38 3.2 The figure shows all 30 antennas on the GMRT with 14 of them gathered in the central one. 41 3.5 Amplitude and phase vs uv distance for 3C286, after initial inspection and marking of the data.
The 21-cm Line of Neutral Atomic Hydrogen (H I )
Therefore, the HI 21-cm absorption allows us to study high-redshift neutral gas systems, which are thought to be the precursors of today's galaxies (Curran & Webb, 2006). However, very few HI 21-cm absorption systems are known to date, leaving many gaps in the understanding of the role these absorption systems may play in galactic evolution.
Molecular Gas
Although the transition is rare, hydrogen atoms are very abundant in the universe, making the HI21-cm line easily detectable. The HI21-cm line has been successfully used as a mapping tool for the distribution and kinematics of atomic hydrogen.
Radiative Transfer
- Emission and Absorption of Photons
- Two Level System: Einstein A and B Coefficients
- Equation of Radiative Transfer
- Emission and Absorption Coefficients in Terms of Einstein Coefficients
- Integration of the Equation of Radiative Transfer
Lightman (2008), "the radiation field becomes the black body radiation field in Local Thermodynamic Equilibrium (LTE)". Therefore, the “source function is the amount to which the specific intensity relaxes” if given a sufficient optical depth (Rybicki & Lightman, 2008).
Spectral Lines: Broadening Mechanism
- Spontaneous Emission and the Natural Line Width
- Thermal Broadening
- Collisional Broadening
- The Voigt Profile
According to Pradhan & Nahar (2011), "the mean free path for collisions,l is defined by,nlpd2 = 1, where d is the effective diameter for a collision close enough to affect the radiation process". For the given transition, a is only a function of the damping constant, the wavelength of the line center, and the thermal width.
Radio Interferometry Techniques
- Van Cittert-Zernike Theorem
- Two Element Interferometry
- Aperture Synthesis
- Interpretation of Aperture Synthesis
The autocorrelation function is the Fourier transform of the power spectrum of the radiation field, this theorem is known as the Wiener-Khinchin theorem (Taylor et al., 1999). The measurement of the spatial coherence function depends only on the relative difference between r1andr2. The correlator output can be reshaped with respect to the radio luminosity integrated over the sky.
Thus, a synthesized image in the thel-m plane represents a projection of the celestial sphere on a plane tangent to their origin (Taylor et al., 1999). Another assumption, if the baselines are coplanar, lies in the direction of the celestial pole so,w⇡0 (Taylor et al., 1999). The basic result of the previous section is that “there is a Fourier transform relationship between the sky brightness and the visibility function” (Taylor et al., 1999).
Editing and Calibration of the Visibility
Antenna-based Calibration
We have argued that visibility measurements need to be calibrated due to noisy interference from various objects. Vi,obsj (t) =Gi,j(t)Vi,jtru(t) +ei,j(t) +hi,j(t), (2.19) where Gi,jis the basis-based complex gain term, the time and observations are given byt,hi,jis a complex stochastic noise term, andei,jis the basis-based complex compensation term (Taylor et al., 1999). The use of complex numbers is a convenience, as it describes the combination of two correlator outputs (often called Real and Imaginary outputs, or cosine and sine correlator outputs) into a complex quantity (Taylor et al., 1999).
To invert Eq. 2.19) and determine the best approximation of the true visibilities Vi,jtruf from the observed visibilities Vi,obsj , we would assume average values in time periods between which known calibration sources would be observed. However, "the best calibration procedure is used to determine the gain factors of individual antennas" (Taylor et al., 1999). The simplest method to determine this term is to observe a part of the radio sky that contains no emission and integrate for a few minutes to reduce the contribution of the stochastic noise sources.
Amplitude and Phase Calibration
It should also be noted that antenna gains can drift over time, and so we need to calibrate at shorter times than the instrument changes. Since the amplitude of the phase calibrator remains constant during the course of the observations, its measured amplitude provides a measure of the system gain (and its variation) for each baseline. The measured amplitudes and phases of these robust calibrators can be used to determine complex antenna-based gains (as described below).
Most data corruption occurs before the signal pairs are correlated, so the baseline-based complex gain Gi j(t) can be approximated by the product of the associated antenna-based complex gains,gi(t)engj(t)(see eq.
Bandwidth Smearing
Deconvolution
The H¨ogbom Algorithm
Data Reduction and Spectral Line Analysis
- Bandpass Calibration
- Self-Calibration
- Continuum Subtraction
- Velocity Reference Frames and CVEL
In spectral line analysis, continuum subtraction is one of the first steps to be performed after all calibrations have been applied to the visibility data set. Most radio telescopes are fixed to the surface of the Earth, i.e. they are in a topocentric stationary frame. For this reason, spectral regridding of the dataset is performed prior to imaging.
The presence of galaxy is determined only from the emission lines detected in the SDSS spectrum of the quasar. Based on the FIRST survey (Faint Images of the Radio Sky at Twenty-Centimeters), "the background quasar at 1.4 GHz has a peak and integrated flux densities of 135 mJy/beam and 140 mJy", respectively (White et al., 1997 ). The purpose of the GMRT observation was to search for H I 21-cm absorption from the foreground galaxy atzg=0.3714.
Data Analysis
RFI Inspection: In this step, we identify and flag radio frequency interference (RFI) in the calibrated baseline-based visibility for 3C286. In this step we make a single channel map of the calibrators and the target source. We use the input parameter mode = mfs which makes a single multi-frequency synthesis image from the specified channels.
In the next step, we create a channel-averaged continuum image of the target source to obtain the continuum map with the best possible signal-to-noise ratio (SNR). At this point the self-calibrated data is stored in the MS in the corrected data column. This task makes adjustments to the line-free channels and subtracts the emission in the UV domain.
Results
Description of GMRT Observations
The standard flux density calibrator 3C286 within ⇠33.7 of the quasar was observed twice while observing the track for both flux and bandpass calibration. The compact radio source J1445+099 within ⇠7.08 of the quasar was chosen as the phase calibrator and observed every 45 minutes for ⇠10 minutes.
Data Analysis
The flux density of 3C286 is slightly different from that obtained in the previous chapter, because we observe at a different frequency, RestFreq MHz. J1445+099 RFI Inspection: In this step we use viewer to plot the data for each baseline as raster plots. We use the input parameter mode = mfs, which creates a single multi-frequency synthesis image from the specified channels.
We use the same image size and pixel size as in the previous chapter, because we use the same array and observe with a similar frequency. The rms of J1445+099 is twice as high as that obtained in the previous map from the same source. Before we do that, we set the resting frequency of the OH 18 cm line in the measurement set.
Results
We open the spectral window table of the MS with tb.open and insert the rest frequency with tb.putcell. Image from the spectral continuum subtracted dataset to obtain the image cube: Next step is to image the line dataset obtained from the previous step. This rms is a bit higher compared to the rms we measured in a search for H I 21-cm absorption to SDSS J1443+0214 (recall that we used the rms of ⇠1.2 mJy/beam/channel in HI21-cm absorption spectrum measured).
This is most likely due to higher levels of RFI in the OH dataset. The upper limit for the OH column densities for detected HI absorption components of 21 cm is given in the table. The rms used here (srms=0.7) was obtained from the spectrum of HI 21-cm absorption smoothed to ⇠15 km/s (corresponding to the average FWHM of two HI21-cm absorption components).
VLA Search for OH Absorption Towards SDSS J0849+5108
Description of VLA Observations
The VLA telescope is very similar to the GMRT, except for the operating frequencies and the fact that VLA is reconfigurable. As mentioned in the previous chapter, GMRT also has a Y-shaped configuration, but consists of fixed antennas spread over 25 km. As discussed by Gupta et al. 2013), “the background radio source of the quasar galaxy pair (QGP) SDSS J0849+5108 has been the subject of considerable debate and speculation about the optical spectrum being affected by lensing/or reddening of foreground galaxies”.
One of the main differences (apart from more IFs) from the previous data sets considered so far is that the data file from the two individual runs is about 100 GB, i.e. otherwise, the VLA data analysis was performed following the same strategy adopted for GMRT observations described in the previous sections. In the following, we will broadly describe the steps carried out in the data reduction.
Data Analysis
The first step in data reduction, as usual, is to obtain a measurement set summary (MS). After that, as a general data editing and screening strategy, at this stage of the data reduction process, you should focus on calibers. The data reduction scheme is to determine various corrections from the calibrator sources, then apply these correction factors to the science target.
In the next step, we use taskgaincal to solve the complex antenna-based gains for J0542+4951 and then determine the bandpasses. All the above-mentioned steps were performed for the data from both observation series, and the calibrated data were combined. From the final self-calibrated continuum image of the target from the merged data set, we measure the flux density⇠208 mJy.
Results
Conclusion
This will also lead to a much better understanding of the global evolution of cold gas in the (e.g. z < 1) Universe, but will also reveal many interesting examples of galaxies at intermediate redshifts in which the ISM can be studied (Zwaan et al. , 2015). In the previous chapters, we presented the results of the HI21-cm and hydroxyl (OH) absorption line searches from two quasar-galaxy pairs (QGPs). For these reasons, while radio absorption line searches have to date provided valuable constraints on the evolution of diffuse atomic gas in galaxies, they have not really been helpful in detecting molecular gas reservoirs (Gupta et al., 2012).
Therefore, a large absorption line blind survey (e.g. the MeerKAT absorption line survey) at radio wavelengths can overcome these limitations and provide an unbiased view of the cold atomic and molecular gas in the galaxy. “The fundamental goal of CRTS is a systematic exploration and characterization of the dim and variable sky” (Drake et al., 2014). These telescopes consist of the Catalina Schmidt Survey (CSS), the Mount Lemmon Survey (MLS) and the Siding Spring Survey (SSS).
Description of SALT Observations
The same configuration was used for all targets and the angle of the slit position was chosen such that a bright star always fell into the slit. We drew a vertical red line to show the curvature of the skyline (see Fig. 5.3). Bias reduction: Reducing the level of bias is the first step in data reduction.
Flat Field: This is a technique used to correct for pixel sensitivity bias due to the internal performance of CCDs and optics. However, the most important task of the system is XCSAO, which implements the cross-correlation method. The sky emission and the reference star have completely occupied the spectrum of the object of interest.
