SPATIAL-DISTRIBUTION OF LOW-SURFACE-BRIGHTNESS GALAXIES

The spatial distribution of low-surface-brightness (LSB) galaxies is important both as a test of theories of large-scale structure formation and for the physical understanding of the environmental effects that influence the evolution of galaxies. In this paper we calculate, using redshift samples, the cross-correlation functions [xi(AB)(r)] of LSB galaxies with normal galaxies in complete samples (i.e. CfA and IRAS). This enables us to compare directly the amplitudes and shapes of the correlation functions for LSB galaxies with those for CfA and IRAS galaxies. For pair separations r greater than or similar to 2 h-1 Mpc, we find xi(AB)(r) is-proportional-to r-gamma with gamma almost-equal-to 1.7. This shape of xi(AB) is in agreement with that of the correlation functions for other galaxies. The amplitudes (A) of xi(AB)(r) are lower than those of the autocorrelation functions for the CfA and IRAS samples, with A(LSB-CfA):A(CfA-CfA) almost-equal-to 0.4 and A(LSB-IRAS):A(IRAS-IRAS) almost-equal-to 0.6. These results suggest that LSB galaxies are embedded in the same large-scale structure as other galaxies, but are less strongly clustered. This offers the hope that LSB galaxies may be unbiased tracers of the mass density on large scales. For r congruent-to 2 h-1 Mpc, the cross-correlation functions are significantly lower than that expected from the extrapolation of xi(AB) on larger scales, showing that the formation and survival of LSB galaxies may be inhibited by interaction with neighbouring galaxies. We show that a simple hierarchical model, in which LSB galaxies are formed only in haloes lacking close interactions with other haloes, reproduces both the deficit of pairs at small separations and the low amplitude of the correlation function. This model suggests that LSB galaxies should, on average, be younger than normal galaxies. consistent with direct observations. The model also suggests that a strong luminosity (mass) segregation in galaxy clustering is not a necessary consequence of biased galaxy formation, unless the effect of surface brightness (collapse time) is taken into account. It is also possible that a significant fraction of the mass density of the Universe resides in galaxies that have not been observed because of their low surface brightness.