Science Highlights

Nancy A. Levenson Science Highlights From standard candles to the serendipitous use of one of the most distant known supernovae to study the interstellar medium in very distant galaxies, learn about four of Gemini’s most recent contributions to the understanding of our universe. The Best Standard Candle for Cosmology Exploding stars offer some of the most precise measurements of cosmic distances. Astrono- mers have long used observations of these supernovae at visible wavelengths for this pur- pose and they provide the basis for the 2011 Nobel Prize in Physics. Supernovae do have some intrinsic differences in visible light, however, so the observations must be corrected; that is, to standardize the candles (to the same absolute luminosity). Visible light also suf- fers from the complication of attenuation Figure 1. by dust anywhere along the line-of-sight, The residual Hubble from the supernova’s host galaxy to our diagram for supernovae vantage point in the Milky Way. observed in the H band (green), compared with In contrast, at near-infrared wavelengths, previous NIR samples Type Ia supernovae serve as the best (blue). The deviation “standard candle” for these determina- of each measurement tions. As Rob Barone-Nugent (University from the overall mean is of Melbourne, Australia) and colleagues plotted against redshift, show in the Monthly Notices of the Royal z, which indicates Astronomical Society, Type Ia supernovae distance. are intrinsically more consistent in their peak luminosity when viewed in the near- infrared (NIR), so they do not require these corrections. Because of this characteristic, the team can measure cosmological distances to an accuracy of 5 percent (Barone-Nugent et al., Monthly Notices of the Royal Astronomical Society, 425: 1007, 2012). Such precise mea- December2012 GeminiFocus 25 Figure 2. surements are essential to the study of the Microwave (orange), expansion history of the universe and hence optical (red, green, to constrain the nature of dark energy. blue), and ultraviolet (blue) image of The supernovae used in the study, discov- Phoenix Cluster. ered with the Palomar Transient Factory, Image courtesy of were confirmed as ordinary Type Ia superno- the Chandra X-ray vae by subsequent spectroscopy and by us- Observatory. ing the Near-Infrared Imager and Spectrom- eter (NIRI) on the Gemini North telescope to follow the characteristic fading of the NIR emission and determine the peak brightness. One further important selection criterion was to restrict the study to supernovae at distances large enough so the overall expan- Michael McDonald (Massachusetts Institute sion of the universe (the Hubble flow) de- of Technology) and colleagues have now de- termines the motion of their host galaxies, tected the first evidence for significant cool- independent of local peculiar motions; i.e., ing-flow-induced star formation in a central redshifts 0.03 < z < 0.09 (Figure 1). While ear- cluster galaxy (McDonald et al., 2012 Nature, lier work had already indicated the greater 488: 349). The cluster itself, designated SPT- uniformity of supernova emission in the NIR, CLJ2344-4243, was detected with the South this is the first large study to obtain high- Pole Telescope. Follow-up spectra obtained quality measurements of more distant su- using the Gemini Multi-Object Spectrograph pernovae. Although these observations are (GMOS) at Gemini South provided some of more difficult because the distant superno- the first hints that the central galaxy was vae appear fainter, this avoids the complica- unlike the red, well-formed elliptical galax- tion of local motions and results in the most ies typical of cluster cores. The researchers precise known standard candle for cosmo- also used additional measurements of other logical measurements. cluster members to determine the baseline redshift (z = 0.6) for comparison of other ob- Beginning to Solve the Cooling servations. Flow Problem The more complete analysis of the so-called Clusters of galaxies are full of hot gas that “Phoenix Cluster” and its central galaxy (Fig- emits copious X-ray radiation. This emis- ure 2) emerges from observations spanning sion should lead to a “cooling flow,” whereby X-ray to far-infrared energies. The central cooling material sinks to the dense center of galaxy possesses an active nucleus in addi- the cluster. In turn, we expect this inflowing tion to star formation at a rate of 740 MSun/ reservoir of relatively cool gas to stimulate year. The star formation rate is still too star formation in the galaxy located at the low to prevent runaway cooling, given the cluster’s core, rather than result in runaway measured cooling flow rate of 3800 MSun/ cooling of the cluster gas. The problem, un- year, suggesting that the feedback mecha- til now, is that observations of such central nism is not fully established in this example. galaxies have revealed them to be quies- Nonetheless, the high star formation rate cent, showing little evidence for ongoing points to this mode of star formation from star formation. intracluster gas as an important element 26 GeminiFocus December2012 of galaxy formation, in addition to galaxy Large Telescope confirmed mergers, which had been widely considered their classification. previously. The team continues to use observations with GMOS-S to complement the Probing the Distant South Pole Telescope survey, so more excit- Interstellar Medium ing results should be forthcoming. The emission of material be- Populating the tween the stars of distant galaxies is often too feeble “Brown Dwarf Desert” to detect directly, so a use- Formation of multiple stellar systems tends ful technique is to observe to favor objects of roughly the same mass, the absorbing effect of this whereas planets tend to be much less mas- material on the light from a sive than their central stars. The result is a background source. Typical- “desert” in the population distribution, with ly, distant quasars and gam- few brown dwarfs as companions of stars, or ma-ray bursts (GRBs) have equivalently, few systems showing mass ra- effectively served as these tios of 1-10 percent. Markus Janson (Prince- sources. Now, Edo Berger ton University) and colleagues report finding (Harvard-Smithsonian Cen- several more candidates in this sparse region ter for Astrophysics) and (The Astrophysical Journal, 758: L2, 2012), but colleagues demonstrate the raise new questions at the same time. utility of extremely luminous supernovae for this work, The newly-identified targets have masses in using PS1-11bam, one of the range of 45-95 times the mass of Jupiter. the most distant confirmed They also lie at relatively large distances from supernovae, as their subject their central stars (angular separations of (The Astrophysical Journal 0.35-1.83 arcseconds, which corresponds to Letters, 755: L29, 2012). In about 40-200 astronomical units). Thus, mass addition to expanding the alone does not distinguish their formation number of galaxies in the history, and the large separations open ad- early universe that may be probed, the new ditional pathways. Their origin may be simi- Figure 3. class of sources hints at differences among lar to those of standard stellar systems (for- NICI images of three the galaxy environments of supernovae, low-mass companions mation in common collapse of dense cloud GRBs, and ordinary star formation. to stars in the Sco- cores), planets (mass build-up through accre- Cen region. The first tion), or none of the above (through capture PS1-11bam was discovered in the Pan- two are likely brown of free-floating external bodies). STARRS1 imaging survey, with spectros- dwarfs, and the third copy showing characteristic features of ul- is probably a very low- The team obtained the high-angular-reso- traluminous supernovae, likely due to the mass star. lution images of the targets in the young core collapse of a massive star. Subsequent (10-12 million year) Scorpius-Centaurus observations using GMOS at Gemini North (Sco-Cen) stellar association using the Near- confirm that it is very distant, with redshift Infrared Coronagraphic Imager on Gemini (z = 1.566). More importantly, galactic emis- South, with multiple observational ep- sion and narrow metal absorption lines ap- ochs to demonstrate genuine association pear at the same redshift, revealing the host through common proper motion (Figure 3). galaxy’s interstellar medium (Figure 4). The Additional spectroscopy using ESO’s Very December2012 GeminiFocus 27 Figure 4. equivalent widths of Mg II and Fe II in this Left: Portion of a case are intermediate between typical ob- Gemini spectrum of servations of quasars (which tend to probe PS1-11bam containing galaxy outskirts) and GRBs (which tend to several interstellar probe the central regions of galaxies), and absorption features of Fe II and Mg II at they are much lower than those of star-form- z = 1.566 (black). ing galaxies at the same redshift. The error spectrum This first direct demonstration that ultralu- is shown in blue, and for comparison, minous supernovae can reveal distant gal- the composite GRB axies suggests that the next generation of spectrum in red. imaging surveys and spectroscopy from ex- Right: A zoom-in tremely large telescopes could be applied to on the relevant galaxies in the earliest days of the universe. Fe II and Mg II lines demonstrates the similarity to GRB absorption spectra. Nancy A. Levenson is Deputy Director and Head The host galaxy also of Science at Gemini Observatory and can be appears in the emission reached at: [email protected] of [O II] 3727. 28 GeminiFocus December2012.

Load more