To continue the unparalleled success of the Very Large Array (VLA) for radio
astronomy, the facility is currently being converted to become the 'Expanded
VLA' (EVLA). The EVLA will radically improve the VLA in order to cover the full
0.93-50 GHz radio wavelength range without gaps, provide up to an order of
magnitude better sensitivity, and to allow observations at much larger
bandwidths and spectral resolution as currently possible. For observations of
the 21 cm line of atomic neutral hydrogen (HI), the EVLA offers thousands of
km/s velocity coverage at sub-km/s resolution for targeted observations as well
as an improved spectral baseline stability. In addition, every L-band (21 cm)
continuum or targeted HI observation can be set-up to simultaneously observe a
full z=0-0.53 HI redshift survey at a velocity resolution of a few km/s. In
turn, every HI observation will also yield deep radio continuum images of the
field. These synergies will deliver a wealth of data which opens up a wide
'discovery space' to study the details of galaxy evolution and cosmology.Comment: to appear in the proceedings to the conference: "The Evolution of
  Galaxies through the Neutral Hydrogen Window", Arecibo, PR, US

Ott, Juergen

Perley, Rick

Rupen, Michael

team, the EVLA

English

arXiv

To continue the unparalleled success of the Very Large Array (VLA) for radio astronomy, the facility is currently being converted to become the 'Expanded VLA' (EVLA). The EVLA will radically improve the VLA in order to cover the full 0.93–50 GHz radio wavelength range without gaps, provide up to an order of magnitude better sensitivity, and to allow observations at much larger bandwidths and spectral resolution as currently possible. For observations of the 21 cm line of atomic neutral hydrogen (HI), the EVLA offers thousands of km s^−1 velocity coverage at sub-km s^−1 resolution for targeted observations as well as an improved spectral baseline stability. In addition, every L-band (21 cm) continuum or targeted HI observation can be set-up to simultaneously observe a full z = 0−0.53 HI redshift survey at a velocity resolution of a few km s^−1. In turn, every HI observation will also yield deep radio continuum images of the field. These synergies will deliver a wealth of data which opens up a wide 'discovery space' to study the details of galaxy evolution and cosmology

Ott, Jürgen

Caltech Authors - Main

The EVLA: Prospects for Hi 
Jurgen Ott^ '* ^ Rick Perley**, Michael Rupen** and EVLA team** 
"National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 
"^ California Institute of Technology, 1200 E California Blvd, Caltech Astronomy 105-24, Pasadena, 
CA91125, USA 
""National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville Road, Socorro, NM 87801, 
USA 
Abstract. To continue the unparalleled success of the Very Large Array (VLA) for radio astron-
omy, the facility is currently being converted to become the 'Expanded VLA' (EVLA). The EVLA 
will radically improve the VLA in order to cover the full 0.93 - 50 GHz radio wavelength range 
without gaps, provide up to an order of magnitude better sensitivity, and to allow observations at 
much larger bandwidths and spectral resolution as currently possible. For observations of the 21 cm 
line of atomic neutral hydrogen (Hi), the EVLA offers thousands of kms^ ' velocity coverage at 
sub-kms^' resolution for targeted observations as well as an improved spectral baseline stability. 
In addition, every L-band (21 cm) continuum or targeted Hi observation can be set-up to simulta-
neously observe a full z = 0 — 0.53 Hi redshift survey at a velocity resolution of a few km s^^ In 
turn, every Hi observation will also yield deep radio continuum images of the field. These synergies 
will deliver a wealth of data which opens up a wide 'discovery space' to study the details of galaxy 
evolution and cosmology. 
Keywords: Radio Lines: ISM, ISM: general. Facilities: EVLA 
PACS: 95.55.Jz, 95.85.Bh, 98.38.Gt 
INTRODUCTION 
The Very Large Array (VLA) has played a key role in exploring the radio Universe for 
almost three decades. To continue the unparalleled success of this facility, the VLA is 
currently undergoing a drastic rejuvenation process to become the 'Expanded VLA or 
EVLA. The conversion is very comprehensive and comprises additional and upgraded 
receivers, new broad-band fiber optics, new online control systems, new digital electron-
ics, and a state-of-the-art correlator called WIDAR (Wideband Interferometric Digital 
Architecture). The upgrades will not only improve the continuum sensitivity by about 
an order of magnitude largely due to a substantial increase of instantaneous receiver and 
correlator bandwidth, but they will also provide extreme spectral resolution and a wide 
coverage of the radio spectrum - for the first time it will be possible to observe at any 
chosen frequency in the entire 1-50 GHz radio window. 
The EVLA specifications and its current status is described on ites webpage^ [see 
also 1]. With its unparalleled sensitivity the EVLA is a true pathfinder for the Square 
Kilometer Array (SKA). Even the EVLAs mapping speed matches that of special-
Jlirgen Ott is a Jansky Fellow of the National Radio Astronomy Observatory; email: jott@nrao.edu 
http://www.aoc.nrao.edu/evla 
CP1035, The Evolution of Galaxies through the Neutral Hydrogen Window, edited by R. Minchin andE. Momjian 
O 2008 American Institute of Physics 978-0-7354-0558-5/08/$23.00 
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purpose, ultra-wide field but lower sensitivity and narrow band SKA pathfinders like 
ASKAP, ATA, or MEERKAT in their early incarnations. 
One of the fields of astronomy that the VLA pioneered is the observation of atomic, 
neutral hydrogen (Hi) in our Milky Way, in nearby galaxies and the intragroup medium, 
as well as at higher redsfiifts. Those observations revealed the complex properties of the 
neutral ISM, its importance on star formation and the physics of the interface between a 
galaxy's disk, its halo and the intergalactic medium. The abundance of Hi and its 21 cm 
line properties also proved to be indispensable tools to derive the complex dynamics of 
galaxies in the form of, for example, density waves, rotation curves (and thus the dark 
matter distribution), tidal interactions, and mergers of galaxies. In this article we like to 
summarize what EVLA offers for future Hi observations. 
EVLA RECEIVERS FOR HI OBSERVATIONS 
The HI hyperfine line rest frequency of -^  1.420 GHz falls into the radio L-band. The 
VLA L-band receivers cover a frequency range of 1.25 - 1.8 GHz. The EVLA receivers 
will widen this to 0.93 - 2.1 GHz. At the lower end this will increase the Hi redshift 
coverage from a current upper limit of z -^  0.14 to z -^  0.53. This is an improvement 
of almost Az -^ 0.4 or an additional -- 3.5 Gyr of look-back time (in a WMAP ACDM 
cosmology). The system temperature over telescope efficiency Tsys/e of the EVLA is 
expected to decrease from -^  75 K to about 60 K or less (Tsys of the EVLA is designed to 
hover around -^  26 K), saving about 30% of integration time to reach the same sensitivity. 
The old VLA L-band feedhom design featured a microwave lens in the optical path. 
With the redesigned EVLA receivers, this lens is not required anymore. Thus, ground 
radiation scattered by the lens (and other structural dish elements) into the feedhom is 
largely reduced and the EVLA L-band system temperature improves substantially over 
the VLA at lower elevations; at an elevation of -^  20° the EVLA system temperature is 
about half that of the VLA which corresponds to a four fold increase in sensitivity. 
As part of the EVLA conversion, the L-band receivers will be equipped with new or-
thomode transducers. Until they become available for all antennas in 2012, an interim 
L-band system is currently being installed (with a slightly lower sensitivity and a min-
imum frequency of 1 GHz, corresponding to z -^  0.42). For more information on the 
performance of the EVLA L-band system upgrade and performance, we would like to 
refer to the EVLA webpages and also to Emmanuel Momjian's contribution [2]. 
At lower frequencies the EVLA offers a P-band receiver which covers frequencies 
of 300 - 340 MHz equaling an Hi z -^  3.2 — 3.7 redshift range. This band remains 
unchanged in the VLA to EVLA conversion. The system is not sensitive enough to 
observe typical, gas rich galaxies at the available redshift in Hi emission. However, 
searches for Hi absorption in P-band have been conducted in the past [e.g. 3] and are 
still an option for the EVLA. 
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THE WIDAR CORRELATOR 
Since the installation of the VLA correlator, Moore's law pushed processing speeds 
of computers by -^  5 orders of magnitude. Taking advantage of this, a new correlator, 
WIDAR, will be commissioned in 2008/2009. However, one should keep in mind that 
the current VLA correlator was designed to be very suitable for Hi observations toward 
nearby galaxies. The bandwidth and resolution almost perfectly matches what is needed 
for such observations (bandwidth of -^  200kms^^ at a resolution of -^  5kms^^). But 
the VLA correlator design severely limits the amount of 'discovery space', e.g., very 
wide and shallow lines [for an example, see 4] would not be discovered with the VLA 
without prior knowledge of these features. The narrow bandwidth typically used for 
galaxies of -^  1.5 MHz has only few, supposedly line-free channels at the band edges. 
Any wide lines extending across these channels would not be discovered because they 
would be removed in the process of continuum subtraction. Another case of limited 
discovery space is that other Hi sources in the field, e.g., companion galaxies, remain 
undetected with the VLA if they are at a slightly different velocity than the main target. 
At the other extreme, very narrow line features, e.g., caused by Hi self-absorption are 
smeared out and would be missed in a typical extragalactic Hi setup with its velocity 
resolution of a few km s^^ To open up new discovery space, wide bands at high spectral 
resolution are desired. Such capabilities are provided by the new WIDAR correlator (for 
a description of the technology, see [5]). WIDAR will have a spectral resolution of down 
to Hz ranges and a bandwidth of up to 8 GHz. The full bandwidth is split up into four 
2 GHz baseband pairs and in each baseband pair up to 16 independent sub-band pairs can 
be selected with bandwidths between 31.25 kHz and 128 MHz. The full 8 GHz baseband 
pairs have a minimum of 16384 channels which will always be available. Recirculation 
trades bandwidth for more channels and the maximum number of channels are of order 
4 million. Such a flexible design will cover virtually any need for setups to observe Hi in 
single targets with thousands of km s^^ bandwidth and sub-km s^^ velocity resolution. 
But the EVLA will be able to do more. The L-band receivers and WIDAR will cover 
the entire HI redshift range of Z = 0 — 0.53 at aresolution of 3.2kms^^ when observed 
with two polarization products, and at 6.4km s^^ when observed at full stokes. Other 
configurations will be able to, e.g., stack multiple radio recombination lines in order to 
improve the signal-to-noise of Zeeman splitting experiments in a single observation. 
PIGGY-BACKING 
The velocity resolution of a few km s^^ over the full 0.93 - 2.1 GHz L-band range en-
ables new, unique synergies with other observations. Every EVLA L-band continuum 
observation will also be an Hi redshift survey and vice versa. Also, virtually every tar-
geted HI observation leaves enough computing power in WIDAR to once more perform 
a simultaneous Hi survey over the entire z = 0 — 0.53 range at good velocity resolution. 
As if this would not be enough, the minimum data dumping time for 1 GHz bandwidth 
L-band data is -^  100 ms which allows monitoring of and searches for transient sources 
in the field while simultaneously observing Hi or radio continuum projects (but note 
that current data output limitations imposed by archiving are -^  25 MB s^^ equaling to 
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dumping times of ^ 20 s, when the maximum number of channels and baselines is read 
out). These are exciting new opportunities that will pave the way toward SKA Hi sur-
veys. For example, without spending any dedicated survey time, the z = 0 — 0.53 redshift 
volume around a VLA standard calibrator will accumulate hundreds of hours prior to the 
commissioning of the SKA. This will provide very deep Hi and radio continuum images 
essentially for free. The up to 8 bit quantization of WIDAR also delivers improved high 
dynamic range imaging capabilities which reduce current sensitivity limitations due to 
the inevitable presence of strong sources in any field. The new digital transmission sys-
tem also removes the infamous "3 MHz ripple" and related spectral baseline instabilities. 
This reduces the systematic uncertainties of deep Hi observations dramatically. 
SUMMARY 
Over the VLA, the EVLA will improve Hi observations in terms of a wider L-band 
frequency range (down to -^  930 MHz), a better Tsys/e sensitivity (in particular at off-
zenith elevations), much improved spectral baseline stability, and, most importantly, 
spectral bandwidth and resolution. The L-band receiver improvement guarantee that 
the EVLA will still remain the most sensitive interferometer for Hi in the world for 
at least a decade, until SKA pathfinders will be expanded far beyond the currently 
planned prototypes. The new WIDAR correlator is flexible enough to allow observations 
of virtually any galaxy at sub-kms^^ resolution with thousands of kms^^ bandwidth. 
At the same time, every L-band continuum or targeted Hi observation will also yield 
a blind z = 0 — 0.53 redshift Hi survey at a velocity resolution of a few kms^^ and 
vice versa. It is clear that these new possibilities have their price in a very large data rate 
and that data reduction will challenge today's computing capabilities. The opportunities, 
however, are tremendous and it is up to the community to develop new strategies on how 
to take advantage of the wealth of EVLA data in order to answer the open questions of 
galaxy evolution and cosmology. 
ACKNOWLEDGMENTS 
The National Radio Astronomy Observatory is a facility of the National Science Foun-
dation operated under cooperative agreement by Associated Universities, Inc. 
REFERENCES 
1, p. J. Napier, in Revealing the Molecular Universe: One Antenna is Never Enough, eds. D. C. Backer, 
J, W, Moran, & J, L, Turner, ASP Conf, Ser 356, ASP, San Francisco, 65 (2006), 
2, E. Momjian & R. Perley, These Proceedings (2008). 
3, A, Taramopoulos, F, H, Briggs, & D, A, Tumshek, AJ 107, 1937 (1994), 
4, R, Morganti, J, Holt, L, Saripalli, T, A, Oosterloo, & C, N, Tadhunter, A& A 476, 735 (2007), 
5, B, R, Carlson & P E, Dewdney, Electron. Lett. 36, 987 (2000), 
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