Gas Properties of Dwarf GalaxiesGas Properties of Dwarf Galaxies

Ayesha Begum (IOA, Cambridge)

Results from FIGGS: FFaint (Maint (MBB>-14.5) >-14.5) IIrregular rregular GGalaxies alaxies GGMRT MRT SSurveyurvey

Jayaram N. ChengalurJayaram N. Chengalur (NCRA), (NCRA), Igor D. KarachentsevIgor D. Karachentsev (SAO), (SAO), Margarita SharinaMargarita Sharina (SAO), (SAO), Serafim S. KaisinSerafim S. Kaisin (SAO) (SAO)

What is FIGGS ?What is FIGGS ?

HI survey of nearby (< 10 Mpc) faint HI survey of nearby (< 10 Mpc) faint

(M(MBB>-14.5)>-14.5) dwarf irregular galaxies dwarf irregular galaxies with the GMRTwith the GMRT

Obtain high-quality observations ofObtain high-quality observations of atomic ISM at high velocity resolution atomic ISM at high velocity resolution (~1.6 km/s) and variety of spatial (~1.6 km/s) and variety of spatial

resolutions (~ 40” – 5”) resolutions (~ 40” – 5”)

A systematic study of physical systematic study of physical characteristics characteristics

of atomic ISM in faint dIrr galaxiesof atomic ISM in faint dIrr galaxies

Giant Metrewave Radio TelescopeGiant 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

What is FIGGS ?What is FIGGS ?

HI survey of nearby (< 10 Mpc) faint HI survey of nearby (< 10 Mpc) faint

(M(MBB>-14.5)>-14.5) dwarf irregular galaxies dwarf irregular galaxies with the GMRTwith the GMRT

Obtain high-quality observations ofObtain high-quality observations of atomic ISM at high velocity resolution atomic ISM at high velocity resolution (~1.6 km/s) and variety of spatial (~1.6 km/s) and variety of spatial

resolutions (~ 40” – 5”) resolutions (~ 40” – 5”)

A systematic study of physical systematic study of physical characteristics characteristics

of atomic ISM in faint dIrr galaxiesof atomic ISM in faint dIrr galaxies

Why FIGGS ?Why FIGGS ?

Interplay between neutral gas and star formation in faintest Interplay between neutral gas and star formation in faintest gas rich galaxies gas rich galaxies ►► SStar formation threshold for faint galaxiestar formation threshold for faint galaxies

Extend the Baryonic Tully-Fisher relation to a regime of very Extend the Baryonic Tully-Fisher relation to a regime of very low mass/luminosity.low mass/luminosity.

Density distribution of dark matter halosDensity distribution of dark matter halos

Create archival dataset (calibrated (u,v) data, data cubes, MOMNT Create archival dataset (calibrated (u,v) data, data cubes, MOMNT

maps, maps, HI spectrum and rotation curves) for the astronomical community.HI spectrum and rotation curves) for the astronomical community.

The FIGGS SampleThe FIGGS Sample

A sample of 65 galaxies (Catalogue of neighboring (10 Mpc) galaxies ) Karachentsev et al. 2004, AJ, 127, 2031

Selection criterion:

MB > -14.5, HI Flux > 1 Jy km/s, optical size > 1 arcmin, δ> -40 deg

HI mass typically MHI ~ 107 MO

Typical Observing time ~ 5 - 6 hours for most galaxies Typical Observing time ~ 5 - 6 hours for most galaxies Noise per channel ~ 2 - 3 mJy/BeamNoise per channel ~ 2 - 3 mJy/Beam

High velocity resolution ~ 1.65 km/s used for observationsHigh velocity resolution ~ 1.65 km/s used for observations Factor of ~ 4 times better than most earlier studiesFactor of ~ 4 times better than most earlier studies

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 MDwarfs fainter than MBB ~ -14.0 have chaotic velocity fields ??? ~ -14.0 have chaotic velocity fields ???

(e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)(e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)

FIGGS

GMRT HI imagesGMRT HI images

HST V and I band optical imagesHST V and I band optical images ►► TRGB distance (accuracy of ~ 10%) known for most galaxiesTRGB distance (accuracy of ~ 10%) known for most galaxies

First large sample of faint dwarfs with accurate distancesFirst large sample of faint dwarfs with accurate distances

HHαα images and rotation curves from the 6m BTA images and rotation curves from the 6m BTA Long slit optical spectroscopic data from WHT/INTLong slit optical spectroscopic data from WHT/INT

The most detailed multi-wavelength study of faint dIrr galaxies

Survey StatusSurvey Status

The GMRT observations over last year. Data The GMRT observations over last year. Data

reduction is now completereduction is now complete

Time to do some science…..Time to do some science…..

Gas Fraction of FIGGS GalaxiesGas Fraction of FIGGS Galaxies

►► Average gas fraction of FIGGS galaxies < ffgasgas >~ 0.7

► ► Trend of increasing gas fraction with Trend of increasing gas fraction with a decrease in luminosity/massa 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 baseline for a comparative study of galaxy properties

Gas FractionGas Fraction f fgasgas= M= Mgasgas/(M/(Mgasgas+M+Mstarstar))

Extended HI disks of FIGGS galaxiesExtended 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

NGC 3741 (MB ~ -13.0 mag)

DDHI ~ 8.8 D ~ 8.8 DHo D DHI HI ~ 6.9 D~ 6.9 DHoHo

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

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

And IV (MB ~ -12.4 mag)

Dwarf Galaxies with Giant HI Disks

NGC 3741 Galaxy with the most extended HI disk Galaxy with the most extended HI disk NGC 3741 Galaxy with the most extended HI disk Galaxy with the most extended HI disk

MD/LB ~ 107 One of the “darkest irregular galaxies known

NGC 3741: Vrot derived to 38 x rd

Classification of sample galaxiesClassification of sample galaxies

Sample galaxies classified in the following categoriesSample galaxies classified in the following categories

G: G: Galaxies with systematic rotation and NGalaxies with systematic rotation and NBB > 2 > 2

D: D: “Chaotic” velocity field and N“Chaotic” velocity field and NB B >> 2 2

GU: GU: Systematic rotation with NSystematic rotation with NB B << 2 2 DU: DU: “Chaotic” velocity field and N“Chaotic” velocity field and NB B << 22

NNBB: Number of resolution elements across the galaxyNumber of resolution elements across the galaxy

Do all faint dIrr galaxies have disturbed kinematics ?

Total galaxies: 65 Non detections:5Total galaxies: 65 Non detections:5

G=35 D=16 GU=3 DU=6 G=35 D=16 GU=3 DU=6

Most of the sample galaxies show systematic rotationMost of the sample galaxies show systematic rotation Faintest known galaxies with such regular kinematics

Are external or internal processes responsible for disturbed kinematics ?

CurCurrent star formation or external environment not correlated with observed kinematics

Mdyn~108 MΘ

Regular Vs. Disturbed KinematicsRegular Vs. Disturbed Kinematics

All (except one) low mass galaxies have disturbed kinematics

Star formation “threshold” in faint dwarfsStar formation “threshold” in faint dwarfs

CCritical threshold column density of ~ 10ritical threshold column density of ~ 102121 cm cm-2 -2 (averaged over 500 pc)(averaged over 500 pc) for for star formation in dwarf galaxiesstar formation in dwarf galaxies (Skillman 1987 NASA conf. Pub.; Hunter et al. (Skillman 1987 NASA conf. Pub.; Hunter et al.

1998 ) 1998 )

►► Critical amount of dust shielding required for star formationCritical amount of dust shielding required for star formation Threshold column density a function of the metallicity of the galaxyThreshold column density a function of the metallicity of the galaxy

Observed column density resolution dependentObserved column density resolution dependent ►► Require maps at angular resolution varying by a factor of ~ 8Require maps at angular resolution varying by a factor of ~ 8

Hybrid configuration of the GMRTHybrid configuration of the GMRT ► ► Produced HI maps corresponding to ~ 300 pc resolutionProduced HI maps corresponding to ~ 300 pc resolution

Unique data setUnique data set ►► Wide range in star formation rateWide range in star formation rate ►► Metallicities lower than for earlier studiesMetallicities lower than for earlier studies

Hybrid configuration of GMRTHybrid 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 acompact array

Star formation “threshold” in faint dwarfsStar formation “threshold” in faint dwarfs

CCritical threshold column density of ~ 10ritical threshold column density of ~ 102121 cm cm-2 -2 (averaged over 500 pc)(averaged over 500 pc) for for star formation in dwarf galaxiesstar formation in dwarf galaxies (Skillman 1987 NASA conf. Pub., Hunter et al. (Skillman 1987 NASA conf. Pub., Hunter et al.

1998 ) 1998 )

►► Critical amount of dust shielding required for star formationCritical amount of dust shielding required for star formation Threshold column density a function of the metallicity of the galaxyThreshold column density a function of the metallicity of the galaxy

Observed column density resolution dependentObserved column density resolution dependent ►► Require maps at angular resolution varying by a factor of ~ 10Require maps at angular resolution varying by a factor of ~ 10

Hybrid configuration of the GMRTHybrid configuration of the GMRT ► ► Produced HI maps corresponding to ~ 300 pc resolutionProduced HI maps corresponding to ~ 300 pc resolution

Unique data setUnique data set ►► Wide range in star formation rateWide range in star formation rate ►► Metallicities lower than for earlier studiesMetallicities lower than for earlier studies

HI distribution at ~ 300 pc resolutionHI distribution at ~ 300 pc resolution

Star formation “threshold” in faint dwarf galaxiesStar formation “threshold” in faint dwarf galaxies

“Threshold” column density varies by a factor of ~4 between sample galaxies

No correlation between “threshold” column density and metallicity

“Threshold” column density varies by a factor of ~4 between sample galaxies

No correlation between “threshold” column density and metallicity

FIGGS galaxies at highest resolution (~20-100 pc)FIGGS galaxies at highest resolution (~20-100 pc)

Substantial fine scale structure visible at high resolution Scales of energy injection into ISM through stellar winds/supernovae

►► No simple, universal relationship seen between Hα emitting gas and high column density neutral gas

ConclusionsConclusions

FIGGS is probing the regime of faintest, lowest mass, gas rich FIGGS is probing the regime of faintest, lowest mass, gas rich galaxiesgalaxies

GMRT Observations and HI data reduction for FIGGS complete.GMRT Observations and HI data reduction for FIGGS complete.

Most faint dwarf Irr show systematic rotation.Most faint dwarf Irr show systematic rotation.

Extended HI disks discovered from FIGGSExtended HI disks discovered from FIGGS

The star formation “threshold” column density varies by a factorThe star formation “threshold” column density varies by a factor of ~ 4 between sample galaxiesof ~ 4 between sample galaxies No universal relation between the high column density HI and No universal relation between the high column density HI and

Halpha emitting gas Halpha emitting gas

Much more to come…Much more to come…

THANKS!THANKS!THANKS!THANKS!

DDO 43 (~ -14.5 DDO 43 (~ -14.5 mag)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

Ayesha BegumAyesha Begum (IOA) (IOA) Jayaram N. ChengalurJayaram N. Chengalur (NCRA), (NCRA), Igor D. KarachentsevIgor D. Karachentsev (SAO), (SAO), Margarita SharinaMargarita Sharina (SAO), (SAO), Serafim S. KaisinSerafim S. Kaisin (SAO) (SAO) Robert KennicuttRobert Kennicutt (IOA) (IOA)

FFaint aint (M(MBB>-14.5)>-14.5) IIrregular rregular GGalaxies alaxies GGMRT MRT SSurveyurvey

FIGGS

Baryonic Tully-Fisher relation for faint galaxies

Dwarf galaxies deviate from the TF relation defined by bright Dwarf galaxies deviate from the TF relation defined by bright galaxiesgalaxies Baryonic correctionBaryonic correction McGaugh et al.(2000) ApJL 533

99

Most earlier studies using single dish observationsMost earlier studies using single dish observations

Inclination corrected 50% profile widths not a good measure for Inclination corrected 50% profile widths not a good measure for faintest dwarf galaxiesfaintest dwarf galaxies Need HI rotation curves corrected for the Need HI rotation curves corrected for the pressure supportpressure support

HI synthesis observations are crucial to obtain BTF relation forHI synthesis observations are crucial to obtain BTF relation for extremely faint dwarf galaxiesextremely faint dwarf galaxies

Tully-Fisher Relation at Faint Tully-Fisher Relation at Faint EndEnd

B Band TF I Band TF

FIGGS galaxies lie below the TF relation defined by bright galaxies

Higher scatter at low luminosity end

BTF RelationBTF Relation

►► BTF very sensitive to IMF (Γ*) at high mass end

BTF holds for low mass galaxies depending on the choice of IMF

Bottemma IMF Salpeter IMF

►► Higher scatter at the low mass end

Properties of the FIGGS Properties of the FIGGS SampleSample

<Distance=4.6 Mpc> <HI mass=4 X 107 MO>

<MHI/LB=1.4> <DHI/DHo= 2.6>

IntroductionIntroduction

Dwarf galaxies form first in hierarchical modelsDwarf galaxies form first in hierarchical models

►► Dwarfs form building blocks of larger galaxiesDwarfs form building blocks of larger galaxies

Dwarf galaxies are dynamically much simpler systems Dwarf galaxies are dynamically much simpler systems compared to bright galaxiescompared to bright galaxies

►► Ideal to study the interplay between neutral ISM Ideal to study the interplay between neutral ISM and star formationand star formation

Study of dwarfs provide a test of various predictions of Study of dwarfs provide a test of various predictions of hierarchical models hierarchical models ►► Shape of dark matter halos, correlations between Shape of dark matter halos, correlations between halo parametershalo parameters

Kinematics of Dwarf Irr GalaxiesKinematics of Dwarf Irr Galaxies

Detailed systematic HI studies limited to dwarfs brighter Detailed systematic HI studies limited to dwarfs brighter

than ~ -15.0 mag.than ~ -15.0 mag. (e.g. Swaters 1999 PhD Thesis Groningen)(e.g. Swaters 1999 PhD Thesis Groningen)

Dwarfs fainter than MDwarfs fainter than MBB ~ -14.0 have chaotic velocity fields. ~ -14.0 have chaotic velocity fields.

(e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)(e.g. Cote et al. 2000 AJ 120 3027, Lo et al. 1993 AJ 106 507)

Do all faint galaxies have disturbed kinematics ?

Is NGC 3741 a unique galaxy ?Is NGC 3741 a unique galaxy ?

Baryonic fraction for NGC 3741 (within the extent of the gas disk) ~ 0.18 Baryonic fraction for NGC 3741 (within the extent of the gas disk) ~ 0.18 ►► comparable to other galaxies with less extended HI diskcomparable to other galaxies with less extended HI disk

No evidence for baryon loss (measured within the extent of gas disk) in No evidence for baryon loss (measured within the extent of gas disk) in faintfaint

dwarf galaxies (dwarf galaxies (contradiction to simulations of galaxy formationcontradiction to simulations of galaxy formation !!) )

MB(mag) MB(mag)

Begum et al. 2005, A&A Lett, 433, 1

To reconcile rotation curve data with theoretical models require baryons in dwarfs to occupy a smaller fraction of their dark matter halos compared to large galaxies

Dissimilar morphology of high NDissimilar morphology of high NHIHI and H and Hαα emission emission

Star formation in dwarf galaxiesStar formation in dwarf galaxies

Bright spiral galaxiesBright spiral galaxies

►► Star formation rate correlated to the gas column density Star formation rate correlated to the gas column density ((Schmidt power-law relation)Schmidt power-law relation) ►► Suppression of star formation below critical column densitySuppression 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 Nature of process regulating star formation in dwarf galaxies poorly

understoodunderstood

CCritical threshold column density of ~ 10ritical threshold column density of ~ 102121 cm cm-2 -2 (averaged over 500 pc)(averaged over 500 pc) for for star star

formation in dwarf galaxiesformation in dwarf galaxies (Skillman 1987 NASA conf. Pub. ) (Skillman 1987 NASA conf. Pub. )

►► Critical amount of dust shielding required for star formationCritical amount of dust shielding required for star formation Threshold column density a function of the metallicity of the galaxy Threshold column density a function of the metallicity of the galaxy

Number of galaxies studied is too small to make any definite conclusionsNumber of galaxies studied is too small to make any definite conclusions