The Stellar Content of the Southern Tail of NGC 4038/9 and a Revised Distance

Saviane, I. (ESO), Hibbard, J.E. (NRAO), & Rich, R.M.R. (UCLA)
2004, The Astronomical Journal, 127, 660 (pdf 2.4MB) (ADS) (astro-ph/0311200)

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/9, the Antennae. We resolve individual stars, and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2-100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I=26.5+/-0.2 from which we derive a distance modulus (m-M)=30.7+\-0.25. The implied distance of 13.8+/-1.7 Mpc is nearly a factor of two closer than commonly quoted distances for NGC 4038/9. In contrast to the previously studied core of the merger, we find no super star clusters. One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of O-B star associations seen in dwarf irregular galaxies. The youngest population has a total stellar mass of ~ 2E5 Mo, while the old population has a stellar mass of ~7E7 Mo. If our smaller distance modulus is correct, it has far-reaching consequences for this proto-typical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central super star clusters have sizes typical of galactic globular clusters rather than being twice as large, and the unusually luminous X-ray population becomes both less luminous and less populous.

Don't find that convincing? Then try this:

A New Red Giant-based Distance Modulus of 13.3 Mpc to the Antennae Galaxies and Its Consequences

Saviane, I. (ESO), Momany, Y. (AO, Padova), Da Costa (RSAA), G. S., Rich, R. M. R. (UCLA) & Hibbard, J.E. (NRAO)
2008, The Astrophysical Journal, 678, 179 (ADS) (astro-ph/0802.1045)

The Antennae galaxies are the closest example of an ongoing major galaxy merger, and thereby represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultra-luminous X-ray sources, super-stellar clusters, etc). In a previous paper HST/WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST/ACS imaging that resolves the composite stellar populations, and most importantly, reveals a well-defined red giant branch. The tip of this red giant branch (TRGB) is unambiguously detected at Io(TRGB)=26.65 +/- 0.09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of (m-M)o= 30.62 +/- 0.17 corresponding to a distance of 13.3 +/- 1.0 Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well determined shorter distance.

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