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Search for OH Megamasers at theredshift of z 1.7 in GOODS-North

Field

Pallavi Patil

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Plan of the talk

What is Maser?

GOODS-North

Interferometry and Synthesis Imaging Data Reduction

Results

Future Plans References

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MASER : Overview

Microwave Amplification by Stimulated Emissionof Radiation

Masers are narrow and intense emission lines.

OH, H2O, CH3OH, SiO and NH3molecules exhibitMaser emission in interstellar space.

Classification (based on the physical origin)

Galactic masers : Associated with comets, starforming regions and evolved stars

Extragalactic masers : Associated with Active GalacticNucleus(AGN) and starburst galaxies

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Maser :

Mechanism

SEYFERT GALAXIES: A REVIEW- Stephen J. Curran (2000)

Pumping

Spontaneous

decay

E2

E1

E3

Maser emission

http://ned.ipac.caltech.edu/level5/Curran/frames.htmlhttp://ned.ipac.caltech.edu/level5/Curran/frames.html

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Megamasers

The line luminosity is billion times more thanGalactic Masers, hence Megamasers.

Not observed in our Galaxy.

OH Megamaser (OHM) is found in thestarburst galaxies (ULIRGs) and H2OMegamasers are found in the molecular

clouds surrounding AGNs. OHMs are useful in studying the early

universe.

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The GOODS-North Field

Great Observatories Origins Deep Survey is a

multi-wavelength campaign which unites

extremely deep observations in optical, IR, UV,

X-ray and radio wavelengths.

Centered around two regions in Northern and

southern hemispheres.

Scientific aim: Study formation and evolution

of galaxies and star formation history.

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The GOODS-North Field

http://hermes.sussex.ac.uk/content/hermes-survey

Location:

RA - 12h 36m 49.4s

Dec - 621258

Very rich and deepobservations are

available at radio, far IR,

mid IR, submm, optical ,

UV and X-ray

wavelengths

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GOODS-North: Observational details

Frequency of observation: 610MHz

Bandwidth: 32 MHz

Number of channels :256 Channel bandwidth: 125 kHz

Angular Resolution : 6 arcsec

Area covered : 1.93 sq.deg Total integration time : 33 hours

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Interferometry and Synthesis Imaging

A single dish antenna: Simplest Radio Telescope. Angular Resolution, = 1.22 Better resolution needs larger aperture.

Signals from small antennas are combined to give an

equivalent large aperture Principle of interferometry

D

Geometric delay,

g=D./c

S

S

Output of two element interferometer observing a

source in the direction S and separated by distance D

Output 1 =

V1cos(2(t- g)) Output 2 = V2cos 2 t

- Voltage multiplier

- Time Average

Correlator

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The correlator output is proportional to

V1V2cos(2Dsin/c) = V1V2cos(2Dsin/) The correlator output (visibility) is a function

position of the source()the brightness(I)

antenna separation (baseline-D). The 90 phase shift in one antenna gives sine wave

output : V1V2sin(2Dsin/)

The complex visibility function is defined byV = R(cos corr)-R(sin corr)

R, Response of an interferometer

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Synthesis Imaging

Relationship between Visibility and Sky Intensity, vanCittert-Zernike Theorem

where (,,) and (,,) are coordinates used to expressthe antenna positions and sky intensity distribution

One baseline One Fourier component Aperture synthesis utilizes Earths rotation to increase

number of baselines.

GMRT(Giant Meterwave Radio Telescope), Pune has 30antennas arranged in Y shape to give maximum UVcoverage.

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Giant Meterwave Radio Telescope

(GMRT)

gmrt.ncra.tifr.res.in

30 antennas, 45-m diameter

Longest baseline 25 km Frequency coverage: 150 MHz to 1450 MHz

http://gmrt.ncra.tifr.res.in/gmrt_hpage/Users/doc/WEBLF/LFRA/node164.htmlhttp://gmrt.ncra.tifr.res.in/gmrt_hpage/Users/doc/WEBLF/LFRA/node164.html

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Data Reduction and Imaging

RAW DATA

Decimation

Scheme

Transfer flags,

selfcal solution toraw data

CorrelatorOutput

RFI MitigationBandpass,Flux, PhaseCalibration

Selfcal

Residual

Analysis

* AIPS package and in-house

software tools have been

used for the data analysis

Imaging

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Calibration

Observed visibility is dependant on the gain of antenna

and is affected by radio frequency interference (RFI),

ionosphere disturbance and correlator offsets.

Visibilities needs to be corrected before an image ismade. Corrupted data can be removed or modified.

Role of calibration is to recover true visibility from

observed visibility

Antenna based gain

Baseline

based gain

Baseline

based offset

Randomnoise

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Calibration and RFI removal

RFI mitigation : Removal of systematic ripplesusing Rfix algorithm

Initial Calibration:

Bandpass : Compensation of gain variation acrossbandwidth of observation

Flux : Estimation of true amplitude of the celestialsource. Flux calibrator used 3C286

Phase : Compensating for temporal variations due toionosphere. Phase calibrator used 1313+675

Data Reduction and Imaging

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Data reduction and imaging

The decimation scheme takes a median of the visibilitywithin a grid 5 frequency channels and 7 timeintegrations.

Reduction in file size by 35 and no loss of sensitivity.

Imaging : Inverse Fourier transformation of visibilityusing CLEAN algorithm.

Self-calibration : Estimation of antenna gains using themodel visibility data calculated from the original image

Residual Analysis : Systematic errors are moreprominent

scheme

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Residual Analysis

Residual visibilities in the UV plane

After flagging points above certain

amplitude

Flagging residualdeviant points in theUV plane

Expected a noise likedistribution foramplitudes in the UVplane.

Bright points in thefirst imagecorrespond tocorrupted data.

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Baseline based correction

Baseline offsets : The long term time average ofreal and imaginary visibilities should be zero.

Any correlator offsets can increase noise in theimage.

Any gain corrections specific to a baselinescannot be corrected by self-calibration

The AIPS task BLCAL was used to correct for anymultiplicative as well as additive baseline basederrors.

A software tool to identify baseline with largeoffsets

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Baseline based correction: Correlator

offsets

Number of baseline vs time average of a baseline per channel

A few baselines have significant offsets.

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Results

Each day was combined to obtain final image.

Consistent results over the five days.

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Results : Flux Scaling Errors

The observed flux ofstrongest sources in the

field is 1.7 times less than

expected flux.

No errors in our dataanalysis routine.

Elevation dependence

ruled out.

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Future Plans

Sufficient sensitivity achieved for detection ofOH megamaser at z 1.7

Higher merging activity, quadratic relation

between far-IR and maser line luminosity Higher probability of detection.

Still some of systematic errors needs to be

corrected. Plan for short observation request to GMRT

The search for OH megamaser.

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Images: Transfer of flags

GDNKZ1 GDNLZ1

Noise at the center 187 Jy 125 Jy

Imaging details : averaged over 125 channels

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Images: Self calibration and UV-planeflagging

GDNKZ1 GDNKZ8

Noise at the center 187 Jy 38 Jy

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Baseline based correction

GDNKZ1 GDNKZ8

Noise at the center 187 Jy 38 Jy

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