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

The Survey of Ionized Gas in the Galaxy, Made with Arecibo (SIGGMA) is using the Arecibo L-band Feed Array (ALFA) to create the most extensive radio recombination line (RRL) survey ever made. SIGGMA will fully sample the entire Galactic plane observable from the Arecibo Observatory in a set of RRLs that fall in the bandpass of the ALFA receiver. RRLs provide a wide range of critical information about the physical state of ionized interstellar gas that is generally not obtainable through other observational means. We will use these data to identifiy new HII regions, compute HII region electron temperatures, investigate photodissociation region physics with carbon RRLs, and investigate the origin of the warm ionized medium.

What are Radio Recombination Lines?

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.

Why survey for Radio Recombination Lines?

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 bn 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.

Which Radio Recombination Lines is SIGGMA targeting?

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.

The frequencies of 12 α-transition RRLs within the 300 MHz band-pass
n Hnα Henα Cnα Central freq.
Note. — Col.1 shows the 12 lower quantum numbers. Col.2-4 are the frequencies of the 12 α lines of H, He and C in MHz. Col.5 gives the adopted central frequencies of each narrow band spectrum.
174 1237.636 1238.130 1238.243 1237.8780
173 1259.150 1259.633 1259.778 1259.4065
172 1281.175 1281.697 1281.815 1281.4360
171 1303.718 1304.249 1304.368 1303.9835
170 1326.792 1327.333 1327.454 1327.0625
169 1350.414 1350.964 1351.088 1350.6890
168 1374.600 1375.161 1375.286 1374.8805
167 1399.368 1399.938 1400.066 1399.6530
166 1424.734 1425.314 1425.444 1425.0240
165 1450.716 1451.307 1451.440 1451.0115
164 1477.335 1477.937 1478.072 1477.6360
163 1504.608 1505.221 1505.359 1504.9145
How is SIGGMA carrying out observations?

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.

SIGGMA pointing grid
The grid of pointings used for SIGGMA, with RA horizontal and Dec vertical. Circles indicate the size (FWHM) of the ALFA beams. Circles with the same fill color are observed in the same ALFA pointing; circles with similar fill colors are part of the same pointing cluster, with N, C and S indicated the north, central and south pointings of the cluster. The red rhomboid indicates the repeating pattern of the pointing clusters, illustrating that they repeat along lines of constant declination.