7 Clump properties

To investigate the clumps in more detail, we have performed photometry with POLYPHOT for the nuclear region, and APPHOT with radii of 20, 22, and 19 pixels for Clumps B-D. The results are shown in Table 5. For the [255-555] numbers, we have used the three-filter selected objects, while for the [555-814] set, we have used the two-filter selected set of objects.

We can immediately see that the nuclear region is the reddest in both colors. Regardless of the age of the latest burst in this region, we expect this to be the case from the color map. Clump B is bluest in [555-814] color, and, perhaps surprisingly, Clump C is bluest in [255-555] color.

We can use these measurements to estimate the efficiency of cluster formation by calculating the flux from clusters vs. the total flux from the region; these results are shown in Table 6. Measurements of the fraction of light from clusters at 2200 Å in a sample of starburst galaxies found an average of $\sim$$20\%$ (Meurer et al. 1995). The numbers here are similar, and range from $13 \%$ near I for the nuclear region, up to $33 \%$ for the mid-UV flux coming from clusters in Clump B. At F555W, the total flux in clusters compared to the total galaxy flux is $7 \%$.

 

 

Table 5: Photometric properties of the clumps.

Region	F255W	F555W	F814W	255-555	555-814
Nuclear	14.22	14.84	14.23	-0.62	0.61
Clump B	14.56	15.81	15.64	-1.25	0.17
Clump C	14.91	16.48	16.07	-1.57	0.41
Clump D	16.33	17.31	16.89	-0.98	0.42

 

 

Table 6: Fraction of flux from clusters vs. total flux.

Region	F255W	F555W	F814W
Nuclear	0.16	0.14	0.13
Clump B	0.33	0.22	0.20
Clump C	0.22	0.26	0.26
Clump D	....	0.16	0.17

The F255W filter gives us information about the stellar emission only, and primarily about the young, massive stars which dominate at this wavelength. One consequence of this is that a measurement of the fraction of the F255W flux coming from the clusters compared to the overall flux gives a direct measurement of the massive star formation occurring below our cluster detection limits. In contrast, the F555W and F814W filters tell us about the combination of stellar and nebular emission.

Clump B has $33 \%$ of its UV emission coming from only 8 clusters, but this number drops to $22 \%$ of visual light coming from 14 clusters, and $20\%$ at F814W. Clump C has $22 \%$ of its UV emission from 12 clusters, but this number rises to $26 \%$ at F555W and F814W with a total of 18 clusters. We interpret this as evidence for an underlying stellar population in Clump B, which is absent in Clump C. Although Clump B is a strong H$\alpha$ source, we can rule out nebular emission not associated with the clusters, as this would make the [255-555] and [555-814] colors too blue and too red, respectively.