Is NGC 3108 transforming itself from an early- to late-type galaxy – an astronomical hermaphrodite?

Abstract

A common feature of hierarchical galaxy formation models is the process of ‘inverse’ morphological transformation: a bulge dominated galaxy accretes a gas disc, dramatically reducing the system's bulge-to-disc mass ratio. During their formation, present-day galaxies may execute many such cycles across the Hubble diagram. A good candidate for such a ‘hermaphrodite’ galaxy is NGC 3108: a dust-lane early-type galaxy which has a large amount of H i gas distributed in a large-scale disc. We present narrow-band Hα and R-band imaging, and compare the results with the H i distribution from the literature. The emission is in two components: a nuclear bar and an extended disc component which coincides with the H i distribution. This suggests that a stellar disc is currently being formed out of the H i gas. The spatial distributions of the Hα and H i emission and the H ii regions are consistent with a barred spiral structure, extending some 20 kpc in radius. We measure an extinction-corrected star formation rate (SFR) of 0.42 M⊙ yr−1. The luminosity function of the H ii regions is similar to other spiral galaxies, with a power-law index of −2.1, suggesting that the star formation mechanism is similar to other spiral galaxies. We measured the current disc mass and find that it is too massive to have been formed by the current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was higher in the past. With the current SFR, the disc in NGC 3108 will grow to be ∼6.2 × 109M⊙ in stellar mass within the next 5.5 Gyr. While this is substantial, the disc will be insignificant compared with the large bulge mass: the final stellar mass disc-to-bulge ratio will be ∼0.02. NGC 3108 will fail to transform into anything resembling a spiral without a boost in the SFR and additional supply of gas.