All Figures

Figure 1: NGC 2859, a prototypical double-barred galaxy first noted by Kormendy (1979). The figure shows logarithmic R-band contours, displaying both the outer bar and the inner bar inside it, along with ellipse fits to the isophotes (upper right) and an unsharp mask (lower right) of the inner bar, with hints of spiral arms just outside it. The dashed circles on the unsharp mask have, from the inside out, radii = , , and  , three different ellipse-fit-based measures of bar size;see Sect. 3.1 for definitions. (See Erwin & Sparke 2003 for moredetails on the observations.)

Open with DEXTER


In the text

Figure 2: Examples of the appearance of "normal'' (i.e., large-scale) bars in unsharp masks. For each galaxy I show logarithmically scaled isophotes (top) and an unsharp mask from the same image (bottom). a) The outer bar of the double-barred SB0 galaxy NGC 4340; traces of a ring surrounding the bar can also be seen (R-band image from the MDM Telescope). b) NGC 4643, an example of an extremely narrow bar in an SB0/a galaxy, with the central "spine'' of the bar showing up clearly in the unsharp mask, along with very faint traces of a thin ring (R-band image from Erwin & Sparke 2003).

Open with DEXTER


In the text

Figure 3: As for Fig. 2, but now showing inner bars from double-barred galaxies. a) NGC 1543, where the narrow inner bar is strikingly similar to the (large-scale) bar of NGC 4643 in Fig. 2 (WFPC2 F814W image). b) NGC 6684, an inner bar with a more typical unsharp mask appearance (WFPC2 F814W image). c) NGC 4340, where the inner bar is surrounded by an elliptical stellar nuclear ring (R-band image from the MDM Telescope).

Open with DEXTER


In the text

Figure 4: Example of a single-barred galaxy where a nuclear ring masquerades as an inner bar. The SBa galaxy NGC 2273 was listed as double-barred by Mulchaey et al. (1997) on the basis of ellipse fits (upper right: black = K-band fits to their image, gray = HST H-band fits) and the appearance of their K-band image (upper left). However, an HST NICMOS2 H-band image (lower left) and its unsharp mask (lower right) show that the "inner bar'' ( ) is actually a nuclear ring, with a nuclear spiral inside; WFPC2 images and color maps confirm this (see Erwin & Sparke 2003; Martini et al. 2003).

Open with DEXTER


In the text

Figure 5: Another example of a nuclear ring masquerading as an inner bar. Here, the SB0 galaxy NGC 4371 shows three distinct peaks in ellipse fits to ground-based images (upper right: black = ground-based R-band fits, gray = WFPC2 V-band fits; both from Erwin & Sparke 1999), which led Wozniak et al. (1995) to suggest that it was triple-barred. The R-band images (upper and lower left) do indeed show a prominent inner elliptical feature with , corresponding to the middle ellipticity peak; but unsharp masking (bottom right) shows that this is due to a stellar nuclear ring. The innermost ellipticity peak (in the ground-based ellipse fits) is actually due to the combination of seeing effects and the superposition of an elliptical ring on top of rounder bulge isophotes (see Erwin & Sparke 1999; Erwin et al. 2001).Circumnuclear dust lanes produce variable ellipticity and positionangles in the HST fits at .

Open with DEXTER


In the text

Figure 6: Another example of how ellipse fits can misleadingly suggest two bars in a single-barred galaxy: in this case, a false "outer bar'' signature is produced by spiral arms in the SABc galaxy NGC 1667. a) Logarithmically spaced K-band isophotes (from the image of Mulchaey et al. 1997), along with ellipse fits b) to the same image (black points) plus ellipse fits to the NICMOS2 H-band image (gray points). Two strong ellipticity peaks are present, suggesting a possible double-barred galaxy (e.g., Laine et al. 2002): an "outer bar'' with and an "inner bar'' with . However, unsharp masking c) with Gaussian pixels shows that the "outer bar'' feature is really due to spiral arms (Mulchaey et al.). In contrast, unsharp masking with pixels d) shows that the "inner bar'' is indeed a bar - almost certainly the galaxy's only bar.

Open with DEXTER


In the text