Starting in the 1970s, the Arecibo 305m telescope has made seminal
contributions in the field of extragalactic spectroscopy. With the Gregorian
upgrade completed in the late 1990s, the telescope acquired a field of view.
Population of that field of view with a seven-feed array at L-band (ALFA)
increased by nearly one order of magnitude its survey speed. As a result, much
of the extragalactic astronomy time of the telescope is now allocated to survey
projects, which are briefly discussed. The next technical development stage for
the 305m telescope is foreseen as that of a 40 beam system that would take
advantage of phased array technology: AO40. This would further speed up the
survey performance of the telescope. It is shown how the figure of merit for
survey speed of AO40 would be comparable with that of SKA-precursor facilities,
planned for operation in the next decade. A number of scientifically desirable
new surveys that would become possible with AO40 are briefly discussed.Comment: To appear in AIP Conference Proceedings, "The Evolution of Galaxies
  through the Neutral Hydrogen Window", Feb 1-3 2008, Arecibo, Puerto Rico,
  eds. R. Minchin & E. Momjian. 6 pages including 1 figur

Emmanuel Momjian

Riccardo Giovanelli

Robert Minchin

English

arXiv

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Extragalactic HI Surveys at Arecibo: the Future
Riccardo Giovanelli
Space Sciences Bldg., Cornell University, Ithaca, NY 14853
Abstract. Starting in the 1970s, the Arecibo 305m telescope has made seminal contributions in
the field of extragalactic spectroscopy. With the Gregorianupgrade completed in the late 1990s,
the telescope acquired a field of view. Population of that field of view with a seven–feed array at
L–band (ALFA) increased by nearly one order of magnitude itssurvey speed. As a result, much
of the extragalactic astronomy time of the telescope is now alloc ted to survey projects, which are
briefly discussed. The next technical development stage forthe 305m telescope is foreseen as that of
a 40 beam system that would take advantage of phased array technology:AO40. This would further
speed up the survey performance of the telescope. It is shownthe figure of merit for survey
speed of AO40 would be comparable with that of SKA–precursorfacilities, planned for operation
in the next decade. A number of scientifically desirable new surveys that would become possible
with AO40 are briefly discussed.
Keywords: 21 cm line, Arecibo
PACS: 95.35.+d, 95.80.+p, 95.85.Bh, 98.52.Nr, 98.52.Sw, 98.52.Wz, 98.58.Ge, 98.58.Nk,
98.62.Py, 98.62.Ve, 98.80.Es
EXTRAGALACTIC SPECTROSCOPY AT ARECIBO: A
TIMELINE.
The 21 cm line of neutral Hydrogen was first detected in 1953 ina galaxy other than
our own. The Arecibo Observatory entered operations a decade later, but it took another
decade and the replacement of the 305m telescope primary mirror with a finer surface,
for the field of extragalactic spectroscopy to become establi hed at that facility. In the
following years, the Arecibo telecope became a protagonisti the field of extragalactic
spectroscopy, with fundamental contributions to the understanding of galactic structure,
the predominance of dark matter in galaxies and the case of nature vs. nurture in their
evolution; the large scale structure of the Universe, its baic parameters and peculiar
velocity field. In the 1990s, a second upgrade of the 305m telescope corrected for the
spherical aberration of its primary, delivering an extended fi ld of view at its focus.
The exploitation of that field of view by populating it with multifeed arrays opened a
new era at Arecibo: that of surveys which would cover large fractions of the 13,000
square degrees of the accessible sky. Currently about to start or already underway,
extragalactic spectroscopy surveys that use the ALFA feed array will deliver results of
unprecedented scope and impact in the field. A progressivelylarger fraction of telescope
time is allocated to survey work.
CURRENTLY ONGOING HI SURVEYS
ALFA is a seven-feed focal plane array; surveys enabled by this device started taking
data in 2005. Among them: AGES (theArecibo Galactic Environment Survey) aims
at the investigation of the HI contents in the periphery of nearby galaxies, groups and
clusters; ZOA (theZone of Avoidance survey) will identify galaxies near the Galactic
plane, helping overcome the limitations imposed by dust extinction; AUDS (theArecibo
Ultra Deep Survey) will carry out a high sensitivity pencil beam exploration to z ∼ 0.25.
The most ambitious among the HI extragalactic surveys is ALFALFA (the Arecibo
Legacy Fast ALFA survey)[1], which in blind mode will cover 7000 square degrees of
sky and will detect more than 25,000 HI sources. While ZOA andAUDS will initiate in
2008, ALFALFA and AGES are well under way.
The mediancz of the ALFALFA survey is near 8000 km s−1, the typical scalelength of
baryonic acoustic oscillations. ALFALFA is the only large–scale HI survey that samples
a fair volume of the Universe (HIPASS’ mediancz is less than 3000 km s−1). Moreover,
a large fraction of HIPASS sources suffer from confusion, making the identification of
optical counterparts difficult and often impossible without follow–up higher resolution
HI observations. The smaller Arecibo beam largely obviatesth problem: more than
95% of ALFALFA sources can be unambiguously associated withthe correct optical
counterpart.
The collective science goals of extragalactic HI surveys atArecibo is described in the
introductory paper by Giovanelli, of these proceedings, and several initial findings of
those surveys have been presented. Here, it is worth refreshing our memory on one of
such findings, briefly discussed in the presentation b Brian Ke t[2]. ALFALFA reports
the discovery of huge gas streams in the outer regions of the Virgo cluster, extending
hundreds of kpc and witnessing to dramatic events in therevolution of galaxies in those
environments. The northern part of the cluster, now fully mapped by ALFALFA to a
detection limit of 2× 107 M⊙, is shown in Figure 1. The orange shading shows the
extent of the X–ray emission centered on M87, by hot intracluster gas (ICM); the blue
dots are HI detections, their areas proportional to their HImasses. The picture clearly
illustrates the impact of the ICM on gas content of galaxies.The red stars show the
location of HI streams, some of which can be tracked over degrees (1◦ ≃ 300 kpc at the
cluster distance. These features are detected near the column density limit of ALFALFA
(∼ 5× 1018 cm−2), suggesting that ALFALFA — while allowing us to seeing these
structures for the first time — may be seeing just “the tip of the iceberg”.
AO40
The Arecibo telescope has the largest collecting area in world radio astronomy, about
1/10 of the future SKA and comparable with or larger than the SKA precursor tele-
scopes currently under design or construction. ALFA has provided a nearly one order
of magnitude survey speed, with respect to single beam operation. However, the focal
plane of the telescope is undersampled by about a factor of 16by ALFA (beam centers
are separated by approximately 2 beamwidths). An improvement on this state of affairs
can be achieved with the development of focal plane phased arrays (FPPA). Two im-
Source: Brian Kent
FIGURE 1. Composite of the Virgo cluster: X ray emission (orange), HI sources (blue circles) and HI
streams (red stars). See text for details.
TABLE 1. Parameters for the Computation of Survey Speed
D(m) Beam(’) Ntel ×Nbeam FoV Tsys(K) BW(MHz) FoM
AO1 225 3.5 1×1 1 25 100 1
ALFA 225 3.5 1×7 7 30 300 14
AO40 225 3.5 1×40 40 50 300 30
APERTIF 25 30 14×25 1800 50 300 33
ASKAP 12 60 30×30 8800 50 300 37
portant advantages of FPPAs are relevant: (i) they can exploit a arger physical area of
the focal plane and (ii) beams can be packed together more clos ly than in feed arrays,
such as ALFA. A FPPA approaching Nyquist sampling of the FOV may allow for 40 in-
stantaneous beams at L–band. We shall refer to such an implementation on the Arecibo
telescope as “AO40”. FPPA technology is not well developed yt, and it is still uncer-
tain that such devices will be able to deliver the low temperaure and broad bandwidth
performances we have become used to. We underscore that mostSKA precursor future
telescopes, such as ASKAP and APERTIF, are betting on the succe s in implementing
such technology.
Using a standard expression for the survey speed figure of merit, FoM, of a telescope
of effective apertureAe f f , system temperatureTsys, total instantaneous solid angle of the
surveyΩ f ov and bandwidthBW :
FoM ∝ (Ae f f/Tsys)2Ω f ovBW, (1)
and using the same expectation values forTsys = 50 K andBW = 300 MHz for FPPAs at
AO40, ASKAP and APERTIF, the relativeFoM for those 3 systems with respect to the
single beam Arecibo are respectively 30, 33 and 37, as shown in Table 1. TheFoM for
ALFALFA is 5. With the implementation of FPPA technology on which future facilities
rely on, Arecibo (AO40) can be as fast a survey machine as any currently planned.
The main disadvantages of Arecibo with respect to those precursor experiments are
that it currently has a limited field of view and that it will always have worse angular
resolution than a comparable collecting area with a distribu ed aperture. The paramount
advantage is, of course, that Arecibo already exists and is awell understood, efficient
photon bucket.
Is there a niche for Arecibo as a competitive HI survey machine? The density of
galaxies per unit solid angle increases obviously withz. At z ∼ 0.1, confusion within
the ∼ 4′ HPFW of the Arecibo beam cannot often be resolved, unless high quality
optical redshifts are available for the sources within the beam, so that confusion can be
overcome kinematically. This limits the pursuit of blind HIsurveys with the Arecibo
telescope at highz. A practical rule of thumb indicates that the median centroiding
accuracy of a source smaller than the telescope beam, detected at signal–to–noiseStN
with a beam of HPFWβ , is approximatelyβ/StN. A source detected at a marginal
Stn ≃ 6 will, at 21 cm wavelength, have an associated positional error somewhat larger
than 0.5’. Identification of the optical counterpart of an HIsource without the benefit
of an accurate optical redshift at even modestz ∼ 0.1 thus becomes doubtful. The
maximum distance at which a given HI mass can be detected increases only ast1/4s ,
wherets is the dwell time per beam. The highestz of any source detected in HI emission
is 0.28, achieved by Catinella and collaborators at Arecibo[3].Detection of even the
brightest galaxies at such redshift requires dwell times ofeveral hours at Arecibo, and
much longer at any other telescope. The survey niche for AO40is thus that ofz ∼ 0,
deep surveys. A number of those have especially strong astrophysical appeal.
FUTURE EXTRAGALACTIC BLIND HI SURVEYS.
A necesary observational activity that will require significant telescope time after com-
pletion of currently ongoing surveys consists in the follow–up corroboration of inter-
esting but weak sources and deeper mapping of a subset of suchobjects. Statistical
exercises (e.g. [1]) indicate that∼20% of the total survey time will be required for those
functions after survey completion. At the current rate of telescope time allocation, AL-
FALFA will require 7 calendar years to complete. The next stage of exploitation of the
capabilities of an AO40 facility may be the extension of suchsurveys as ALFALFA and
AGES to fuller sky coverage. The extension ofALFALFA++ to cover the 13000 square
degrees of sky accessible with the telescope would require approximately 500 hours of
AO40 time. An extension ofAGES++ to a more extensive set of targets, including
clusters and groups of galaxies withcz < 10000 km s−1, to cover 3000 square degrees
with 300 seconds of dwell time per beam, would require 1500 hours with AO40. These
would constitute a veritable legacy of complete extension.
More interesting, however, would be deeper surveys that would reach significantly
fainter levels of the HI mass function. As mentioned before,a significant new discovery
of ALFALFA consists in the detection of numerous sources with no optical counter-
part and giant streams of HI, very near the detection limits of the survey. A survey of
the cluster with 10 times the sensitivity of ALFALFA would bec rtain to reveal new
marvels and, bringing the HI detection limit down to 2× 106 M⊙, would provide an
unprecedented investigation of the low mass end of the HI mass and luminosity func-
tions. A Virgo Deep survey that would cover 200 sq. deg. with an effective dwell time
of 4000 sec per beam would require 2000 hours with AO40. Such Virgo Deep survey
would deliver genuinely new science and provide a fundamental scientific reference.
TheλCDM paradigm of galaxy formation predicts the presence of low mass systems
in mini–filaments within cosmic voids. Such structures havenever been detected. The
nearest large–scale void in the local universe lies in the foreground of the Pisces–Perseus
supercluster. With a diameter of order of 60 Mpc, its center is nearcz ≃ 2500 km s−1.
A Deep Void survey that would cover 400 square degrees of the void solid angle
with 10 times the sensitivity of ALFALFA would require some 4000 hours of AO40
time. Projected along the line of sight to the central regions f the Local Group (LG),
such a survey would permit detection of LG sources with as little as 5×103 M⊙ and
unprecedented column density sensitivity (near 3× 1017 cm−2) in resolved sources in
the LG and High Velocity Cloud complexes.
FUTURE EXTRAGALACTIC TARGETED HI SURVEYS
HI can be currently detected in emission from extragalacticobjects out to az ∼ 1/3,
with dwell times of order of 10 ksec at AO. HI sources at distances> 200 Mpc are
most unlikely starless, as the masses required for their detection with AO correspond
squarely in the category of giant galaxies, for which no theoretical expectation exists of
any inhibition to profuse star forming activity. Thus the study of such objects in the HI
line is best done via targeted, rather than blind searches.
GASS is an approved, targeted Arecibo survey to obtain complete “ID cards” for a
sample of galaxies in the local Universe (z < 0.04). It uses the SLOAN and GALEX
surveys as target finders for objects for which cold gas content and kinematics are to be
obtained. The collective information will be used to measure galaxy parameters, such as
masses, current and past star formation activity, etc. The survey sample includes some
1000 objects, of which a fraction of∼15% will be provided by ALFALFA. A possible
extension to GASS++, which would include objects toz ≃ 0.3 (a lookback time of
about 4 Gyr), would be able to survey significant predicted changes in scaling laws of
disks, relying on the uniquely high quality of the kinematicinformation in the HI data.
From observations of damped Lyman Alpha systems we know thatΩHI ≃ 10−3 at
z > 2; estimated values atz = 0 are about a factor of 3 to one order of magnitude lower.
Theoretical estimates of theΩHI(z) differ substantially ([4],[5],[6]Baughet al. 2004;
Cenet al. (2003); Nagaminet al. 2005; Millenium run). The steeper part of the change
in ΩHI(z) may have taken place between look-back times of 1–7 Gyr. The obs rvational
characterization of that function is of special importance, with respect to SKA plans for
the “Billion HI Galaxy” surveys, the measurement of baryon acoustic oscillations and
the elucidation of the nature of dark energy. As pointed out before, in spite of its large
collecting area, sensitivity issues and confusion preventthe effective use of the Arecibo
telescope for observations of individual galaxies at even higher redshift. However, the
Arecibo beam is well matched to the angular size of intermediat redshift clusters of
galaxies. For example, A1835, atz = 0.25, has an SZ signature 4’ in diameter. Pointed
observations would aim at the detection of the integrated HIemission of the whole clus-
ter. An HI mass of 1012 M⊙ spread over a width of 1500 km s−1 could in principle
be detectable in less than 10 hours with the Arecibo telescope. A number of possible
complications could make such an experiment difficult, however: the ability to detect a
broad spectral signature against system standing waves, the increase in the continuum
noise especially by radio galaxies in the cluster itself, easily come to mind. Yet, the
concept offers substantial promise, which may lead to the characterization ofΩHI(z)
through much of the interval of cosmic history to which ground–based DLA work is
blind (z < 1.6). This is an experiment that will require much tender loving care in the
understanding of telescope systematics. A dual beam optionand sophisticated rfi identi-
fication/excision techniques would be required for such a HICluster Integral,HIClint ,
survey.
ACKNOWLEDGMENTS
This work has been supported by NSF grants AST–0307661, AST–0435697, AST–
0607007. The Arecibo Observatory is part of the National Astronomy and Ionosphere
Center which is operated by Cornell University under a cooperative agreement with the
National Science Foundation.
REFERENCES
1. Giovanelliet al. 2005,AJ 130, 2589
2. Kent, B.R. 2008,these proceedings
3. Catinella, B. 2008,these proceedings
4. Baugh, C.et al. 2004,NewAR 48, 1239
5. Cen, R.et al. 2003,ApJ 598, 741
6. Nagamine, K.et al. 2005,IAU Symp 216, 266


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