Scientific Results in Press

Design and characterization of 225-370 GHZ DSB and 247-360 GHz SSB full height waveguide SIS mixers

B. Lazareff(¹), D. Billon-Pierron(¹), A. Navarrini(¹) and I. Peron(¹)
(¹)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d'Hères, France

Abstract:
We report on the design and characterization of two full height waveguide SIS mixers for astronomical applications: a Double Side Band (DSB) fixed-tuned mixer covering the 225-370 GHz band ( ${\rm\approx 50~\%}$ of relative bandwidth), and a tunable Single Side Band (SSB) mixer covering the 247-360 GHz frequency range. The DSB receiver noise temperature we have measured is below 50 K over a bandwidth larger than 100 GHz for the DSB mixer and has a minimum of 27 K (uncorrected) at 336 GHz; to our knowledge this is the lowest noise ever reported at this frequency. A receiver noise temperature below 80 K and an image band rejection around -14 dB were measured over most of the band of the SSB mixer.
Both mixers use similar chips that integrate a parallel tuning inductor with a radial microstrip stub to compensate for the junction capacitance of 75 fF (junction size ${\rm 1~\mu m^2}$). A stability criterion for intrinsically DSB and SSB mixers under typical operating conditions has been derived. The receiver designs have been optimised in order to guarantee a low mixer noise temperature while maintaining adequate gain and stable operation over the whole frequency bands of interest.


Apperared in: Proceedings of the 5th. European Workshop on Low Temperature Electronics (WOLTE 5), J. Phys. IV France 12 (2002), Pr3-161

Widespread HCO Emission in the Nuclear Starburst of M82

S.García-Burillo(¹), J.Martín-Pintado(¹), A.Fuente(¹), A.Usero(¹) and R.Neri(²)
(¹)Observatorio Astronómico Nacional, Campus Universitario, Apartado 1143, Alcalá de Henares, E-28800 Madrid, Spain, (²)Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France

Abstract:
We present a high-resolution ( $\sim 5\hbox{$^{\prime\prime}$ }$) image of the nucleus of M82 showing the presence of widespread emission of the formyl radical (HCO). The HCO map, the first obtained in an external galaxy, reveals the existence of a structured disk of $\sim 650$ pc full diameter. The HCO distribution in the plane mimics the ring morphology displayed by other molecular/ionized gas tracers in M82. More precisely, rings traced by HCO, CO, and H II regions are nested, with the HCO ring lying in the outer edge of the molecular torus. Observations of HCO in Galactic clouds indicate that the abundance of HCO is strongly enhanced in the interfaces between the ionized and molecular gas. The surprisingly high overall abundance of HCO measured in M82 [ ${\rm X(HCO)} \sim 4\times 10^{-10}$] indicates that its nuclear disk can be viewed as a giant photon-dominated region (PDR) of $\approx 650$ pc size. The existence of various nested gas rings, with the highest HCO abundance occurring at the outer ring [ ${\rm X(HCO)} \sim 0.8\times 10^{-9}$], suggests that PDR chemistry is propagating in the disk. We discuss the inferred large abundances of HCO in M82 in the context of a starburst evolutionary scenario, picturing the M82 nucleus as an evolved starburst.
Based on observations carried out with the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. IRAM is supported by the Institut National des Sciences de l'Univers/Centre National de la Recherche Scientifique (France), the Max-Planck-Gesellschaft (Germany), and the Instituto Geográfico Nacional (Spain).


Appeared in: ApJ 575, L55

On the Heating Source of the Orion KL Hot Core

P.de Vicente(¹), J.Martín-Pintado(¹), R.Neri (²), A.Rodríguez-Franco
(¹)Observatorio Astronómico Nacional, Campus Universitario, Apartado 1143, Alcalá de Henares, E-28800 Madrid, Spain, (²)Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France, (³)Escuela Universitaria de Optica, Departmento de Matemática Aplicada, Universidad Complutense de Madrid, Av. Arcos de Jalón s/n, 28037 Madrid, Spain

Abstract:
We present images of the J=10-9 rotational lines of HC3N in the vibrationally excited levels 1v7, 1v6, and 1v5 of the hot core (HC) in Orion KL. The images show that the spatial distribution and the size emission from the 1v7 and 1v5 levels are different. While the J=10-9 1v7 line has a size of $4\hbox{$^{\prime\prime}$ }\times 6 \hbox{$^{\prime\prime}$ }$ and peaks $1.1\hbox{$^{\prime\prime}$ }$ northeast of the 3 mm continuum peak, the J=10-9 1v5 line emission is unresolved ( $< 3 \hbox{$^{\prime\prime}$ }$) and peaks $1.3 \hbox{$^{\prime\prime}$ }$ south of the 3 mm peak. This is a clear indication that the HC is composed of condensations with very different temperatures (170 K for the 1v7 peak and >230 K for the 1v5 peak). The temperature derived from the 1v7 and 1v5 lines increases with the projected distance to the suspected main heating source I. Projection effects along the line of sight could explain the temperature gradient as being produced by source I. However, the large luminosity required for source I ( $>5 \times 10^5 \mbox{M$_\odot$ }$) to explain the 1v5 line suggests that external heating by this source may not dominate the heating of the HC. Simple model calculations of the vibrationally excited emission indicate that the HC can be internally heated by a source with a luminosity of $10^5 \mbox{M$_\odot$ }$, located 1.2 $^{\prime\prime}$southwest of the 1v5 line peak (1.8 $^{\prime\prime}$south of source I). We also report the first detection of high-velocity gas from vibrationally excited HC3N emission.
Based on excitation arguments, we conclude that the main heating source is also driving the molecular outflow. We speculate that all the data presented in this Letter and the IR images are consistent with a young massive protostar embedded in an edge-on disk.


Appeared in: ApJ 574, L163

Molecular Gas and Dust at z = 2.6 in SMM J14011+0252: A Strongly Lensed, Ultraluminous Galaxy, Not a Huge, Massive Disk

D.Downes(¹) and P.M.Solomon(²)
(¹)Institute de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 St Martin d'Hères, France, (²)Astronomy Program, State University of New York, Stony Brook,NY 11794

Abstract:
We used the IRAM Interferometer to detect CO(3-2), CO(7-6), and 1.3 mm dust continuum emission from the submillimeter galaxy SMM J14011+0252 at a redshift of 2.6. Contrary to a recent claim that the CO was extended over $6.6\hbox{$^{\prime\prime}$ }$ (57 kpc), the new data yield a size of $2 \times 0.5\hbox{$^{\prime\prime}$ }$ for the CO and the dust. Although previous results placed the CO peak in a region with no visible counterpart, the new maps show the CO and dust are centered on the J1 complex seen on K-band and optical images. We suggest the CO is gravitationally lensed not only by the foreground cluster A1835, but also by an individual galaxy on the line of sight. Comparison of measured and intrinsic CO brightness temperatures indicates the CO size is magnified by a factor of $25 \pm 5$.
After correcting for lensing, we derive a true CO diameter of $\approx 0.08 \hbox{$^{\prime\prime}$ }$ (700 pc), consistent with a compact circumnuclear disk of warm molecular gas similar to that in Arp 220. The high magnification means the true size, far-IR luminosity, star formation rate, CO luminosity, and molecular gas mass are all comparable with those in present-epoch ultraluminous IR galaxies, not with those of a huge, massive, early-universe galactic disk.


Accepted for publication in ApJ

The Opacity of nearby Galaxies from Counts of background Galaxies: II. Limits of the Synthetic Field Method

R.A.González(¹), L.Loinard(¹), R.J. Allen(²) and S.Muller(³)
(¹)Instituto de Astronomía, UNAM, Unidad Morelia, Michoacán, México, C.P. 58190, (²)Space Telescope Science Institute, Baltimore, MD 21218 (³)Institut de Radio Astronomie Millimétrique, F-38406 St. Martin d'Hères, France

Abstract:
Recently, we have developed and calibrated the Synthetic Field Method to derive the total extinction through disk galaxies. The method is based on the number counts and colors of distant background field galaxies that can be seen through the foreground object; it is the only method capable of determining extinction without a priori assumptions about the dust properties or its spatial distribution, and has been successfully applied to NGC 4536 and NGC 3664, two late-type galaxies located, respectively, at 16 and 11 Mpc.

Here, we study the applicability of the Synthetic Field Method to HST images of galaxies in the Local Group, and show that background galaxies cannot be easily identified through these nearby objects, even with the best resolution available today. In the case of M 31, each pixel in the HST images contains fifty to one hundred stars, and the background galaxies cannot be seen because of the intrinsic granularity due to strong surface brightness fluctuations. In the LMC, on the other hand, there is only about one star every six linear pixels, and the lack of detectable background galaxies results from a ``secondary'' granularity, introduced by structure in the wings of the point spread function.

The success of the Synthetic Field Method in NGC 4536 and NGC 3664 is a natural consequence of the reduction of the intensity of surface brightness fluctuations with distance. When the dominant confusion factor is structure in the PSF wings, as is the case of HST images of the LMC, and would be the case of M 31 images obtained with a 10-m diffraction-limited optical telescope, it becomes in principle possible to improve the detectability of background galaxies by subtracting the stars in the foreground object. However, a much better characterization of optical PSFs than is currently available would be required for an adequate subtraction of the wings. Given the importance of determining the dust content of Local Group galaxies, efforts should be made in that direction.


To appear in Astronomical Journal

The line-of-sight distribution of the gas in the inner 60 pc of the Galaxy

B. Vollmer(¹), R. Zylka(^2,4) and W.J. Duschl(^3,1)
(¹)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, (²)Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany, (³)Institut für Theoretische Astrophysik der Universität Heidelberg, Tiergartenstraße 15,69121 Heidelberg, Germany, (⁴)IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France

Abstract:
2MASS K$_{\rm S}$ band data of the inner 60 pc of the Galaxy are used to reconstruct the line-of-sight distances of the giant molecular clouds located in this region. Using the 2MASS H band image of the same region two different populations of point sources are identified according to their flux ratio in the two bands. The population of blue point sources forms a homogeneous foreground that has to be subtracted before analyzing the K$_{\rm S}$ band image. The reconstruction is made using two basic assumptions: (i) an axis-symmetric stellar distribution in the region of interest and (ii) optical thick clouds with an area filling factor of $\sim$1 that block all light of stars located behind them. Due to the reconstruction method the relative distance between the different cloud complexes is a robust result, whereas the absolute distance of structures located more than 10 pc in front of Sgr A^* might be up to 30% larger than the one we derived from the data. It is shown that all structures observed at 1.2 mm continuum and in the CS(2-1) line are present in absorption. We place the 50 kms^-1 cloud complex close to, but in front of Sgr A^*. The 20 kms^-1 cloud complex is located in front of the 50 kms^-1 cloud complex and has a large LOS distance gradient along the direction of the galactic longitude. The Circumnuclear Disk is not seen in absorption. This leads to an upper limit of the cloud sizes within the Circumnuclear Disk of $\sim$0.06 pc.


Accepted for publication in A&A