New Life to Horn Antenna

Abstract: In this article, we present some results obtained with our horn antenna for receiving the 21-cm emission of neutral hydrogen. The goal is to map our galaxy as much as possible to highlight the spiral structure of the Milky Way. This is a long-term project; this post will be updated as new results become available.

Introduction

Details on the design and construction of the horn antenna and receiver for the 21 cm emission of neutral hydrogen are contained in the following posts already present on the site:
Horn Antenna for the 21cm Neutral-Hydrogen Line
Low-Noise SDR-Based Receiver for the 21cm Neutral-Hydrogen Line
GNURadio Software for 21cm Neutral-Hydrogen Line

While the surveys already carried out in the past are described in the following articles
Milky Way Structure detected with the 21 cm Neutral-Hydrogen Emission
Measurement of the Milky Way Rotation with Doppler Shift of 21cm Emission

Our antenna has now been upgraded, creating an alt-azimuth mount with wheels, as shown in the images in Fig.1 and Fig.2. With this new, more functional configuration, we aim to repeat some measurements already performed and add others, to obtain more data on the 21 cm emission from neutral hydrogen clouds in our galaxy. This article is therefore a “work in progress” that will be updated with the results obtained from new measurement campaigns.

Fig.1 – Horn antenna with alt-azimuth mount

Fig.2 – Horn antenna with alt-azimuth mount

Per verificare e controllare i dati acquisiti con il nostro sistema è possibile utilizzare un’interfaccia web, Profile Search, resa disponibile dal dipartimento di astronomia dell’università di Bonn, che permette di avere il “profilo HI” espresso in termini di temperatura di brillanza versus velocità radiale [km/s], per la specifica direzione di puntamento del radiotelescopio (espressa in coordinate galattiche). Questi dati sono ottenuti con le osservazioni fatte dai radiotelescopi professionali da 25 m di Dwingeloo, Olanda, e da 30 m a Villa Elisa, Argentina. Dal confronto con questi dati è possibile valutare la “bontà” della nostra misurazione.
Oltre al “HI profile” questa pagina web rende disponibili anche le curve di velocità radiale delle nubi di idrogeno al variare della distanza [kpc] dall’osservatore e dal centro della galassia. Se dalla nostra misura individuiamo un picco di velocità, possiamo associare questo picco ad una nube di idrogeno e dalla sua velocità radiale possiamo ricavare la sua distanza dal centro della galassia. Queste informazioni possono quindi essere utilizzate per determinare la struttura a spirale della galassia.

To verify and check the data acquired with our system, you can use a web interface, Profile Search, made available by the Department of Astronomy at the University of Bonn. This interface allows you to obtain the “HI profile,” expressed in terms of brightness temperature versus radial velocity [km/s], for the specific pointing direction of the radio telescope (expressed in galactic coordinates). These data are obtained from observations made by the 25-m professional radio telescopes at Dwingeloo, Netherlands, and the 30-m at Villa Elisa, Argentina. By comparing these data, we can evaluate the “quality” of our measurement. In addition to the “HI profile,” this web page also provides radial velocity curves of hydrogen clouds related to the distance [kpc] from the observer and from the centre of the galaxy. If our measurement identifies a velocity peak, we can associate this peak with a hydrogen cloud. From its radial velocity, we can calculate its distance from the centre of the galaxy. This information can then be used to determine the spiral structure of the galaxy.

Measures

Below are the results of some measurements. To point the radio telescope, we used galactic coordinates, which are converted to alt-azimuth coordinates with the Stellarium application. The galactic coordinate system is a celestial coordinate system centred on the Sun and aligned with the centre of the Milky Way. The galactic equator is thus aligned with the galactic plane. Similar to geographic coordinates, galactic coordinates are based on longitude and latitude.

For each measurement, we report the pointing direction of the radio telescope, the RF spectrum of the signal, the radial velocity profile of the emitting cloud, and the Gaussian fit. Where possible, we also marked the position of the signal sources on the equatorial plane of the galaxy.

Radial velocity is calculated using the Doppler effect and corrected for the proper motion of our reference system, which includes:
– the Earth’s rotation
– the Earth’s revolution around the Sun
– the Sun’s motion within the local star cluster.
When we’ve taken these three motions into account, we’re in what’s called the local reference system (standard local rest). To obtain the value to subtract (or add) for the correction, refer to the following link: https://www.gb.nrao.edu/cgi-bin/radvelcalc.py

Galactic coordinates longitude l=180° – latitude b=0°

 

Fig.3 – Pointing direction for galactic longitude l=180°

Fig.4 – RF signal spectrum for galactic longitude l=180°

Fig.5 – LSR Radial velocity profile for galactic longitude l=180°

Fig.6 – Gaussian fit of radial velocity for galactic longitude l=180°

Galactic coordinates longitude l=113° – latitude b=9°

 

Fig.7 – Pointing direction for galactic longitude l=113°

Fig.8 – RF signal spectrum for galactic longitude l=113°

Fig.9 – LSR Radial velocity profile for galactic longitude l=113°Fig.10 – Gaussian fit of radial velocity for galactic longitude l=113°

Fig.11 – Position of signal sources in equatorial plane for galactic longitude l=113°

Galactic coordinates longitude l=90° – latitude b=0°

 

Fig.12 – Pointing direction for galactic longitude l=90°

Fig.13 – RF signal spectrum for galactic longitude l=90°

Fig.14 – LSR Radial velocity profile for galactic longitude l=90°

Fig.15 – Gaussian fit of radial velocity for galactic longitude l=90°

Fig.16 – Position of signal sources in equatorial plane for galactic longitude l=90°

Galactic coordinates longitude l=45° – latitude b=0°

 

Fig.17 – Pointing direction for galactic longitude l=45°

Fig.18 – RF signal spectrum for galactic longitude l=45°

Fig.19 – LSR Radial velocity profile for galactic longitude l=45°Fig.20 – Gaussian fit of radial velocity for galactic longitude l=45°

Galactic coordinates longitude l=7° – latitude b=0°

 

Fig.21 – Pointing direction for galactic longitude l=7°

Fig.22 – RF signal spectrum for galactic longitude l=7°

Fig.23 – LSR Radial velocity profile for galactic longitude l=7°Fig.24 – Gaussian fit of radial velocity for galactic longitude l=7°

Conclusions

In this post, we described some measurements of the 21-cm RF emission from neutral hydrogen. The goal is to highlight the spiral structure of the galaxy. With our receiving apparatus, we obtain consistent data, but the small size of the antenna severely limits spatial resolution. Despite these limitations, the initial results are encouraging, but there is still much work to be done.

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