Name: Chiara Ferrari
Email: Chiara.Ferrari@uibk.ac.at
Institution: Institute of Astrophysics, Innsbruck University
Postal_address:
Technikerstrasse 25, 6020 Innsbruck (AUSTRIA)
Collaborators: Dick Hunstead (Sydney University, AUSTRALIA), Sophie Maurogordato and Christophe Benoist (Nice Observatory, FRANCE)
Title: Dynamics and star formation properties of the galaxy cluster A3921
Phone_numbers: (02) 9351 3871 (Hunstead) +39 349 77 14 167 (Ferrari)
Instrument: 2dF
Detector: Tektronix_CCD
Number_of_spectrographs: 2
Central_wavelength_(Angstroms): 6000
Spectral_resolution_(Angstroms): 8-9
Spectral_features: [OII]3727; CaII H,K; Hd; G; Hb; [OIIIa,b]; MgIb; Ha (note: z=0.094)
Preferred_2dF_grating: 300B
Alternative_2dF_gratings: None
2dF_filter: not used
CCD_readout_speed: Slow
Source_list:
Field RA(J2000) DEC(J2000) Number of Mag-range S/N Nexp Texp ident
hh:mm:ss.ss dd:mm:ss.s Targets Targets min
A3921 22:49:00 -64:22:00 ~400 V=16-20 >15 3-5 50-30
Magnitude_range_guide_stars: 13-15
Astrometry: We will prepare the input fields using positional information from the galaxy catalogue obtained from our deep VRI-bands imaging observations (WFI@NTT ESO telescope). This catalogue covers 1.5 square degrees of the 2dF field. We will complete the external regions using the SuperCOSMOS and APM catalogues. The positional accuracy of these catalogues is adequate.
Last_date: 2004/10/31
Worst_seeing: 2.5
Moon_phase: grey
Photometric_conditions: no
Extra_information:
Optimum time for obtaining galaxy spectra would be in August, September and beginning of October.
PATT_ATAC: no
Figures: 2
Justification:
A3921 is a rich cluster at z=0.094. From our spectroscopic and photometric survey of its central ~15' region [3], we detected the presence of two dominant clumps of galaxies with a mass ratio of ~4: a main cluster centred on the BCG (A3921-A), and a NW sub-cluster (A3921-B) hosting the second brightest cluster galaxy (Fig.1). By applying a two-body dynamical formalism to the two sub-clusters [3], and comparing our optical results with the analysis of XMM X-ray observations [1], we conclude that A3921-B is probably traversing the main cluster tangentially in a SW/NE direction. Analysis of the spectral features of the confirmed cluster members reveals that galaxies characterized by recent or ongoing star formation have significantly different spatial and velocity distributions with respect to passive objects. The fraction of active objects, moreover, is significantly higher than observed in lower redshift clusters [2]. The merging event seems, therefore, to have affected the kinematics and spectral properties of the active galaxies in A3921, but a wider spectral coverage is essential for testing this hypothesis. Interestingly, although we observe very few bright post-SF objects (k+a) in comparison with higher redshift clusters, we do detect a higher fraction of faint (18.5<V<20) k+a galaxies. Similar results have been obtained in the Coma cluster (z=0.02) [4]. It has been suggested that the evolution with redshift of the luminosity distribution of k+a galaxies may be due to the fact that the maximum mass of actively star-forming galaxies decreases at lower redshift (`downsizing effect', [4]); our proposed 2dF observations will test this hypothesis. Our recent ATCA observations of A3921 have revealed the presence of an unusual, extended, steep-spectrum radio source located in A3921-B. Fig.2 shows that this source occupies a 'hole' in the ICM distribution traced by XMM, and that it has no bright optical counterparts. The origin of this peculiar radio source, and its possible connection with the properties of the surrounding ICM, will be analysed in more detail using our forthcoming Chandra and ATCA observations (July 2004). Observing Plan: Our previous spectroscopic observations have already sampled the densest part of the cluster. We propose now to observe ~400 galaxies over a more extended area, covering the whole central region of A3921 at least out to one virial radius (Rv>~35') with a single 2dF pointing. We aim to observe all galaxies with 16<V<20 having the same colour as the cluster red-sequence or bluer. With a total integration time of 2.5h, we can obtain high (>=10) S/N spectra for all the galaxies in this magnitude range. The spectral resolution is sufficient to provide a reliable determination of both radial velocity and spectral type. This will allow us to: 1) test our hypothesis about the dynamical state of the cluster by analysing the velocity distribution of a larger and more spatially extended sample of cluster galaxies; 2) compare the spatial and velocity distributions of passive and active galaxies over a larger area of the cluster; and 3) ensure an adequate spectral coverage of the galaxies in the vicinity of the extended radio source detected in our ATCA observations (Fig.2). The combination of X-ray and radio images with 2dF spectra will provide the tools necessary to probe the environment and kinematics of A3921, and explore in detail the effect of the merger on these properties. [1] Belsole, E. et al., 2004, submitted to A&A [2] Dressler, A. 1987, in Nearly Normal Galaxies: From the Planck Time to the Present, ed S. Faber (New York: Springer), 276 [3] Ferrari, C. et al., 2004, submitted to A&A [4] Poggianti, B. et al., 2004, ApJ, 601, 197