The Outer Disks of Early-type Galaxies. II. Surface-brightness Profiles of Unbarred Galaxies and Trends with Hubble Type

• Published in: The Astronomical journal 2011-11, Vol.142 (5), p.145-jQuery1323896781130='48'
• Main Authors: Gutiérrez, Leonel, Erwin, Peter, Aladro, Rebeca, Beckman, John E
• Format: Article
• Language: English
• Subjects: ASTROPHYSICS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BRIGHTNESS; DUSTS; EVOLUTION; GALACTIC EVOLUTION; GALAXIES; OPTICAL PROPERTIES; PHYSICAL PROPERTIES; PHYSICS
• ISSN: 1538-3881; 0004-6256; 1538-3881
• DOI: 10.1088/0004-6256/142/5/145

We present azimuthally averaged radial profiles of R-band surface brightness for a complete sample of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred galaxies. Following very careful sky subtraction, the profiles can typically be determined down to brightness levels well below 27 mag arcsec--2 and in the best cases below 28 mag arcsec--2. We classified the profiles according to the scheme used previously for the barred sample: Type I profiles are single unbroken exponential radial declines in brightness; Type II profiles ('truncations') have an inner shallow slope (usually exponential) which changes at a well-defined break radius to a steeper exponential; and Type III profiles ('antitruncations') have an inner exponential that is steeper, giving way to a shallower outer (usually exponential) decline. By combining these profiles with previous studies, we can make the first clear statements about the trends of outer-disk-profile types along the Hubble sequence (including both barred and unbarred galaxies), and their global frequencies. We find that Type I profiles are most frequent in early-type disks, decreasing from one-third of all S0-Sa disks to barely 10% of the latest-type spirals. Conversely, Type II profiles (truncations) increase in frequency with Hubble type, from only ~25% of S0 galaxies to ~80% of Sd-Sm spirals. Overall, the fractions of Type I, II, and III profiles for all disk galaxies (Hubble types S0-Sm) are 21%, 50%, and 38%, respectively; this includes galaxies (~8% of the total) with composite Type II+III profiles (counted twice). Finally, we note the presence of bars in 10 galaxies previously classified (optically) as 'unbarred.' This suggests that ~20% of optically unbarred galaxies are actually barred; the bars in such cases can be weak, obscured by dust, or so large as to be mistaken for the main disk of the galaxy.