Extragalactic Science Jim Condon

Twelfth Synthesis Imaging Workshop It’s been a long week filled with talks starting to blur together in your head. Also, I realize am the last lecturer standing between you and the workshop dinner. So instead of trying to impress you with how much I know, it would be better to pose only one question and provide one take-home answer. Twelfth Synthesis Imaging Workshop

How can synthesis imaging help me do better science? This is the one question. More general than “extragalactic” science. To answer it, consider the costs and benefits of synthesis imaging? Twelfth Synthesis Imaging Workshop

Science benefits of synthesis imaging Higher angular resolution: diffraction limited by the size of the array, not by the size of each telescope Correlation zeros or differentiates out most unwanted effects (e.g., varying atmospheric emission, ground radiation, “1/f” noise, RFI, …) Higher sensitivity is reached via longer practical integration times and lower “confusion” caused by unresolved background sources Higher spectral resolution: lag correlators measure frequencies very accurately with clocks, not wavelengths with rulers. Higher dynamic range is possible because the point-source response can be controlled and modified (e.g., selfcal, clean) and is nearly independent of mechanical pointing errors. Higher astrometric accuracy by using clocks instead of rulers to determine angles, and eliminating plane-parallel atmospheric refraction Single-dish telescopes see “everything”, most of it unwanted. Correlation multiplies the voltages from different antennas, uncorrelated signals have zero average. First two bullets: multiplying interferometers usualy work as advertised; single dishes, sometimes. Spectral resolution versus optical spectrometers (e.g., mechanical gratings) Twelfth Synthesis Imaging Workshop

Beating Confusion (GB 300-ft at 1.4 GHz) 1.4 GHz image made with the 300-foot telescope. 12 arcmin FWHM, peak source shown ~ 1 Jy, rms fluctuations ~ 30 mJy caused by sources, not by receiver noise, so all of the bumps are “real”. Ring shows area in next slide.

NVSS (45 arcsec beam) grayscale under GB 300-ft (12 arcmin beam) contours σc ~ 1 μJy/beam × (θ / 5 arcsec)2 × (ν / 1.4 GHz)-0.7 “RMS” confusion c  Note multiple NVSS sources contributing to one GB “source”, effect on GB positions and angular sizes. Single dishes are badly confusion limited at cm wavelengths, so interferometers are needed for sensitive continuum observations. Still a problem with GBT observers not understanding this. The EVLA is sensitive enough that confusion can be a problem.

22 GHz H2O maser disk imaging and astrometry with the HSA = GBT + VLBA Angular resolution: 0.0003 arcsec Spectral resolution: 1 km/s Differential astrometric precision: 0.000002 arcsec ≈ 10-11 radians Extreme example: 200X resolution jump from Galileo telescope (10 arcsec) to HST (.05 arcsec); another 200X resolution from HST to HAS (.0003 arcsec). 1e-11 rad is angle subtended by thickness of human hair (1e-4 m) in Moscow (1e07 m away), seen from Socorro.

Maser rotation curve of UGC 3789 Distance = 50 ± 7 Mpc so H0 = 69 ± 11 km/s/Mpc 1.09 × 107 solar mass BH or dense “star” cluster? Plummer distribution: ρ(r) = ρ0 (1 + r2/ c2)−5/2 Evaporation if N small Collisions if N large beam ρ0 > 4 × 1011 Msun/pc3 m* < 0.08 Msun N* > 108 (Braatz et al. ApJ in press) Distance and H0: ArXiv1005:1955 Spectral resolution separates images of narrow maser lines << beam apart on sky. Unprecedented angular resolution resolves the gravitational “sphere of influence” of extragalactic SMBHs, so we can see a clearly Keplerian rotation curve. Planets, brown dwarfs all r ~ 10^10 cm ~ 0.1 solar, star radius propto M^2, so no star cluster possible. Collision time sacle < 2 million yr << galaxy age.

Science costs of synthesis imaging Loss of “zero spacing” flux on extended sources (this is primarily a problem for nearby Galactic sources) Poor surface-brightness sensitivity at high angular resolution because the array area “filling factor” is low Computational costs may limit total bandwidth, spectral resolution, time resolution, field-of-view, … Complexity also limits multibeaming, pulsar observations, etc. Quantum noise limits sensitive synthesis imaging to radio frequencies! Coherent amplification is the “Roth IRA” of astronomy: You have to pay the quantum noise tax (which is proportional to frequency) up front one time, but all subsequent gains from replicating photons are tax free. Computational costs, but these are going down, so the EVLA and ALMA have bandwidths ~ single dish bandwidths and will gradually eat the lunches of single dishes except for very high frequency continuum (wideband bolometers), high-time-resolution observations (e.g., pulsars, bistatic radar, …), and very extended sources (single dishes provide the “zero spacing” data). There is no optical analog of the VLA. Twelfth Synthesis Imaging Workshop

Resolution versus surface-brightness sensitivity Use different arrays, wavelengths to match observations to phenomena over orders-of-magnitude in angular/linear size and surface brightness.

The quantum noise limit for coherent amplification T / ν = h / k = 48 K / THz e.g., ~ 150 K at λ = 100 μm ~ 15000 K at λ = 1 μm Reference: Zmuidzinas, J. “Progress in Coherent Detection Methods” at http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.9.217 Quantum noise makes coherent amplification noisy at high ( >> THz) frequencies, so low-noise coherent optical amplfiers are impossible in principle. Incoherent (e.g., photon counting) detectors will always be more sensitive at high frequencies. Coherent amplification is the “Roth IRA” of astronomy: You have to pay the quantum noise tax (which is proportional to frequency) up front one time, but all subsequent gains from replicating photons are tax free. Computational costs, but these are going down, so the EVLA and ALMA have bandwidths ~ single dish bandwidths and will gradually eat the lunches of single dishes except for very high frequency continuum (wideband bolometers), high-time-resolution observations (e.g., pulsars, bistatic radar, …), and very extended sources (single dishes provide the “zero spacing” data). Twelfth Synthesis Imaging Workshop

What is the main limitation of radio astronomy? Twelfth Synthesis Imaging Workshop

Normal galaxies example: Mouse vs. elephant Normal galaxies FIR peak / radio luminosity ratio > mass ratio of Elephant (IR astronomer) 5000 kg to mouse (radio astronomer) ~ 25 g. The radio power is just a completely insignificant tracer and has no use except insofar as it can be connected to the FIR. MPI Heidelberg 2010 Feb 22

VLBA/HSA Image of the Starburst Nuclei in the ULIRG Arp 220 Arp 220 = ultraluminous starbursts in nuclei of two merging galaxies with 1 arcsec separation. This VLBA/HSA image clearly shows that the radio sources are multiple SN3, not an AGN. Note ~ 1000 mag of visual extinction, angular resolution possible only with aperture synthesis.

Jet Energy via Radio Bubbles in Hot Cluster Gas 1.4 GHz VLA contours over Chandra X-ray images. X-ray brightness gives pressure, energy = work to evacuate bubbles = pressure X volume. This reduces the long-standing “cooling flow” problem, demonstrates “AGN feedback”. Twelfth Synthesis Imaging Workshop

Radio Spectral Lines: Cold Gas HI and cold (10s of K) molecular gas are both most visible at radio wavelengths. HI extends beyond the galaxy of stars and is an excellent tracer of mass and galaxy dynamics. Flat HI rotation curves implied dark-matter halos of galaxies. HI data indicate galaxy distances via the Tully-Fisher relation, map out the large-scale structure of the “local” universe, and sensitively trace the debris of galaxy-galaxy and galaxy-cluster interactions. HI only probe of “dark ages” and EOR. Molecular gas (e.g., CO, HCN,..) traces dense molecular clouds where stars form.

The EOR Quasar at z = 6.42 Optical Image Walter et al. 2003 z = 6.42 source in EOR, high mass of molecular gas and supermassive black hole only ~ 870 Myr after big bang. CO(3-2) 345 GHz / 7.642 = 46 GHz. Black holes come first (before bulge)—Carilli VLA correlator could barely do this, EVLA correlator makes it easy. GN20: A Unique Laboratory for Studying Early, Clustered Massive Galaxy Formation, Chris Carilli et al. Use the wide bandwidth of the WIDAR correlator to observe a proto-cluster of massive galaxies at z = 4. This is a particularly dense cluster, with very high levels of star formation, and it includes the brightest known sub-millimeter galaxies in this redshift range. The CO observations proposed by this group would showcase the full power of the EVLA by combining sensitive, high resolution imaging and the broad spectral coverage of the WIDAR correlator to provide for ground-breaking studies of galaxies in the early universe and their environment.

EVLA and ALMA together EVLA continuous frequency coverage from 1 GHz to 50 GHz Detect CO at almost any redshift Study excitation of star-forming gas in distant galaxies EVLA for low-excitation temps (low J) lines of CO, HCN at high z, and massive molecules in cold molecular clouds at low z. EVLA and ALMA very complementary. Original VLA was primarily for HI and continuum from radio galaxies. EVLA with continuous frequency coverage is a powerful instrument for other lines: molecular, RRLs, etc.

Parts of external galaxies: SNe and GRBs SNe are the ultimate energy source of most synchrotron CRs in “normal” galaxies, although discrete SNRs account for only about 10% of the synchrotron luminosity. Individual SNe study circumstellar environment and hence pre-explosion winds. z = 0.835 GRB scintillation implies size at one month ~ 3 micro arcsec ~ 10^{17} cm implies superluminal expansion with gamma ~ a few. Relativistic to nonrelativistic, calorimetry of burst energy. Get geometry: lack of VLBA proper motion rejects “cannonball” model in favor of “fireball” model. Get circumburst density. Frail, Waxman, & Kulkarni (2000).

Twelfth Synthesis Imaging Workshop Take-away message: Synthesis imaging is the secret weapon of radio astronomy Note large number N of radio telescopes. Correlation requires coherent (phase conserving) amplification to replicate (in both amplitude and phase) signal photons from each antenna to multiply with the signals from the (N-1) other antennas without loss of sensitivity. Unlike optical interferometers where light signal from each telescope is split up and divided among the other telescope(s) before detection. Quantum noise: sensitive only at radio wavelengths. Remarkably, the highest angular resolution in astronomy is at the longest wavelength band—radio. Twelfth Synthesis Imaging Workshop

The end … End of talk. End of workshop. But it will have succeeded only if you now use what you have learned as a beginning to write a great proposal to make great observations with the E/VLA, VLBA, and ALMA and do great astronomy. Not!