Galactic centre

Everyone wants to point at the centre first. It is worth doing once, mostly so you understand why it is not where the clean results come from.

What the model says. At l = 0° the line of sight runs straight at the Galactic centre, so every parcel of gas in a circular orbit moves across your beam, not along it. The flat-curve model therefore predicts zero radial velocity at every distance. The panel below shows a flat line, and that is correct for the model.

What the sky does instead. The inner few kiloparsecs are not circular orbits. The Galaxy has a bar, and the gas near the centre moves on elongated paths, so real spectra toward l = 0° are broad, with emission spread over hundreds of km/s in large surveys. One well-known feature is the near side of the 3-kpc arm, seen in absorption at about −53 km/s against the bright continuum behind it. A small antenna with a beam several degrees wide averages over all of this, so expect a wide hump, not a tidy peak.

Continuum. The centre is one of the brightest patches of 21 cm continuum in the sky. Your total power will rise noticeably as it drifts through the beam, before you even look at the line. That is useful as a pointing check: if the total-power trace doesn’t bump at the predicted transit time, your pointing or your clock is off.

Practicalities. From mid-northern latitudes the centre never gets high. At 40° N it culminates at about 21°, which puts the beam’s lower edge into ground noise and trees, and drift scans are short. From the southern hemisphere it passes nearly overhead. Northern observers get better data from l = 30° to 80° and should treat the centre as a demonstration.