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Gas Properties of Dwarf Galaxies

Gas Properties of Dwarf Galaxies. Results from FIGGS: F aint (M B >-14.5) I rregular G alaxies G MRT S urvey. Ayesha Begum (IOA, Cambridge). Jayaram N. Chengalur (NCRA), Igor D. Karachentsev (SAO), Margarita Sharina (SAO), Serafim S. Kaisin (SAO). What is FIGGS ?.

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Gas Properties of Dwarf Galaxies

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  1. 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)

  2. 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

  3. 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

  4. 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

  5. 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.

  6. 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

  7. ΔV ~ 1.6 km/s Begum & Chengalur 2004 A&A, 413, 525 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

  8. 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

  9. Survey Status The GMRT observations over last year. Data reduction is now complete Time to do some science…..

  10. Gas Fraction of FIGGS Galaxies ►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 Gas Fraction fgas= Mgas/(Mgas+Mstar)

  11. 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.

  12. NGC 3741 (MB ~ -13.0 mag) Dwarf Galaxies with Giant HI Disks And IV (MB ~ -12.4 mag) DHI ~ 8.8 DHo DHI ~ 6.9 DHo NGC 3741: Vrot derived to 38 x rd 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

  13. Classification of sample galaxies Do all faint dIrr galaxies have disturbed kinematics ? NB: Number of resolution elements across the galaxy Total galaxies: 65 Non detections:5 G=35 D=16 GU=3 DU=6 Sample galaxies classified in the following categories 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 Most of the sample galaxies show systematic rotation  Faintest known galaxies with such regular kinematics

  14. Are external or internal processes responsible for disturbed kinematics ? Regular Vs. Disturbed Kinematics • Current star formation or external environment not correlated with observed kinematics Mdyn~108 MΘ All (except one) low mass galaxies have disturbed kinematics

  15. 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 ►Require maps at angular resolution varying by a factor of ~ 8 • Hybrid configuration of the GMRT ►Produced HI maps corresponding to ~ 300 pc resolution • Unique data set ►Wide range in star formation rate ►Metallicities lower than for earlier studies

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

  17. 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 ►Require maps at angular resolution varying by a factor of ~ 10 • Hybrid configuration of the GMRT ►Produced HI maps corresponding to ~ 300 pc resolution • Unique data set ►Wide range in star formation rate ►Metallicities lower than for earlier studies

  18. HI distribution at ~ 300 pc resolution

  19. 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

  20. 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

  21. 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…

  22. THANKS!

  23. DDO 43 (~ -14.5 mag) Typical data products from the survey 46” X 42” 33” X 22” 15” X 13” 11” X 10” 46” X 42” 32” X 22” 12” X 10” 6” X 5”

  24. 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)

  25. Baryonic Tully-Fisher relation for faint galaxies • Dwarf galaxies deviate from the TF relation defined by bright galaxies • Baryonic correctionMcGaugh 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

  26. Tully-Fisher Relation at Faint End I Band TF B Band TF FIGGS galaxies lie below the TF relation defined by bright galaxies Higher scatter at low luminosity end

  27. 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

  28. Properties of the FIGGS Sample <Distance=4.6 Mpc> <HI mass=4 X 107 MO> <DHI/DHo= 2.6> <MHI/LB=1.4>

  29. 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

  30. 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 ?

  31. 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

  32. Dissimilar morphology of high NHI and Hα emission

  33. 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

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