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

Topics to be addressed What factors govern star formation in faint gas rich dwarf galaxies ? 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 compared to bright galaxies  Ideal to study the interplay between neutral ISM and star formation. interplay between neutral ISM and star formation. Baryon fraction in faint gas rich dwarf galaxies Baryon fraction in faint gas rich dwarf galaxies ☛ Theory/numerical simulations vs observations ☛ Theory/numerical simulations vs observations

Faint Irregular Galaxies GMRT Survey (FIGGS)  A survey of the neutral hydrogen (HI) emission in a large sample (~65) of nearby ( -14.5) dwarf irregular galaxies with the GMRT  The faintest sample galaxies ~ 10 4 times less luminous than typical L * galaxy  The faintest sample galaxies ~ 10 4 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 FIGGS Sample Median M HI ~ 3 X 10 7 M sun, M B ~ mag, Median M HI ~ 3 X 10 7 M sun, M B ~ mag, HI flux > 1.0 Jy km/s HI flux > 1.0 Jy km/s 65 galaxies with GMRT HI cubes, optical broad band and H  images 65 galaxies with GMRT HI cubes, optical broad band and H  images Sub samples with TRGB distances, optical spectroscopy, GALEX images,… Sub samples with TRGB distances, optical spectroscopy, GALEX images,…

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 14 dishes in a compact array 14 Dishes in compact array

Faint Irregular Galaxies GMRT Survey (FIGGS)  A survey of the neutral hydrogen (HI) emission in a large sample (~65) of nearby ( -14.5) dwarf irregular galaxies with the GMRT  The faintest sample galaxies ~ 10 4 times less luminous than typical L * galaxy  The faintest sample galaxies ~ 10 4 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 FIGGS Sample Median M HI ~ 3 X 10 7 M sun, M B ~ mag, Median M HI ~ 3 X 10 7 M sun, M B ~ mag, HI flux > 1.0 Jy km/s HI flux > 1.0 Jy km/s 65 galaxies with GMRT HI cubes, optical broad band and H  images 65 galaxies with GMRT HI cubes, optical broad band and H  images Sub samples with TRGB distances, optical spectroscopy, GALEX images,… Sub samples with TRGB distances, optical spectroscopy, GALEX images,…

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

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

NGC 3741 (M B ~ mag) HI disk extends to 8.8 times the Holmberg radius HI disk extends to 8.8 times the Holmberg radius  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 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 Begum et al. 2008b, MNRAS 383, 809, Begum et al. 2005, A&A, 433, 1L

HI extends to ~ 7 times Holmberg radius HI extends to ~ 7 times Holmberg radius Do “dark” galaxies have anomalously low baryon fractions? And IV (M B ~ mag) Dwarf Galaxies with Giant HI Disks M D /L B ~ 237  One of the “darkest irregular galaxies known Radius (kpc) V rot (km/s)

Baryon fraction in numerical simulations Small halos are less efficient at capturing baryons 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 Baryon fraction expected to vary inversely with galaxy mass (Gnedin, 2000, ApJ, 542, 533; (Gnedin, 2000, ApJ, 542, 533; Hoeft et al. 2006, MNRAS, 371, 401) Hoeft et al. 2006, MNRAS, 371, 401) Data from Hoeft et al. 2006, MNRAS, 371, 401

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

Interplay between HI and star formation in dwarf galaxies

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 (Kennicutt -Schmidt power-law relation) (Kennicutt -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 ★ 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) (Oey & Clarke, 2005)  UV a better tracer of SF in faint dwarf galaxies  UV a better tracer of SF in faint dwarf galaxies

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  Study of globally averaged and “pixel-to-pixel” correlation between SF and HI in each of these FIGGS galaxies between SF and HI in each of these FIGGS galaxies ■ Observed column density resolution dependent  Produced HI maps corresponding to ~ 400 pc & 200 pc resolution  Produced HI maps corresponding to ~ 400 pc & 200 pc resolution

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

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

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

Small scale correlation between Σ SFR and Σ gas at 400 pc resolution  Power law dependence of Σ SFR on Σ gas  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

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

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

Conclusions on star formation study in faint galaxies  There is, initially at least, a near universal relation between Σ SFR Σ gas.  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

THANKS!THANKS!

Pixel by pixel correlation

Introduction ■ Dwarf galaxies form first in hierarchical models ► Dwarfs form building blocks of larger galaxies ► Dwarfs form building blocks of larger galaxies ■ Study of dwarfs provide a test of various predictions of hierarchical models hierarchical models ► Shape of dark matter halos, correlations between halo ► Shape of dark matter halos, correlations between halo parameters parameters ■ 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

Leo T: Faintest gas rich galaxy

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

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

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 )

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 inCompact array

Baryon fraction in dwarf galaxies Small halos are less efficient at capturing baryons 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 Baryon fraction expected to vary inversely with galaxy mass Gnedin, 2000, ApJ 542, 533

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 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