Optical and near infrared photometry of Butcher-Oemler clusters

Rich clusters of galaxies at moderate redshifts (z approx. .3) have a larger proportion of optically blue galaxies than their low redshift counterparts. Spectroscopic examination of the blue galaxies by various authors has shown that the blue galaxies are generally Seyferts, show evidence for recent star formation, or are foreground objects. Unfortunately, spectroscopy is too time consuming to be used on large samples. Thus, we have looked for a way to separate Seyferts, starbursts, ellipticals and nonmembers using photometry alone. Five moderate redshift clusters, Abell numbers 777, 963, 1758, 1961 and 2218, have been observed in the V, R and K bands. We model the spectral energy distributions of various kinds of galaxies found in clusters and derive observed colors. We have modeled the spectral energy distributions (SED) of several kinds of galaxies and compute their colors as a function of redshift. We expect to see ellipticals, spirals, starbursts, post-starburst and Seyfert galaxies. The SED of elliptical and Sbc galaxies was observed by Rieke and Rieke. The SEDs for the starburst galaxies was created by adding a reddened 10(exp 8) year old burst to a spiral galaxy SED. The post-starburst (E+A) galaxy SEDs are composed of a slightly reddened 10(exp 9) year old burst and elliptical galaxy SED. SEDs for the Seyferts were created by adding a v(exp -1.1) power law, and a hot dust thermal spectrum to the Sbc. From the SEDs the colors of galaxies at various redshifts with assorted filters were computed. Lilly & Gunn (1985) have optical and infrared photometry for a sample of galaxies in CL0024+1654 observed spectroscopically by Dressler, Gunn and Schneider (1985). We have used this data to choose the most appropriate SEDs for our starburst and post-starburst models. The most likely explanation for the optically blue colors in most cluster galaxies is star formation. Very few galaxies lie in the Seyfert locus. Abel 1758 has more Seyfert candidates than the other clusters, we observed. It seems possible to roughly sort types of galaxies in clusters by color alone. The cluster population seems to vary considerably between clusters, but our K selected sample has few Seyferts in any cluster

The potential for high performance HgCdTe arrays at 4 microns

The potential of existing technology at Rockwell International in terms of the goals for astronomical detector arrays in the 3 to 5 micron interval is evaluated. Measurements have been obtained for a number of samples of HgCdTe diodes manufactured by Rockwell International. All the diodes reported on here had cutoff wavelengths at high temperatures of 4.6 to 4.7 microns. Although no confirming measurements were made, the cutoff wavelength is expected to move to 5 microns or beyond at the low temperatures of our tests. Diode sizes ranged from 20 to 150 microns. The test program yielded full diode curves and relative response at 3.4 microns for the sample diodes as a function of temperature. Dark currents are quoted below as the current passing through the diode with a back bias of 50 mV. The various diode types showed a wide range of behavior, both with regard to dark current and responsibility. The test results for one of the best diode types are illustrated. This detector has a size of 148 microns and a cutoff wavelength of 4.61 microns

Search for IR Emission from Intracluster Dust in A2029

We have searched for IR emission from the intracluster dust (ICD) in the galaxy cluster A2029. Weak signals of enhanced extended emission in the cluster are detected at both 24 and 70 micron. However, the signals are indistinguishable from the foreground fluctuations. The 24 versus 70 micron color map does not discriminate the dust emission in the cluster from the cirrus emission. After excluding the contamination from the point sources, we obtain upper limits for the extended ICD emission in A2029, 5 x 10^3 Jy/sr at 24 micron and 5 x 10^4 Jy/sr at 70 micron. The upper limits are generally consistent with the expectation from theoretical calculations and support a dust deficiency in the cluster compared to the ISM in our galaxy. Our results suggest that even with the much improved sensitivity of current IR telescopes, a clear detection of the IR emission from ICD may be difficult due to cirrus noise.Comment: 5 pages, 4 figures, accepted by ApJ

Massive Star Formation in Luminous Infrared Galaxies: Giant HII Regions and their relation to Super Star Clusters

We have used HST/NICMOS H-band narrow-band Pa-alpha (at rest 1.87micron) images to identify star clusters and HII regions respectively in a sample of 8 luminous infrared galaxies (LIRGs). These observations have revealed the presence of a large population of super star clusters (SSC) and bright HII regions. A significant fraction of the HII regions shows H-alpha luminosities above that of 30 Doradus, the prototypical giant HII region. The excess of extremely luminous HII regions in LIRGs has been confirmed by comparison with normal galaxies observed at similar spatial resolutions. Despite the large numbers of identified star clusters and HII regions in LIRGs, we only find a small fraction of coincidences, between 4% and 30%. Using synthesis models we have reproduced the relative fractions of young HII regions, intermediate and old star clusters observed in Arp299 and the central region NGC3256 using a Salpeter IMF and instantaneous star formation. HII regions with no detected near-infrared cluster counterpart (25-39%) represent the youngest sites of star formation, with ages of up to approximately 5Myr and mostly intermediate mass (~10^5Msun) ionizing clusters. For these two galaxies, and within the present detection threshold we can only detect coincidences (4-10%) between an HII region and a near-infrared star cluster for the most massive star clusters (~10^6Msun) during the first 7Myr of their evolution. The identified near-infrared SSCs with no detectable Pa-alpha emission represent the ``old'' population (53-66% of the detected sources), with ages of between 7 and 20-40Myr. Older clusters possibly created in this or previous episodes of star formation are likely to exist in these systems but cannot be identified with the present detection threshold. (Abridged)Comment: Accepted for publication in AJ (July issue). Figure 2 not included. Go to: http://nicmos2.as.arizona.edu/~aalonso/work/papers/lirghii_v2.ps for a complete version of pape

Laboratory and telescope use of the NICMOS2 128 x 128 HgCdTe array

The second generation of Hubble Space Telescope (HST) instruments will include a near-infrared instrument. This choice has driven the development of near-infrared arrays to larger sizes and lower read noises. Rockwell International has delivered an array for use in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument; this array has been dubbed NICMOS2. NICMOS2 is a 128x128 array of HgCdTe diodes In-bonded to a switched MOSFET readout. The readout was specifically designed for astronomical use with the HST requirement of low read noise a prime goal. These arrays use detector material which is similar to that used by Rockwell in previous arrays (e.g., HgCdTe produced on a sapphire substrate), but the NICMOS2 devices differ substantially from other 128x128 arrays produced by Rockwell in having a read noise of only 30 electrons when read out using appropriate correlated sampling. NICMOS2 has now been characterized in the laboratory, and it has been used on groundbased telescopes

Low noise HgCdTe 128 x 128 SWIR FPA for Hubble space telescope

Large area focal plane arrays of unprecedented performance were developed for use in Near Infrared Camera and Multi-Object Spectrometer (NICMOS), a proposed Hubble Space Telescope refurbishment instrument. These FPAs are 128x128-element, HgCdTe hybrid arrays with a cutoff wavelength of 2.5 microns. The multiplexer consists of a CMOS field effect transistor switch array with a typical mean readout noise of less than 30 electrons. The detectors typically have a mean dark current of less than 10 electrons/s at 77 K, with currents below 2 electrons measured at 60 K (both at 0.5 V reverse bias). The mean quantum efficiency is 40 to 60 percent at 77 K for 1.0 to 2.4 microns. Functional pixel yield is typically greater than 99 percent, and the power consumption is approximately 0.2 mW (during readout only)

PhoSim-NIRCam: Photon-by-photon image simulations of the James Webb Space Telescope's Near-Infrared Camera

Recent instrumentation projects have allocated resources to develop codes for simulating astronomical images. Novel physics-based models are essential for understanding telescope, instrument, and environmental systematics in observations. A deep understanding of these systematics is especially important in the context of weak gravitational lensing, galaxy morphology, and other sensitive measurements. In this work, we present an adaptation of a physics-based ab initio image simulator: The Photon Simulator (PhoSim). We modify PhoSim for use with the Near-Infrared Camera (NIRCam) -- the primary imaging instrument aboard the James Webb Space Telescope (JWST). This photon Monte Carlo code replicates the observational catalog, telescope and camera optics, detector physics, and readout modes/electronics. Importantly, PhoSim-NIRCam simulates both geometric aberration and diffraction across the field of view. Full field- and wavelength-dependent point spread functions are presented. Simulated images of an extragalactic field are presented. Extensive validation is planned during in-orbit commissioning