From B. J. Smith, C. Struck, P. N. Appleton, V. Charmandaris, W. Reach, and J. J. Eitter (2005), Astronomical Journal, 130, 2117.

An broadband optical R band image of Arp 107 from the Fick Observatory in Boone Iowa. Note the strong primary arm/partial ring in the south, the tail to the northwest, the bridge, and the plume to the northeast of the companion.

An continuum-subtracted H-alpha image of Arp 107 from the Fick Observatory. Note the clumps of star formation in the ring. The artifacts near the companion are due to imperfect continuum-subtraction.

Spitzer images of Arp 107 (UGC 5984). North is up and east to the left. Left to right, top to bottom: 1) IRAC band 1 (3.6 microns), 2) IRAC band 2 (4.5 microns), 3) IRAC band 3 (5.8 microns), 4) IRAC band 4 (8.0 microns), 5) MIPS 24 microns. The field of view is the same as in the optical images shown above. The morphology of this system appears very different at these three wavelengths. The bridge is clear at 3.6 microns but almost disappears at 8.0 microns. The knots in the ring are more prominent at 8.0 microns than at 3.6 microns. The elliptical companion is very faint at 24 microns but the ring is clearly visible.

In the 8 micron image, 29 clumps were identified by eye in the vicinity of Arp 107. These are identified in this color view of the 8 micron image, encircled by 5" radii regions. Note that some fainter clumps are not included. Also, in some cases, when a second fainter clump was very close to a brighter clump, so that their regions strongly overlapped, the second clump was not included. The regions are numbered by increasing declination, with the labels being directly above the respective region.

The IRAC colors of the brightest 8 micron clumps in Arp 107 (black crosses). These clumps are labeled. This plot also contains the colors of M0III stars (open dark blue square) (from M. Cohen 2005, private communication) and the mean colors for the field stars in Whitney et al. (2004) (magenta triangle). The green diamonds mark the predicted values for the Li and Draine (2001) dust models, for a very large range of interstellar radiation field intensities. The red circles show the locations of the Sloan Digitized Sky Survey quasars in the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) Elais N1 field (Hatziminaoglou et al. 2005). These quasars have redshifts between 0.5 and 3.65; since their spectral energy distributions are power laws, their infrared colors do not vary much with redshift.

The photometry was done using the IRAF daophot routine, with 4.1 pixel (5") radii apertures (as shown in the above figure). The sky was determined using the mode in an annulus surrounding the source, with an inner radius of 5 pixels (6") and an outer radius of 10 pixels (12"). The errorbars include both statistical uncertainties and an uncertainty in the colors due to varying the sky annuli (see paper). No color corrections were made. Aperture corrections were included, although they have only a tiny effect on the IRAC colors of these clumps. As given in the IRAC Data Manual, the aperture corrections are 1.06, 1.06, 1.07, and 1.09 for IRAC bands 1 - 4, thus the color change due to the aperture correction are very small.

Notes on the colors:

• 1. The colors of Source #23 (the bright source to the west of the galaxy) are similar to those of the quasars. Thus it is likely a background quasar, unassociated with Arp 107.
• 2. The nucleus of the elliptical companion, source #28, is very blue, with colors like that of M0III stars at all wavelengths. Thus it contains almost all old stars and little ISM. This is consistent with its absorption-line optical spectrum (W. Keel 2005, private communication).
• 3. The nucleus of the ring galaxy, #14, is also quite blue, so also has an old stellar population, but not as old as #28. It clearly has some interstellar matter, as shown by its large [5.8] - [8.0] color.
• 4. Source #9, a concentration inside the radius of the ring, also has colors indicative of an old star, suggesting it is a foreground star. This is confirmed by its optical spectrum, which is that of an M1-2V star (W. Keel 2005, private communication).
• 5. The clumps in the upper part of the second plot have [5.8] - [8.0] colors close to that predicted by the Li and Draine (2001) dust model, indicating these wavelengths are dominated by dust. But the [4.5] - [5.8] colors are much bluer, indicating stellar contributions at [4.5] microns.
• 6. In the [4.5] - [5.8] vs. [5.8] - [8.0] plot, there is a sequence of sources at an almost constant [5.8] - [8.0] ~ 2, with increasing [4.5] - [5.8] micron colors. This sequence from left to right is a sequence in increasing ratio of young stars/old stars.
• 7. Note that this sequence, #17, #4, #5, #7, #10, #21, and #26 is the order these clumps appear in the ring, going in a counterclockwise direction from #17 to #26. The northern part of the ring (clumps #21 and #26) is younger than the southern part (#4 and #5). This trend is shown better in the plot of position angle vs. color shown below.

Plots of position angle vs. Spitzer IRAC colors for the clumps in the ring. As noted above, there is a strong azimuthal trend for the [4.5] - [5.8] color. No trend is visible in the [3.6] - [4.5] color. For the [5.8] - [8.0] color, there is a slight variation (0.2 magnitudes) compared to that in the [4.5] - [5.8] color (1 magnitude). The lack of a strong variation in the [5.8] - [8.0] ratio suggests that these two bands are dominated by dust emission, and the dust spectrum does not vary strongly around the ring. This plot shows that the large variation in the [3.6] - [8.0] color around the ring is mainly due to variations in [4.5] - [5.8]. Clumps #20 and #22 are not shown in the second and third panels since their 5.8 micron uncertainties are very large.

These plots show the color-color diagrams of the fainter Arp 107 clumps. Note the positions of:

• 1. Source #19, which is located off the western side of the galaxy near source #23 (see the finding chart above). The colors of this source are also similar to those of quasars, thus it also is probably a background quasar, like source #23.
• 2. Sources #1, #2, and #3, which are outside the ring to the south, source #12 to the west of the ring, and sources #27 in the bridge, #29 at the northern tip of the tail, and source #25, to the northwest of the ring. These sources have [3.6] - [4.5] and [4.5] - [5.8] colors like those of old stars, but have excess emission in the 8.0 micron band compared to old stars (that is, reddish [5.8] - [8.0] color). This suggests these are either very old star formation regions associated with the galaxy, with some PAH (polycyclic aromatic hydrocarbon) emission but evolved stars, or they are foreground stars, and dust emission from the galaxy contributes at 8 microns.

  
  The [5.8] - [8.0] vs. [8.0] - [24] colors for the clumps. The clumps (black crosses) are labeled to the upper left of the corresponding datapoint. The green diamonds are the locations of the Li and Draine (2001) dust models, with various interstellar radiation fields scaled to the solar neighborhood radiation field. The red circles are the locations of the Hatziminaoglou et al. (2005) quasars. Stellar photospheres are expected to lie near [5.8] - [8.0] ~ 0, [8.0] - [24] ~ 0.

  The 24 micron fluxes were measured within a 5" radius aperture, and a background annulus with an inner radius of 10" and an outer radius of 15". The errorbars include both the statistical uncertainties, and a term due to varying the background annuli (see text). The 5" radius aperture used in the photometry is too small to include all of the light from the clump, given the ~6" FWHM PSF for MIPS 24 micron observations and the 2.45"/pixel plate scale. Using a larger aperture radius, however, is not a viable option, since the clumps are crowded. Thus a substantial aperture correction is required.

  According to the MIPS Data Handbook, a 5" aperture includes only ~50 percent of the total light from a point source. Testing this using large aperture data for the isolated source #23 (which may be a background quasar) and two bright sources in the field far from the galaxy, we get consistent aperture corrections. Thus we have adjusted the [8.0] - [24] colors using this correction. Including the 8.0 micron aperture correction given above, the aperture corrections shift the data points upwards on this plot by 0.65 magnitudes in [5.8] - [8.0] color. This plot includes this correction.

  No color corrections were included in determining these colors.

  Note:

• As noted above for the IRAC colors, the [8.0] - [24] color of Source #23 is similar to those of the quasars. This further supports the hypothesis that this is a background quasar.
• As expected, the foreground star #9 has colors like a star.
• The nucleus of the companion shows a small excess relative to the expected [8.0] - [24] color of stars (~0.0), thus there may be some cold dust associated with this elliptical.
• The Seyfert nucleus, clump #14, has colors similar to a quasar, with very red [8.0] - [24] colors.
• The [8.0] - [24] micron colors of the brightest ring clumps, #10, #21 and #26 (and the other bright ring clumps) are similar to those for standard dust models with relatively weak radiation fields (approximately like the solar neighborhood). This is consistent with their relatively low observed mid-infrared surface brightness (see paper).
• The clumps in the southern part of the ring, #4, #5, #6, and #7, have [5.8] - [8.0] colors like those of the dust models, but somewhat redder [8.0] - [24] colors, thus appear to have weaker interstellar radiation fields.

The [8.0] - [24] micron color of the ring clumps, as a function of position angle. As noted above, the clumps in the southern part of of the ring have slightly bluer [8.0] - [24] colors than the bright clumps in the north. Thus the northern/western clumps may have stronger interstellar radiation fields.

For the clumps in Arp 107, the ratio of the H-alpha fluxes to those in the Spitzer 3.6 and 8.0 micron bands, compared to the 8.0 micron magnitudes.

The ratio of H-alpha to 3.6 micron, and the ratio of H-alpha to 8 microns, vs. position angle for clumps in the arm/ring. Note the azimuthal trend. This trend may be caused by extinction of the H-alpha emission; alternatively, it may be caused by PAH excitation by non-ionizing photons.

To see more Spitzer results on interacting galaxies, click here.

Last updated 10/31/05 by B. J. Smith.