Cosmology with the GMRT

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Cosmology with the GMRT
Jayaram N Chengalur NCRA/TIFR
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Outline

• Brief introduction to the Giant Metrewave Radio
  Telescope (GMRT)
• Constraining the variation in Fundamental
  Constants
• Near field cosmology from Dwarf Galaxy
  observations

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The Giant Metrewave Radio Telescope

• The Giant Metre-wave Radio Telescope (GMRT) is a
  large aperture synthesis radio telescope
  optimized for operation at low frequencies
• Wavelengths of 21cm and longer
• Designed and built (near Pune) primarily by NCRA,
  a national centre of TIFR.
• Array telescope consisting of 30 antennas, each
  45m across
• Novel SMART antenna design
• The most sensitive synthesis radio telescope in
  the world at most of its frequencies of operation,

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GMRT Antenna Layout
Unique hybrid configuration with mix of long and
short baselines allows simultaneous imaging of
extended as well as compact emission
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1km
Low and high resolution images of CH3CHO emission
from SgrB2 made from a single GMRT observation
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Using the GMRT

• Time is allocated to proposals by an independent
  time allocation committee
• Two calls per proposals per year
• At present time allocation is roughly evenly
  split between Indian and Foreign PI proposals

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Fundamental Constants
Nissim Kanekar Tapasi Ghosh
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Introduction

• Low energy fundamental constants (ae2/hc,
  µmp/me) are expected to show spatio-temporal
  evolution
• 
  Uzan, 2003, Rev. of
  Modern Physics
• Timescales of these changes are poorly
  constrained
• Need to search for changes over as wide a range
  of timescales as possible
• Terrestrial methods place extremely tight limits,
  but over very short timescales
• e.g. lt 10-17/yr (over 1 year)
  
  Rosenband et
  al. Science 319, 1808, (2008)
• Astrophysical techniques offer less precision,
  but probe much longer timescales

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Astrophysical methods

• Precise spectral line frequency depends on the
  values of various fundamental constants
• Comparing the line frequency in a distant source
  to that observed on earth will allow one to
  measure variations in the values of the
  fundamental constants
• The redshift of the distant source is unknown a
  priori
• one needs at least two lines (with different
  dependence on the fundamental constants) to
  measure any possible change.
• Narrow absorption lines from cold gas are best
  suited for precise frequency (redshift)
  measurements.

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Optical Spectral Lines

• The fractional separation between the alkali
  doublet (e.g. Si IV, MgII) lines ??/? a2
• Current limits ?a/a lt 1-2 x 10-5 (2 lt z lt 3 )
• Murphy et
  al. MNRAS, 327, 1237, 2001 Chand et al. AA,
  430, 47, 2005
• (MM) Relativistic first order corrections lead to
  different fine structure transitions in different
  species having different dependencies on a
• Average over a large number of transitions to
  reduce statistical errors
• Current limits ?a/a few x 10-6, but with
  conflict between groups
• Murphy et
  al. MNRAS 345,609,2003Chand et al. AA,417, 853,
  2004
  Levshakov et al. AA, 466.
  1077, 2007
• The MM method gives lower statistical errors, but
  larger systematic ones, e.g.
• calibration errors on different echelle orders,
• kinematical velocity shifts between species,
• Isotopic abundance variations

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HI 21cm vs fine structure lines

• Comparisons of the HI 21cm hyperfine line
  frequency with fine structure (e.g. MgII) line
  frequency constrains X gpµa2
• Radio spectral line frequencies can be easily
  measured to high precision.
• Current published constraints ?X/X lt 2 x 10-5
  (0.23 ltz lt 2.35)
• HI 21cm absorption studies were one of the first
  major projects at the GMRT (Kanekar, PhD thesis)
• But early, commissioning phase observations had
  small, systematic errors because of imprecise
  correction for the earths motion
• Preliminary results from fresh, high precision,
  have substantially improved precision
• are in the process of obtaining more precise
  optical redshifts
• Large number of absorbers need to be averaged
• Because of the possibility of kinematical shifts
  between the HI 21cm absorbing gas and the MgII
  absorbing gas.
• HI 21cm absorption comes from cold ( 80K) gas,
  while MgII absorbing gas is often warm (
  5000-10,000K)

Kanekar et al. 2008
Kanekar, Chengalur Lane 2007
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OH 18cm radio lines
Selection rule ?F0,1

• The OH molecule has 4 spectral lines with
  wavelengths 18cm
• These lines arise from a combination of ?
  doubling and hyperfine interaction
• Observations of redshifted OH 18cm line
  absorption from GMRT was also part of Nissim
  Kanekars PhD thesis
• Some of the earliest observations of OH at
  cosmological redshifts.

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Constraints from OH measurements

• Comparison of the redshifts of the OH main (?F0)
  lines with redshifts of the HI 21cm (hyperfine)
  transition and CO mm (rotational) transitions
  allows one to simultaneously constrain ?a/a and
  ?µ/µ and ?gp/gp
• Constraints are relatively weak unless one
  assumes ?gp/gp is small
• Results are subject to possibility of kinematical
  shifts between HI, OH and CO absorbing gas.
• If one assumes that ?gp/gp is small (e.g.
  Langacker et al. 2002) then one gets
• ?a/a -5 1.5 x10-6
• ?µ/µ -7.8 2.4 x 10-6

Chengalur Kanekar PRL 91, 241302 (2003)
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Conjugate OH lines
OH 18cm lines from Centaurus A (z 0)
van Langevelde et al. (1995)

• The OH satellite (?F1) lines are often
  conjugate i.e. have same spectral shape,but
  opposite signs
• Consequence of selection rule driven competitive
  pumping
• Since line shape is the same, non parametric,
  cross correlation techniques can be used to
  determine spectral shifts
• High level of independence from systematic
  effects (kinematic doppler shifts, isotopic
  variations, calibration errors)
• Can be applied to a single object
• Cross correlation of Centaurs A (z0) lines gives
  ?V 0.05 0.11 km/s

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Conjugate OH lines at cosmological distances

• First detection of conjugate lines at
  cosmological distances was for PKS1413135, (z
  0.247)
• Data constrain Ggpa2µ1.849
• Original data leads to
• ?G/G 2.2 3.8 x 10-5

Kanekar, Chengalur Ghosh PRL 93, 051302, (2004)
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Current constraints from PKS1413134

• Limits from the new high sensitivity data are
  ?G/G lt 1.4 x 10-6
• ?a/a 3.1 x 10 -6 (2s, if ?µ/µ is constant)
• ?µ/µ 3.1 x 10 -6 (2s, if ?a/a is constant)
• There have been theoretical suggestions that
  changes in ?µ/µ are correlated with changes in
  ?a/a with ?µ/µ 50 ?a/a
• Calmet
  Frisch Eur. Phy. J. C, 24, 639, 2002
• In such a model, our data constrains ?a/a lt
  10-7
• One of the most sensitive existing limits

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Comparison of OH based and optical spectra based
constraints

• OH constraints are offer similar precision, but
• Apply to a single object (optical results are
  averages over large redshift range)
• Not subject to the same systematics
• Currently probe a complementary redshift range

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Near field Cosmology Dwarf Galaxies as
Cosmological Probes
Ayesha Begum Sambit Roychowdhury I. D.
Karachentsev S. Kaisin M. Sharina
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Cosmic Evolution The quick tour

• The universe starts in a hot big bang and expands
  and cools steadily.
• Inflation makes the density distribution very
  (but not perfectly) smooth
• Perturbations observed to be 10-5 at the epoch
  when protons and electrons combine
• Small perturbations collapse to form the first
  stars and blackholes
• Energy release from these objects reionizes the
  universe
• Perturbations continue to grow to form galaxies
  and clusters of galaxies

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Hierarchical Galaxy Formation

The smallest objects collapse first, bigger
objects form by the merger of smaller ones
Kauffman White 1993
The growth of galaxies by mergers is driven by
the gravity of the non baryonic dark matter the
baryonic matter (stars, gas) occupy a small
region in the center of a much larger dark matter
halo
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Near field cosmology from Dwarf Galaxies
Numerical simulation Each large Dark
Matter halo is surrounded by several, as yet
unmerged, smaller halos.

• The process of galaxy merger is highly
  inefficient
• Every large galaxy should be surrounded by dozens
  of left over dwarf galaxies which are remnants
  of the primordial galaxy population
• As the earliest formed systems, with relatively
  simple internal structure, properties of dwarfs
  are sensitive to cosmology.

3D map of the local group Two large
galaxies (Milkyway,Andromeda) surrounded by
several small dwarf galaxies
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Dwarf Galaxies as cosmological probes

• Since dark matter is typically dominant even in
  the central regions, the dark matter density
  distribution in dwarfs should reflect that
  predicted by numerical simulations
• Details of baryon physics, e.g.
• the mass to light ratio of the stellar
  population,
• feedback from baryonic cooling and collapse on
  the structure of the Dark Matter Halo
• make it difficult to accurately determine the
  dark matter density profile in big galaxies.
• Baryons are easily lost from the shallow dark
  matter potential wells of small galaxies
• Reheating during the epoch of reionization, as
  well as from feedback from star formation should
  lead to dwarf galaxies having baryon fractions
  smaller than the cosmic mean.

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The Faint Irregular Galaxy GMRT Survey FIGGS

• A survey of the neutral hydrogen (HI) emission
  in a large, systematically selected, sample of
  dwarf galaxies.
• Faintest sample galaxies are 104 times
  less luminous than the Milkyway
• HI 21cm observations are preferred because
• Accurately trace the dark matter potential
  because the gas is cold compared to the stars
• Dark matter potential can be traced to large
  galacto-centric distances because the gas disk is
  extended compared to the stellar disk
• Doppler shifts can be easily measured to high
  accuracy.
• Accurate distances are known for a large fraction
  of the sample
• complimentary multi-wavelength data is also
  available with our collaborators or in the public
  domain.

By far the largest such study of dwarf galaxies,
possible due to high sensitivity of the GMRT
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Dark matter in faint dwarf galaxies
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DDO 210 (MB -10.6 mag)

• Need to observe the circular velocity in order to
  reconstruct the underlying density
• distribution
• Earlier, less sensitive, observations indicated
  that gas in the faintest dwarf galaxies
• has chaotic velocity fields
• Fresh, high sensitivity GMRT observations
  established that even the faintest dwarfs
• have well defined coherent large scale velocity
  fields

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Dark matter density profiles
Traditionally used (phenomenological) dark halo
models have constant density cores (psuedo
isothermal halos) ?(r)?0/1(r/rc)2 Numer
ical simulations of hierarchical CDM models
predict cusped density core (NFW) dark
matter halos ?NFW (r)?i /
(r/rs)(1r/rs)2 (Navarro et al.
1997 ApJ 490 493) From measurements of the
circular velocity as a function of
galacto-centric radius (rotation curve) one can
reconstruct the underlying mass distribution
Rotation curves of FIGGS galaxies can be
used to check if dark matter density
distribution matches numerical predictions
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GMRT Observations of Camelopardalis B (MB
-12.3)
Velocity (km/s)
Galactocentric distance (arcsec)
a
a
One of the faintest galaxies with a well measured
rotation curve
Begum, Chengalur Hopp New Ast, 2003, 8, 267
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Dark matter in Camelopardalis B
Begum et al. New Ast, 2003, 8, 267

• Halos with constant density cores provide a
  good fit, but cuspy halos do not
• NFW halos in general do not provide a
  good fit to our sample galaxies.
• Rotation curve derived at a range of spatial
  resolutions
• results are not a consequence of limited
  angular resolution

Tension between the predictions of CDM
heirarchichal galaxy formation numerical
simulations and observations is probably
indicative of baryonic processes (e.g. cooling
and collapse) shaping the centers of the dark
matter halos even in dwarf galaxies.
Alternatively it has been taken as evidence for
WDM
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• Serendipitous discoveries of extremely gas rich
  galaxies
• (The baryon fraction in the faintest dwarfs)

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NGC 3741 A dwarf galaxy with a giant HI disk
Rotation curve measured to a record 38 optical
disk scale-lengths Mass/Luminosity 107 one of
the darkest galaxies known.
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HI in Andromeda IV
HI disk extends out to more than 6 Holmberg
radii Mass/Luminosity 237 ! Do dark galaxies
also have anomalously low baryon fractions?
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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

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Baryon fraction Theory vs Observation

• Since baryons cool and collect at the center of
  the halo, the baryon fraction increases with
  decreasing radius
• Simulations give baryon fraction as measured 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

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Baryon fraction in gas rich galaxies
Large scatter in baryon fraction for all
galaxies Dwarf galaxies dont have systematically
smaller baryon fractions AndIV and N3741 are not
particularly baryon deficient but for some
reason they have been unable to convert gas into
stars (See Roychowdhury et al 08 for star
formation recipes in dwarfs)
Baryon fraction in galaxies with well measured HI
rotation curves
Discrepancy between predicted and observed
baryon fractions is probably again indicative of
our lack of understanding of the detailed
processes involved in baryon capture and cooling,
star formation etc.
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Summary

• Radio spectral lines from redshifted absorbers
  provide very competitive constraints on the
  cosmic variation of fundamental constants
• Can be applied to a single object
• Are not subject to the same systematics as
  optical lines
• Probe a complementary redshift range
• Detailed observations of nearby, extremely faint
  dwarf galaxies allow one to do near field
  cosmology
• Dark matter distribution in these galaxies does
  not conform to predictions of CDM numerical
  simulations
• Baryon content also does not decrease with halo
  mass as expected
• While these discrepancies could be interpreted as
  being problems related to the CDM model, it is
  more likely that they are a consequence of our
  poor understanding of baryonic processes
• Cooling and collapse of gas into stars, feed back
  from star formation etc.

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Thank you