FIGGS: The Faint Irregular Galaxy GMRT Survey

Jayaram N Chengalur

NCRA/TIFRAyesha BegumI. D. Karachentsev

Sambit Roychowdhury

S. Kaisin, M. Sharina

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.

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

MB > -14.5, HI Flux > 1 Jykm/s,

Dopt > 1', δ > -40o

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

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

Properties of the FIGGS sample

<MHI> ~ 3 x 107 Msun<MB> ~ -13

<D>~ 4Mpc

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

FIGGS Data Products

HI SpectrumVelocity FieldHI (45”)

+

CCD R+B

HI (28”)

+

CCD V + B

HI (11”)

+

GALEX UV

HI (5”)

+

H

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…

Scaling relations for gas disks

• 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 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)

Dark Matter

DDO 210 (MB -10.6 mag)

V ~ 6.5 km/s (VLA Darray)

Lo et al. 1993 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

Large Scale Kinematics of the HI

All FIGGS galaxies show large scale velocity gradients

- Not always consistent with that expected from a rotating disk

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

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

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

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

Bar

yoni

c M

ass

I ban

d M

agni

tude

Vrot

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)

Star formation in extremely faint dwarfs

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/1Msunpc−2)1.4±0.15Msunyr−1kpc−2

(Kennicutt ApJ 498, 541, 1998)

(Kennicutt ApJ 498, 541, 1998)

Unclear if recipes derived from large spirals apply to dwarf (“primordial”) galaxies

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. 2006 MNRAS 365 1220)

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. 2006 MNRAS 365 1220)

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

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)

Fine scale structure of the ISM in dwarf galaxies

Fine Structure in the HI gas

The HI emission is clumpy and shows considerable structure even on scales of 20-100 pc.

Expected in turbulent ISM models – BUT Can this be quantified?

Power spectrum of HI fluctuations in the faintest dwarf galaxies

• HI signal is very weak – estimators used earlier won’t 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 worke.g. Hobson et al. MNRAS 275, 863 e.g. Hobson et al. MNRAS 275, 863

(1995)(1995)

HI fluctuation PowerSpectrum in DDO 210 (MB ~ -10.9)

• The power spectrum has a slope ~ -2.75 ± 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)

Intrinsic axial ratio of dwarf HI disks

aa

ac

aaa

bb

Face on

Edge on

Intermediate

p

qpq

dqqpp

02/1222/12 )()1(

)()(

Observed Distribution

Intrinsic Distribution

Invert directly, or using Lucy deconvolution

q=b/a

p=c/a

Intrinsic axial ratio of HI disks

Dwarfs have quite fat HI disks <q> ~ 0.57 --Consistent with their higher /Vrot ratios

Observed axial ratios Intrinsic axial ratios

Thank you

Caveat’s

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)

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)