Crystal Keddie-Hill
Agnes Scott College

REU Summer Program 2007
Univ. of Wisconsin - Madison
Madison, WI 53706

Advisers:
Dr. Eric Wilcots
Laura Chomiuk
Emily Freeland

NGC 3631:A Multiwavelength Study

Introduction and Objectives

Our galaxy is believed to reside in something known as the Local Bubble, a region of space filled with diffuse hot gas but lacking any higher density neutral gas, with the hot gas possibly surrounded by a shell of dense neutral material. In order to better understand our own galactic environment we began a search for similar structures in other spiral galaxies, including NGC 3631 and NGC 3184.

The Game Plan

To find these holes we needed to look at our galaxy in multiple wavelengths:

HI: 21cm line data from the VLA provide information on the HI features of the galaxy. (note to non-astrophysics folks: HI = neutral hydrogen. This is the neutral gas in which we are looking for bubbles, holes, or shells. The two will frequently be used interchangeably, so keep this in mind).
H&alpha data provide information on hot, young star forming regions (like O and B associations) that could be the cause for the areas of evacuated Neutral Hydrogen.
[SII] data show areas in the galaxy that have experienced some kind of event that causes a shock, such as a supernova.
Low energy X-ray emissions idicate areas of diffuse hot gas, which is what we would expect to find in areas that are devoid of HI.

In order to determine the presence of HI holes we planned to compare the NGC 3631's HI emission to H&alpha, [SII], and X-ray emission. Regions that are dim in HI but very bright in H&alpha, for example, could be experiencing star formation that has blown away all of the neutral hydrogen and created a bubble. If these voids in HI are also bright in X-ray, then this would be consistent with the predicted conditions in the holes. The same is true for [SII]: as supernovae are also thought to be a cause for HI holes, any areas that have strong [SII] emission but very little HI emission are potential candidates for HI holes as well.

My Galaxy: NGC 3631

NGC 3631 is a grand design spiral galaxy with a Hubble classification of SASc located ~16 Mpc away. It is also nearly face-on, with an inclination of 17° (Fridman 2001). This orientation is ideal for our purposes, and in terms of distance on astronomical scales it is also not too terribly far away. According to Knapen (1997) the galaxy doesn't have any obvious companions and is relatively isolated.

NED Search Results <--- This page will give you information like right ascension, declination, systemic velocity, redshift, and even images.

The Data

Radio Data

In November of 2002, the Very Large Array was used to observe the HI in NGC 3631. The VLA, one of the best interferometers in the world, has 27 radio antennas each with a diameter of 25 feet. The antennas are spread out in a Y shape, and in its most extended configuration (A), each arm extends 21 kilometers. In this configuration the VLA approximates a single dish with a diameter of 36 kilometers. The most compact configuration is the D configuration, where all of the dishes are within 0.6 km of the center. Due to the nature of interferometry, longer baselines are better for observing small-scale structure, and shorter baselines are better for observing large-scale structure. During our observing run the VLA was in its C configuration with 27 operational antennas, and this gave us an angular resolution of 14 arcseconds (our beam was 14.08" by 13.67"). This corresponds to a spatial resolution of 1.1 kpc on the galaxy, which basically means that we can't resolve any structure that is smaller than 1.1 kpc. My colleague for the summer, Matthew Richardson, had a very similar project to mine, only he had B array data as well as C array data. When the two are combined this improves the angular resolution, so to read about Matthew's higher-resolution HI analysis, check out his page on NGC 3184. The radio data were reduced in AIPS (Astronomical Image Processing System), which is available on the VLA website. If you ever find yourself reducing data in AIPS and in dire need of assistance, check out emily and Laura's blog. Laura and emily, my two brilliant grad student advisors from this summer (along with other Wisconsin grad students) created this rather useful forum, and if you find yourself without the link you can just do a quick google search for "AIPS help." Their site is first on the list of search results.

Our observations were of the neutral hydrogen 21 cm line, which is in the radio section of the electromagnetic spectrum. This line is ideal for studying HI in galaxies because it provides tremendous amounts of information about the distribution of HI in our galaxy. How do we get this information? Neutral hydrogen atoms hanging out in our galaxy have a proton and electron, each with parallel spins. Occassionally the electron will flip its spin so that they are antiparallel (this is a lower energy state), and when this happens the electron emits light that has a wavelength of 21 cm. When this hydrogen atom has a certain velocity, however, this wavelength gets slightly Doppler shifted away from the 21 cm line. When you have an entire galaxy full of HI that has a range of different velocities you get a slightly broadened 21 cm line. The broadened line is essentially a range of wavelengths, and these wavelengths correspond to the different velocities of the hydrogen atoms. We use this information to make what we call a data cube in radio astronomy. In this cube you have RA and dec on the x and y axes and velocity on the z-axis. Moving along the z-axis corresponds to moving through the cube, and each velocity represents what we call a "channel." So if you stop at a point on the z-axis (i.e. at a particular velocity) the RA and dec will tell you about the distribution of HI at that velocity. It helps to think about the cube as a book, and each channel is a page in the book with each page having a specific velocity. If we lay out a few channels we get what we call a channel map(fig. 1), which will show you information about the distribution of HI at several different velocities.

Channel Map

Fig. 1: Channel map of NGC 3631. Each box shows the distribution of HI moving at the velocity indicated in the top right corner of the box. This nifty map was made using the AIPS task KNTR.

Once you have a data cube you can use it to determine even more information about your galaxy. For example, all of the channels can be added together to get an image of the HI distribution. Integrating the flux of the HI in the galaxy along the velocity axis of our cube creates a moment 0 map, which provides useful information about the structure and distribution of neutral hydrogen. Figure 2 is my color-coded moment 0 map. The bright regions are areas of high intensity while the bluer regions are areas of lower intensity, and in terms of HI intensity can be thought of as the number of HI atoms in a region. Something interesting to note is the HI "fluff" at the bottom/bottom-right of my galaxy. We are not exactly sure as to the cause of this fluff, because this sort of thing os usually caused by tidal interactions between interacting galaxies. But as I said before,according to the literature NGC 3631 has no known companions. What, then, is causing the appearance of the fluff? That question (among others) is under investigation.

Moment 0 map

Fig.2 Moment 0 map.

While moment 0 maps provide information about the intensity of the HI, moment 1 maps provide information about the velocity of those particles. So while moment 0 maps are just the integration of the intensity along the velocity axis, moment 1 maps are created by integrating the intensity in each channel times the velocity (to the first power) over all of the velocities. Moment 1 maps effectively represent the rotation of gas in the galaxy. In my color-coded moment 1 map in figure 3 the blue-ish gas is moving towards us and the pink/white-ish gas is moving away from us. The fluff is also very pronounced in this image, and judging from the color-coding it appears to be rotating at not terribly different velocities than the gas within the galaxy that it is in proximity to. If the fluff were being tidally pulled from the galaxy it seems as though we wouldn't see it rotating in such a way.

Moment 1 Map

Fig. 3 Moment 1 map.

Those are the basics of my radio data, and we've already come across one peculiarity. Let's see what's going on in my optical data.

Optical Data

During a three-night observing run in May of 2007, the WIYN 3.5m telescope was used to obtain optical images of NGC 3631. Located at Kitt Peak, the telescope is operated by the WIYN consortium, which consists of the University of Wisconsin, Indiana University, Yale University, and the NOAO. For the observations we used OPTIC on the WIYN, an imager with a 10' field of view and a fairly short read-out time, which is ideal for optimizing time on source. The images were taken in three filters: H&alpha, [SII], and continuum. As I mentioned previously H&alpha is ideal for viewing things like regions of star formation while S[II] is good for seeing areas that have been shocked, possibly by supernovae.The continuum images are pretty much just for subtracting from the H&alpha and the S[II], so that we are left with emission that is JUST from H&alpha or S[II].

For example, this is a stacked image that was taken with the H&alpha filter but also has continuum emission (stacked just means that we combined multiple exposures in the same filter, it's like "stacking" 3 ten-minute exposures to get something similar to a 30-ish minute exposure):

Fig. H&alpha

And this image is just continuum emission:

Fig NGC 3631 continuum image.

When the second image is subtracted from the first we get only emission that is due to the H&alpha that we want to observe:

Fig.Just H&alpha, no continuum.

And this one is JUST the S[II] emission:

Fig.Just S[II] emission.

I was a little freaked out the first time I saw them, too. I was certain that I had done something wrong, but don't worry, this is how they're supposed to look. As you have probably noticed, there is hardly any emission at all that is just due to S[II], compared with the amount of H&alpha that we see. This is not terribly surprising since S[II] is caused by rather significant events like supernovae while H&alpha is due to things like star formation.

As the first people at Wisconsin to reduce data from OPTIC (and what seems like some of the only people ever), we ran into plenty of problems. We worked with our images in a multi-extension format, which means that we worked with both amplifiers from both CCDs separately, giving us four extensions for each image before eventually recombining them. We worked with these mosaics in IRAF with the MSCRED package. If you want gory details please email me at the email address at the top of the page and I will be happy to share. For now I will spare those who are not interested (hi mom!)

I didn't reduce any x-ray data, but part of the original plan was to compare our radio and optical data with existing x-ray images of NGC 3631. This didn't happen for various reasons, but we still have plenty of data to work with. Now that we have images in our two wavelengths, let's investigate.

Analysis

One important thing to note at this stage is that we decided late in the summer that our HI resolution was not good enough for our specific purposes. With 1.1 kpc resolution on the galaxy, it really just wasn't good enough to look for the hole/bubble/shell structure that we wanted to study (and this is part of the reason that we didn't bother working with the x-ray data). But don't worry, we are working on getting better resolution data (you can read all about it in the upcoming "Future Plans" section). While this temporarily impacts the initial goal of our project, it doesn't mean that I didn't get to do some good science. I had data in two wavelengths, and after weeks and weeks of working with all of it I started to notice a few things. This is when my project veered from "Looking For Holes in HI in NGC 3631" to "Looking at Cool Structures in NGC 3631." (Those aren't actual titles, they're just to give you some idea of the different directions that I found my work taking me.) So that's where we're headed now: interesting and peculiar things about NGC 3631.

Aside from the aforementioned "fluff" in my radio data, the first thing I began to notice is that NGC 3631 appears to have a straight spiral arm. This is mostly obvious in the optical images, and it seems rather strange, and delightfully ironic, for a spiral galaxy to have a straight arm. The straightness of the arm could be caused by any number of things. Anomalies of this sort are usually explained as the result of some interaction with a companion galaxy, but according to the literature there is no companion galaxy. What else could be the cause? We're not sure. We're also not sure if it's even real structure or just an optical illusion. We could just be detecting non-arm emission more than we are detecting emission that is actually in the spiral arm. It is also entirely possible that the region we think is the spiral arm actually isn't, and that we aren't adequately detecting the actual spiral arm. Again, we're not entirely sure why, but we are looking into it..

Fig. The straight spiral arm is indicated by the red arrows, as if that wasn't already obvious.

The straight spiral arm in the optical images is made all the more perplexing by the right angle that seems to occur in the HI in my moment 0 map:

Fig. The right-angled arm is indicated by the arrows.

Fig. A closer look.

One of the best ways to compare images in multiple wavelengths is to make a contour map of one of your images and overlay those contours on another image. This helps you see if the same structures occur at different wavelengths, or simply how different galactic structures compare at multiple wavelengths. Let's see if the straight arm in my optical image coincides with the right angle in my radio image.

This is my moment 0 map (in grayscale rather than in nice colors), with contours from my continuum-subtracted H&alpha image:

Fig. H&alpha contours on HI.

It appears that the right angle in my HI is somewhat, but not extensively populated by some H&alpha emission, while my straight arm in H&alpha actually appears to spiral somewhat in the HI. The spiraling of the so-called straight arm at a different wavelength gives us reason to believe that the linearity of that particular arm isn't real structure. We're still looking into it.

Another peculiar feature that you may have noticed is the hole in the middle of the continuum-subtracted H&alpha image. This hole also appears in a 6 cm and 20 cm radio continuum image that Laura reduced. The best part is that the holes coincide if you look at a contour map of H&alpha on radio continuum.