The Survey of Ionized Gas in the Galaxy, Made with Arecibo

In hot gas, atoms can become ionized, leaving ions and free electrons. These can then recombine. Recombination lines are caused by the electrons cascading down the energy levels of the atom after recombining. As the electron drops down through the energy levels, the energy it looses at each step is given off as a photon, this has a well-defined energy for any given transition, allowing us to calculate the frequency at which these recombination line photons will be detected. When this falls in the radio part of the spectrum, these lines are known as radio recombination lines (RRLs).

Recombination lines are identified with the chemical symbol for the atom (e.g. H for hydrogen, He for Helium, C for carbon), the number of the level in which the electron ends up, and a greek letter indicating the number of levels the electron has moved (α = 1, β = 2, γ = 3, etc.). The RRL H170α, for example, is given off when an electron falls from the 171st level of a hydrogen atom to the 170th level. The energy loss in this transition gives this RRL its characteristic frequency of 1326.792 MHz.

A detailed description of RRLs and the mathematics behind them is given in NRAO's Essential Radio Astronomy course.

RRLs are powerful probes of ionized gas in the interstellar medium because:

1. they do not suffer from interstellar extinction and can therefore probe the Galactic disk to great distances (see Anderson et al. 2012);
2. they are always optically thin and thus complications due to radiative transfer do not occur (see Lockman & Brown 1978);
3. the coefficients that characterize departures from thermodynamic equilibrium (the b_n factors) are known for the entire range of relevant electron temperatures and densities and thus corrections for stiumulated emission can be made and volume densities can be obtained.

SIGGMA will enable us to identify new HII regions, which are the archetypal tracers of Galactic spiral structure. Increasing their census will help us to better understand the structure of the Milky Way. SIGGMA will be the first unbiased spectral line survey of Galactic HII regions.

The RRL line to continuum ratio has a simple relationship with the electron temperature (in local thermal equilibrium). By combining spectral line data from SIGGMA with continuum data from the GALFACTS survey, we will be able to easily measure the electron temperature distribution. Since we are observing both the inner and outer Galaxy visible from Arecibo, SIGGMA will allow us to test for variation in the electron temperature gradient between these regions, as well as deriving maps of electron temperature for the first time over so large an area. The gradient in electron temperature is thought to be the result of a gradient in the abundances of the primary coolants (O, N, Ne, etc.), thus these measurements will help constrain models of Galactic chemical evolution.

Arecibo has the sensitivity to map carbon RRLs in a substantial number of sources with a wide range of metallicities. These probe the properties of nebular photo-dissociation regions (PDRs) and can be used to test PDR models. We will be able to create maps of carbon RRL emission, allowing a details study of the PDF characteristics that give rise to the strongest lines.

With SIGGMA, we will be able to trace the strength of the ionized gas from known giant HII regions until it transitions into the warm interstellar medium (WIM). By comparing our results with previous observations, we will be able to isolate the source of the WIM.

SIGGMA takes the spectral line data from the Mock spectrometers, which cover the full 300 MHz bandpass of ALFA, and splits it into 12 narrow-band spectra centered near the 12 α transitions within the bandpass (see table). Of the 12 transitions, two (169α and 174α) are often affected by RFI and are discarded. The other transitions are co-added to give the equivalent of ~50 minutes integration per point.

In the inner Galaxy, SIGGMA is a commensal partner to PALFA, while in the outer Galaxy SIGGMA is commensal with ALFA ZOA. From July 2013, it is planned that SIGGMA will take over as the primary project in the outer Galaxy, supporting observations by ALFA ZOA and PALFA.

Observations are carried out in a ‘leapfrog’ mode (see Spitzak & Schneider 1998) where the telescope points at one point on the sky for a period as it drifts past, then ‘leaps’ to a second point that is offset in right ascension so that the telescope tracks along the same path in azimuth and elevation as it did when following the first source. Having the telescope in the same physical configuration for both sources means that one can be used as the ‘off’ scan for the other.

SIGGMA observers a pre-planned grid of pointings, designed to cover the sky using this ‘leapfrog’ observing mode. Three sets of pointings are made in order to cover the ALFA beam pattern. In the inner Galaxy (where PALFA is prime), these are all 270s pointings and the final coverage is beam-sampled. In the outer Galaxy, the pointings are 180s each and each set of pointings is repeated three times with a slight offset in order to make a Nyquist-sampled map of depth equivalent to ~300s per point.