When I was taking one of my first astronomy courses as an undergraduate, the TA told our class that if something didn’t make sense in astronomy if it was because it was “either quantum mechanical or historical”. This statement was quite true for me for several years, especially the second half. Astronomy has a lot of interesting terminology that has been collected over the years. Being immersed in the field, I sometimes forget how much seeminly random jargon is thrown around. So I decided I would start a series of posts describing some of this terminology. That way, when I use it without thinking, I’ve already explained what I mean.

Since ALFALFA is a radio survey, I thought I would start by describing the jansky. The jansky is a unit of flux density named for Karl Jansky, who was the first person to detect radio waves of an astronomical origin. (He detected radiation from the center of our Galaxy.) Flux is a measure of energy per area, and the typical SI units used for measuring it are watts per square meter. Flux density is a measure of flux per unit frequency; if a source emits over a range of frequencies, the flux density describes the flux at specific part of the spectrum, rather than the total flux summed over the spectrum. One jansky (abbreviated Jy) is $10^{-26} \,\mathrm{W \,m^{-2}\, Hz^{-1}}$.

The jansky was originally designated for use in radio astronomy because it was the right order of magnitude for early radio sources. It is much easier to report a source with a flux density of 17 Jy as opposed to $1.7 \times 10^{-25}\, \mathrm{W\, m^{-2} \,Hz^{-1}}$. A jansky also gives a good intuitive sense of how strong a source is. If I see an object with a flux density greater than 1 Jy, I consider it a “bright” or strong source. We routinely find galaxies that are a hundred times fainter, on the order of 10 mJy (milli jansky). Developing an intuition for source strength is important as it provides a reference frame: Is this a good (believable) detection? Is the source interestingly bright (or faint)?