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Spiral Galaxy Messier 88
Chris Spedden

Our universe is vaster and more amazing than we can understand, filled with spectacular structures of remarkable complexity that leave us in awe.  Among these, many would argue that the crowned jewels of the cosmos are galaxies.  Galaxies are enormous collections of stars, planets, dust, and gas that formed billions of years ago when the universe was still young.  Galaxies come in innumerous shapes and sizes, ranging from small dwarf galaxies only a fraction of the size of our Galaxy and consisting of only 5 or so billion stars, to giant elliptical galaxies dozens of times the size of our Galaxy and consisting of many trillions of stars.  Although each galaxy is unique in its features and details, galaxies can be classified based on their general shape, size, and traits.  Our own galaxy, the Milky Way, contains some 300 billion stars, and is classified as a spiral galaxy.  Spiral galaxies are easily defined by their distinct spiral pattern, with a number of spiral arms branching out from a central, bulging core, the galactic center.  The arms themselves taper off in thickness, so when spiral galaxies are viewed from the side, they look like thin disks with round, bulging central cores.

Spiral galaxy M88 One particularly beautiful spiral galaxy is called Messier 088, or M88. Based on calculations of my image, it is approximately 39 kpc, or kilo-parsecs, in diameter, roughly the same size as the Milky Way, and it is about 35,300 kpc away from us.  One kilo-parsec is 1,000 parsecs in length, and one parsec is about 3.09x10^18 m in length.  So the distance from us to M88 is around 5.9x10^23 m, or about 62.4 million light years, a light year being the distance light can travel in a year.  For perspective, light is fast enough to travel around the Earth 7 ½ times in a single second, so M88’s distance from us is nearly incomprehensible.  It is located in the Virgo Cluster, which is a cluster of some 2,000 galaxies whose center is approximately 50,000 kpc away from us, and which appears in the sky in the constellation Virgo.  The Virgo Cluster is the central cluster of the larger Virgo Supercluster, of which our Local Group (our local galaxy cluster) is an outlying member.  So M88, despite how tremendous its distance from us is, is actually very close considering the scale of the entire universe.

M88 was one of the first galaxies discovered, found by Charles Messier on March 18, 1781, and it was one of the first spiral galaxies to be recognized as such, listed by Lord Reese as one of the 14 “spiral nebulae.”  It was called a spiral nebula because the existence of other galaxies were not known at the time, but rather were discovered some time later.  At an apparent magnitude of 9.52, it is also one of the brightest galaxies in the sky. 

Although M88 is a spiral galaxy, its spiral arms are vague.  However, there are two somewhat distinct blue arms branching out from what appears to be an intermediate ring around the core.  The arms are accented by orange, darker regions between the arms.  This is because the galaxy is rotating, and the orange regions are on the front of the arms, while the bluer regions are towards the back of the arms.  This pattern is caused by the dust in the front of the arms being compressed by the force of the arm’s rotation, creating denser dust regions that appear orange and darker in comparison. 

The arm branching off of the left of the core along the major axis ends fairly quickly, reaching the outer edge of the galactic disk in about a quarter of the galaxy’s circumference.  The other arm, however, is more elongated; starting from the right side of the core along the major axis, it is wound more tightly, and branches nearly halfway around the galaxy before it reaches the disk’s edge.  The elongation of the upper arm could be due to a number of things, although the most likely is that it is from M88’s interactions with other galaxies in the Virgo Cluster.

 

Colors and Structures:

M88 is beautiful to look at, but one might wonder what makes up its structure, and why it exhibits the colors that it does.  The galaxy appears mostly blue, with wisps of orange throughout, and it has an almost entirely orange core.  The blue color indicates predominantly young, hot stars, especially towards the edge of the galaxy’s disk. 

Closer to and in the bulge of M88, the mostly orange color could indicate the presence of many cooler and older stars, or it could be a sign of reddening due to dust extinction, which is far more likely.  To explain, longer wavelengths of light, like infrared light, is unaffected by dust, while optical light on the other hand is scattered by dust.  Blue light, since it has a smaller wavelength, is scattered more easily by dust than is red light, so regions with a high density of dust will appear redder.

Messier 088

To the left of the galactic core along the major axis and inside the ring, there is a darker region, and this darkness seems to be from an extremely dense dust cloud, which is blocking out most of the light from behind it. M88 has virtually no bulge, and its core is very small and compact, a more unusual trait for spiral galaxies.

The ring around the core is in an indeterminate state, somewhere between being a ring and not a ring.  Fully formed rings in spiral galaxies have very little between them and the cores, and the spiral arms branch directly off of the ring.  While the arms branch off of the ring, there is a lot of dust between the ring and the core, thus its intermediate designation.

According to NED, the NASA/IPAC Extragalactic Database, M88’s classification is SA(rs)b.  The S means that it is indeed a spiral galaxy, the A means that it has no bar formation near the core, the rs means that it is intermediate between having and not having a ring around the core, as we have observed, and the b means that the spiral arms are wrapped moderately tightly around the core, again as we observed.

 

Multi-Wavelength Comparison:

M88 in midInfrared                M88 in microwave
M88 in Mid-Infrared (24 micron); Spitzer          M88 in Microwave (2.6 mm); NMA

The hottest of stars (O and B stars) emit ultraviolet, or UV, light, and when this light hits dust, it is absorbed by the dust, warming it to the point where it emits light of its own in the far infrared spectrum, with wavelengths around 12 to 100 microns.  When the UV light comes in contact with gas, it excites, or ionizes, the molecules in the gas, causing them to emit light in the mid-infrared range, or around a wavelength of 8 microns. 

When observed in the mid-infrared wavelength of 24 microns, M88’s appearance is very similar to its appearance in the optical wavelength.  This is due to the heating of dust grains from the UV emissions from the many hot, blue stars in the galaxy, causing the dust to re-emit brightly in the mid-infrared. 

If we compare the mid-infrared image to the optical image, we can see that the darkest region in the optical, the region to the left of the core and inside the ring, is the brightest region in the mid-infrared.  This suggests that this is a region of active star formation; the new, hot stars in this region are warming the dust around them through their UV emissions.

The image on the right shows M88 in the microwave part of the spectrum, at 2.6 mm wavelength.  This particular wavelength is emitted by hydrogen, from giant molecular clouds in the galaxy.  Giant molecular clouds are regions of cool molecular gas mostly composed of molecular hydrogen, and they have the potential to become future regions of star formation.  The microwave image shows that the highest concentration of giant molecular clouds is at the core of the galaxy and at the densest part of the ring around the core.  In this microwave image, one can see the less dense regions of the galaxy, which helps to more clearly see the shape of the spiral arms, which supports the observations made in the optical and infrared.

 

Calculations of Dust Extinction:

M88If you look at the region of the galaxy within the ring and outside the galactic core, you will notice a darker region to the left of the core. This region, as mentioned earlier, is darkened due to the dust grains reflecting and blocking optical light, especially blue light, causing it to appear redder. This effect is called reddening, and the process of the dust blocking the light is called extinction. For those more familiar with the technical aspects of galactic calculations, the following are calculations to determine the relative extinction of the dust region to the left of the galactic core as compared to the region the same distance away to the right of the core.

 

The equation for calculating the change in intensity due to dust extinction is m-mo = 2.5log(Io/I) = 2.5log(1/e) = 1.086τ, where m is the apparent magnitude of the region with greater extinction, mo is the apparent magnitude of the region with less extinction, Io is the intensity of the region without extinction, I is the intensity of the region with extinction, and τ is the optical depth of the dust region. The intensity of the light is a measure of its brightness, and the optical depth is a dimensionless parameter that measures how opaque the dust cloud is. Intensity can also be expressed as I = Ioe, and the optical depth can also be expressed as τ = σN/A = σNL, where σ is the cross sectional area of each dust grain causing scattering, generally assumed to be about 10^-9 cm^2. The dust cloud is treated as a column, and A is the area of that column, N is the total number of dust particles in the dust column, and L is the length of the dust column.

Using the computer program MaxIm DL 5, which allows for in-depth analysis of astronomical images, I determined that the pixel count dropped to 167 counts at the area of greatest extinction from 359 at the point opposite the point of greatest extinction. Plugging these two values in for the intensities in the first equation, I found that T =0.765. The extinction, A, equals 1.086T, so I found that the dust caused an extinction of 0.831 magnitudes relative to the other, less reddened area on the opposite side of the core. Further, I calculated the column density of the dust in the region with greater extinction, using the equation that column density = nL = T/σ. Plugging in the T value found above and the typical cross section for dust, I found that the column density of the dust cloud was about 7.65x10^7 particles/cm^2.

M88 dust profile

The chart above shows the number of pixel counts recorded in MaxIm across the core and through the regions of more and less dust extinction. The core is clearly visible as the large peak near the center of the graph, while the region of more extinction is the dip to the left of the peak at around x = 144. This is essentially an intensity chart, and it clearly shows the effects of dust extinction on the intensity of light in a region. The intensity of a galaxy without dust extinction would have a smooth, symmetrical distribution across the galaxy, and similarly the graph's shape would be symmetrical if no extinction was present, but the unevenness of the graph demonstrates that dust extinction is indeed occurring. One thing to note is that the graph shows the pixel count of the galaxy image with the background subtracted, meaning that background noise is removed from the situation so that the effects of dust extinction are clearly visible.


For the Eager Learner: A Scholarly Article

For individuals especially knowledgeable in galaxy dynamics, below is a summary of the article Pre-Peak Ram Pressure Stripping in the Virgo Cluster Spiral Galaxy NGC 4501 by B. Vollmer, M. Soida, A. Chung, J.H. van Gorkom, K. Otmianowska-Mazur, R. Beck, M. Urbanik, and J. D. P. Kenney, which discusses the pressure and velocity dynamics of M88, also known as NGC 4501.

This article discusses how the environment of a galaxy cluster, namely the Virgo Cluster, can influence galaxies within that cluster, namely M88. Galaxies in the Virgo Cluster are H1 deficient, meaning they have lost a significant amount of their interstellar medium (ISM) in their interactions with the surrounding galaxies. These interactions could, for example, help explain the elongated shape of M88's spiral arms. The authors basically argue that the gas in the galaxy's outer disk is being removed much faster than expected through the classic ram pressure criterion. This process of gas being lost at the outer edge of a spiral galaxy's disk via compression is called ram pressure. In the image below, we can see a sharp outer edge on an overdense region on the southwest side of the galaxy, which could be a sign of ram pressure compression.

M88 in H1

Surface Density Distribution of M88

Observations of M88 in other wavelengths, among them 6 cm radio continuum and the H band, confirm these observations, showing clearly defined, higher density regions to the southwest side of the galaxy, a condition in line with ram pressure compression. Faint Hα emissions can also be seen at the ridge of this dense gas region, which the authors argue is from nearly edge-on ram pressure stripping, which would generate enough energy to emit in Hα.

The presence and position of ram pressure stripping suggests that M88 is heading towards the center of the Virgo cluster, and is interacting with the gravitational forces of the surrounding galaxies, which causes the ram pressure compression to occur.


References:

Bendo, Geoge, ed. "Public to get access to spectacular infrared images of galaxies." The University of Manchester, School of Physics and AstronomyJodrell Bank Centre for Astrophysics, 26 Mar. 2012. Accessed 1 May 2013 http://meadeds114telescope.blogspot.com/2012/03/public-to-get-access-to-spectacular.html

Powell, Richard. "The Virgo Cluster." The Atlas of the Universe, 2006. Accessed 1 May 2013. <http://www.atlasoftheuniverse.com/galgrps/vir.html>.

Vollmer,B., M. Soida, A. Chung, J.H. van Gorkom, K. Otmianowska-Mazur, R. Beck, M. Urbanik, and J. D. P. Kenney. "Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501." 2008 Astronomy and Astrophysics 483, 89

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Right Ascension (J2000) 12:31:59.00
Declination (J2000) +14:25:11.00
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 5x300 seconds in C, 9x300 seconds in B, 5x300 seconds in V, 2x300 seconds in R
Date observed

March 25, 2013

Data Reduction:

For anyone interested, below is the process I followed to analyze my data images.

The basic parameters for my observations are listed above, including the filters the images were taken in, the length of the exposure in each filter, and the number of images taken in each filter. In order to calibrate the images, I first had to set the bias, dark, and flat for each filter.

The bias must have the same binning and camera temperature (within a few degrees) as the data image, and it is taken when the shutter is closed, so the filter does not matter. The exposure time is also unimportant, because the bias is taken with zero exposure time.

The dark must have the same binning and camera temperature (within a few degrees) as the data image, and the exposure time should be similar to the exposure time of the data image, although it does not have to be perfectly identical. The filter used is unimportant because the dark is taken when the shutter is closed.

The flat must have the same binning as the data image, although it can have a different exposure time or temperature than the data image. It is more important for flats to be taken in the same filter as the data image, although being closer to the same date is more important still.

After these three calibration images were set in MaxIm, I calibrated the data using the calibration data, and then combined the individual images from each filter together into one image. I made sure here to align stars in each image so that each filter's composite was accurate. I then combined the composite images from each filter together, and set the color balance for the image. The goal was to try to make the background stars appear mostly white in color, and I found that the best balance was Red 6, Green 9, Blue 50.

I then refined the brightness scale for the image, which allowed me to better display the faint regions of the galaxy without washing out the bright regions. In the screen stretch window, I tinkered with the values that displayed the most detail in M88, and settled on a minimum of 6245 and a maximum of 8503, with a gamma value of 0.5.

I then saturated the image, setting the saturation at 600% with a Min of Values between 40, meaning only pixels with 40 counts or more would be displayed.

Next, I used a tool called unsharp mask to sharpen up the edges and details of the galaxy, and settled at a cutoff of 5%, a mask weight of 50%, a feather distance of 20, and a reduce radius by value of 0. I also set it to display pixels with values between 70 and 30,000 counts.

Finally, I cropped the image, decreasing its width from 2184x1472 to 464x471 with a x offset of 830 and a y offset of 497.

 

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