Double barred galaxies at intermediate redshifts: A feasibility study

Despite the increasing number of studies of barred galaxies at intermediate and high redshifts, double-barred (S2B) systems have only been identified in the nearby (z<0.04) universe thus far. In this feasibility study we demonstrate that the detection and analysis of S2Bs is possible at intermediate redshifts (0.1 < z < 0.5) with the exquisite resolution of the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a novel method. The radial profile of the Gini coefficient -- a model-independent structure parameter -- is able to detect bars in early-type galaxies that are large enough that they might host an inner bar of sufficient angular size. Using this method and subsequent examination with unsharp masks and ellipse fits we identified the two most distant S2Bs currently known (at redshifts z=0.103 and z=0.148). We investigate the underlying stellar populations of these two galaxies through a detailed colour analysis, in order to demonstrate the analysis that could be performed on a future sample of intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to push the limit for S2B detection and analysis out by a factor of ten in redshift and lookback time (z=0.5, t=5Gyr) compared to the previously known S2Bs. This in turn would provide new insight into the formation of these objects.Comment: 9 pages + 10 figures. Accepted for publication in MNRAS. Main change from version 1 is an extension of the introduction/motivation and discussion section. A full resolution version including colour figures is available at http://www.astro.unibas.ch/~tlisker/papers/lisker2006_s2b.pd

SDSS J122958.84+000138.0: A Compact, Optically red galaxy

We report a new compact galaxy, SDSS J122958.84+000138.0 (SDSS J1229+0001), which has unique morphological and stellar population properties that are rare in observations of the nearby universe. SDSS J1229+0001 has an $r$-band absolute magnitude (M$_{r}$) and half-light radius (R$_{h}$) of $-$17.75 mag and 520 pc, respectively. Located in a fairly low density environment, morphologically it is akin to a typical early-type galaxy as it has a smooth appearance and red colour. But, interestingly, it possesses centrally concentrated star forming activity with a significant amount of dust. We present an analysis of structural and stellar population properties using archival images and VLT/FORS2 spectroscopy. Analysis of UKIDSS H-band image shows that the observed light distribution is better fitted with two components S\'ersic function with inner and outer component effective radii 190 and 330 pc, respectively. Whereas, overall half-light radius measured in H-band is much smaller compared to optical, i.e 290 pc. We prepared a Spectral Energy Distribution (SED) from optical to FIR and interpret it to derive star-formation rate, dust mass and stellar mass. We find that the SDSS J1229+0001 has dust mass M$_{dust}$ = 5.1 $\times$ 10$^{5}$ M_{\sun} with a dust to stellar mass ratio log(M$_{dust}$/M$_{*}$) = $-$3.5. While the observed stellar population properties are -- to some extent -- similar to that of a typical S0 galaxy, a unified view from stellar population and structural properties may suggests that SDSS J1229+0001 is a {\it smoking gun} example of a compact early-type galaxy in formation.Comment: 10 pages, Accepted MNRA

On the Color Magnitude Relation of Early-type Galaxies

In this letter we present a study of the color magnitude relation of 468 early-type galaxies in the Virgo Cluster with Sloan Digital Sky Survey imaging data. The analysis of our homogeneous, model-independent data set reveals that, in all colors (u-g, g-r, g-i, i-z) similarly, giant and dwarf early-type galaxies follow a continuous color magnitude relation (CMR) that is best described by an S-shape. The magnitude range and quality of our data allows us to clearly confirm that the CMR in Virgo is not linear. Additionally, we analyze the scatter about the CMR and find that it increases in the intermediate-luminosity regime. Nevertheless, despite this observational distinction, we conclude from the similarly shaped CMR of semi-analytic model predictions that dwarfs and giants could be of the same origin.Comment: Accepted by ApJL; 5 pages, 3 figures; added missing line to Figure