Bars from the Inside Out: An HST Study of their Dusty Circumnuclear Regions

Martini, P. 2004, to appear in "Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning Fork strikes a New Note" held June 7-12th, 2004 in Pilanesberg National Park, South Africa.

The results of bar-driven mass inflow are directly observable in high-resolution HST observations of their circumnuclear regions. These observations reveal a wealth of structures dominated by dust lanes, often with a spiral-like morphology, and recent star formation. Recent work has shown that some of these structures are correlated with the presence or absence of a bar. I extend this work with an investigation of circumnuclear morphology as a function of bar strength for a sample of 48 galaxies with both measured bar strengths and ``structure maps'' computed from HST images. The structure maps for these galaxies, which have projected spatial resolutions of 2 -- 15 pc, show that the fraction of galaxies with grand-design (GD) circumnuclear dust spirals increases significantly with bar strength, while tightly wound dust spirals are only present in the most axisymmetric galaxies. In the subset of galaxies classified SB(s), SB(rs), or SB(r), GD structure is only found at the centers of SB(s) or SB(rs) galaxies, and not SB(r). Bar strength measurements of 45 SB(s), SB(rs), and SB(r) galaxies show that SB(s) galaxies have the strongest bars, while SB(r) galaxies have the weakest bars. As SB(s) galaxies are also observed to most commonly possess dust lanes along their leading edges, this is further support of a connection between GD structure and bar-driven inflow on larger scales. There is also a modest increase in the fraction of loosely wound dust spirals at later morphological types, and a corresponding decrease in the fraction of chaotic structures. This trend may reflect an increase in the fraction of galaxies with circumnuclear, gaseous disks. The trend appears to reverse at type Scd, where the fraction of galaxies with chaotic circumnuclear dust structure increases dramatically, although these data are of poorer quality.