¹Department of Physics and Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
²National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, USA

Passive spiral galaxies - unusual galaxies with spiral morphologies but without any sign of on-going star formation - have recently been shown to exist preferentially in cluster infalling regions. This discovery directly connects passive spiral galaxies to cluster galaxy evolution studies, such as the Butcher-Oemler effect or the morphology-density relation. Thus, detailed study of passive spiral galaxies could potentially yield new insight on the underlying physical mechanisms governing cluster galaxy evolution.

However, in previous work, passive spiral galaxies were selected from low-resolution optical images with ~1.5 arcsec seeing. Passive spirals could therefore be mis-identified as S0 galaxies, or as dusty-starburst galaxies which are not passive at all. To address this issue we performed deep, high-resolution, near-infrared imaging of 32 passive spiral galaxies with UKIRT.

We selected our target galaxies from 73 passive spiral galaxies presented by Goto et al. (2003). None of the 73 galaxies have any emission in [OII] or H&alpha (< 1&sigma in equivalent width) though all have a disc-like morphology. Of the 73 passive spiral galaxies, the 32 targets accessible during our run in September 2003 were observed in the K band using the UKIRT Fast Track Imager (UFTI). Data were taken during periods of good atmospheric transparency and with excellent seeing of ~0.5 arcsec. In Figure 1, we show K-band images of 16 of the 32 passive spiral galaxies. Although selected in poorer conditions, the deep and high resolution imaging capability of UKIRT clearly shows the discs and spiral arm structures. Thus, passive spiral galaxies are not S0s, but truly are spiral galaxies.

FIGURE 1: UKIRT K band images of passive spiral galaxies. Each image is 35x35 arcsec in size.

We used the restframe optical-infrared (r-K) colour distribution for the observed passive spiral galaxies to investigate whether they are dusty starburst galaxies or truly passive galaxies. Since the K band is less affected by dust extinction than the r band, dusty starburst galaxies are known to have redder colours in r-K by ~1 mag (Smail et al. 1999). Figure 2 plots g-i colour against r-K colour. Optical photometry (g, r, and i) is from the SDSS. The black circles are for passive spiral galaxies observed with UKIRT. The red squares are for early-type galaxies in the control sample. For a reference, we plot the distribution of all galaxies in the volume limited sample with K magnitudes measured with the Two Micron All Sky Survey (2MASS; Jarrett et al. 2000) as the contour.

FIGURE 2: Restframe g-i vs. r-K two-colour diagram. The circles are for passive spirals. The squares are for the early-type galaxies in the control sample. The contours represent all galaxies in the volume limited sample with 2MASS K magnitudes.

Interestingly, compared with all galaxies (the contour), passive spiral galaxies (circles) are not redder in r-K colour. Indeed, the r-K colours of the passive spiral galaxies are indistinguishable from the early-type galaxies (squares). These results support the truly passive nature of these galaxies, since dusty starburst galaxies should have r-K colours redder by 1 magnitude than normal galaxies.

Thus, our results support the truly "passive" and "spiral" nature of these galaxies. It is very likely that passive spiral galaxies are indeed transition objects currently undergoing cluster galaxy evolution. Further studies of passive spiral galaxies will reveal the physical mechanisms governing cluster galaxy evolution.

References
Goto T., Okamura S., Sekiguchi, M., et al. 2003, PASJ, 55, 757
Jarrett T. H., Chester T., Cutri R., Schneider S., Skrutskie M., Huchra J. P., 2000, AJ, 119, 2498
Smail I., Morrison G., Gray M. E., Owen F. N., Ivison R. J., Kneib J.-P., Ellis R. S., 1999, ApJ, 525, 609

Leiden Observatory, The Netherlands

The first direct detection of an extrasolar planet will constitute a true milestone in astronomy. Although now more than 130 planets have been discovered indirectly, the star/planet contrasts have so far proven to be too high to reach this goal. Most attempts have concentrated on probing star-light reflected from the atmospheres of hot Jupiters using optical echelle spectroscopy. This can in principle be disentangled from direct star light by its slowly varying Doppler shift over the orbital period of the planet. Contrasts down to 10^-4-5 have been reached, but these have proven to be insufficient to detect reflected light from even the most promising candidates (e.g. Leigh et al. 2003).

For a few years it has been known that the orientation of the orbit of the hot Jupiter HD209458b is such that it transits its host star (Charbonneau et al. 2000). This can be of great benefit when trying to detect direct emission from this planet, since it implies that the planet will be eclipsed by the star on regular intervals. Instead of trying to disentangle a planetary component which only varies slowly over a several day period, in this case the light from the planet completely disappears within 26 minutes of the ingress time, to be blocked off by the host star for 2.2 hours, after which time it reappears again. At optical wavelengths, the depth of the secondary eclipse is expected to be at the 10^-5 level, beyond the reach of current instrumentation. However, in the near- and mid-infrared the contrast is significantly better. Here the contribution from the planet is dominated by its thermal emission, resulting in an eclipse depth of perhaps as much as a few milli-magnitude in the K-band.

 

FIGURE 1: During a transit (T1 -- T3) a small fraction of the star's light is blocked off by the planet, allowing the determination of its size. The secondary eclipse (S1 - S3) is the moment the planet moves behind the host star. The small decrement is due to the missing light contribution from the planet.

Using spectroscopy, Richardson et al. (2003) have tried to detect the disappearance and reappearance of planetary absorption features around the secondary eclipse, though without success. Here we report on a different approach, using high precision K-band photometry with UIST. This project is highly challenging! To be able to detect the planetary signal of HD209458b a precision of ~0.5 milli-magnitude per hour needs to be achieved; normally, the accuracy of near-infrared photometry is limited to the 1% level. In addition, the target is of K=6.3, and therefore can only be observed - without saturating the array - by defocusing the telescope. The target and two nearby reference stars were observed in cycles of 15 minutes, each observation consisting of a repeated 5-point jitter, with the target and the reference stars falling onto the same area of the array.

The first test observations were taken in July 2004. After careful flat fielding and sky subtraction two clear patterns emerged. Firstly, a dependence of the measured flux on the position in the jitter sequence. This is most likely caused by a residual dark current effect. The second is a dependence of the measured flux on the width of the star profile. This is probably caused by a combination of a residual non-linearity and an aperture effect. Both effects can be calibrated in an ad-hoc manner, resulting in a 1&sigma scatter of 4.0 milli-magnitude per jitter point, and a ~1-1.5 milli-magnitude uncertainty per 15 minute cycle.

Two secondary eclipses of HD209458b were subsequently observed on 23 August 2004 and 13 September 2004. The preliminary photometry of the first epoch is shown in Figure 2. Although this formally shows a ~3&sigma detection, we are too uncertain about possible slowly varying systematic effects during the 4 hour observation, so we do not claim this as a detection. In addition, the second epoch observation suffers from a sharp change in seeing, complicating the analysis and thereby not allowing a confirmation (Snellen 2005, in prep.).

FIGURE 2: Preliminary result of the K-band photometry of HD209458 (blue crosses) and the two reference stars (red diamonds and squares) during the secondary eclipse of August 23, 2004. The data are corrected for non-linearity, aperture effects, and change in airmass. The lower panel shows the relative photometry of HD209458. Although a tentative signal is observed, at the present time we are too uncertain about systematic effects to claim this as a detection (Snellen 2005, in prep.).

However, these first observations are promising, and indicate that in the near future a genuine planet signal may be detected using this method. Further developments in observing strategy, to improve the photometry on HD209458b or even more suitable transiting exoplanets, are underway.

References
Charbonneau et al. 2000, ApJ, 529, 45
Leigh et al. 2003, MNRAS, 346, L16
Richardson et al. 2003, ApJ, 597, 581