The influence of reddening

(You should have found this page from SED plots or Color-Magnitude and Color-Color plots.)

Introduction

By now, you may have realized that:

• Color-magnitude diagrams (CMDs) or for that matter color-color diagrams are a good way of looking at the ensemble distribution of points for a set of objects, e.g., looking at the properties of many objects at once in 2 to 4 bands at once.
• Spectral energy distributions (SEDs) are a good way of looking at the ensemble distribution of the set of detections for a single object, e.g., looking at the properties of one object in many bands (usually all the bands you have) at once.

So, they are two different facets of the same information -- you could have a suite of CMDs looking at a set of objects in several different color-mag (and color-color) spaces, and a suite of SEDs looking at several different objects in a set of several SEDs. Both ways of looking at the information are useful, and we will use them both.

In some cases, the influence of reddening can be important, and that is what this page is about. (Wikipedia entry on reddening/extinction)

Reddening is the same phenomenon we observe here on Earth when watching a sunrise or sunset -- the Sun appears redder than usual because the blue (higher energy) light is preferentially scattered by the atmosphere out of the line of sight, leaving mostly red light to reach your eye. In that case, the more junk in the atmosphere (like from a volcano), the more dramatic the reddening. Same thing happens in space, just due to junk inbetween us and the source we're observing.

As you might imagine, this happens rather a lot in star forming regions, because the stars form in cocoons, in clouds of gas and dust. So we do often have to worry about this, formally. However, the influence of reddening is strongest at bluer bands (e.g., U band is very sensitive to it) and becomes less strong the farther into the IR you go -- JHK can suffer some effects, and you kind of have to be behind a brick wall before it matters for IRAC bands. But it can happen.

Reddening is commonly abbreviated Ax or A_x or A subscript(x), and means the amount of reddening at band x, e.g., Av is reddening at V band.

Effect of Av on SEDs

OK. Now let's try and understand how Av affects SEDs.

Remember this YouTube video