A spectroscopic analysis of the starburst galaxies NGC 3395 and NGC 3396.

摘要

We have obtained ultraviolet and visible wavelength spectra of 31 bright star forming knots in the interacting galaxies NGC 3395 and NGC 3396 using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. The knots are possible super star clusters on the order of ∼100 pc diameter with measured metallicities on the order of 0.5–0.6 $Z\odot$. The spectra are consistent with a massive production of hot young stars in a starburst.; Ages of the starburst knots were calculated using several diagnostics from the Leitherer et al. Starburst 99 code (SB99) using an Initial Mass Function (IMF) with a power law coefficient α = 2.35 and an upper mass limit of 100 $M\odot$. We modeled our star forming knots as instantaneous starbursts with the measured metallicity and we obtained consistent and reasonable estimates of the starburst age. The UV-brightest knots are ∼5 Myr old in both galaxies. We found no age gradient in the galaxies implying the starburst does not propagate across the galaxy but rather occurs simultaneously everywhere. The data are also consistent with the interpretation that the starburst is not only happening more or less simultaneously within each galaxy, it is also occurring simultaneously in both galaxies. If true, the fact that it is occurring simultaneously in both galaxies gives credence to the interaction being the source of the star formation in line with current theory.; While our starforming knots were spatially resolved, at high redshift one cannot resolve individual knots and instead has to rely on spatially unresolved spectra. To assess the representativeness of these spectra of the underlying structure, we simulated the spectra one would observe by defining the entire portion of each galaxy observed as an unresolved knot. We found the metallicities for the unresolved knots were very representative of the resolved knots that made them up. We also found that the ages we derived for the unresolved knots were representative of the underlying resolved knot ages to approximately a factor of 2–3.; We measured the flux in the broad He II λ4686 emission line characteristic of Wolf-Rayet (WR) stars. Uncorrected for reddening, we estimated ∼250 WR and ∼1100 O stars in NGC 3395 and ∼400 WR and ∼2500 O stars in NGC 3396. We were unable to subcategorize the WR stars, although the spectra do have some characteristics of WN stars.; We discovered a super bubble in NGC 3395 with a radius of approximately 24 pc containing ∼80 WR stars uncorrected for reddening. We calculate that it also contains ∼25 O stars although we suspect that we have severely underestimated this value and a more representative value may be as many as 420 O stars uncorrected for reddening.