Radio Spectroscopy : Young Hive of Astronomy

“The sky is everywhere, even in the dark beneath your skin.” — Wisława Szymborska

Astronomy is a wonderful field where we get to observe the stars, galaxies and the universe at large — a hobby of immense interest for our ancestors! Famous American astronomer Carl Sagan once said that knowing about the cosmos is like knowing ourselves, since the cosmos is both outside and inside us. Probably, that is why we feel an innate attraction towards the cosmos!

Radio Spectroscopy is a specific field in ‘Observational Astronomy’ where celestial objects which emit electro magnetic rays in the radio frequencies can be detected.

The Very Large Array (VLA) in central New Mexico (Source: Katie Stone, KUNM.org)

Unlike X-Ray and Optical Spectroscopy, which have been going on for decades now, the advent of radio spectroscopy is relatively new. The first detection of radio waves from an astronomical object was in 1932, when Karl Jansky at Bell Telephone Laboratories observed radiation coming from the Milky Way, while inclusion of Radio Spectroscopy in day-to-day observational astronomy took place much later — in the 1980s. Radio Astronomy in India was brought in by Govind Swarup in late 1980s — 1990s of TIFR. These methods, like X-Ray, Optical and Radio can be used in observing the cosmos, due to them facing subsequent low opacity in the atmosphere.

Opacity or Opaqueness is lowest for Radio waves, X-Rays and Visible Light in the EM Spectrum (Source:Wikimedia)

There are two types of radio-spectral lines, which are generally used in radio spectroscopy:

1. The H I 21 cm Line : It is possibly the most used in radio astronomy, and is used to calculate the atomic gas mass of high red-shift galaxies.
2. The ionized carbon CII-158 μm line: It is used to do research on high red-shift disk galaxies, in particular.

The charms of Radio Spectroscopy:-

Here are some of the reasons you should absolutely love radio spectroscopy and also why it is so important:

> Both radio waves and visible light have the least atmospheric opacity, as mentioned earlier, hence can be easily detected terrestrially.

> Observations can be done from the Earth for both optical as well as radio with big telescopes, however, Radio has even lesser atmospheric opacity so it serves better! The radio sky: frequency of nearly 10 MHz — 1THz; wavelength of around 30m to 0.3mm. On the other hand, the optical sky: wavelength around 3000 Å — 10,000 Å.

> Observatories all around the world allow anyone to do radio spectrography from any of the observatories.

Sunset at Pan-STARRS Observatory in Science City, Maui, Hawaii (Source : Wikimedia)

> Optical and UV spectroscopy are mammothly affected by inter-stellar or cosmic dust during space observations, but it does not happen for Radio rays.

A popular example of this fact is that of Barnard 68 or Boötes void. A complete black-out happens in optical, due to blockage caused by dusts and silicates. However, as soon as IR and Radio Spectroscopy are used, stars are faintly observed. This happens because when inter-stellar dust is heated by UV rays of stars, it emits in IR-Radio spectral region.

On the left is the image of Barnard 68, with VLT Optical while the right one is the image of it when observed through VLT Near-IR. You can clearly notice the difference. (Source : StarDate)

> Radio Inferometry has far better angular resolution than optical’s. Resolution(R) is basically wavelength to diameter ratio (λ/D where λ is the wavelength and D, the telescope diameter). Examples include:

Hubble: R nearly 0.05 arcsec

Very Large Array (VLA): R nearly 0.04 arcsec

ALMA: 4 milli arcsec

Very Large Baseline Array: R nearly 0.15 milli arcsec

Two of the Atacama Large Millimeter/submillimeter Array (ALMA) 12-metre antennas gaze at the sky at the observatory’s Array Operations Site (AOS), high on the Chajnantor plateau at an altitude of 5000 metres in the Chilean Andes. (Source: Wikimedia)

Understanding Space By Radio:

The main constituents of galaxies are dark matter, stars, galaxies and gas. Understanding galaxies requires us to understand both stars and the Inter-stellar Medium (ISM): Galaxies look very different in stars and gas! Whatever you see in optical images, you see a lot more with radio spectroscopy! Some examples are:

Well-resolved optical and HI emissions come from a face-on galaxy named NGC 6964. (Source: SlicerAstro)

This false-color image shows the galaxy M87: optical light is shown in white/blue, the radio emission in yellow/orange (Source: Francesco de Gasperin / LOFAR collaboration)

The first galaxy discovered to host a repeating fast radio burst was a tiny dwarf galaxy. Now a second host galaxy has been found (arrow points to the burst’s location), and it’s a massive spiral similar to the Milky Way. (Source: SHRIHARSH TENDULKAR/GEMINI OBSERVATORY)

The Spiral Galaxy M51: Left, as seen with the Hubble Space Telescope;
Right, radio image showing location of Carbon Monoxide gas. (Source: National Radio Astronomy Observatory)

The critical ISM spectral lines (e.g. the HI 21cm line, CO rotational lines,etc) all lie at radio wavelengths.

Through radio inferometry, one can make 3-D spectral line images (“cubes”) and can measure velocity fields and trace kinematics.

A Few Good lines for Observations:

1. 21 cm Line: It is a hyperfine line in atomic hydrogen (HI), at nearly 1420.4 MHz. Possibly the most important line in astronomy is the 21cm line. It helps measure the atomic gas mass of galaxies, galaxy rotation curves, etc.

The HI radial velocity field of the nearby spiral galaxy M33 is shown here by colors corresponding to Doppler redshifts and blueshifts relative to the center of mass; brightness in this image is proportional to HI column density. (Source: CV.NRao.Edu)

2. CO Lines: Rotational lines in the CO Molecules are used for this, at 115.27 x J GHz, helping to measure molecular gas mass of galaxies. It is a specific varsity of the series of molecular spectral lines.

A CV.NRao.Edu graph of different molecular spectra.

3. C II — 158 micrometre line: Fine structure transition in ionized Carbon ( C ) at 1901 GHz. Strongest cooling in galaxies of nearly 0.5% of a galaxy’s luminosity! It is though, not really a radio line!

4. H2O megamasers: It helps measure black hole masses using the Hubble Constant. Twenty-one H2O masers have been identified in the nuclei of active galaxies. The detection rate is about 7 percent.

Stacking:

21 cm line emission is perhaps, as we have noticed, is the most widely used radio line nowadays. However, 21 cm line emission’s strength is directly proportional to the atomic hydrogen gas mass but there’s a catch. 21 cm line is hyperfine — kinda, weak. It is very hard to detect the line at cosmological distances, until you are ready to put in hours of observation time. Therefore, a method called “stacking” is used as a method of averaging to save time as well as make things more efficient. The task can be achieved in various ways and you can use any software that is at your disposal. It is advised to use TOPCAT for catalog manipulations, DS9 for image viewing, and Python for stacking and more extensive calculations.

Stacking is a method that does signal averaging. It owes its success to two reasons: the signal is added coherently while the noise is added incoherently. In order words, if you take the signal to be a rich man while the noise to be a poor man, it makes the poor poorer while the rich richer. The result is that the signal-to-noise ratio is increased by √ n, where n is the number of times observations are taken.

An example of stacking on a simulated inclined disk. (Source: Katherine Rosenfeld and Nathan Sanders)

Stacking has previously been carried out using optical (e.g. Zibetti et al. 2005) and infrared (e.g. Zheng et al. 2006, 2007) images, as well as in the radio. The large coverage area of the Faint Images of the Radio Sky at Twenty-cm (FIRST; Becker, White & Helfand 1995) survey has led to a number of stacking experiments being carried out (e.g. Wals et al. 2005; de Vries et al. 2007; White et al. 2007; Hodge et al. 2008), looking at the radio properties of quasars and low-luminosity active galactic nuclei (LL-AGN), and some studies of star-forming galaxies have taken place using smaller, but deeper radio surveys (e.g. Boyle et al. 2007; Ivison et al. 2007; Beswick et al. 2008; Carilli et al. 2008).

Also, a research collaboration between Nissim Kanekar of TIFR, Aditya Chowdhary and others have talked about H I 21-centimetre emission from an ensemble of galaxies at an average redshift of one. Kanekar even received the Shanti Swarup Bhatnagar Prize from the President of India in 2017.

In the end, never forget to look at the sky! With the radio, it just gets a bit better!