Keck spectroscopy of the faint dwarf elliptical galaxy population in the Perseus Cluster core: mixed stellar populations and a flat luminosity function

We present the result of a photometric and Keck low-resolution imaging spectrometer (LRIS) spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of   M _B =−12.5  . Spectra were obtained for 23 dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra, we confirm 12 systems as cluster members, with the remaining 11 as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour–magnitude relation for the Perseus Cluster down to   M _B =−12.5  . We confirm an increase in the scatter about the colour–magnitude relationship below   M _B =−15.5  , but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between   M _B =−18  and −12.5, finding  α=−1.26 ± 0.06  , which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with α-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal-poor population with ages ∼8 Gyr and metallicities  [Fe/H] < −0.33  , and a young, metal-rich population with ages <5 Gyr and metallicities  [Fe/H] > −0.33  . Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.     

Looking inside stars and looking for planets

Asteroseismology and extrasolar planets are the main science goals of the Eddington mission, now approved by ESA for a 2007 launch. Alan Penny presents a summary of the January 2002 RAS meeting that discussed the sciences of this wide-field high-precision photometric space telescope. Since the date of this meeting, ESA has decided to implement the mission in the framework of a 2007–08 launch.     

The Matuyama-Brunhes reversal at Tecopa basin, southeastern California, revisited again

Numerous records of the Matuyama-Brunhes geomagnetic transition have been obtained from paleomagnetic studies. Because few of the reversal records are of acceptable reliability, however, the exact behavior of the field during the transition has remained enigmatic. To provide confirmation of one of the more reliable records, we have re-examined the transition at two sites, 150 m apart, in lake sediments of Tecopa basin, southeastern California. The two sites are geographically very close to that of Valet et al. [10], who previously obtained a record from that site indicating that the transitional field was non-dipolar and axisymmetric.

The Matuyama-Brunhes reversal is recorded differently at each of our two sites and at that of Valet et al. [10]. Zones of mixed polarities and/or intermediate directions occur at all three sites but they differ greatly in polarity character, thickness and stratigraphic position. It appears that all three sites have provided mutually contradictory records of the transition. It is unlikely, therefore, that any of the records is acceptable for establishing the nature of the transition at this locality.

Obliteration of the transition is apparently the result of acquisition of a stable, normal-polarity overprint that appears to consist of two remanence components, one acquired during post-depositional compaction and dewatering, and one during later sediment diagenesis.