Alex Orchard
University of Wisconsin-Whitewater

REU program-Summer 2014

Wtih Dr. Bob Benjamin, Dr. Matt Haffner, and Martin Gostisha

# The Velocity Structure of Diffuse Ionized Gas in the Direction of the Scutum-Centaurus Spiral Arm

 Intro WHAM Data Reduction Maps Results

## WIM, the [S II] line, and Gaussian Distributions

The galaxy is a complex system. Not only does it contain countless stars, but it also contains an "atmosphere" of diffuse gas. This gas can take several forms, but what I looked at was the Warm Ionized Medium, or WIM. This is originally neutral gas, which has been ionized and heated through one of several possible methods including photoionization from hot stars and heating from cosmic rays. The WIM emits a constant but very faint amount of light in the visible spectrum. In my project I traced the [S II] emission line to explore the WIM. One important thing to note is that the light from these emission lines is spread out into a Gaussian-like distribution (similar to a "bell curve"). As atoms collide and move thermally throughout the gas, their velocities become distributed. If you record the wavelengths of the emission from that gas some of the light will be red-shifted because the atom emitting that wave will be moving away from you, while some will be blue-shifted because that specific atom is moving towards you. The majority of the particles will be moving at about the same velocity as the entire cloud, and they will differentiate their velocities in an approximately symmetric way. Because of this, a Gaussian distribution becomes a good fit for the spectrum you are likely to get for an emission line from the WIM.

## Local Standard of Rest Velocity and the Flat Rotation Curve

When viewing objects in the sky, one can determine the velocity of that object through red- or blue-shifting of a spectral line. It is important to remember that when you do find this velocity, it is a line-of-sight velocity. No transverse velocity can be determined by this method. This can make it difficult to actually determine the motion of an object, unless you know something about how the object moves already. Luckily, in Galactic astronomy, we do know how objects move relative to the galaxy. The galaxy follows what's called a flat rotation curve. In this model, all objects orbit the center of the galaxy at a speed that is independent of radius (240 km/s). Using this information, and a little math, Dr. Benjamin made this helpful map to understand how line-of-sight velocity can be converted to distance, shown below.

In this map, the lines are lines of equal Local Standard of Rest (LSR) velocities. LSR velocities are the official name of the line of sight velocities found for objects in the Galaxy. They are so named because it's the velocity found relative to our rest frame. In the map, blue lines are negative velocities which means objects there are moving away from us, and red are positive, meaning they are moving towards us. If you want to convert velocity to distance, you'll find that there is a problem inside the solar circle (the circle of zero LSR velocity). In one direction, there are two possibilities for each individual velocity. So, if you were to read -80 at 330 degrees, there are two approximate places at which that object could be placed. The other thing to notice is where the Scutum-Centaurus Spiral Arm is, and what velocity ranges it resides in. If you look between -80 and -20 inside the solar circle, you will see a spiral arm, part of which lies between these velocities. This is the Scutum-Centaurus Arm. Take note of the velocity ranges it lies in, as this will become relevant later.