ALFA Extragalactic HI Studies


Introduction Highlights General Information for
the New Observer
The E-ALFA Consortium
E-ALFA Projects Documentation Publications and
Data Release
Links & Contacts


Since the installation of ALFA, extragalactic HI astronomy with Arecibo has entered a new phase. While previously Arecibo was often used to observe galaxies that had been found elsewhere, mainly with optical telescopes, now the science starts with galaxies discovered at Arecibo. The reason behind the change is that ALFA allows Arecibo to carry out surveys, covering large areas of sky rapidly and with unprecedented sensitivity. This also makes it possible to cover areas around selected sources, allowing for the detection of structure too big to be seen with interferometers or mapped efficiently with single-pixel detectors.


* First NSF-sponsored undergraduate ALFALFA workshop held at Arecibo, January 13-14, 2008.
* ALFALFA discovery of an HI cloud complex in the Virgo Cluster.
* Arecibo's sensitive new eye begins massive sky survey (Press Release)
* Detection of HI rich galaxies with the Ultra Deep Survey of ALFA.

General Information for the New Observer

The first consideration if you are thinking about HI observations is whether ALFA or L Band Wide (LBW) is the best receiver for you. LBW has higher sensitivity for pointed observations and lower side-lobe levels, thus if you want a single pixel spectrum of a galaxy, you should use it rather than ALFA. However, if you want to map an area of sky then read on...

Assuming that you do want to map a region of sky, you next need to check whether you need separate observation or whether the science can be done within one of the existing E-ALFA projects (see below). In particular, if you only need shallow data on a region of sky (noise levels of a couple of mJy per beam) then you should join the ALFALFA project team and carry out the science within the collaboration, and similarly if you want to work on one of the areas being targetted by AGES you should join that project.

If nobody else is mapping the area you want to to the depth you require, then you need to look at how to plan your observations. The AGES observing strategy is probably the easiest to adapt to your requirements, and it is relatively simple to calculate the observing time needed for a given depth and sky coverage. In this strategy, drift scans are used to cover the sky with a displacement of 1.5′ between subsequent scans. One pass like this gives a little under 100 s beam-1 integration time, multiple passes can be used to build this up as required. Obviously, the time for each drift scan matches the RA range that is to be covered, while the number of scans in declination is given by (dec-range/1.5) + 1 (for dec-range in arcminutes). Thus the on-source time is (ra-range)×((dec-range/1.5) + 1).

An allowance also has to be made for repositioning the telescope between drifts. This is normally around 20% of the drift time (minimum around 1 minute), although for long drifts (> 15 mins) observing becomes more efficient. As normal, around 10-15 minutes per night should be allowed for start-up time and getting the telescope into position. Note also that when planning your observations, you should attempt to stay above 15° zenith angle as scans taken below this will be substantially noisier and the beam-shapes more distorted. Do not forget when calculating the LST range requested that your observations will start earlier and end later than your central RA by half of your drift-length.

The AGES strategy is normally executed using command-file observing within CIMA. Tools exist at Arecibo to help in creating command files for this observing mode. Data reduction is normally done using the AIPS++ programs LiveData and Gridzilla, which were originally developed for HIPASS and have since been modified for use by AGES, AUDS and ZOA at Arecibo. It is, however, possible to adapt the command files for reduction using the IDL routines developed for ALFALFA if this is preferred.

The E-ALFA Consortium

* Consortium Guidelines

The Consortium Guidelines lay out the general rules for the running of the E-ALFA Consortium. Following the adoption of these guidelines, a Coordinating Committee was elected consisting of representatives of E-ALFA Consortium projects. The current Coordinating Committee consists of Jon Davies (Cardiff), Wolfram Freudling (ESO), Riccardo Giovanelli (Cornell), Trish Henning (New Mexico) and Judith Irwin (Queen's, Canada)

* Membership

Membership in the E-ALFA Consortium is open to all interested parties irrespective of institutional affiliation, career level, expertise or scientific background. All members of E-ALFA projects are members of the consortium, but not all consortium members are necessarily members of project teams.

List of members

E-ALFA Projects


* Overview:

The Arecibo Legacy Fast ALFA (ALFALFA) survey is covering 7000 sq. deg. of sky at high galactic latitude with an integration time of 48 s/beam. With a wide areal coverage and higher sensitivity than earlier wide-area surveys on other telescopes, this will probe the faint end of the HI mass function as well as supporting a wide range of other science. The survey is divided into a number of 'cubes' which will be released incrementally as the survey progresses through tools developed in collaboration with the US National Virtual Observatory.

See the ALFALFA web site for more details.

* Observing Technique:

ALFALFA uses the “fixed-azimuth-drift” observing mode. The telescope is held stationary with ALFA at a fixed azimuth, normally on the meridian except where this would bring the instrument too close to the zenith, and the sky is allowed to drift past while data is taken. The array is rotated to an angle of 19° (on the meridian), giving an equal spacing between all the ALFA beams of 2.1′. In order to better reject spurious signals and rfi and to avoid the scalloping that would arise from unequal pixel gain, the survey has two passes, with the second pass interleaving the beams of the first.


* Overview:

The Arecibo Galaxy Environment Survey (AGES) is the second tier of the high galactic latitude “wedding cake”, covering a smaller area of 200 sq. deg. with a longer integration time of 300 s/beam. Thirteen areas of sky have been selected, covering the full range of environments from the Local Void through isolated galaxies, galaxy pairs and galaxy groups to the Virgo Cluster.

See the AGES web site for more details.

* Observing Technique:

AGES uses the “drift-and-chase” observing mode, a modified form of the fixed-azimuth-drift where the telescope is held at a fixed azimuth for a short period (typically 20 minutes or less) before being moved back to the starting RA to begin drifting again, thus allowing the same RA range to be covered a number of times in a single night. As with ALFALFA, the array is rotated to give an equal separation between the beams, with the necessary rotation angle depending on the position on the sky. Between each scan, there is an offset of 1.5′ in declination, which has the effect that every beam individually makes a Nyquist-sampled map of the sky, thus minimizing any effects due to differences between the beams. This scan pattern is repeated three times to build up the required integration time.


* Overview:

The Arecibo Ultra Deep Survey (AUDS) is the third tier of the “wedding cake”, covering a very small area (0.36 sq. deg.) with incredible sensitivity. This survey is aimed at detected galaxies beyond z=0.1, and the number detected will indicate how strong the evolution of HI mass with redshift is. In a no-evolution scenario, 36 galaxies are expected, but if there is strong evolution then half as many again should be found.

See the AUDS web site for more details.

* Observing Technique:

AUDS uses a drift-and-chase technique similar to AGES (see above), but with one major difference: in AUDS the beams are rotated in such a way that three of them cross the same point in the sky. This sacrifices areal coverage in favour of three times greater integration time on the region of interest, allowing AUDS to reach the great depth necessary for the project in a minimum of telescope time.


* Overview:

The Zone Of Avoidance (ZOA) survey is aimed at discovering structure behind the Milky Way, in the part of the sky where galaxies cannot be observed in the optical due to the dust in our galaxy. Radio waves, however, can penetrate the galaxy with ease, thus galaxies can be detected that are completely hidden to other telescopes, revealing large scale structure that would otherwise remain unknown.

See the ZOA web site for more details.

* Observing Technique:

ZOA takes advantage of the ability of Arecibo to perform comensal observing by taking all of its data during observations by other projects. To date commensal observing has been carried out with G-ALFA projects; once the E-ALFA backend arrives commensal observing with P-ALFA projects will begin.

* NGC 2903 ALFA Survey

* Overview:

The NGC 2903 ALFA Survey was carried out during the shared-risk c ommissioning period of ALFA. It surveyed NGC 2903 and the surrounding field in a manner similar to that eventually adopted by AGES and also made beam maps at the AZ and ZA positions of the drift scans which are being used to clean the data to remove the effects of ALFA sidelobes from the map of the galaxy.

See the NGC 2903 Project web site for more details.


* E-ALFA Memos and Documents
* ALFA Memos
* CIMAFITS documentation
* CIMA (telescope control software) homepage
* Drift mapping precursor experiment documentation
* ALFALFA survey technial documents

Publications and Data Release

* ALFALFA Data Release
* AGES Data Release
* E-ALFA Papers
* E-ALFA at IAU Symposium 244
* E-ALFA at AAS 207
* ALFALFA publications(from the Cornell Extragalactic Group website)
  Title Location Author
* E-ALFA Coordinating Committee Cornell University Martha Haynes
* Arecibo Legacy Fast ALFA Survey (ALFALFA) Cornell University Martha Haynes
* Undergraduate ALFALFA Team Cornell University Becky Koopmann
* Arecibo Galaxy Environment Survey (AGES) NAIC Robert Minchin
* Arecibo Ultra Deep Survey (AUDS) ESO Wolfram Freudling
* The GALEX Arecibo SDSS Survey (GASS) MPA Barbara Catinella
* Zone of Avoidance Survey (ZOA) NAIC Trish Henning
* NGC2903 Survey Queen's University Judith Irwin
* NGC2903 Survey Data Archive Cornell University Adam Brazier
* 1st E-ALFA Meeting (March 2003) NAIC Robert Minchin
* 2nd E-ALFA Meeting (May 2004) NAIC Robert Minchin

People to Contact for More Information

* Project Details

For more details on a specific survey project, please contact that project's Principal Investigator:

* ALFALFARiccardo Giovanelli
* AGESJon Davies
* AUDSWolfram Freudling
* ZOATrish Henning
* NGC2903Judith Irwin

* General E-ALFA Information

For further information on E-ALFA generally, please contact Emmanuel Momjian or Robert Minchin.