Presentation on theme: "Cosmology with the GMRT Jayaram N Chengalur NCRA/TIFR."— Presentation transcript:
Cosmology with the GMRT Jayaram N Chengalur NCRA/TIFR
Outline Brief introduction to the Giant Metrewave Radio Telescope (GMRT) Constraining the variation in Fundamental Constants Near field cosmology from Dwarf Galaxy observations
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,
GMRT Antenna Layout Unique hybrid configuration with mix of long and short baselines – allows simultaneous imaging of extended as well as compact emission 25 1km Low and high resolution images of CH 3 CHO emission from SgrB2 made from a single GMRT observation
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
Fundamental Constants Nissim Kanekar Tapasi Ghosh
Low energy fundamental constants (α≡e 2 /ħc, μ≡m p /m e ) 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. < /yr (over 1 year) [Rosenband et al. Science 319, 1808, (2008)] Astrophysical techniques offer less precision, but probe much longer timescales Introduction
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.
The fractional separation between the alkali doublet (e.g. Si IV, MgII) lines Δλ/λ~ α 2 –Current limits Δα/α < 1-2 x (2 < z < 3 ) [Murphy et al. MNRAS, 327, 1237, 2001; Chand et al. A&A, 430, 47, 2005] (MM) Relativistic first order corrections lead to different fine structure transitions in different species having different dependencies on α –Average over a large number of transitions to reduce statistical errors –Current limits Δα/α ~ few x 10 -6, but with conflict between groups [Murphy et al. MNRAS 345,609,2003;Chand et al. A&A,417, 853, 2004; Levshakov et al. A&A, , 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 Optical Spectral Lines
HI 21cm vs fine structure lines Comparisons of the HI 21cm hyperfine line frequency with fine structure (e.g. MgII) line frequency constrains X ≡ g p μα 2 –Radio spectral line frequencies can be easily measured to high precision. –Current published constraints ΔX/X < 2 x (0.23
OH 18cm radio lines The OH molecule has 4 spectral lines with wavelengths ~ 18cm –These lines arise from a combination of Λ doubling and hyperfine interaction Observations of redshifted OH 18cm line absorption from GMRT was also part of Nissim Kanekar’s PhD thesis –Some of the earliest observations of OH at cosmological redshifts. Selection rule ΔF=0,±1
Constraints from OH measurements Comparison of the redshifts of the OH main (ΔF=0) lines with redshifts of the HI 21cm (hyperfine) transition and CO mm (rotational) transitions allows one to simultaneously constrain Δα/α and Δμ/μ and Δg p /g p Constraints are relatively weak unless one assumes Δg p /g p is small Results are subject to possibility of kinematical shifts between HI, OH and CO absorbing gas. If one assumes that Δg p /g p is small (e.g. Langacker et al. 2002) then one gets –Δα/α = -5 ± 1.5 x10 -6 –Δμ/μ = -7.8 ± 2.4 x Chengalur & Kanekar PRL 91, (2003)
Conjugate OH lines The OH ‘satellite’ (ΔF=±1) 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 (z~0) lines gives ΔV = 0.05 ± 0.11 km/s OH 18cm lines from Centaurus A (z ~ 0) [van Langevelde et al. (1995)]
Conjugate OH lines at cosmological distances First detection of conjugate lines at cosmological distances was for PKS , (z= 0.247) Data constrain G≡g p [α 2 μ] Original data leads to ΔG/G = 2.2 ± 3.8 x Kanekar, Chengalur & Ghosh PRL 93, , (2004)
Current constraints from PKS Limits from the new high sensitivity data are ΔG/G < 1.4 x –Δα/α = 3.1 x (2σ, if Δμ/μ is constant) –Δμ/μ = 3.1 x (2σ, if Δα/α is constant) There have been theoretical suggestions that changes in Δμ/μ are correlated with changes in Δα/α with Δμ/μ ~ 50 Δα/α [Calmet & Frisch Eur. Phy. J. C, 24, 639, 2002] In such a model, our data constrains Δα/α < ~ –One of the most sensitive existing limits
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
Near field Cosmology Dwarf Galaxies as Cosmological Probes Ayesha Begum Sambit Roychowdhury I. D. Karachentsev S. Kaisin M. Sharina
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 ~ 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
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”
Near field cosmology from Dwarf Galaxies 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. Numerical simulation: Each large Dark Matter halo is surrounded by several, as yet unmerged, smaller halos. 3D map of the local group: Two large galaxies (Milkyway,Andromeda) surrounded by several small dwarf galaxies
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.
A survey of the neutral hydrogen (HI) emission in a large, systematically selected, sample of dwarf galaxies. Faintest sample galaxies are ~ 10 4 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. The Faint Irregular Galaxy GMRT Survey: FIGGS By far the largest such study of dwarf galaxies, possible due to high sensitivity of the GMRT
Dark matter in faint dwarf galaxies
DDO 210 (M B mag) V ~ 6.5 km/s (VLA) Lo et al AJ, 106, 507 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 V ~ 1.6 km/s (GMRT) Begum & Chengalur 2004 A&A, 413, 525
Dark matter density profiles Traditionally used (phenomenological) dark halo models have constant density cores (‘psuedo isothermal’ halos) ρ(r)=ρ 0 /[1+(r/r c ) 2 ] Numerical simulations of hierarchical CDM models predict cusped density core (“NFW”) dark matter halos ρ NFW (r)=ρ i / [(r/r s )(1+r/r s ) 2 ] (Navarro et al ApJ ) 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
GMRT Observations of Camelopardalis B (M B ~ -12.3) Galactocentric distance (arcsec) Velocity (km/s) aa Begum, Chengalur & Hopp New Ast, 2003, 8, 267 One of the faintest galaxies with a well measured rotation curve
Dark matter in Camelopardalis B 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 Begum et al. New Ast, 2003, 8, 267 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
Serendipitous discoveries of extremely gas rich galaxies ( The baryon fraction in the faintest dwarfs)
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.
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?
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 Gnedin ApJ 542, 535, (2000)
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
Baryon fraction in gas rich galaxies Large scatter in baryon fraction for all galaxies Dwarf galaxies don’t 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.
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.