ALFA Extragalactic HI Studies

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.

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.

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

ALFALFA

AGES

AUDS

ZOA

NGC 2903 ALFA Survey

Project Details

	ALFALFA – Riccardo Giovanelli
	AGES – Jon Davies
	AUDS – Wolfram Freudling
	ZOA – Trish Henning
	NGC2903 – Judith Irwin

General E-ALFA Information