The elliptical galaxy NGC 6109 was observed in the Sloan Digital Sky Survey DR6 in the ugriz filters. We measured the magnitude as it changed along the semi-major axis, henceforth referred to as radius, and created surface brightness profiles in a ll band passes. We then attempted to fit these profiles of our galaxy with the Sersic, de Vaucouleurs, and exponential models. Color profiles of NGC 6109 were then created in four colors (u-g, g-r, r-i, i-z) to examine the co lor gradient of the galaxy. As a final look at the color gradient, we also created a color composite image of the target field. Using these methods, we were able to see that the surface brightness profiles were best fit with an exponential model, as is common for small elliptical galaxies. We also saw that the color gradients indicated a gradient in stellar age, with older stars in the center and younger stars in the outer regions.
The Sersic profile can be described by
Color gradients in elliptical galaxies can be interpreted in two ways, as a gradient in metallicity, or in age. While astrophysicists have grappled with this so called ‘age-metallicity degeneracy,’ it is possible to disentangle the two. One way of doing this is to take measurements in colors that are known to be mostly sensitive to one of the two influencing parameters (Li & Han 2008). However, it is also thought that the cause of color gradients in elliptical galaxies could be determined by galactic s ize, with gradients in giant ellipticals due to metallicity, and gradients in dwarf ellipticals caused by an age gradient (Eunhyeuk, Lee & Geisler 2000).
Our object of interest, NGC 6109, is a small, faint galaxy of type E/S0 located in the constellation Corona Borealis. This galaxy is also known to be a strong source of radio waves. While few studies have focused on this galaxy alone, it has been includ ed in a number of wide surveys. Basic information can be found on this object, but information of this specific nature is not well known.
As we planned to create a color composite image, it was necessary to make sure each frame lined up precisely with the others. This was made possible with the xregister task which uses stars in each frame to determine how the image must be shifted to line up with a reference image. Once that these frames had been shifted it was necessary to trim the edges that did not overlap; this was accomplished with IRAF’s imcopy task.
To create a color image of our galaxy, NGC 6109, we used the export task in IRAF and told it to act on the g, r, and i images. Since we did not have data in a blue filter, we created a shifted-color image instead by mapping the i band image to red, the r band image to green, and the g band to blue. This resulted in a real color image of our galaxy shown in Figure 1.
For this project we planned to examine the surface brightness of NGC 6109 as a function of radius, as well as examine the color gradients of this galaxy. To fit a set if isophotes to the images, we used the ellipse task in IRAF; this task can be found in the isophote subdirectory of the stsdas’s analysis package. First we edited the ‘maxrit’ parameter set in geompar to 150 (pixels), which determines the maximum radius. We then edited the parameters for the ellipse program to work on a specific frame, a nd set it to interactive mode. When the program finished fitting ellipses, it output a table including both the radius of the ellipse (in pixels), and the flux of that isophote (in counts).
Once we had this information we multiplied each of the measurements of the radius by 0.4 to translate from pixels to arc seconds. Next it was necessary to calibrate our measured fluxes into actual ugriz magnitudes. To accomplish this, we first ne eded to zero-point correct our obtained flux measurements. This was done by dividing the total number of counts at a given radius by the number of counts for an object at 20th magnitude,
To create a surface brightness profile of the galaxy in each waveband, we plotted the calibrated magnitude of the galaxy in the relevant filter against the radius, in arc seconds. To see how these profiles compared to other small, faint elliptical galaxi es, we fit this data to Sersic profiles with n=2 and n=3, as well as to the de Vaucouleurs, and the exponential fit. We then plotted color profiles of NGC 6109 with each color, in magnitudes, plotted against the radius, in arc seconds.
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| Figure 1. 13.6’ x 9.86’ color composite image of NGC 6109 using gri bands. |
The surface brightness profiles, shown in Figure 2, indicate that the galaxy is brightest in the i band, and dimmest in the u band, as would be expected for a faint galaxy that is known to be a strong radio source. It is also obvious from t hat the surface brightness continues to increase into the center rather than flattening out over the central arc seconds.
We can also see an interesting feature in these radial profiles, particularly in the i band. At approximately 35 arc seconds from the galactic center, there is a slight increase in magnitude before it continues to decrease in the outer regions. T
his is not apparent in the u band as information in this band only exists out to approximately 25 arc seconds.
| Figure 2. Surface brightness profiles of NGC 6109 in ugriz with the magnitude plotted against the radius (in arcsec). It can be seen that this galaxy is brightest at long wavelengths and dimmer in shorter wavelengths. |
After plotting surface brightness profiles in ugriz, we made fits to the data with three different models, the Sersic, de Vaucouleurs, and the exponential. The agreement of these fits can be seen in the ugriz profiles in Figure 3, below. F
rom these fits it is apparent that the observed surface brightness profiles are best described by an exponential, as described above in Section 1.
| Figure 3. Surface brightness profile in u shown with magnitude plotted against radius (in arcsec). Also plotted are two Sersic profiles (n=2 and n=3), the de Vaucouleurs model, and an exponential fit. |
As can be seen in the color profiles of Figure 4, each color shows a negative gradient indicating that the galaxy becomes bluer with increasing radius. The exception to this trend is the r-i color which has an almost flat, but slightly positive gr
adient. This indicates that the magnitudes in r and i fall off at almost the same rate, with i falling off somewhat quicker.
| Figure 4. Color Profiles of NGC 6109 in u-g, g-r, r-i, and i-z. The magnitude is plotted against the radius (in arcsec) on a log scale and each gradient is shown with its linear fit. All gradients show that NGC 6109 ge ts increasingly bluer with radius, except for the r-i gradient, which indicates an increase in red light with increasing radius. |
All of the surface brightness profiles show that the surface brightness increases into the center, rather than reaching a plateau. This is to be expected as this behavior is typical of dim elliptical galaxies, (Sparke & Gallagher 2007). These profiles w ere best fit by an exponential curve, agreeing well for at least the inner 10 arc seconds. Again, according to Spark & Gallagher (2007), this sort of agreement is expected as faint ellipticals tend to fit better to an exponential curve than to either the Sersic or de Vaucouleurs models.
The color profiles we were able to measure show a slight, but observable negative gradient, with the exception of the flat or positive gradient shown in the r-i profile. Interpreting these as gradients in age, we can see that the older, redder sta rs are largely contained in the center regions of the galaxy, whereas the young, blue stars extend out farther from the center.