Large Radial Distances Research Articles

Lost and found dark matter in elliptical galaxies

The kinematical properties of elliptical galaxies formed during the mergers of equal mass, stars+gas+dark matter spiral galaxies are compared to the observed low velocity dispersions found for planetary nebulae on the outskirts of ellipticals, which have been interpreted as pointing to a lack of dark matter in ellipticals (which poses a problem for the standard model of galaxy formation). We find that the velocity dispersion profiles of the stars in the simulated ellipticals match well the observed ones. The low outer stellar velocity dispersions are mainly caused by the radial orbits of the outermost stars, which, for a given binding energy must have low angular momentum to reach their large radial distances, usually driven out along tidal tails.

Dynamical consequences of two modes of centrifugal instability in Jupiter's outer magnetosphere

Important global‐scale properties of the middle and outer portions of the Jovian magnetosphere can be interpreted in terms of plasma experiencing either marginal or explosive centrifugal instability as a response to rotational stresses. The process accounts for characteristics of the plasmasheet, particularly a strong dawn‐dusk asymmetry, inferred from magnetic field data. Confining forces externally imposed at the magnetopause are critical to this analysis. Because of the vast length of magnetospheric flux tubes, most plasma‐disk ions do not bounce between mirror points as a flux tube rotates through local time sectors. Rotational stresses can lead to irreversible heating and increase plasma anisotropy, creating the conditions for firehose instability. The outer edge of the plasma disk appears marginally stable prenoon, but there is weak loss of plasma from its outer edge. The plasma disk thickens further postnoon as it assimilates the empty flux tubes of the outer magnetosphere where small perpendicular scale instability enables Bohm diffusion of plasma to refill depleted flux tubes. On the nightside, the plasmasheet becomes strongly unstable, material flows out down tail, the plasma disk thins, and its outer portions can break off leaving depleted closed flux tubes behind. This outflow must dominate solar wind‐controlled reconnection in the magnetotail. The depleted flux tubes at large radial distance on the morningside form a distinct low‐density plasma/magnetic regime that overlays a thin plasma disk. The dawn‐dusk asymmetry in the UV aurora is likely associated with the larger tapping of the ionospheric flywheel in the afternoon sector to drive the heating and instability that the model proposes.

A Minor-Axis Surface Brightness Profile for M31

We use data from the Isaac Newton Telescope Wide-Field Camera survey of M31 to determine the surface brightness profile of M31 along the southeast minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4° (≈55 kpc), where μV ~ 32 mag arcsec-2; this is the first time the M31 minor-axis profile has been mapped over such a large radial distance using a single data set. We confirm the finding by Pritchet & van den Bergh that the minor-axis profile can be described by a single de Vaucouleurs law out to a projected radius of 14 or ≈20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power law of index about -2.3 or an exponential of scale length 14 kpc. The fraction of the total M31 luminosity contained in this component is ≈2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean color of the stellar population remains roughly constant at V - i ≈ 1.6 from 05 to 35 along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.

Po‐Poster ‐ 18: Investigation of tilted dose kernels for portal dose prediction in a‐Si electronic portal imagers

The effect of beam divergence on dose calculation in an amorphous silicon electronic portal imaging device (EPID) was investigated with Monte Carlo generated dose kernels. The flat‐panel detector was simulated in EGSnrc (user code DOSXYZnrc) with a 3.0 cm water buildup; the model included details of the detector's imaging cassette and the front cover upstream of it. To approximate the effect of the EPID's rear housing, a 21 mm air gap and 10 mm water slab were introduced into the simulation as equivalent backscatter material. Kernels were generated with monoenergetic 2, 6, and 18 MeV photons with the orientation of the pencil beam varying from 0 to 14 degrees in 2 degree increments. Dose was scored in the phosphor layer of the detector. To reduce statistical fluctuations at large radial distances from the incident pencil beam, the kernels were first averaged bilaterally and then combined into square half rings. Profiles of the kernels were observed to demonstrate increasing asymmetry with increasing angle and energy, while the total energy deposited in the phosphor by the 2 MeV pencil beam decreased by greater than 2% at larger angles. Further investigation via comparison of superposition to convolution dose calculation methods is required to determine the effect these angled kernels have on calculation accuracy in clinical beam geometry.

Erratum: “Observations of Water Vapor Outflow from NML Cygnus” (ApJ, 610, 427 [2004

For the values of the outflow velocities vinner and radius Rinner that we adopted at the inner edge of the outflows, and vf , the final velocities we cited for large radial distances from the stars, this requires revision of the calculated stellar masses. The corrected masses for NML Cyg and VY CMa become MNML 53 M and MVY 27 M , respectively. If we assume parabolic outflow, instead of FWHM velocities, the terminal velocities vf can be taken to be 26 and 20 km s , respectively, instead of 19 and 17 km s , and the corresponding stellar masses become MNML 41 M and MVY 22 M . Neither the calculated mass-loss rates that we cite in our paper, ṀNMLk 2:4 ; 10 4 M yr 1 and ṀVYk 3:3 ; 10 4 M yr , nor any other aspects or conclusions of the paper are affected by this correction.

Can the galactic rotation curves be explained in brane world models?

We consider solutions with conformal symmetry of the static, spherically symmetric gravitational field equations in the vacuum in the brane world scenario. By assuming that the vector field generating the symmetry is nonstatic, the general solution of the field equations on the brane can be obtained in an exact parametric form, with the conformal factor taken as parameter. As a physical application of the obtained solutions we consider the behavior of the angular velocity of a test particle moving in a stable circular orbit. In this case the tangential velocity can be expressed as a function of the conformal factor and some integration constants only. For a specific range of integration constants, the tangential velocity of the test particle tends, in the limit of large radial distances, to a constant value. This behavior is specific to galactic rotation curves and is explained usually by invoking the hypothesis of dark matter. The limiting value of the angular velocity of the test particle can be obtained as a function of the baryonic mass and radius of the galaxy. The behavior of the dark radiation and dark pressure terms is also considered in detail, and it is shown that they can be expressed in terms of the rotational velocity of a test particle. Hence all the predictions of the present model can be tested observationally. Therefore the existence of the nonlocal effects, generated by the free gravitational field of the bulk in a conformally symmetric brane, may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center.

Exospheric distributions of minor ions in the solar wind

We investigate the acceleration of heavy solar wind ions on the basis of an exospheric Lorentzian model and show that the heavy ions can flow faster than the protons when their temperatures in the corona are more than proportional to their mass. The Lorentzian kinetic exospheric model [Pierrard and Lemaire, 1996], initially developed only for electrons and protons of the solar wind in a monotonic potential energy, is here generalized for the case of a nonmonotonic potential energy and applied to the heavy solar wind ions. We study how the mass, the charge, and the coronal temperature of the heavy ions influence their motion. The present work indicates that the temperature of the ions in the corona characterizing the dispersion of their velocities is a crucial factor determining their bulk velocity at large radial distances. Pierrard and Lamy [2003] showed that the velocity filtration effect can produce very high values of ion temperatures in the solar corona, considering that ion velocity distribution functions are enhanced with suprathermal tails in the low corona. We show that for sufficiently high ion temperatures at the exobase, our exospheric model can account for large bulk speeds of the heavy solar wind ions and for their relative abundances at 1 AU.

The solar wind in the outer heliosphere

We review recent plasma and magnetic field results stemming from Voyager 1 and 2 observations of the solar wind in the outer heliosphere. The solar wind speed profiles in 2002 are dominated by shock-related speed increases followed by long ramps of decreasing speed. Significant variability in the density and temperature are seen on all time scales. Transient events near solar maxima drive the formation of MIRs which are increasingly important at large radial distances. Both the density and magnetic field magnitude are consistent with the predictions of Parker's solar wind model. The solar wind pressure changes at all latitudes over the solar cycle, resulting in an oscillation of the heliospheric boundary positions. As the Voyagers move farther out, the effect of the interstellar medium on the solar wind increases. Observed speeds at Voyager 2 are about 60 km/s lower than those in the inner heliosphere due to interaction with the interstellar neutrals.

Modelling and numerical simulation of hydrodynamical processes in a confined rotating electrode configuration

This work presents an extensive hydrodynamic study of the rotating disk electrode in a confined electrochemical cell configuration. The deviation, particularly on the axis, with a classical Von Cochran expression is evaluated. The mathematical model of the hydrodynamic properties is presented and demonstrated. The numerical resolution of the equation system obtained is made using the Patankar finite volumes method using in the Fluent ® code. The three stationary and transient velocity components of the flow, the gauge pressure and the stream function cartography have been calculated using nonstationary algorithms for rotation velocities ω∈{10 0;5×10 0;10 1;5×10 1;10 2} rad s −1 over the whole reactor volume. These results have been validated in the vicinity of the rotating disk by means of the classical analytical Cochran expressions. The results are in good agreement for large radial distances from the axis and for small axial distances from the electrode, which is important for electrochemical species fluxes, whereas an almost 40% difference has been calculated for the axial velocity profile component on the axis. This point is important because the first term of the truncated Cochran equation is widely used to estimate convective reactive species transport at the electrode surface. These expressions are good evaluations for peripheral profiles and there are few consequences for large surface electrodes. Validation “far from the electrode”, in the bulk has been made using LDA measurements. Good accordance between numerical and experimental data are obtained with problems arising at high rotation speeds. Very large turbulence intensities were measured at the chemical boundary layer frontier.

The Far‐Infrared Spectral Energy Distributions of X‐Ray–selected Active Galaxies

[Abridged] We present ISO far-infrared (IR) observations of 21 hard X-ray selected AGN from the HEAO-1 A2 sample. We compare the far-IR to X-ray spectral energy distributions (SEDs) of this sample with various radio and optically selected AGN samples. The hard-X-ray selected sample shows a wider range of optical/UV shapes extending to redder near-IR colors. The bluer objects are Seyfert 1s, while the redder AGN are mostly intermediate or type 2 Seyferts. This is consistent with a modified unification model in which the amount of obscuring material increases with viewing angle and may be clumpy. Such a scenario, already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications, allows for different amounts of obscuration of the continuum emission in different wavebands and of the broad emission line region which results in a mixture of behaviors for AGN with similar optical emission line classifications. The resulting limits on the column density of obscuring material through which we are viewing the redder AGN are 100 times lower than for the standard optically thick torus models. The resulting decrease in optical depth of the obscuring material allows the AGN to heat more dust at larger radial distances. We show that an AGN-heated, flared, dusty disk with mass 10^9 solar and size of few hundred pc is able to generate optical-far-IR SEDs which reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength, cooler part of the IR continuum.

Dynamic and structural behavior of different rigid nonpolarizable models of water

The local structure and the dynamical behavior of water have been analyzed in two different regimes (the dense isochore of 0.995 g/cm2 and the supercritical isotherm of 673 K) through four rigid nonpolarizable models of water. An important change in the slope of temperature dependence of the self-diffusion coefficient at ρ=0.995 has been observed at T≈450 K, showing two main regions that are related to a change on the activation energy of the process (originated for a change of the structure of the first solvation shell from a tetrahedral to dodecahedral arrangement). The local orientational structure of water has been analyzed through the use of tetrahedral order parameters q. A direct relation between q and D has been observed for all models showing some kind of master curve up to 450 K at the 0.995 g/cm3 isochore. The structure of the system at short and large radial distances has been analyzed through a decomposition of the site–site radial distribution functions in terms of spherical harmonics, and a three-dimensional picture of the total pair distribution function has been reconstructed from this set of spherical harmonic projections.

Modeling the inner plasma sheet protons and magnetic field under enhanced convection

In order to understand the evolution of the protons and magnetic field in the inner plasma sheet from quiet to disturbed conditions, we incorporate a modified version of the Magnetospheric Specification Model (MSM) with a modified version of the Tsyganenko 96 (T96) magnetic field model to simulate the protons and magnetic field under an increasing convection electric field with two‐dimensional (2‐D) force balance maintained along the midnight meridian. The local time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer (LLBL). We previously used this model to simulate the inner plasma sheet under weak convection corresponding to a cross polar cap potential drop (ΔΦPC) equal to 26 kV and obtained 2‐D quiet time equilibrium for proton and magnetic field that agrees well with observations. We start our simulation for enhanced convection with this quiet time equilibrium and time‐independent boundary particle sources and increase ΔΦPC steadily from 26 to 146 kV in 5 hours. Simulations are also run separately to steady states by keeping ΔΦPC constant after it is increased to 98 and 146 kV. The magnitudes of proton pressure, number density, and temperature and their increase from quiet to moderate activity (ΔΦPC = 98 kV) are consistent with most observations. Our simulation at high activity (ΔΦPC = 146 kV) underestimates the observed pressure and temperature. This disagreement indicates possible dependence of the boundary particle sources on activity and possible effects of solar wind dynamic pressure enhancements that have not yet been included in our simulation. The simulated equatorial pressures and temperatures show stronger enhancement on the dusk side than on the dawn side as convection is increased, while density profiles show an increase on the dawn side and a decrease on the dusk side. The simulated proton flow speed at the equatorial plane increases with enhancing convection while the overall flow direction does not change significantly, a result of enhancement in both the earthward electric drift and the azimuthal diamagnetic drift. The equatorial magnetic field strength decreases more in the near‐Earth plasma sheet than at larger radial distances as ΔΦPC increases, resulting in an increasing flat radial profile with enhancing convection. The feedbacks from diamagnetic drift and magnetic fields to increasing convection are found to restrain the pressure increase. Based on the good agreement between our results and observations at moderate activity, our magnetic field indicates that the plasma and magnetic field in the plasma sheet can be in a state far from possible force balance inconsistency during periods of moderately enhanced convection. A scale analysis of our results shows that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet for moderate activity.

An Overview of Results From rpi on Image

The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft was designed as a long-range magnetospheric radio sounder, relaxation sounder, and a passive plasma wave instrument. The RPI is a highly flexible instrument that can be programmed to perform these types of measurements at times when IMAGE is located in key regions of the magnetosphere. RPI is the first radio sounder ever flown to large radial distances into the magnetosphere. The long-range sounder echoes from RPI allow remote sensing of a variety of plasmas structures and boundaries in the magnetosphere. A profile inversion technique for RPI echo traces has been developed and provides a method for determining the density distribution of the plasma from either direct or field-aligned echoes. This technique has enabled the determination of the evolving density structure of the polar cap and the plasmasphere under a variety of geomagnetic conditions. New results from RPI show that the plasmasphere refills in slightly greater than a day at L values of 2.8 and that ion heating is probably playing a major role in the overall density distribution along the field-line. In addition, RPI's plasma resonance observations at large radial distances over the polar cap provided in situ measurements of the plasma density with an accuracy of a few percent. For the first time in the magnetosphere, RPI has also observed the plasma D resonances. RPI's long antennas and its very low noise receivers provide excellent observations in the passive receive-only mode when the instrument measures the thermal plasma noise as well as natural emissions such as the continuum radiation and auroral kilometric radiation (AKR). Recent passive measurements from RPI have been compared extensively with images from the Extreme Ultraviolet (EUV) imager on IMAGE resulting in a number of new discoveries. For instance, these combined observations show that kilometric continuum can be generated at the plasmapause from sources in or very near the magnetic equator, within a bite-out region of the plasmasphere. The process by which plasmaspheric bite-out structures are produced is not completely understood at this time. Finally, RPI has been used to successfully test the feasibility of magnetospheric tomography. During perigee passages of the Wind spacecraft, RPI radio transmissions at one and two frequencies have been observed by the Waves instrument. The received electric field vector was observed to rotate with time due to the changing density of plasma, and thus Faraday rotation was measured. Many future multi-spacecraft missions propose to use Faraday rotation to obtain global density pictures of the magnetosphere.

Soft X-ray observation of a large-scale coronal wave and its exciter

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfven speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3.

Constraints on MOND from the lensing cluster MS2137-23

The cluster of galaxies MS2137-23 presents the most constrained lensing configuration of gravitational images ever detected in a distant cluster of galaxies. With a set of two arc systems with known redshifts and many other arclets spread between ≈30 h −1 kpc and 1 h −1 Mpc, MS2137-27 is well suited to probe its mass profile and to challenge theoretical expectations from CDM scenario with more speculative alternatives, like MOND models. In this note, I describe the recent lensing analysis I carried out on this cluster using together arcs and CHANDRA X-ray data. Even with this unique data set, I cannot reject neither an isothermal model nor an NFW model. The MOND model is not compatible with the observations, unless a significant fraction of the baryonic matter has not been detected by CHANDRA. The need for much more baryons in the MOND model than for the dark matter paradigm implies significant dynamical differences between these models which can be explored at very large radial distance. In particular, MOND lensing signal becomes isotropic beyond 1 h −1 Mpc, in contrast with collisionless DM halos which produce elliptical potentials. This difference can in principle be used to test MOND prescriptions, regardless the exact relation between matter and the gravitational deflection law assumed for MOND.

Dominant contributions to lateral distribution functions in ultra-high energy cosmic ray air showers

In hadron induced air showers of highest energies ( E>10 18 eV), the lateral distribution functions of electrons and muons are a superposition of many separate electromagnetic sub-showers, initiated by meson decay at different altitudes and energies. The lateral distribution function is the primary tool for reconstructing the energy of the primary in a UHE cosmic ray shower, so understanding it in detail is a prerequisite for having confidence in the energy determination. We analyze in this paper the dominant contributions to the ground level lateral distribution functions, as a function of the altitude and energy at which the sub-showers are initiated. Far from the core, the dominant contribution to the density of electrons comes from sub-showers initiated at low altitudes and low energies ( E<100 GeV). The dominant sub-showers are initiated at large radial distance from the core and at a large angle with respect to the main shower axis. This demonstrates the need for careful treatment of low energy hadron physics modeling even for ultrahigh energy primaries.

Global comparison of magnetospheric ion fluxes and auroral precipitation during a substorm

Integrated fluxes from global images taken by the High Energy Neutral Atom (HENA) and the far ultraviolet (FUV) imagers on the IMAGE spacecraft were compared for a six‐hour period, during which a reasonably intense substorm occurred. HENA and the FUV proton auroral imager (SI‐12) monitor emissions which are representative of trapped and precipitating magnetospheric proton fluxes, respectively. For several hours prior to substorm onset, measurements of the fluxes of lower energy (10–16 and 16–27 keV) magnetospheric Energetic Neutral Atoms (ENA‐s) by HENA and precipitating auroral protons by FUV SI‐12 show strong similarities, with the implication that, in general, proton precipitation is controlled by a steady pitch angle diffusion process. Less similarity is seen between ENA‐s and the auroral electron precipitation, which is monitored with the FUV Wideband Imaging Camera. Prior to substorm onset, ENA intensity at large radial distance (L &gt; 8) is reduced while the overall integrated ENA flux increases signifying earthward motion and accumulation of the plasma. About 20 minutes before onset, the auroral fluxes decrease while the ENA intensity continues to grow. The observations are consistent with a pre‐onset increase in plasma pressure in the inner magnetosphere without an increase in precipitation showing more efficient trapping perhaps by the distorted nightside magnetosphere. At substorm onset the increase in precipitation intensity is very sudden while the more gradual intensification of the energetic ENA‐s continues. At onset the electron aurora shows an increase in intensity of one order of magnitude, while the increase in precipitating proton flux is only 50%. The intensification of the precipitation is relatively short lived (∼10 minutes) while the ENA substorm enhancements last about an hour.

Distributions of current and auroral precipitation in Jupiter's middle magnetosphere computed from steady-state Hill–Pontius angular velocity profiles: solutions for current sheet and dipole magnetic field models

We present an analysis of the magnetosphere–ionosphere coupling current system which is associated with the maintenance of plasma corotation in Jupiter's middle magnetosphere. The formulation follows Cowley and Bunce (Planet. Space Sci. 49 (2001) 1067), but here the angular velocity of the equatorial plasma is computed using the steady-state theory of Hill (J. Geophys. Res. 84 (1979) 6554) and Pontius (J. Geophys. Res. 102 (1997) 7137) rather than being determined by a simple empirical model. Results are compared both for a realistic magnetospheric current sheet magnetic field model and for a simple dipole field. The results for the current sheet model confirm previous conclusions concerning current and auroral precipitation distributions based on the empirical angular velocity model. Specifically, we find that upward field-aligned currents flow from the ionosphere to the equatorial current sheet over the whole radial range from ∼20 R J to the outer boundary of the region considered at 100 R J, thus returning from the current sheet to the ionosphere at larger radial distances outside this region. The upward currents are of sufficient intensity to require the existence of field-aligned voltages which accelerate magnetospheric electrons down into the atmosphere, thus creating intense auroras. At ionospheric heights, the upward field-aligned current densities are several tenths of a μA m −2 , flowing in a narrow latitudinal band less than ∼1° wide, located at ∼16° dipole co-latitude. The associated field-aligned voltages are several tens of kV, resulting in precipitated electron energy fluxes capable of exciting UV auroras of hundreds of kR intensity. The model thus forms an appropriate basis on which to understand the origin of Jupiter's ‘main auroral oval’ emissions. For the dipole field, the steady-state equatorial angular velocity profile is found to be very similar to that for the current sheet model. Nevertheless, the different mapping of the field lines to the ionosphere results in a magnetosphere–ionosphere coupling current system which has significantly different properties. In particular, the height-integrated ionospheric Pedersen current intensity is found to be factors of ∼2–3 less than that for the current sheet model, and since the field-aligned current is also found to be spread over a much broader latitudinal band, ∼5° wide centred at ∼10° dipole co-latitude, the field-aligned current density is reduced by more than an order of magnitude. Only small (few kV) field-aligned voltages are thus required to carry the upward current in this case, resulting in much weaker electron precipitation, and much weaker, few kR, UV auroras. Our results thus emphasise that while the magnetic field model may not greatly effect the steady-state angular velocity profile, it nevertheless is of crucial significance in determining the consequent magnetosphere–ionosphere coupling current system, and the resulting auroral precipitation.

The Extent of NGC 6822 Revealed by Its C Star Population

Using the CFH12K camera, we apply the four band photometric technique to identify 904 carbon stars in an area 28' x 42' centered on NGC 6822. A few C stars, outside of this area were also discovered with the Las Campanas Swope Telescope. The NGC 6822 C star population has an average I of 19.26 mag leading to an average absolute I magnitude of -4.70 mag, a value essentially identical to the mean magnitude obtained for the C stars in IC 1613. Contrary to stars highlighting the optical image of NGC 6822, C stars are seen at large radial distances and trace a huge slightly elliptical halo which do not coincide with the huge HI cloud surrounding NGC6822. The previously unknown stellar component of NGC 6822 has a exponential scale length of 3.0' +/- 0.1' and can be traced to five scale lengths. The C/M ratio of NGC 6822 is evaluated to br 1.0 +/- 0.2.

U-, B- and r-band luminosity functions of galaxies in the Coma cluster

We present a deep multicolour CCD mosaic of the Coma cluster (Abell 1656), covering 5.2 deg(2) in the B and r bands, and 1.3 deg(2) in the U band. This large, homogeneous data set provides a valuable low-redshift comparison sample for studies of galaxies in distant clusters. In this paper we present our survey, and study the dependence of the galaxy luminosity function (LF) on passband and radial distance from the cluster centre. The U-, B- and r-band LFs of the complete sample cannot be represented by single Schechter functions. For the central area, r <245 h(100)(-1) kpc, we find best-fitting Schechter parameters of M-U* = -18.60(-0.18)(+0.13) and alpha(U) = -1.32(-0.028)(+0.018), M-B* = 19.79(-0.17)(+0.18) and alpha(B) = -1.37(-0.016)(+0.024) and M-r* = -20.87(-0.17)(+0.12) and alpha(r) = -1.16(-0.019)(+0.012). The LF becomes steeper at larger radial distances from the cluster centre. The effect is most pronounced in the U band. This result is consistent with the presence of a star-forming dwarf population at a large distance from the cluster centre, which may be in the process of being accreted by the cluster. The shapes of the Us of the NGC 4839 group support a scenario in which the group has already passed through the centre.