the gas column density ( Schmidt power-law relation) ( Schmidt power-law relation) ► Suppression of star formation below critical column density ► Suppression of star formation below critical column density ( Tomre's instability criterion) ( Tomre's instability criterion)  Nature of process regulating star formation in dwarf galaxies poorly understood  Critical threshold column density of ~ cm -2 (averaged over 500 pc) for star formation in dwarf galaxies formation in dwarf galaxies (Skillman 1987 NASA conf. Pub. ) (Skillman 1987 NASA conf. Pub. ) ► Critical amount of dust shielding required for star formation ► Critical amount of dust shielding required for star formation  Threshold column density a function of the metallicity of the galaxy  Threshold column density a function of the metallicity of the galaxy  Number of galaxies studied is too small to make any definite conclusions