Star Formation in Extremely Faint Gas Rich Dwarf Galaxies

Sambit Roychowdhury (NCRA), Jayaram N. Chengalur (NCRA), 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 ?

☛ 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

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 107 Msun, 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,…

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 acompact 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 (< 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 107 Msun, 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,…

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)

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.

DHI (1 X 1019 cm-2) DHo (26.5 mag arcsec-2)

DHI/DHo

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

NGC 3741 (MB ~ -13.0 mag)

HI disk extends to 8.8 times the Holmberg radius

A unique opportunity to trace the large scale mass distribution around dwarf galaxies

Dwarf Galaxies with Giant HI Disks

NGC 3741 Galaxy with the most extended HI disk

MD/LB ~ 107

Begum et al. 2008b, MNRAS 383, 809, Begum et al. 2005, A&A, 433, 1L

HI extends to ~ 7 times Holmberg radius

Do “dark” galaxies have anomalously low baryon fractions?

And IV (MB ~ -12.4 mag)

Dwarf Galaxies with Giant HI Disks

MD/LB ~ 237 One of the “darkest irregular galaxies known

Radius (kpc)

Vro

t (km

/s)

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

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. 2009

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 (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

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 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 haveΣSFR below that predicted by Kennicutt (1998)

relation Σgas 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 ΣSFR and Σgas for 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

Small scale correlation between ΣSFR and Σgas at 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

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.

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

Conclusions on star formation study in faint galaxies

There is, initially at least, a near universal relation between ΣSFR Σgas.

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!

Pixel by pixel correlation

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

Leo T: Faintest gas rich galaxy

DDO 43 (~ -14.5 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 ► 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

DDO 210 (MB ~ -10.6 mag)

Lo et al. 1993 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 MB ~ -14.0 have chaotic velocity fields ??? (e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)

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 Compact array

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, 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

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