Presentation on theme: "FIGGS: The Faint Irregular Galaxy GMRT Survey"— Presentation transcript:
1 FIGGS: The Faint Irregular Galaxy GMRT Survey Jayaram N ChengalurNCRA/TIFRAyesha BegumI. D. KarachentsevSambit RoychowdhuryS. Kaisin, M. Sharina
2 Near field cosmology from Dwarf Galaxies Extremely faint dwarfs are particularly interesting in the context of hierarchical galaxy formation modelsThe smallest objects collapse first, larger galaxies from by merger of smaller onesThe process of galaxy merger is highly inefficientEvery 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 The Faint Irregular Galaxy GMRT Survey: FIGGS GMRT observations of the neutral hydrogen (HI) in a well defined sample of nearby dwarf galaxies.Selected from KK catalog65 galaxies identified using the selection criteriaMB > -14.5, HI Flux > 1 Jykm/s,Dopt > 1', δ > -40oAccurate distances are known for a large fraction of the samplecomplimentary multi-wavelength data (HST, Ground based optical broad band, Ha, GALEX UV…)By far the largest interferometric HI study of dwarf galaxies
4 The Giant Meterwave Radio Telescope (GMRT) Hybrid configuration12 dishes in central compact arrayRemaining along 3 “Y” armsAllows one to simultaneously make low and high resolution images14 km1 km x 1 kmAperture Synthesis Radio Telescope (interferometer)30 Antennas each 45m in diameterAbout 70 km N of Pune, 160 km E of Mumbai.
5 Properties of the FIGGS sample <D>~ 4Mpc<MHI> ~ 3 x 107 Msun<MB> ~ -13
6 FIGGS vs. other large interferometric HI surveys Average gas fraction of FIGGS galaxies < fgas >~ 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 (11”) + GALEX UV CCD R+BHI (5”)+HaHI (28”)+CCD V + B
8 FIGGS: Expected Outcomes Study the relationship between dark and baryonic matter in the smallest gas rich galaxiesDark 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 formationStar formation thresholds, recipes, effective yield…
9 Scaling relations for gas disks Begum et al. MNRAS,386,1667 (2008)HI mass correlates tightly with DHI (1 Msun/pc2)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 SHICorrelation of HI mass with optical diameter weaker than for spiralsCoupling between gas and star formation in dwarfs not as tight as in spirals?
11 DDO 210 (MB mag)Is there an ordered component to the gas motions in thefaintest dwarf galaxies?V ~ 1.6 km/s (GMRT)Begum & Chengalur 2004 A&A, 413, 525DV ~ 6.5 km/s (VLA Darray)Lo et al AJ, 106, 507
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 (MB ~ -12.0)Begum, Chengalur & Hopp New Ast, 2003, 8, 267
14 Cam B : Rotation CurveBegum et al. New Ast, 2003, 8, 267Peak rotation velocity comparable to velocity dispersionImportant to correct for pressure support before determining the dynamical mass (Model Dependent!)
15 Mass model for CamBBegum et al. New Ast, 2003, 8, 267Constant 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
16 TF relationsBegum et al., MNRAS,386,138, (2008)VrotBaryonic MassI band MagnitudeCompare 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
17 Scatter about the TFBegum et al., MNRAS,386,138, (2008)HSTFIGGSThe scatter about the BTF/TF (normalized by the measurement error) is much larger for the FIGGS sample as compared to the HST sampleStar formation efficiency is lower and shows more scatter in dwarf galaxies as compared to large spirals
19 Star formation recipes for spirals and starburst galaxies (Kennicutt ApJ 498, 541, 1998)Star formation occurs only above a fixed threshold gas column densityStar formation rates determined from Ha observationsRelated 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 densitySSFR = (2.5 ± 0.7)10−4(Sgas/1Msunpc−2)1.4±0.15Msunyr−1kpc−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 Ha) (Begum et al MNRAS )Star formation recipes have traditionally used Ha as the tracer of star formation rate SsfrSince observed gas density SHI varies with resolution, all HI images have to be constructed at the same linear resolutionLinear resolution used here is 300pcThere is no one to one correspondence between HI column density and star formation (as traced by Ha)The observed “threshold” column density is much lower than that predicted from disk instability model
21 Ha vs. HI on fine (~ 20pc scales) (Begum et al MNRAS )On very small scales Ha emission is sometimes co-incident with HI peaks, and sometimes with HI “holes”
22 Measuring SFR in faint Galaxies At low SFR, Ha measurements of SFR are susceptible to stochastic errorsHa is produced by massive OB stars, i.e. traces the instantaneous SFRThe number of OB stars in small clusters is susceptible to large Poisson fluctuations even if the IMF is universalUV may be a better indicator of SFR
23 Pixel by Pixel Correlation with FUV Roychowdhury et al. MNRAS,397,1435, (2009)Pixel by Pixel Correlation with FUVSeveral galaxies show “Schmidt Law” type behaviour but the slope isGenerally much steeper than 1.4Highly variable from galaxy to galaxyPower law continues until one reaches the sensitivity limit of the GALEX dataAt the highest Sgas, the SSFR approaches the value predicted by the “Kennicutt-Schmidt” law
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 won’t workSuggest a new estimator, based on correlation between visibilities on adjacent baselines which is well suited to dragging the signal out of the noiseSimilar estimator is used in interferometric CMBR worke.g. Hobson et al. MNRAS 275, 863 (1995)
27 HI fluctuation PowerSpectrum in DDO 210 (MB ~ -10.9) The power spectrum has a slope a ~ ± 0.45over scales from 80 – 500 pcthe intensity fluctuations appear to come from modulation of the densitysimilar slope to that seen in the MilkywayIn thin disks expect a ~ -1.6Since fluctuations happen in 2DDDO210 must have a relatively fat diskBegum et al. MNRAS,372,L33 (2006)See also Dutta et al. MNRAS,398,887 (2009)
28 Intrinsic axial ratio of dwarf HI disks bFace onEdge onIntermediateq=b/aObserved DistributionIntrinsic DistributionInvert directly, or using Lucy deconvolutionp=c/a
29 Intrinsic axial ratio of HI disks Observed axial ratiosIntrinsic axial ratiosDwarfs have quite fat HI disks <q> ~ 0.57--Consistent with their higher s/Vrot ratios
31 Caveat’sRoychowdhury et al. MNRAS,397,1435, (2009)Correcting the SFR calibration for low metalicity will increase the deviation from the “Kennicutt-Schmidt” SFR lawCorrecting Sgas for possible molecular gas content will have the same effectTo bring agreement with the “Kennicutt-Schmidt” law one will need truncation of the IMF at the upper end.
32 Star formation recipes and galaxy formation simulations Numerical simulations accurately follow the gravity driven merger of the dark matter halosPhysics of the baryonic material is complex and poorly understoodheating/cooling, collapse to form stars, feed back from star formationMost simulations of galaxy formation use “recipes” for following the star formation processRecipes 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)