ALMAHow does it work and how to use it?

Dr. Lizette Guzman-RamirezESO Fellow for ALMA

What is ALMA?

• Atacama Large Millimeter and sub-millimeter Array.

ALMA as an Interferometer

• 66 antennas working as one radiotelescope.• Is as if we had a 15km radiotelescope.

ALMA as an Interferometer

• ALMA is 10-100 times more sensitive and have 10-100 times better angular resolution than existing instruments.

• Total cost US$ 1.3 billions

• One antenna the resolution goes λ/D, where D is the diameter of the antenna.

How interferometry works

• Two antennas the resolution goes λ/B, where B is the baseline between the two antennas.

D

B

How interferometry works

Interferometry

• The 66 antennas and electronics working with a precision of one picosecond (10-12s).

• The signals from the different antennas must be combined in a way that the path followed from each antenna until it is combined at the central computer (the correlator) must be known with an accuracy equal to the diameter of a human hair (10-9m).

• Seven weather stations, and specially-built Water Vapor Radiometers (WVR) to measure the amount of line-of-sight water vapor present in the atmosphere, will be used to correct for the atmospheric effects.

How interferometry works

The antennas are moved using a transporter

Main parts of an ALMA antenna

• Antenna– It captures and concentrates

the radio waves coming from the astronomical source at the point known as the focus. The light is reflected by a second reflecting surface –called a subreflector- to a point behind the parabolic surface, where there is a receiver geared to capture the signal concentrated by the antenna.

54 (12m) and 12 (7m) antennas12m Array

Baseline (BL)

7m Array Atacama Compact Array (ACA)

The Front End is where the signal gets amplified, it has to be cooled to 4K

The Back End to the central computer (correlator), and subsequently transmits it to the central building.

The correlator is the fastest supercomputer that take the signals from the antennas and combine them

ALMA bands

ALMA bandsALMA Band Frequency (GHz)

1 31 - 45

2 67 - 90

3 84 - 116

4 125 - 163

5 163 - 211

6 211 - 275

7 275 - 373

8 385 - 500

9 602 - 720

10 787 - 950

ALMA bands

Main molecules per bandALMA Band Frequency (GHz) Main Lines

1 31 - 45

2 67 - 90

3 84 - 116 CO(1-0)

4 125 - 163 H2O

5 163 - 211

6 211 - 275 CO (2-1)

7 275 - 373 CO (3-2), [CII] z=5

8 385 - 500 CO (4-3), [CII] z=3

9 602 - 720 CO (6-5), [CII] z=2

10 787 - 950 CO (7-6), CO (8-7)

Splatalogue

• The splatalogue catalog is maintained by NRAO and is a compilation from several databases for molecular and atomic lines. Many spectral lines are listed in it, so it is a very useful tool in planning observations for ALMA.

• http://www.splatalogue.net

Splatalogue

Observing modes

• The telescope can be set in “frequency division mode” (FDM), which means that high spectral resolution can be reached but with bandwidth, quantization, and Nyquist sampling restrictions.

• In the “time division mode” (TDM), the entire 2 GHz bandwidth is used at 3 bit level quantization. This mode is used to observe continuum

Velocity resolution per band per observing mode

Spatial resolution per frequencyThis depends on the baseline, for this case 1km baseline was used for bands 3-7 and 1.5km for 8 & 9. For Cycle 3

longer baselines will be offered, up to 15km (resolution is roughly an order of magnitude better)

For a baseline of 16km at 650GHz the resolution is 0.005” (5marcsec)

Science goals

• ALMA Deep Field – High-z galaxies• Star and Planet formation• Chemistry in molecular clouds• Map gas and dust in the Milky Way and other

galaxies• Evolved starts – dust formation• The Sun – origin of solar wind

ALMA discoveries• Multiwavelength

composite of the Antennae, showing their namesake tidal tails in radio (blues), past and recent starbirths in optical (whites and pinks) ALMA’s first mm/submm test views, in Bands 3 (orange), 6 (amber), & 7 (yellow), showing detail surpassing all other views in these wavelengths.

ALMA discoveries• The dust ring around the

bright star Fomalhaut. The underlying blue picture shows an earlier picture obtained the HST. The new ALMA image has given astronomers a major breakthrough in understanding a nearby planetary system and provided valuable clues about how such systems form and evolve.

ALMA discoveries

• A spiral structure around the old star R Sculptoris. This feature has never been seen before and is probably caused by a hidden companion star orbiting the star. This slice through the new ALMA data reveals the shell around the star, which shows up as the outer circular ring, as well as a very clear spiral structure in the inner material.

ALMA discoveries• This composite shows the region around the massive star-forming region

SDC 335.579-0.292. The Spitzer view is at infrared wavelengths (3.6, 4.5 and 8.0 microns) and the ALMA view is at wavelengths around three millimeters. The yellow blob at the center of the ALMA images is a stellar womb with over 500 times the mass than the Sun. The embryonic star is feeding on the material that is racing inwards. It is expected to give birth to a 100Mo star.

ALMA discoveries

• Observations of CO fog by hunting for a different molecule known as diazenylium (N2H+). This fragile molecule is easily destroyed in the presence of CO gas, so would only appear in detectable amounts in regions where CO had frozen out, and is hence a proxy for CO ice.

ALMA discoveries• This image from ALMA

shows the distribution of molecular gas close to the supermassive black hole at the centre of the galaxy NGC 1433. As well as discovering the spiral structure ALMA observations have also revealed an unexpected small outflow of material from the central black hole.

ALMA discoveries

• ALMA data (in red) shows newly formed dust in the center of the remnant. HST (in green) and Chandra (in blue) show the expanding shockwave.

ALMA discoveries• Dust and gas disk around

HD142527. The dust and gas distributions observed by ALMA are shown in red and green, respectively. Near-infrared image taken by the NAOJ Subaru Telescope is shown in blue. The image clearly shows that the dust is concentrated in the northern (upper) part of the disk.

ALMA discoveries

• Detection of radio emission from GRB host galaxies, which had been a long-sought goal for astronomers, was finally made possible by ALMA with its unprecedentedly high sensitivity.

ALMA discoveries

• ALMA Pinpoints Pluto to Help Guide NASA’s New Horizons Spacecraft. The cold surface of Pluto and its largest moon Charon as seen with ALMA on July 15, 2014.

ALMA discoveries• The sharpest image ever

taken by ALMA. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.