Revealing the Double Nucleus of NGC 4490
  NGC 4490/85 (UGC 7651/48) or Arp 269 is well known for being one of the
closest interacting/merging galactic systems. NGC 4490 has a high star
formation rate (SFR) and is surrounded by an enormous HI feature stretching
about 60 kpc north and south of the optically visible galaxies. Both the driver
for the high SFR in NGC 4490 and the formation mechanism of the HI structure
are puzzling aspects of this system. We have used mid-infrared Spitzer data to
show that NGC 4490 has a double nucleus morphology. One nucleus is visible in
the optical, while the other is only visible at infrared and radio wavelengths.
We find the optical nucleus and the potential infrared visible nucleus have
similar sizes, masses, and luminosities. Both are comparable in mass and
luminosity to other nuclei found in interacting galaxy pairs and much more
massive and luminous compared with typical non-nuclear star-forming complexes.
We examine possible origin scenarios for the infrared feature, and conclude
that it is likely that NGC 4490 is itself a merger remnant, which is now
interacting with NGC 4485. This earlier encounter provides both a possible
driver for extended star formation in NGC 4490, and multiple pathways for the
formation of the extended HI plume.
