Experimental radio cosmology

Abstract:

The design and preliminary results from testing the performance of a 230GHz unilateral finline SIS mixer, fabricated on a silicon substrate are presented. The mixer will be employed in the single baseline heterodyne interferometer-GUBBINS (220-GHz Ultra-BroadBand INterferometer for S-Z), which aims to measure the null frequency in the Sunyaev-Zel'dovich spectrum at ~227GHz. The mixer is operated in the frequency range of 180GHz~280GHz with an IF bandwidth of 1-13 GHz. An important feature of this mixer is its ultra-wide IF bandwidth, so as to achieve very high brightness sensitivity in the observation of the galaxy clusters in the faint cosmic microwave background radiation. This SIS mixer described is deposited on an 80um silicon substrate with the dielectric constant of 11.9. The incoming RF signal from the feed horn is coupled via the waveguide mode to a unilateral finline, which is tapered to a 2.5um slotline. The signal is coupled from the slotline to the microstrip with the aid of two radial stubs. The employment of a silicon substrates gives a slotline impedance of 36Ω, which is ideal for coupling to the microstrip, where the SIS junction is fabricated. Also, the employment of silicon allows the generation of the trenches around the device so that individual devices can be separated from the batch without dicing. A 2-stage notch was fabricated at the front end of the substrate to match the loaded waveguide to the free space. The capacitance of the SIS junction was tuned out using a wide band circuit consisting of two stubs and a 3-stage Chebyshev transformer. This is followed by a 3-stage RF choke to prevent RF signal from leaking into the IF output port. The mixer chip was fabricated at KOSMA, Cologne using Nb-AlOx-Nb tunnel junction defined with E-beam lithography. The device used in this experiment had a normal resistance of 20Ω, a current density of 14KA/cm2, an area of 1μm2 and an intrinsic capacitance of 75fF. Measurement of the mixer performance was done using a local oscillator which gives sufficient power to pump the mixer in the frequency range 200GHz~260GHz using 12μm beam splitter. Measurement of the mixer sensitivity was done using the hot/cold Y-factor method. Uncorrected noise temperature of 75K was obtained at 208GHz, with 30 K contributed by mismatch to the IF system. Future characterization and improvements of the IF system and the measurement with better-tuned devices will also be reported.

Abstract:

An anomalous radio continuum component at cm-wavelengths has been observed in various sources, including dark clouds. This continuum component represents a new property of the interstellar medium. In this work, we focus on one particular dark cloud, the bright reflection nebula M78. The main goal of this work is to investigate the cm-wave continuum emission in a prominent molecular cloud, nearby and with complementary observational data. We acquired Cosmic Background Imager (CBI) visibility data of M78 at 31 GHz with an angular resolution of ∼ 5.8arcmin, and CBI2 data at an angular resolution of ∼ 4.2arcmin. A morphological analysis was undertaken to search for possible correlations with templates that trace different emission mechanisms. Using data from Wilkinson Microwave Anisotropy Probe and the Rhodes/HartRAO 2326-MHz survey, we constructed the spectral energy distribution (SED) of M78 in a 45-arcmin circular aperture. We used results from the literature to constrain the physical conditions and the stellar content. The 5-31 GHz spectral index in flux density (α= 1.89 ± 0.15) is significantly different from optically thin free-free values. We also find closer morphological agreement with IR dust tracers than with free-free sources. Dust-correlated cm-wave emission that is not due to free-free is significant at small scales (CBI resolutions). However, a free-free background dominates at cm-wavelengths on large scales (∼ 1°). We correct for this uniform background by differencing against a set of reference fields. The differenced SED of M78 shows excess emission at 10-70 GHz over free-free and a modified blackbody, at 3.4σ. The excess is matched by the spinning dust model from Draine & Lazarian. © 2010 Universidad de Chile. Journal compilation © 2010 RAS.

Abstract:

The diffuse cm wave IR-correlated signal, the 'anomalous' CMB foreground, is thought to arise in the dust in cirrus clouds. We present Cosmic Background Imager (CBI) cm wave data of two translucent clouds, ζ Oph and LDN 1780 with the aim of characterizing the anomalous emission in the translucent cloud environment. In ζ Oph, the measured brightness at 31GHz is 2.4σ higher than an extrapolation from 5-GHz measurements assuming a free-free spectrum on 8 arcmin scales. The SED of this cloud on angular scales of 1° is dominated by free-free emission in the cm range. In LDN 1780 we detected a 3σ excess in the SED on angular scales of 1° that can be fitted using a spinning dust model. In this cloud, there is a spatial correlation between the CBI data and IR images, which trace dust. The correlation is better with near-IR templates (IRAS 12 and 25μm) than with IRAS 100μm, which suggests a very small grain origin for the emission at 31GHz. We calculated the 31-GHz emissivities in both clouds. They are similar and have intermediate values between that of cirrus clouds and dark clouds. Nevertheless, we found an indication of an inverse relationship between emissivity and column density, which further supports the VSGs origin for the cm emission since the proportion of big relative to small grains is smaller in diffuse clouds. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

An analysis of large-area CO J = 3-2 maps from the James Clerk Maxwell Telescope for 12 nearby spiral galaxies reveals low velocity dispersions in the molecular component of the interstellar medium. The three lowest luminosity galaxies show a relatively flat velocity dispersion as a function of radius while the remaining nine galaxies show a central peak with a radial fall-off within 0.2-0.4r25. Correcting for the average contribution due to the internal velocity dispersions of a population of giant molecular clouds, the average cloud-cloud velocity dispersion across the galactic discs is 6.1 ± 1.0 kms-1 (standard deviation of 2.9 kms-1), in reasonable agreement with previous measurements for the Galaxy and M33. The cloud-cloud velocity dispersion derived from the CO data is on average two times smaller than the HI velocity dispersion measured in the same galaxies. The low cloud-cloud velocity dispersion implies that the molecular gas is the critical component determining the stability of the galactic disc against gravitational collapse, especially in those regions of the disc which are H2 dominated. The cloud-cloud velocity dispersion shows a significant positivecorrelation with both the far-infrared luminosity, which traces the star formation activity, and the K-band absolute magnitude, which traces the total stellar mass. For three galaxies in the Virgo cluster, smoothing the data to a resolution of 4.5 kpc (to match the typical resolution of high-redshift CO observations) increases the measured velocity dispersion by roughly a factor of 2, comparable to the dispersion measured recently in a normal galaxy at z = 1. This comparison suggests that the mass and star formation rate surface densities may be similar in galaxies from z = 0 to 1 and that the high star formation rates seen at z = 1 may be partly due to the presence of physically larger molecular gas discs. © 2010 The Authors. Journal compilation © 2010 RAS.