4 Counterparts at other wavelengths

We performed a search for possible counterparts at other wavelengths using available online databases (NED and Simbad). Results from a more extensive search are beyond the scope of the present paper and will be presented in a future work.

We find a point source in the online 2MASS catalog at 1.72'' from NGC 4168-ULX1. Magnitudes are $16.8\pm 0.2$, $16.1\pm 0.1$, and $15.3\pm 0.1$ in the J, H, and K bands, respectively. ULX1 in NGC 4168 also has a possible counterpart, 2.7'' away (EO1385-0040047), detected with the Automatic Plate Measuring Machine. In this case, the magnitude in the R band is 17.56, while in the B band it is 19.76.

At $\sim$10'' from NGC 4565-ULX3, we find the dust cloud NGC 4565-D-064-016 (Howk & Savage 1999). The cloud has dimensions $210\times 400$ pc and a column density $N_{\rm H}>2\times 10^{21}$ cm^-2. At the distance of 9.7 Mpc, the angular dimensions are $4.5'' \times 8.5''$, so that this association could be possible.

Inside the error box of NGC 4565-ULX4, we find two objects: one is the planetary nebula NGC 4565-19, separated by 8.4'' (Jacoby et al. 1996), and the other is the globular cluster KAZF 4565-7, separated by 8.6'' (Kissler-Patig et al. 1999).

Both ULX in NGC 4639 are close to H II regions (see Evans et al. 1996). NGC 4639-ULX1 is close to the region NGC 4639-07 (9.7'' angular separation), while NGC 4639-ULX2 is near NGC 4639-64 (2.2'') and NGC 4639-81 (8.2''). The angular areas of these H II regions are 3.9 arcsec², 1.4 arcsec², 2.5 arcsec², respectively.

For NGC 4698-ULX1 we found radio counterparts with the VLA at 6 cm, with an approximate flux density of 0.8 mJy (Ho & Ulvestad 2001). Also in the optical band there are counterparts detected with the DSS (EO0041-0230928) and by the Hubble Space Telescope. A more detailed study on this source is in preparation (Foschini et al., in preparation).