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Next Generation Radio Astronomy

Capabilities Upcoming with the eVLA and ALMA

The Expanded Very Large Array (EVLA) project will truly revolutionize chemical mapping capabilities at frequencies below 50 GHz in various astronomical environments.  Combined with the GBT, we will now have the capability to determine the spatial distributions and morphology of the molecular emission of complex species from spatial scales from 10’s of arcmins to milli-arcsecs.  Furthermore with the spectral frequency range now available with the EVLA, it is now possible to obtain a complete inventory of known interstellar molecules and their transitions that can be evaluated and synergistically used as multiple probes of the physical, chemical, kinematic and dynamic conditions present various astronomical environment. To fully understand the role of chemistry, the molecular inventory and overall distribution and morphology of molecular species is necessary to ascertain the trends in molecular abundances and distributions which provides the necessary data to test predicted formation routes and reaction dynamics.


The EVLA program modernizes the electronics of the Very Large Array (VLA) in order to improve several key observational parameters by an order of magnitude or more.  The EVLA project will be completed on time and on budget in 2012, 11 years after it began. The total investment to the EVLA project from NSF funding is $59 million with $16 million in redistributed effort from the NRAO Operations budget (in FY2006 dollars).  Its key observational goals are (1) complete frequency coverage from 1 to 50 GHz; (2) continuum sensitivity improvement by up to an order of magnitude (nearly two orders of magnitude in speed) by increasing the bandwidth from the VLA’s 100 MHz per polarization to 8 GHz per polarization; and (3) implementation of a new correlator that can process the large bandwidth with a minimum of 16,384 spectral channels per baseline. A comparison of some of the EVLA performance parameters with those of the VLA is provided in Table 1.  As you can see, the relative increase of many key parameters to the success of molecular imaging (i.e., not mapping a single transition but rather mapping the entire ensemble of transitions of a particular molecular species to obtain a true picture of the molecular concentration)  is orders of magnitude greater than what was available before the EVLA.  These parameters include maximum bandwidth, number of frequency channels and the frequency coverage now available.

Note: The "Factor" gives the factor by which the EVLA parameter will be an improvement over the equivalent VLA parameter.


An example of the capabilities of the EVLA can be seen in the spatially resolved spectra seen below from Orion K-L.



Above 30 GHz, the Atacama Large Millimeter/Submillimeter Array (ALMA) will be the premier ground based observational facility in the world for at least the next 40 years.  The increase in frequency and resolution at these wavelengths will enable us to map the distribution and morphology of molecular emission in regions of the universe at spatial scales smaller that what is currently possible with even the Hubble Space Telescope (HST).  The absolute operational limit of the Wide Field and Planetary Camera 2 (WFPC2) on the HST is 310nm which provides a spatial resolution of 0.014”.  ALMA, in the largest telescope configurations and highest operational frequencies, will be able to resolve objects with a spatial resolution of 0.004”, an increase of 3.5 times over the HST and more than 30 times larger than current operating millimeter wave telescopes.  With this resolution, it will be possible with ALMA to investigate the molecular emission in the disks around newly forming planetary systems and to determine chemical gradients within the pre-solar nebulae of these systems. 


When completed at the end of 2012, ALMA will consist of 50 antennas in its main array and 16 more in the ALMA Compact Array (ACA).  This corresponds to a collecting area of over 5655 m2 in the main array and 914 m2 for the ACA. By comparison, the Combined Array for Research in Millimeter Astronomy (CARMA), which operates 6, 10-m dishes and 9, 6.1-m dishes has a total collecting area of  772m2, a factor of 8.5 times less than ALMA.  The ALMA array will consist of a variety of configurations, a large number of observing modes (standard interferometry, mosaicing, fast-switching, etc.) and complete frequency coverage of the mm/submm windows up to 1 THz.  Below is a figure that illustrates the frequency coverage available to ALMA users and the atmospheric transmission of the ALMA site.  This is the largest frequency coverage available for any astronomical observatory ever built.  By comparison, the CARMA array’s operational frequency is from 85-115 GHz and 215-270 GHz, a factor of more than 10 times less than the available frequency coverage with ALMA.   As such, ALMA will bring to the scientific community, frequency coverage and spatial scales that cannot be probed by any current facility and provides orders of magnitude increases in sensitivity, frequency coverage and resolution than any existing millimeter/submillimeter observatory in the world.


A timeline of some of ALMA's performance parameters is provided below.



Early Science

Array Completion


≥16 x 12m

At least 54 x 12m & 12 x 7m


Bands 3, 6, 7, 9

Bands 3, 4, 6, 7, 8, 9 & 10

Maxiumum Bandwidth

16 GHz (2 polarizations x 8 GHz

Correlator Configurations

21 (0.02 - 40 km/s)

71 (0.01 - 40 km/s)

Maximum Angular Resolution

Maximum Baseline

250m (possibly 500m)

15 km

Continuum Sensitivity (60 sec, Bands 3-9)

~0.2-4.2 mJy

~0.05 - 1 mJy

Spectral Line Sensitivity (60 sec, 1km/s, Bands 3-9)

~30-250 mJy

~7-62 mJy



ALMA is running tests and working well. The figure to the left is recent test data observing the hot core of G34.26+0.15 at 3mm. The spectrum of the selected region gives many molecular lines. ALMA has given the call for early science and proposals will be accepted from June 1st – 30th. More information about ALMA early science Cycle 0 can be found here.



Images from NRAO / AUI / NSF