Below are past "Pictures of the Month" (P.O.T.M.) that have appeared on the Astrophysics Group homepage.

January 2003
May 2002 April 2002 February 2002 January 2002 December 2001
November 2001 October 2001 September 2001


January 2003: ACBAR CMB power spectrum


May 2002: XMM-Newton Observation of the Distant (z=0.6) Cluster RX J1120.1+4318

CMU scientists Kathy Romer and Bob Nichol, in collaboration with colleagues in France, have recently published (Arnaud et al. 2002) results from a 20 ksec XMM-Newton observation of the distant and luminous X-ray cluster RX J1120.1+4318, which was discovered as part of the SHARC survey (Romer et al. 2000). Using these data it has been possible to measure the temperature of the hot, X-ray emitting, gas that fill's the cluster's potential well. It has been possible to measure the temperature as a function of position (see below) and, as a result, put contraints on the value of the mass density of the universe and set a lower limit to the redshift at which the so-called entropy floor in clusters becomes established.


April 2002: Possible discovery of the epoch of Helium re-Ionization.

CMU scientist Dr Mariangela Bernardi, working with collaborators on the
Sloan Digital Sky Survey, has uncovered evidence that the Universe went through a phase of ionization - prompted by the high energy output from an early generation of Quasars - at a redshift of ~3. This discovery is especially striking in that it supports recent claims by theorists that the ionization epoch should have occurred at that time!

Background: Currently favored models of structure formation in the Universe assume that the first stars formed before the first quasars. The photons from these first stars are expected to have been energetic enough to ionize (i.e. remove an electron from an atom) almost all the Hydrogen in the universe, and to ionize Helium (which ordinarily has two electrons) once but not twice. Quasars are expected to emit the more energetic photons required to remove the second electron from a Helium atom. So it is interesting to see if the time when most quasars formed (redshift z~3), is also the time when Helium became twice ionized. The Universe is expanding, so one expects the gas in it to cool as it expands. The kinetic energy associated with the random motions of electrons liberated by ionization can heat up the surrounding gas. So one signature of ionization is a sudden increase in the temperature of the gas in the Universe. One way to measure this temperature change is to measure what is known as the optical depth, tau. The solid line in the figure shows how one expects tau to evolve if the temperature decreases smoothly with time. The symbols in the figure show the evolution of tau measured in a sample of about 1,000 objects drawn from the SDSS database. The feature at z~3 may be the signature we were looking for.


February 2002: A Sunyaev-Zel'dovich image of a low redshift cluster, made with the Viper telescope at the South Pole.

During the Antarctic winter of 1999, the Viper telescope at the South Pole made observations of the low redshift (z=0.055) cluster of galaxies A3667 using a 40 GHz receiver called Corona. Here we describe the analysis of these data and present a map of a 3.6 by 2 degree region centered on the cluster. We go on to use the map in conjunction with a deep ROSAT PSPC (X-ray) image to derive a value for the Hubble's Constant. We find H0=64-30+96 km s-1 Mpc-1 (68\% confidence interval). These results are available as an ApJ Preprint (Cantalupo, Romer, Peterson, Gomez et al. 2002; contact romer@cmu.edu). Viper has gone on to make other SZ images of clusters using the ACBAR instrument.


January 2002: The Deepest X-ray Look at the Sky

The Hubble Deep Field-North (HDF-N) and the surrounding area is one of the two fields with deepest (approximately 1 Ms) observations with the Chandra X-ray Observatory. Takamitsu Miyaji and Richard Griffiths investigated the subtle fluctuations in the off-source area of this deepest X-ray field and found that the number density of sources above a flux (N(>S)) continues to grow even at the faintest flux level. The source density at the faintest fluxes is well above the expectation from a model of AGN populations and consistent with an additional population of star-forming galaxy population. For details, see Miyaji & Griffiths 2002 (Astrophysical Journal Letters, January 1, 2002 issue, 564, L5).


December 2001: Graviational Lenses Discovered by the MDS

A Sample of strong gravitational lenses found in the Medium Deep Survey (see Ratnatunga, Griffiths & Ostrander 1999 for more information). For each lens the first panel shows the lens as seen by the Hubble space telescope, the second panel shows a maximum likelihood model of the lens (see Knudson, Ratnatunga, & Griffiths 2001), and the third panel is the difference between the two. By modeling gravitational lenses the mass and distribution of mass in these galaxies can be calculated.


November 2001: Finding Clusters of Galaxies with the SDSS

The top panels are two sections of the SDSS imaging data filtered using the C4 algorithm (see Nichol et al. 2000). This algorithm looks for galaxies that live in dense regions of like-colored galaxies under the hypothesis that clusters of galaxies are dominated by galaxies of the same color. At the bottom, we show two of the strongest clusters of C4 galaxies in these areas which coincide with two well-known clusters of galaxies thus illustrating the power of this approach in finding massive clusters of galaxies e.g. RXJ0256 has an Xray luminosity of 4e+44 erg/s, while A1882 is a richness class 3 cluster. This also illustrates that the SDSS can detect clusters over a wide range in redshift from z~0.05 to z~0.6.


October 2001: Numerical Simulations of the Large Scale Structure in the Universe (Taken from Croft, Hernquist, Springel, Westover & White;in Prep)

Results from a hydrodynamic computer simulation of of galaxy formation, showing structure in the space around galaxies at redshift 3. Each row is for a different galaxy; the top two rows are for dwarf galaxies, and the bottom two are large, with masses more like the Milky Way. The different panels in each row show the density field (the bluish panel), the temperature of the gas, and the stars. The galaxy in each case is in the middle of the picture, and is small compared to the size of the plot. In this study, we are interested in the intergalactic gas that has yet to condense and form into stars (and is therefore not directly viewable observationally). In the simulation plot, it can be seen as a ``web'' of filaments surrounding the galaxies. This gas can be detected by looking at absorption in the spectra of background quasars. The leftmost panel shows what absorption spectra would look like passing close to the galaxies. We can think of these spectra as one-dimensional maps of the gas in intergalactic space.


September 2001: Evidence For ``Baryon Wiggles'' In The Large Scale Structure of the Universe using new Statistical Tools

We plot the CMB data from the MAXIMA and BOOMERANG experiments (left) along side the matter-density data (right). The solid line is the best fit model (Omega_m = 0.24, \Omega_b = 0.06, and n_s = 1.08 with H_0 = 69) using the matter-density data alone. The amplitudes in both plots remain a free parameter. The solid line in (A) is not a fit to the CMB data (although the chi^2 is 34 for 32 data points). It is the resultant cosmological model using the best-fit parameters from (B) and Omega_vacuum=0.8$, consistent with the Type Ia supernovae results (click here for pre-print).