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FIGGS: The Faint Irregular Galaxy GMRT Survey Jayaram N Chengalur NCRA/TIFR Ayesha Begum I. D. Karachentsev Sambit Roychowdhury S. Kaisin, M. Sharina.

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Presentation on theme: "FIGGS: The Faint Irregular Galaxy GMRT Survey Jayaram N Chengalur NCRA/TIFR Ayesha Begum I. D. Karachentsev Sambit Roychowdhury S. Kaisin, M. Sharina."— Presentation transcript:

1 FIGGS: The Faint Irregular Galaxy GMRT Survey Jayaram N Chengalur NCRA/TIFR Ayesha Begum I. D. Karachentsev Sambit Roychowdhury S. Kaisin, M. Sharina

2 Near field cosmology from Dwarf Galaxies Extremely faint dwarfs are particularly interesting in the context of hierarchical galaxy formation models The smallest objects collapse first, larger galaxies from by merger of smaller ones The process of galaxy merger is highly inefficient Every large galaxy should be surrounded by dozens of left over dwarf galaxies Detailed study of these galaxies could lead to insights into the formation and evolution of galaxies in the early universe. Numerical simulation: Each large Dark Matter halo is surrounded by several, as yet unmerged, smaller halos.

3 GMRT observations of the neutral hydrogen (HI) in a well defined sample of nearby dwarf galaxies. Selected from KK catalog 65 galaxies identified using the selection criteria M B > -14.5, HI Flux > 1 Jykm/s, D opt > 1', δ > -40 o Accurate distances are known for a large fraction of the sample complimentary multi-wavelength data (HST, Ground based optical broad band, H, GALEX UV…) The Faint Irregular Galaxy GMRT Survey: FIGGS By far the largest interferometric HI study of dwarf galaxies

4 The Giant Meterwave Radio Telescope (GMRT) Aperture Synthesis Radio Telescope (interferometer) 30 Antennas each 45m in diameter About 70 km N of Pune, 160 km E of Mumbai. Hybrid configuration 12 dishes in central compact array Remaining along 3 Y arms Allows one to simultaneously make low and high resolution images 14 km 1 km x 1 km

5 Properties of the FIGGS sample ~ 3 x 10 7 M sun ~ -13 ~ 4Mpc

6 FIGGS vs. other large interferometric HI surveys Average gas fraction of FIGGS galaxies ~ 0.7 FIGGS probes the regime of faint, very low mass, gas rich galaxies extending the baseline for a comparative study of galaxy properties

7 FIGGS Data Products HI Spectrum Velocity Field HI (45) + CCD R+B HI (28) + CCD V + B HI (11) + GALEX UV HI (5) + H

8 FIGGS: Expected Outcomes Study the relationship between dark and baryonic matter in the smallest gas rich galaxies Dark Matter Halos, Tully-Fisher relation, Baryonic Tully Fisher relation, baryon fraction…. Study the modes of conversion of gas into stars in the smallest units of star formation Star formation thresholds, recipes, effective yield…

9 Scaling relations for gas disks HI mass correlates tightly with D HI (1 Msun/pc 2 ) Relationship matches within error bars with that found for large spirals (Broeils & Rhee 1997) –Over 3 orders of magnitude of HI mass, disks of galaxies can be described as being drawn from a family with fixed average HI Correlation of HI mass with optical diameter weaker than for spirals –Coupling between gas and star formation in dwarfs not as tight as in spirals? Begum et al. MNRAS,386,1667 (2008)

10 Dark Matter

11 DDO 210 (M B mag) V ~ 6.5 km/s (VLA Darray) Lo et al AJ, 106, 507 Is there an ordered component to the gas motions in the faintest dwarf galaxies? V ~ 1.6 km/s (GMRT) Begum & Chengalur 2004 A&A, 413, 525

12 Large Scale Kinematics of the HI All FIGGS galaxies show large scale velocity gradients - Not always consistent with that expected from a rotating disk

13 HI in Cam B (M B ~ -12.0) Begum, Chengalur & Hopp New Ast, 2003, 8, 267

14 Cam B : Rotation Curve Peak rotation velocity comparable to velocity dispersion Important to correct for pressure support before determining the dynamical mass (Model Dependent!) Begum et al. New Ast, 2003, 8, 267

15 Mass model for CamB Constant density core provides a good fit, but NFW type halo does not » NWF halos in general do not provide a good fit to our sample galaxies. Rotation curve derived at resolutions ranging from 300pc to 750 pc » beam smearing does not seem to be important Begum et al. New Ast, 2003, 8, 267

16 TF relations Compare FIGGS sample with HST large spiral Sample (both samples have accurate distances) FIGGS galaxies are slightly under luminous for their velocity width, but have about the right amount of baryons (Model dependent conclusion!) Dwarf galaxies have been less efficient at converting gas into stars Begum et al., MNRAS,386,138, (2008) Vrot Baryonic Mass I band Magnitude Vrot

17 Scatter about the TF FIGGS The scatter about the BTF/TF (normalized by the measurement error) is much larger for the FIGGS sample as compared to the HST sample Star formation efficiency is lower and shows more scatter in dwarf galaxies as compared to large spirals HST Begum et al., MNRAS,386,138, (2008)

18 Star formation in extremely faint dwarfs

19 Star formation recipes for spirals and starburst galaxies Star formation occurs only above a fixed threshold gas column density –Star formation rates determined from H observations –Related to instabilities in thin disks? (Safronov AnAp 23, 979, 1960;Toomre ApJ 139, 1217, 1964) Above this threshold column density, the star formation rate has a power law dependence on the gas column density SFR = (2.5 ± 0.7)10 4 ( gas /1M sun pc 2 ) 1.4±0.15 M sun yr 1 kpc 2 (Kennicutt ApJ 498, 541, 1998) Unclear if recipes derived from large spirals apply to dwarf (primordial) galaxies

20 Star formation in Dwarfs ( As traced by H ) Star formation recipes have traditionally used H as the tracer of star formation rate sfr Since observed gas density HI varies with resolution, all HI images have to be constructed at the same linear resolution Linear resolution used here is 300pc There is no one to one correspondence between HI column density and star formation (as traced by H ) The observed threshold column density is much lower than that predicted from disk instability model (Begum et al MNRAS )

21 H vs. HI on fine (~ 20pc scales) On very small scales H emission is sometimes co-incident with HI peaks, and sometimes with HI holes (Begum et al MNRAS )

22 Measuring SFR in faint Galaxies At low SFR, H measurements of SFR are susceptible to stochastic errors H is produced by massive OB stars, i.e. traces the instantaneous SFR The number of OB stars in small clusters is susceptible to large Poisson fluctuations even if the IMF is universal UV may be a better indicator of SFR

23 Pixel by Pixel Correlation with FUV Several galaxies show Schmidt Law type behaviour but the slope is Generally much steeper than 1.4 Highly variable from galaxy to galaxy Power law continues until one reaches the sensitivity limit of the GALEX data At the highest gas, the SFR approaches the value predicted by the Kennicutt-Schmidt law Roychowdhury et al. MNRAS,397,1435, (2009)

24 Fine scale structure of the ISM in dwarf galaxies

25 Fine Structure in the HI gas The HI emission is clumpy and shows considerable structure even on scales of pc. Expected in turbulent ISM models – BUT Can this be quantified?

26 Power spectrum of HI fluctuations in the faintest dwarf galaxies HI signal is very weak – estimators used earlier wont work Suggest a new estimator, based on correlation between visibilities on adjacent baselines which is well suited to dragging the signal out of the noise Similar estimator is used in interferometric CMBR work e.g. Hobson et al. MNRAS 275, 863 (1995)

27 HI fluctuation PowerSpectrum in DDO 210 (M B ~ -10.9) The power spectrum has a slope ~ ± 0.45 over scales from 80 – 500 pc the intensity fluctuations appear to come from modulation of the density similar slope to that seen in the Milkyway In thin disks expect ~ -1.6 Since fluctuations happen in 2D DDO210 must have a relatively fat disk Begum et al. MNRAS,372,L33 (2006) See also Dutta et al. MNRAS,398,887 (2009)

28 Intrinsic axial ratio of dwarf HI disks a a a c aa a b b Face on Edge on Intermediate Observed Distribution Intrinsic Distribution Invert directly, or using Lucy deconvolution q=b/a p=c/a

29 Intrinsic axial ratio of HI disks Dwarfs have quite fat HI disks ~ Consistent with their higher /Vrot ratios Observed axial ratios Intrinsic axial ratios

30 Thank you

31 Caveats Correcting gas for possible molecular gas content will have the same effect To bring agreement with the Kennicutt-Schmidt law one will need truncation of the IMF at the upper end. Correcting the SFR calibration for low metalicity will increase the deviation from the Kennicutt-Schmidt SFR law Roychowdhury et al. MNRAS,397,1435, (2009)

32 Star formation recipes and galaxy formation simulations Numerical simulations accurately follow the gravity driven merger of the dark matter halos Physics of the baryonic material is complex and poorly understood –heating/cooling, collapse to form stars, feed back from star formation Most simulations of galaxy formation use recipes for following the star formation process –Recipes are generally derived from observations of nearby galaxies. Numerical simulations of galaxy formation in LCDM cosmology are able to produce realistic disk galaxies. While the gravity driven infall and merger are accurately computed, star formation and feedback are incorporated largely via recipes (e.g. Fabio et al. MNRAS 2007, 374,1479)

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