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Star Formation in Extremely Faint Gas Rich Dwarf Galaxies

Star Formation in Extremely Faint Gas Rich Dwarf Galaxies. Ayesha Begum (UW, Madison). Sambit Roychowdhury (NCRA), Jayaram N. Chengalur ( NCRA ), Igor D. Karachentsev (SAO ), Margarita Sharina (SAO). Topics to be addressed.

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Star Formation in Extremely Faint Gas Rich Dwarf Galaxies

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  1. Star Formation in Extremely Faint Gas Rich Dwarf Galaxies Ayesha Begum (UW, Madison) SambitRoychowdhury (NCRA), JayaramN. Chengalur(NCRA), Igor D. Karachentsev (SAO), Margarita Sharina (SAO)

  2. Topics to be addressed • What factors govern star formation in faint gas rich dwarf galaxies ? ☛ Dwarf galaxies are dynamically much “simpler” systems compared to bright galaxies  Ideal to study the interplay between neutral ISM and star formation. • Baryon fraction in faint gas rich dwarf galaxies ☛Theory/numerical simulations vs observations

  3. Faint Irregular Galaxies GMRT Survey (FIGGS) • A survey of the neutral hydrogen (HI) emission in a large sample (~65) of nearby (< 10 Mpc), faint (MB>-14.5) dwarf irregular galaxies with the GMRT The faintest sample galaxies ~ 104 times less luminous than typical L* galaxy • Obtain high-quality observations of atomic ISM at high velocity resolution (~1.6 km/s) and variety of spatial resolutions (~ 40” – 5”) • FIGGS Sample • Median MHI ~ 3 X 107Msun, MB ~ -13.0 mag , HI flux > 1.0 Jy km/s • 65 galaxies with GMRT HI cubes, optical broad band and Ha images • Sub samples with TRGB distances, optical spectroscopy, GALEX images,…

  4. 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 14 Dishes in compact array 14 dishes in a compact array

  5. Faint Irregular Galaxies GMRT Survey (FIGGS) • A survey of the neutral hydrogen (HI) emission in a large sample (~65) of nearby (< 10 Mpc), faint (MB>-14.5) dwarf irregular galaxies with the GMRT The faintest sample galaxies ~ 104 times less luminous than typical L* galaxy • Obtain high-quality observations of atomic ISM at high velocity resolution (~1.6 km/s) and variety of spatial resolutions (~ 40” – 5”) • FIGGS Sample • Median MHI ~ 3 X 107Msun, MB ~ -13.0 mag , HI flux > 1.0 Jy km/s • 65 galaxies with GMRT HI cubes, optical broad band and Ha images • Sub samples with TRGB distances, optical spectroscopy, GALEX images,…

  6. 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 Fractionfgas= Mgas/(Mgas+Mstar) Begum et al. 2008a,MNRAS,386,1887

  7. 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. Begum et al. 2008a,MNRAS,386,1887

  8. NGC 3741 (MB ~ -13.0 mag) Dwarf Galaxies with Giant HI Disks HI disk extends to 8.8 times the Holmberg radius MD/LB ~ 107 NGC 3741 Galaxy with the most extended HI disk  A unique opportunity to trace the large scale mass distribution around dwarf galaxies Begum et al. 2008b, MNRAS 383, 809, Begum et al. 2005, A&A, 433, 1L

  9. Dwarf Galaxies with Giant HI Disks And IV (MB ~ -12.4 mag) HI extends to ~ 7 times Holmberg radius Vrot (km/s) Radius (kpc) MD/LB~ 237  One of the “darkest irregular galaxies known Do “dark” galaxies have anomalously low baryon fractions?

  10. Baryon fraction in numerical simulations • Small halos are less efficient at capturing baryons • Hot baryons escape during the epoch of reionization • Feed back from star formation drives baryons out of shallow dwarf galaxy potential wells • Baryon fraction expected to vary inversely with galaxy mass (Gnedin, 2000, ApJ, 542, 533; Hoeft et al. 2006, MNRAS, 371, 401) Data from Hoeft et al. 2006, MNRAS, 371, 401

  11. Baryon fraction in gas rich galaxies Begum et al. 2009 • Large scatter in baryon fraction for all galaxies • Dwarf galaxies don’t have systematically smaller baryon fractions • AndIVand N3741 do have less than the average cosmic baryon fraction – but so do many other galaxies Cosmic baryon fraction Baryon fraction in galaxies with well measured HI rotation curves

  12. Interplay between HI and star formation in dwarf galaxies

  13. Star formation in dwarf galaxies ★Bright spiral galaxies ► Star formation rate correlated to the gas column density (Kennicutt -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 ★ Number of extremely faint dIrr galaxies studied is too small to make any definite conclusions ★ Stochasticity in the Hα based SFR for extremely faint dwarf galaxies (Oey & Clarke, 2005) UV a better tracer of SF in faint dwarf galaxies

  14. Star formation in FIGGS galaxies using HI and UV ■ Sample of 23 FIGGS galaxies with publicly available GALEX data Study of globally averaged and “pixel-to-pixel” correlation between SF and HI in each of these FIGGS galaxies ■ Observed column density resolution dependent ProducedHI maps corresponding to ~ 400 pc & 200 pc resolution

  15. Comparison of HI and FUV emission in FIGGS Galaxies at ~ 400 pc resolution Roychowdhury et al. 2009, MNRAS (in press)

  16. “Global” Kennicutt-Schmidt Law • FIGGS galaxies haveΣSFR below that predicted by Kennicutt (1998) relation • Σgas are around or below the expected “threshold density” (Kennicutt (1989); Martin & Kennicutt (2001) Slope=-1.4) Slope=-2.47) Slope=-1.4 Dwarfs and spirals have a steeper dependence of SFR on the gas densities than predited by KS law Environmental dependence of star formation efficiency

  17. “Pixel by pixel” comparison of ΣSFR and Σgasfor FIGGS galaxies at 400 pc resolution • At low Σgas, ΣSFR constant, followed by a power-law increase in ΣSFR with Σgas • No evidence for “threshold” density below which the star formation is completely quenched in any galaxy • The ΣSFR generally lies below that predicted by Kennicutt relation. • At high gas densities, the observed ΣSFR begins to approach the predicted rate

  18. Small scale correlation between ΣSFR and Σgasat 400 pc resolution • Power law dependence of ΣSFR on Σgas • Parameters of the power law fit vary significantly from galaxy to galaxy • The index of power law steeper than 1.4 from Kennicutt (1998) relation • No evidence for “threshold” density below which the star formation is completely quenched in any galaxy

  19. Deviant Galaxies • For 5 galaxies, no good power-law parametrization of (ΣSFR ,Σgas) relation • Substantial offsets between HI and UV peaks • Same galaxies outliers in the “global” (ΣSFR ,Σgas) relation • HI masses near the lower end of our sample distribution.

  20. Pixel-by-pixel comparison at 200 pc resolution • HI and FUV images for 10 galaxies at 200 pc resolution • More pronounced offsets between HI and FUV peaks  Feedback from star formation or molecular gas • Power-law parametrization of (ΣSFR, Σgas) for 5/10 galaxies. • Similar trend as seen at 400 pc resolution • The power law index at 200 pc flatter than at 400 pc resolution • Average Σgas higher at 200 pc than 400 pc

  21. Conclusions on star formation study in faint galaxies There is, initially at least, a near universal relation between ΣSFRΣgas. Feedback and other effects may lead to a breakdown of this relation.  Dwarfs have a lower star formation efficiency both because sufficiently dense regions are rarer, and affect of feedback is more important

  22. THANKS!

  23. Pixel by pixel correlation

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

  25. Leo T: Faintest gas rich galaxy

  26. 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”

  27. 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. ►Test the predictions on the shape of dark matter halos and various correlations between halo parameters

  28. 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

  29. 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 Compact array ➡GMRT’shybrid configuration allows one to make both low resolution (~ 40”) and high resolution (~ 3”) images from a single observing run

  30. Baryon fraction in dwarf galaxies • Small halos are less efficient at capturing baryons • Hot baryons escape during the epoch of reionization • Feed back from star formation drives baryons out of shallow dwarf galaxy potential wells • Baryon fraction expected to vary inversely with galaxy mass Gnedin, 2000, ApJ542, 533

  31. Baryon fraction: Theory vs Observation • Since baryons are concentrated at the center of the halo the baryon fraction increases with decreasing radius • Simulations give baryon fraction at the virial radius • Observations determine the baryon fraction up to the last measured point of the rotation curve • Simulations suggest that the baryon fraction within the last measured point of the rotation curve should vary inversely with halo mass Data from Hoeft et al. 2006, MNRAS, 371, 401

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