At a distance of 50 million light years from earth, there spins the lovely NGC 4244, a 66,000 light year long spiral galaxy known as the Silver Needle. It is an edge-on, spiral galaxy, which we see from an angle, causing it to resemble a needle, suspended in the blackness of space. Spiral galaxies are a large collection of stars, gas, dust, and other matter, that rotate around a central, extremely dense region. Often, they develop a central bulge and dense regions of star formation which resemble rotational arms.
NGC 4244, being an edge-on galaxy, is a slightly difficult to characterize by eye. We can clearly see its bulge, and the central disk that gives it its signature needle look. The NASA/IPAC Extragalactic Database (NED) has classified NGC 4244 as an SA(s)cd galaxy. The S indicates the the galaxy is a spiral, characterized by its rotational dynamics. The A classifies galaxies that do not have a bar running through the central bulge. We can clearly see this in the optical image; the bulge is very small.. (s) galaxies are purely spiral, and do not have any extra ring like structures. Lastly, cd spirals rotate more slowly than a or b type spirals, allowing for a an open bulge/arm structure. cd galaxies tend to have a smaller central bulge. These types of galaxy are less massive then their lower lettered brothers and sisters, and hence have been forming stars recently, in the later periods of their lives.
The predominant blue light, seen in the outer regions of the galaxy seems to indicate that NGC 4244 is actively forming stars. This however has been shown not to be the case. It seems that active star formation may have just recently ceased. The blue light being radiated in the galaxy is radiated from young stars, which indicate that star formation has taken place in the recent past.
The inner regions of the galaxy are radiating much redder optical light, indicating that there is a much greater population of older, low mass stars in this region. They have probably been drawn inward gravitationally, and they now dominate the light of these regions, as the higher mass stars have since burned out.
As you continue to look even further towards the core of the galaxy, there is a sharp, bright object at the very center of the galaxy. This object has been observed to be a central nuclear stellar cluster. Nuclear star clusters have formed due to the swift rotations of the inner regions near a black hole at the core of the galaxy. They tend to be flat clusters, morphing to the disk of their parent galaxy. The nuclear cluster in NGC 4244 is extremely small compared to most central clusters in spiral galaxies. However, the rotation of this galaxy, and the clear flattening of this particular cluster, make it a unique case. We are seeing, very clearly, the inner stars of a galaxy morph to the domineering forces of a central black hole. These stars are spun at extremely high rotational velocities, and the cluster is flattened like a pancake around the deep gravitational well at their center.
There are no other galaxies in the field of view that we have observed, though other observations have found that NGC 4244 is part of the small CVn I or B4 group of 21 galaxies.
Beyond the Optical
To further understand NGC 4244, we need to explore it in wavelengths outside of the optical range. We have studies images at two other frequencies in order to better understand this galaxy.
The first image was taken in radio by the Very Large Array. It is an image of the HI regions of the galaxy, which radiate at a 21cm wavelength. HI regions are regions of neutral atomic hydrogen. This form of hydrogen seems to be very well distributed throughout the galaxy, however it is most prominent in the thin disk spanning the center of the galaxy. This shows us that the galaxy is fairly dominated by gas. HI gas does not indicate star formation, and it is known that there is very little star forming molecular hydrogen gas in the NGC 4244 (Olling et al.). Gases which would indicate the formation of stars seems to have been used up. We are currently seeing NGC 4244 as a galaxy in which star formation has recently ceased, leaving only this large amount of neutral hydrogen gas to fill the spaces between the newly formed stars.
The other interstellar mass we are able to detect comes in the form of dust. Most all spiral galaxies will contain a good amount of dust. Thankfully, 21cm radio waves travel right through dust clouds, allowing us to view gas which would otherwise be obscured. Now however, we want to view the dust directly. To do so we must switch our lens to an infrared telescope. This second non-optical image is taken by Spitzer at a wavelength 24 um. It shows us that most of the dust in the galaxy is gathered in the thin disk extending through the center of the galaxy. This is the same disk in which the neutral atomic hydrogen is located. However the dust is mostly restricted to this area, and is not as prominent in the outer region of the galaxy.
Peculiarities of NGC 4244
This edge-on spiral seemed like a case which the astronomical community was curiously interested in. To learn more, there is no better place to start than in professional literature. There was a particularly interesting paper out of Columbia University in 1996 by Rob P. Olling, “NGC 4244: A Low Mass Galaxy With a Falling Rotation Curve and a Flaring Gas Layer”. This paper contains a wealth of information regarding the rotational dynamics of NGC 4244.
In this paper, Olling continues our discussion of the gas in this galaxy. He notes that, as we can see in the 21cm image, the gas in NGC 4244 is very symmetric. This symmetry gives astronomers a very concrete way to study the rotational motion of this edge-on spiral. The technique used by Olling to derive the rotational dynamics of the great Silver Needle involves measuring how the thickness of a galaxy’s gas layers varies with the distance the gas is located from the center. Through these measurements, they are able to find the shape of the dark matter halo which is so dominating in galaxy dynamics. Olling took this method and applied it directly to NGC 4244. He has measured that the galaxy as a whole is rotating at a speed of around 100 km/s at a distance of 5 kpc from the galactic center. This speed remains consistent until a distance of 10 kpc, but, unlike most spiral galaxies, it decreases from this point out. Olling has deduced, from the structure of the HI gas, that there is a region of swift, uniform rotation between 5 and 10 kpc. These dynamics have helped him better understand the dark matter halo of NGC 4244. It is peaks in the center like most galaxies, however it is unusually flat for a large range of radii in the central regions of the galaxy. It then tapers off slowly as would be expected.
We have learned, from three different wavelengths of light, that NGC 4244 is a galaxy in which recent star formation has taken place in its outer regions, while old, red stars dominate the more central regions. This is very standard for spiral galaxies in general. Unique to this galaxy, however, is the tiny nuclear star cluster at the center, rotating around a super-dense black hole. We have found that the gas of the cluster is primarily neutral atomic hydrogen, which is spread fairly thoroughly throughout the galaxy, but is very dense inside of the thin disk spanning length the the needle. Also packed into this thin disk is most of the galaxy’s dust. Amongst all this, our picture of NGC 2422 is not complete without considering the dark matter halo within which it resides. Most of the matter in any spiral, including our NGC 2422, is dark matter. This matter extends far out beyond the boundaries which we can see with out eyes and cameras. We can detect dark matter only by its gravitational effects on the baryonic matter we are able to directly observe.
Braun, R., “Resolved atomic super-clouds in spiral galaxies.” Astronomy and Astrophysics Supplement 114, 409 (1995)
Dale, D. A., Cohen, S. A., Johnson, L. C., Schuster, M. D., Calzetti, D., Engelbracht, C. W., Gil de Paz, A., Kennicutt, R. C., Lee, J. C., Begum, A., Block, M., Dalcanton, J. J., Funes, J. G., Gordon, K. D., Johnson, B. D., Marble, A. R., Sakai, S., Skillman, E. D., van Zee, L., Walter, F., Weisz, D. R., Williams, B., Wu, S.-Y., Wu, Y., “The Spitzer Local Volume Legacy: Survey Description and Infrared Photometry”, The Astrophysical Journal 703, 517 (2009)
Kutner, Marc L. Astronomy: A Physical Perspective, 2nd ed. Cambridge: Cambridge University Press, 2003.
Olling, Rob J., "NGC 4244: A Low Mass Galaxy With a Falling Rotation Curve and a Flaring Gas Layer " Astrophysical Journal 112, 457 (1996).
Seth, Anil C., Blum, Robert D., Bastian, Nate, Caldwell, Nelson, Debattista, Victor P., Puzia, Thomas H., "A Rotating Compact Nuclear Stellar Cluster in NGC 4244" Astrophysical Journal 687, 997 (2008).
Seth, Anil, Neumayer, Nadine, “Nuclear Star Clusters and Black Holes” Web. Accessed May 2011.
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)||12:17:30|
|Filters used||blue(B), green(V), red(R), and clear(C)|
|Exposure time per filter||11x300 seconds in C, 10x300 seconds in B, 3x300 seconds in V, 2x300 seconds in R|
March 3 and 4, 2011 (CBVR)
The images produced by Calvin College on this page were reduced using Maxim. We used standard bias, dark, and flat correction methods. The images from each filter were alligned via star matching and then sigma clip combined to create composite images. These composite images were then color combined with the a color ratio of 1 red (R) to 1.5 green (V) to 7 blue (B). The newly combined color image was then slightly saturated to more richly show the colors within the galaxy.