Star Formation in Cluster Galaxies at 0.2<z<0.55

The rest frame equivalent width of the [OII]3727 emission line, W(OII), has been measured for cluster and field galaxies in the CNOC redshift survey of rich clusters at 0.2<z<0.55. Emission lines of any strength in cluster galaxies at all distances from the cluster centre, out to 2R_{200}, are less common than in field galaxies. The mean W(OII) in cluster galaxies more luminous than M_r^k<-18.5 + 5\log h (q_o=0.1) is 3.8 \pm 0.3 A (where the uncertainty is the 1 sigma error in the mean), significantly less than the field galaxy mean of 11.2 \pm 0.3 A. For the innermost cluster members (R<0.3R_{200}), the mean W(OII) is only 0.3 \pm 0.4 A. Thus, it appears that neither the infall process nor internal tides in the cluster induce detectable excess star formation in cluster galaxies relative to the field. The colour-radius relation of the sample is unable to fully account for the lack of cluster galaxies with W(OII)>10 A, as expected in a model of cluster formation in which star formation is truncated upon infall. Evidence of supressed star formation relative to the field is present in the whole cluster sample, out to 2 R_{200}, so the mechanism responsible for the differential evolution must be acting at a large distance from the cluster centre, and not just in the core. The mean star formation rate in the cluster galaxies with the strongest emission corresponds to an increase in the total stellar mass of less than about 4% if the star formation is due to a secondary burst lasting 0.1 Gyr.Comment: aasms4 latex, 3 postscript figures, accepted for publication in ApJ Letters. Also available at http://astrowww.phys.uvic.ca/~balogh

Star Formation in Cluster Galaxies at

The rest frame equivalent width of the [OII]3727 emission line, W ffi (OII), has been measured for cluster and field galaxies in the CNOC redshift survey of rich clusters at 0:2 ! z ! 0:55. Emission lines of any strength in cluster galaxies at all distances from the cluster centre, out to 2R 200 , are less common than in field galaxies. The mean W ffi (OII) in cluster galaxies more luminous than M k r ! \Gamma18:5 + 5 log h (q ffi =0.1) is 3:8 \Sigma 0:3 A (where the uncertainty is the 1oe error in the mean), significantly less than the field galaxy mean of 11:2 \Sigma 0:3 A. For the innermost cluster members (R ! 0:3R 200 ), the mean W ffi (OII) is only 0:3 \Sigma 0:4 A. Thus, it appears that neither the infall process nor internal tides in the cluster induce detectable excess star formation in cluster galaxies relative to the field. The colour--radius relation of the sample is unable to fully account for the lack of cluster galaxies with W ffi (OII) ? 10 A, as expected in a model o..

Pre-Heated Isentropic Gas in Groups of Galaxies

We confirm that the standard assumption of isothermal, shock–heated gas in cluster potentials is unable to reproduce the observed X-ray luminosity–temperature relation of groups of galaxies. As an alternative, we construct a physically motivated mode

The Dependence of Cluster Galaxy Star Formation Rates on the Global Environment

A comparison of star formation properties as a function of environment is made from the spectra of identically selected cluster and field galaxies in the CNOC 1 redshift survey of over 2000 galaxies in the fields of fifteen X-ray luminous clusters at 0.18<z<0.55. The ratio of bulge luminosity to total galaxy luminosity (B/T) is computed for galaxies in this sample, and this measure of morphology is compared with the galaxy star formation rate as determined from the [OII]3727 emission line. The mean star formation rate of cluster galaxies brighter than M_r= -17.5 + 5 log h is found to vary from 0.17 +- 0.02 h^{-2} M_sun/yr at R200 (1.5-2 Mpc/h) to zero in the cluster center, and is always less than the mean star formation rate of field galaxies, which is 0.39 +- 0.01 h^{-2}M_sun/yr. It is demonstrated that this significant difference is not due exclusively to the difference in morphological type, as parameterized by the B/T value, by correcting for the B/T-radius relation. The distribution of [OII] equivalent widths among cluster galaxies is skewed toward lower values relative to the distribution for field galaxies of comparable physical size, B/T and redshift, with a statistical significance of more than 99%. The cluster environment affects not only the morphological mix of the galaxy population, but also suppresses the star formation rate within those galaxies, relative to morphologically similar galaxies in the field.Comment: 7 pages, figures included; accepted for publication in ApJ Letters Also available from http://astrowww.phys.uvic.ca/~balogh

Galaxy Evolution in the Z = 0.4274 Cluster MS 1621.5+2640

We discuss the galaxy population of the rich cluster MS1621.5+2640 at z=0.4274, based on spectra and imaging in a field of size 9 arcmin by 23 arcmin (~2 by 5 h**-1 Mpc). The sample comprises 277 galaxies, of which 112 are cluster members, 7 are `near-members', and 47 are field galaxies in the redshift range 0.37<=z<=0.50. The results are analyzed and compared with the z=0.2279 rich cluster Abell 2390. MS1621.5+2640 has a higher blue fraction, a younger stellar population, and is a less evolved cluster. We do not find strong evidence of significant excess star formation compared with the field, although there is a small population of outlying near-members that is unusually blue and that may be affected by the cluster. There is a substantial population of red galaxies with significant Hdelta absorption, which are not easily explained by any simple form of modeled star formation history. We detect two distinct populations of cluster galaxies: those where star formation ceased some time ago, and those with a gradual decrease over many Gyr. Our data suggests that the cluster formed by accretion from the field, with truncation of the star formation beginning at very large radii (>2 times the virial radius). The truncation process does not seem to be a sharp one though, in that lower-luminosity early-type galaxies in the inner core of the cluster are seen with significant Hdelta absorption, indicating some star formation within the last 1-2 Gyr. Some combination of stripping of gas from the outer parts of the galaxy, together with gradual exhaustion of the gas in the inner parts would be consistent with our data.Comment: 46 pages, 20 figures. Accepted for publication in ApJ. Full text and figures available at http://www.hia.nrc.ca/STAFF/slm/ms1621

The Dependence of Cluster Galaxy Star Formation Rates on the Global Environment

A comparison of star formation properties as a function of environment is made from the spectra of identically selected cluster and field galaxies in the CNOC 1 redshift survey of over 2000 galaxies in the fields of fifteen X--ray luminous clusters at 0:18 ! z ! 0:55. The ratio of bulge luminosity to total galaxy luminosity (B/T) is computed for galaxies in this sample, and this measure of morphology is compared with the galaxy star formation rate as determined from the [OII]3727 emission line. The mean star formation rate of cluster galaxies brighter than M r = \Gamma17:5+5 log h is found to vary from 0:17 \Sigma 0:02h \Gamma2 M fi yr \Gamma1 at R 200 (1.5--2 h \Gamma1 Mpc) to zero in the cluster center, and is always less than the mean star formation rate of field galaxies, which is 0:39 \Sigma 0:01h \Gamma2 M fi yr \Gamma1 . It is demonstrated that this significant difference is not due exclusively to the difference in morphological type, as parameterized by the B/T value,..

Galaxy Evolution in the z=0.4274 Cluster MS1621.5+2640

We discuss the galaxy population of the rich cluster MS1621.5+2640 at z=0.4274, based on spectra and imaging in a field of size 9 arcmin by 23 arcmin (¸2 by 5 h \Gamma1 Mpc). The sample comprises 277 galaxies, of which 112 are cluster members, 7 are `near-members&apos;, and 47 are field galaxies in the redshift range 0.37z0.50. The results are analyzed and compared with the z=0.2279 rich cluster Abell 2390. MS1621.5+2640 has a higher blue fraction, a younger stellar population, and is a less evolved cluster. We do not find strong evidence of significant excess star formation compared with the field, although there is a small population of outlying near-members that is unusually blue and that may be affected by the cluster. There is a substantial population of red galaxies with significant Hffi absorption, which are not easily explained by any simple form of modeled star formation history. We detect two distinct populations of cluster galaxies: those where star formation ceased some time a..

A magnetometer for the Solar Orbiter mission

The magnetometer is a key instrument to the Solar Orbiter mission. The magnetic field is a fundamental parameter in any plasma: a precise and accurate measurement of the field is essential for understanding almost all aspects of plasma dynamics such as shocks and stream-stream interactions. Many of Solar Orbiter’s mission goals are focussed around the link between the Sun and space. A combination of in situ measurements by the magnetometer, remote measurements of solar magnetic fields and global modelling is required to determine this link and hence how the Sun affects interplanetary space. The magnetic field is typically one of the most precisely measured plasma parameters and is therefore the most commonly used measurement for studies of waves, turbulence and other small scale phenomena. It is also related to the coronal magnetic field which cannot be measured directly. Accurate knowledge of the magnetic field is essential for the calculation of fundamental plasma parameters such as the plasma beta, Alfvén speed and gyroperiod. We describe here the objectives and context of magnetic field measurements on Solar Orbiter and an instrument that fulfils those objectives as defined by the scientific requirements for the mission