The specific angular momenta of superthin galaxies: Cue to their origin?

Abstract

Superthin galaxies are low surface brightness (LSB) bulgeless disc galaxies having stellar discs with unusually high planar-to-vertical axes ratio $b/a$ $>$ 10 - 20, the formation and evolution of which is not well-understood. We calculate the specific angular momenta of a sample of six superthins and nine other bulgeless LSBs using stellar photometry, atomic hydrogen (HI) surface density and high resolution HI rotation curves available in the literature. We find that the stellar specific angular mometum $j_{\rm{s}}$, and hence the stellar disc size given by the exponential stellar disc scale length R$_D$, of three superthins and seven LSBs lie above the 95.4 $\%$ confidence band of the $j_{\rm{s}}$ - $V_{\rm{rot}}$ regression line for ordinary bulgeless disc galaxies, $V_{\rm{rot}}$ being the asymptotic rotational velocity. Further, we find that superthins and LSBs have higher $j_{\rm{s}}$ and R$_D$ values for a given value of stellar mass $M_{\rm{s}}$ at high values of statistical significance, compared to ordinary disc galaxies. Therefore, we conclude, a superthin is may be distinguished by a characteristically larger disc size which could possibly explain the origin of its large planar-to-vertical axes ratio. Interestingly, we find that the median spin parameter $\lambda = \frac{ j_{\rm{stars}} } { {\sqrt(2) V_{\rm{vir}} R_{\rm{vir}} } }$, $V_{\rm{vir}}$ and $R_{\rm{vir}}$ being the virial velocity and virial radius of the galaxy respectively, is 0.13 $\pm$ 0.01 for superthin galaxies which is an order of magnitude higher than those of LSBs and ordinary disc galaxies, which may have important implications for the existence of superthin stellar discs in these low surface brightness galaxies.