Recent ALMA Science Highlights
Infall in a Highly Optically Thick Protostellar Disk
ALMA is proving to be a powerful instrument in the assessment of gas dynamics in young protostellar disks. In a recent paper, Luis Zapata (UNAM) et al. present 0.2 arcsec ALMA observations of sub-millimeter continuum and line emission from a disk around the low mass, young star IRAS 16293-2422B. The 0.45mm continuum emission is observed to be compact, with a spatial extent of 50 AU (~ 0.4 arcsec). Surrounding the continuum emission is a molecular ring, as traced by H13CN, HC15N and CH3OH. The ring-like structure of the gas and the lack of substructure in the continuum emission are indications that the central regions of the disk are optically thick at these wavelengths. The emission lines of H13CN, HC15N and CH3OH show an inverse P-Cygni profile indicative of infall; these observations are consistent with lower resolution, lower frequency observations of infall by Submillimeter Array and ALMA. The estimated inflow rate of 0.6 km/s is higher than that estimated from the lower frequency observations, indicating that these new observations are detecting gas associated with faster and more turbulent regions closer to the star.
Reference: Luis A. Zapata (UNAM), Laurent Loinard (UNAM, MPIfR), Luis F. Rodriguez (UNAM, King Abdulaziz Univ), Vicente Hernández-Hernández (UNAM), Satoko Takahashi (ASIAA), Alfonso Trejo (ASIAA), and Bérengère Pariseet (MPIfR), 2013, ApJL 764, 14.
High Redshift, Gravitationally Lensed, Dusty Starburst Galaxies Revealed by the SPT and ALMA
Even in Cycle 0, ALMA has already proven itself to be a powerful tool for cosmological studies. In a recent study of millimeter-wave detected sources from the 1,300 square degree South Pole Telescope (SPT) SZ survey, J.D. Vieira (Caltech) and his collaborators used ALMA to image 47 SPT sources with 1.4mm flux densities greater than 20 mJy. These 47 sources were selected after filtering out nearby IRAS galaxies and flat spectrum radio galaxies. At these bright flux densities, the remaining discrete 1.4mm emission is light from cosmologically distant galaxies that have been gravitationally lensed by massive, foreground elliptical galaxies. The candidate lenses were observed with ALMA at 3 mm and 870 microns, with the latter observations achieving a resolution of 0.5". Even with short integrations (1 minute), many of the submillimeter sources are clearly observed to be lensed.
The imaging observations were followed up with blind spectroscopic observations of 26 of the sources. Detections of CO emission from 23 lenses were made (see figure below), with at least 10 having redshifts in excess of 4 and infrared luminosities in the 1012-13 Lsun range. This Cycle 0 ALMA study has doubled the number of z > 4 ultra-luminous infrared galaxies detected thus far from submillimeter spectroscopic surveys, and it has shown that the fraction of high-redshift dusty starburst galaxies is higher than previously thought.
View Publication: J. D. Vieira (Caltech), D. P. Marrone (U. Arizona), S. C. Chapman (Dalhousie University) et al. 2013 Nature 495, 344
ALMA Observes a Flow of Gas Through a Protoplanetary Disk
One area of astronomical research that drove the ALMA design specifications is the study of protoplanetary disks. In a recent Nature paper (493, 191), Simon Casassus of the University of Chile and his collaborators present Early Science ALMA observations of the young star HD 142527. The star was previously known from infrared observations to have a large gap between its 10 AU radius inner disk and the edge of a disrupted outer disk located 140 AU from the star (see Figure 1c in the paper).
The disrupted nature of the outer disk is believed to be the result of a planet-sized body orbiting 90 AU from the star. With ALMA, the authors observed the CO (3-2), 345 GHz dust continuum and the dense gas tracer HCO+ (4-3). While the CO shows that some gas suffuses the gap region, the HCO+ (4-3) is observed to trace denser filaments of gas bridging the outer disk to the inner disk (Figure 1d). Further, the gas along the filaments is blue-shifted in the direction of the star on the near (eastern) edge and redshifted on the far (western) edge.
The authors speculate that the gap-crossing accretion flow may be material straddling the putative protoplanet also responsible for clearing the gap. The authors use the HCO+ emission and the spur of dust continuum observed to be co-spatial with the gas filament (Figure 1a), to estimate the mass in the inflow. They conclude that the rate at which material is being delivered in the inner disk is sufficient to offset the rate at which the inner disk is being depleted by accretion onto the star.
View publication: Simon Casassus (Universidad Chile), Gerrit van der Plas (Universidad Chile), Sebastian Perez M. (Universidad Chile) et al. , 2013, Nature, 493, 191
Surprising Spiral Structure Spotted by ALMA
ALMA has made high resolution, high sensitivity observations of the CO emission around the asymptotic giant branch (AGB) star R Sculptoris (R Scl). The presence of a detached shell of emission from dust and from the CO molecule at a radius of about 20" around the star has been known for many years. The shell results from a thermal pulse that briefly increased the stellar mass loss. The physical parameters of the pulse, common in AGB star evolution, have been difficult to constrain, though they are critical to our understanding of the lifetime of this phase of stellar evolution and of the degree to which processed material enriches the interstellar medium.
The ALMA images reveal the shell to contain a previously unknown spiral structure, though such structure has been associated with binary stars in other systems. The ALMA observations coupled with hydrodynamical models show R Scl to be a binary which underwent a thermal pulse ~1800 years ago and which lasted about 200 years. The mass loss increased thirty-fold during this episode. The measurements indicate that about three times more mass is returned to the interstellar medium during the pulse than previously thought. Stars such as R Scl are major contributors to the galaxy's reservoir of raw materials critical for the creation of new stars, planets and for life.
View Publication: M. Maercker (ESO, Univ Bonn), Mohamed, S.; Vlemmings, W. H. T.; Ramstedt, S.; Groenewegen, M. A. T.; Humphreys, E.; Kerschbaum, F.; Lindqvist, M.; Olofsson, H.; Paladini, C., Wittkowski, M., de Gregorio-Monsalvo, I., Nyman, L.-A. 2012 Nature 490, 232
The First Year of ALMA Science: The High Redshift Universe
The Atacama Large Millimeter/submillimeter Array (ALMA) began Early Science observations 30 Sep 2011. A number of the accepted Cycle 0 proposals make use of the superior sensitivity and spatial resolution of ALMA to study the submillimeter properties of starburst galaxies in the early Universe. These galaxies likely have infrared luminosities in excess of 1012 Lsun, and are the dominant contributors in the infrared luminosity density at redshifts greater than 2.
Early Science data have started to appear in the literature, with investigators of high-redshift galaxy programs making use of ALMA to achieve three primary science goals. The first goal is the identification of high-redshift starburst galaxies via ALMA continuum observations. Most “submillimeter” galaxies identified to date were discovered in wide field bolometer array surveys with single-dish telescopes such as the James Clerk Maxwell 15m Telescope (JCMT) on Mauna Kea, Hawaii and the IRAM 30m Telescope on Pico Veleta, Spain. By ALMA standards, these bolometer array observations produce extremely low-resolution (>7”) data, making it difficult to determine which of the multiple optical or near-infrared galaxies within the FWHP beam are the counterparts to the submillimeter emission. ALMA follow-up provides the best opportunity to pinpoint the position of the submillimeter source. The image above shows an example of this, in which Karim et al. (2012, MNRAS, in press: http://arxiv.org/abs/1210.0249) make use of the order of magnitude improvement in angular resolution with ALMA (1.5” versus 19”) to resolve the regions into a small number of submillimeter bright galaxies. It is worth noting that an optical/near-infrared counterpart is not always observed, and thus our knowledge of this subset of galaxies will be completely derived from continuum and emission line observations with ALMA.
The second goal of Early Science high-redshift observations is the derivation of the infrared luminosity of high-redshift galaxies. Wang et al. (2012: http://arxiv.org/abs/1210.0242) make use of ALMA to observe a sample of redshift 6 quasar host galaxies optically identified in the Sloan Digital Sky Survey and the Canada-France High Redshift Quasar Survey. At such high lookback times, the Universe was less than a Gyr old. From 250 GHz continuum observations of the sample, Wang et al. derive far-infrared luminosities for the quasar hosts of 1012-13 Lsun and, under the assumption that the starburst powers half of the far-infrared luminosity, corresponding star formation rates of 600-2000 Msun per year. By comparison, the extreme starburst galaxy Arp 220 has a star formation rate of 50 Msun under the same assumptions.
The third primary science goal for high-redshift programs is to study the star formation and galaxies dynamics through the analysis of molecular and atomic line emission. An example is given in the spectrum above: Wagg et al. (2012, ApJL, 752, 30:http://adsabs.harvard.edu/abs/2012ApJ...752L..30W) detect [C II] 158 micron emission from the z = 4.69 quasar BR 1202-0725 and its companion submillimeter galaxy. The [C II] emission line is a major cooling line in molecular clouds and is produced in the PDR and neutral regions of galaxies. Further, it is typically 1000 times brighter than CO, with a line luminosity 0.1 - 1.0 % of the far-infrared luminosities of galaxies. The brightness of [C II] makes it ideal for tracing the kinematics of galaxies (see Wang et al. 2012) and for identifying possible outflows. Wagg et al. conclude that the [C II] / far-infrared luminosity ratio they derive for both the quasar host (southern source) and the submillimeter galaxy (northern source) is consistent with prior high-redshift studies which show a high ratio for starburst galaxies than quasar host galaxies. Wang et al. (2012) also detect [C II] emission from their high-redshift quasar hosts with ALMA and estimate similar [C II] / far-infrared luminosity ratios as Wagg et al.. These and complementary results were recently presented at the NRAO conference The Interstellar Medium in High Redshift Galaxies Comes of Age which was held in Charlottesville on 13-15 Sep 2012 in honor of the career of Paul Vanden Bout.
ALMA: Detection of Sugars around a Proto-star
Glycolaldehyde (HCOCH2OH) is the simplest sugar and an important intermediate in the path toward forming more complex biologically relevant molecules and understanding their full chemistry. Joergensen et al present the first detection of 13 transitions of glycolaldehyde around a solar-type young star, using Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Class 0 protostellar binary IRAS 16293-2422 at 220 GHz (6 transitions) and 690 GHz (7 transitions). The glycolaldehyde lines have their origin in warm (200-300 K) gas close to the individual components of the binary. The data also show a tentative detection of ethylene glycol, the reduced alcohol of glycolaldehyde.
View Publication: Jes K. Jørgensen (Copenhagen), Cécile Favre (Aarhus), Suzanne E. Bisschop (Copenhagen), Tyler L. Bourke (Harvard-Smithsonian Center for Astrophysics), Ewine F. van Dishoeck (Leiden, MPIfE), and Markus Schmalzl (Leiden), 2012, ApJ Letters, 757, L4.
ALMA: Imaging Gas in a Proto-planetary Disk
Rosenfeld et al. present a detailed analysis of the spatially and spectrally resolved 12CO J = 2-1 and J = 3-2 emission lines from the TW Hya circumstellar disk, based on Atacama Large Millimeter/submillimeter Array (ALMA) science verification data, and trace molecular gas as close as 2 AU from the central star. These data are reasonably modeled by a warped disk on AU scales. Such a warp could be generated by an embedded massive planet in the disk.
View Publication: Katherine A. Rosenfeld, Chunhua Qi, Sean M. Andrews, David J. Wilner (Harvard-Smithsonian Center for Astrophysics); Stuartt A. Corder (NRAO); C. P. Dullemond (Heidelberg); Shin-Yi Lin (UC-San Diego); A. M. Hughes (UC-Berkeley); Paola D'Alessio (UNAM); and P. T. P. Ho (ASIAA), 2012, ApJ, 757,129.
[C II] Line Emission in Massive Star-forming Galaxies at z = 4.7
The 158 μm [CII] fine structure line is the best tracer of cool atomic gas in high z galaxies. With just 16 antennas in 20 min at 340 GHz, the Atacama Large Millimeter/submillimeter Array (ALMA) has acquired the deepest observations at these frequencies by 10X, observing the dust continuum and [CII] line emission from BR1202-0725, a canonical starburst group galaxies at z = 4.7. Three galaxies were detected – a FIR-luminous quasar, a hyper-starburst galaxy, and a normal star-forming galaxy – revealing early massive galaxy formation in a merging, starburst group within 1.3 Gyr of the Big Bang.
View Publication: Wagg, J. (ESO), Wiklind, T. (JAO), Carilli, C. L. (NRAO, Cambridge), Espada, D. (NAOJ), Peck, A. (JAO), Riechers, D. (Caltech), Walter, F. (MPIfA, NRAO), Wootten, A. (NRAO),Aravena, M. (ESO), Barkats, D. (JAO) et al., 2012, ApJ Letters, 752, L30.