Seymour N.,Mullard Space Science Laboratory |
Symeonidis M.,Mullard Space Science Laboratory |
Page M.J.,Mullard Space Science Laboratory |
Huynh M.,California Institute of Technology |
And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010
In this paper, we examine the contribution of galaxies with different infrared (IR) spectral energy distributions (SEDs) to the comoving IR luminosity density (IRLD), a proxy for the comoving star formation rate (SFR) density. We characterize galaxies as having either a cold or hot IR SED depending on whether the rest-frame wavelength of their peak IR energy output is above or below 90 μm. Our work is based on a far-IR selected sample both in the local Universe and at high redshift, the former consisting of IRAS60 μm-selected galaxies at z < 0.07 and the latter of Spitzer70 μm selected galaxies across0.1 < z≤ 1. We find that the total IR luminosity densities for each redshift/luminosity bin agree well with results derived from other deep mid-/far-IR surveys. Atz < 0.07, we observe the previously known results that moderate luminosity galaxies(LIR < 1011 L⊙) dominate the total luminosity density and that the fraction of cold galaxies decreases with increasing luminosity, becoming negligible at the highest luminosities. Conversely, abovez= 0.1, we find that luminous IR galaxies(LIR > 1011 L⊙), the majority of which are cold, dominate the IRLD. We therefore infer that cold galaxies dominate the IRLD across the whole0 < z < 1 range, hence appear to be the main driver behind the increase in SFR density up toz∼ 1 whereas local luminous galaxies are not, on the whole, representative of the high-redshift population. © 2010 The Authors. Journal compilation © 2010 RAS.
Smith N.,Steward Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2013
What sort of supernova (SN) gave rise to the Crab nebula? While there are several indications that the Crab arose from a sub-energetic explosion of an 8-10M⊙ progenitor star, this would appear to conflict with the high luminosity indicated by historical observations. This paper shows that several well-known observed properties of the Crab and SN 1054 are well matched by a particular breed of Type IIn SN. The Crab's properties are best suited to the Type IIn-P subclass (Type IIn spectra with plateau light curves), exemplified by SNe 1994W, 2009kn and 2011ht. These events probably arise from relatively low energy (1050 erg) explosions with low 56Ni yield that may result from electron-capture SN (ecSN) explosions, but their high visual-wavelength luminosity and Type IIn spectra are dominated by shock interaction with dense circumstellar material (CSM) rather than the usual recombination photosphere. In this interaction, a large fraction of the 101050 erg of the total kinetic energy can be converted to visual-wavelength luminosity. After about 120 d, nearly all of the mass outside the neutron star in the CSM and ejecta ends up in a slowly expanding (1000-1500 km s-1) thin dense shell, which is then accelerated and fragmented by the growing pulsar wind nebula in the subsequent 1000 yr, producing the complex network of filaments seen today. There is no need to invoke the extended, invisible fast SN envelope hypothesized to reside outside the Crab. As differentiated from a normal SN II-P, SNe IIn-P provide a much better explanation for several observed features of the Crab: (1) no blast wave outside the Crab nebula filaments, (2) no rapidly expanding SN envelope outside the filaments, (3) a total mass of ~5M⊙ swept up in a thin slow shell, (4) a low kinetic energy of the Crab at least an order of magnitude below a normal core-collapse SN, (5) a high peak luminosity (-18 mag) despite the low kinetic energy, (6) chemical abundances consistent with an 8-10M⊙ star and (7) a low 56Ni yield. A number of other implications are discussed, concerning other Crab-like remnants, the origin of dust in the Crab filaments, diversity in the initial masses of SNe IIn, and the putative association between ecSNe and SN impostors. This model predicts that if/when light echoes from SN 1054 are discovered, they will exhibit a Type IIn spectrum, probably similar to SNe 1994W and 2011ht ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Shirley Y.L.,Steward Observatory |
Shirley Y.L.,Max Planck Institute for Astronomy
Publications of the Astronomical Society of the Pacific | Year: 2015
The optically thin critical densities and the effective excitation densities to produce a (Formula Presented.) spectral line are tabulated for 12 commonly observed dense gas molecular tracers. The dependence of the critical density and effective excitation density on physical assumptions (i.e., gas kinetic temperature and molecular column density) is analyzed. Critical densities for commonly observed dense gas transitions in molecular clouds (i.e., HCN 1−0, HCOþ 1−0, N2Hþ 1−0) are typically 1–2 orders of magnitude larger than effective excitation densities because the standard definitions of critical density do not account for radiative trapping and 1 Kkm=s lines are typically produced when radiative rates out of the upper energy level of the transition are faster than collisional depopulation. The use of effective excitation density has a distinct advantage over the use of critical density in characterizing the differences in density traced by species such as NH3, HCOþ, N2Hþ, and HCN, as well as their isotopologues; but, the effective excitation density has the disadvantage that it is undefined for transitions when Eu=k ≫ Tk, for low molecular column densities, and for heavy molecules with complex spectra (i.e., CH3CHO). © 2015. The Astronomical Society of the Pacific. All rights reserved.
When astronomers point their telescopes up at the sky to see distant supernovae or quasars, they’re collecting light that’s traveled millions or even billions of light-years through space. Even huge and powerful energy sources in the cosmos are unimaginably tiny and faint when we view them from such a distance. In order to learn about galaxies as they were forming soon after the Big Bang, and about nearby but much smaller and fainter objects, astronomers need more powerful telescopes. Perhaps the poster child for programs that require extraordinary sensitivity and the sharpest possible images is the search for planets around other stars, where the body we’re trying to detect is extremely close to its star and roughly a billion times fainter. Finding earth-like planets is one of the most exciting prospects for the next generation of telescopes, and could eventually lead to discovering extraterrestrial signatures of life. Detectors in research telescopes are already so sensitive that they capture almost every incoming photon, so there’s only one way to detect fainter objects and resolve structure on finer scales: build a bigger telescope. A large telescope doesn’t just capture more photons, it can also produce sharper images. That’s because the wave nature of light sets a limit to the telescope’s resolution, known as the diffraction limit; the sharpness of the image depends on the wavelength of the light and the telescope’s diameter. As optical scientists, our contribution to the next generation of telescopes is figuring out how to craft the gargantuan mirrors they rely on to collect light from far away. Here’s how we’re perfecting the technology that will enable tomorrow’s astrophysical discoveries. The question is how to build something substantially bigger than the current generation of telescopes, which have effective diameters of 8 to 12 meters (26 to 40 feet). One of the biggest challenges is making a bigger mirror to collect the light. First, it helps to know the basic optical layout of a telescope, illustrated here by the Giant Magellan Telescope (GMT) that is being built in Chile. A large primary mirror collects incoming light and reflects it to a focus. The light is reflected a second time by the smaller secondary mirror, to form an image on an instrument located at a safe, accessible place below the primary mirror, where the image is recorded. A mirror much larger than eight meters, made of a single piece of glass, would be too expensive and too hard to handle. Everyone involved in building giant telescopes agrees that the solution is to make the primary mirror out of multiple smaller mirrors. Multiple pieces of glass are shaped and aligned to form one gigantic mirror, called a segmented mirror. Gaps between the segments are acceptable as long as the segments' surfaces lie on a continuous nearly parabolic surface, called the parent surface. The three extremely large telescope (ELT) projects now in development have made very different decisions about the design of this segmented primary mirror. Two of the ELTs, the European ELT and the Thirty Meter Telescope, have adopted the approach pioneered by the 10-meter Keck Observatory telescopes in Hawaii — they’ll make a giant mirror out of hundreds of 1.5-meter segments. The third project, the Giant Magellan Telescope, takes a different tack. Its 25-meter primary mirror will have only seven segments. They’re the largest single mirrors that can be made, the 8.4-meter (28-foot) honeycomb mirrors we produce here at the Richard F. Caris Mirror Lab at the University of Arizona. The GMT’s 3-meter secondary mirror also has seven segments, each paired with one of the primary mirror segments. Artist’s representation of the seven giant mirrors installed in the Giant Magellan Telescope. Giant Magellan Telescope — GMTO Corporation Big mirror segments guarantee a smooth surface over their entire large areas. The more segments there are in the primary mirror, the more its accuracy depends on their precise alignment to keep them on the parent surface. Because of the pairing of primary and secondary mirror segments in the GMT, the fine control needed to form sharp images can be done by moving the small, agile segments of the secondary mirror rather than the 8.4-meter primary segments. A second advantage of the 8.4-meter honeycomb mirrors is their strong legacy, including use in what is currently the world’s largest telescope, the Large Binocular Telescope here in Arizona. One of the challenges of using a large mirror is that it tends to bend under its own weight and the force of wind. The mirror is exposed to wind like a sail on a yacht, but it can only bend by about 100 nanometers before its images become too blurry. The best way to overcome this problem is to make the mirror as stiff as is practical, while also limiting its weight. We accomplish this feat by casting the mirror into a lightweight honeycomb structure. Each mirror has a continuous glass facesheet on top and an almost continuous backsheet, each about one inch thick. Holding the two sheets together is a honeycomb structure consisting of half-inch-thick ribs in a hexagonal pattern. Our honeycomb mirrors are 70 centimeters thick, making them stiff enough to withstand the forces of gravity and wind. But they’re 80 percent hollow and weigh about 16 tons each, light enough that they don’t bend significantly under their own weight. Mold for casting an 8.4-meter honeycomb mirror for the GMT. The glass will melt around the hexagonal boxes to form the honeycomb. Ray Bertram, Steward Observatory We start by melting glass into a complex mold that’s the negative of the honeycomb mirror we want to end up with. While the glass is molten, the furnace spins at five revolutions per minute; the centrifugal force pushes the glass' surface into the concave parabolic shape that can focus light from a distant star. Watch the video below to see the construction of the honeycomb mold and the spin-casting process. The spin-cast mirror surface doesn’t yet have the optical quality needed to make sharp images. But spinning gives it the right overall curvature and saves our having to grind out 14 tons of glass from a flat surface — almost as much glass as is left in the finished mirror. Next we need to polish the surface to an accuracy of a small fraction of the light’s wavelength, so it will form the sharpest images possible. The mirror surface has to match the ideal, nearly parabolic surface to about 25 nanometers — about 3 ten-thousandths of the width of a human hair. That’s really, really smooth; if the mirror were scaled up to the size of North America, the tallest mountain would be one inch high and the deepest canyon would be one inch low. To guide our polishing, the first step is to create a superfine contour map of the mirror’s surface, with steps of less than 10 nanometers. As our “ruler,” we use red laser light; its divisions are the light’s wavelength — about 630 nanometers — and it can be read to about one hundredth of a division. The measuring instrument illuminates the mirror surface, collects the reflected light, and compares the path lengths of the rays reflected by different locations on the mirror. A ray that reflects off a high spot will have a shorter path than a ray that hits a low spot. The instrument uses this information to construct the contour map of the mirror’s surface. The basic principle of polishing is to rub the surface with a disk-shaped tool, removing glass selectively from the spots that are too high. A fine abrasive such as rouge (iron oxide) slowly removes glass, atom by atom, through mechanical and chemical processes. Figuring is removing glass explicitly from high spots identified in the contour map, for example by having the tool rub there longer. This is effective on scales larger than about 10 centimeters. Smoothing is what happens when you rub a stiff tool over a rough surface: the tool naturally sits on the high spots and removes more material there, even without any guidance from a contour map. This is effective on scales smaller than 10 centimeters. Both methods are more difficult when the mirror surface is aspheric, meaning its curvature changes from point to point, which is very much the case for the GMT segments. An 8.4-meter mirror for the Large Synoptic Survey Telescope being polished at the Richard F. Caris Mirror Lab. Steward Observatory, CC BY-ND We’ve developed several new polishing tools to address the challenges of polishing large mirrors for telescopes. One essential feature of any polishing tool is that it match the shape of the mirror surface to an accuracy of around 1 micron. The larger tool in the background is a complex electro-mechanical system that changes the shape of a stiff aluminum disk as it moves over the surface, so it always matches the local curvature of the mirror. The smaller tool in the foreground is much simpler. Similar to Galileo’s reinvention of a carnival toy as an astronomical telescope, our new idea came from Silly Putty — a non-Newtonian fluid that flows like a liquid over a long period of time but acts like a solid on short timescales. We harness those intrinsic properties to achieve both figuring and smoothing. Our tool, containing Silly Putty enclosed by a thin rubber diaphragm, slowly moves over the surface of the mirror while simultaneously rapidly orbiting around itself. The Silly Putty is stiff over the quick period of the orbit, which smooths out small-scale irregularities in the mirror surface. Over the longer time it takes to move across the mirror, the Silly Putty flows easily, so the tool always matches the surface’s shape. As a result, it removes glass at a predictable rate and in a predictable pattern that doesn’t vary as it moves across the mirror. The Giant Magellan Telescope as it will look after construction on Cerro Las Campanas in Chile. Giant Magellan Telescope — GMTO Corporation, CC BY Here at the Mirror Lab, we finished making the first Giant Magellan Telescope segment in 2012. After a pause for work on two other mirrors, the lab is in the process of grinding Segments 2 and 3. Segment 4 has just finished cooling to room temperature after spin-casting in September 2015. We are well on the way to manufacturing the full 25-meter primary mirror. Getting these near-perfect mirrors from our lab in Arizona to a mountaintop in Chile presents another set of challenges. They travel by tractor-trailer on land, and by freight ship from California to Chile. The keys to safe transport are distributing the weight of the mirror over hundreds of support points and having several layers of suspension between the mirror and the road or ship deck. The GMT project schedule calls for a preliminary first light, with four segments installed in the telescope, in 2022. We expect all seven segments to be scanning the cosmos starting in 2024. Many of us who work on the GMT see it as the way to open new windows into the universe, as the Hubble Space Telescope (HST) has done over the last 25 years. That orbiting telescope was a generous gift to the next generation from the people who worked on the project for decades before it launched. HST’s deep space images amazed, motivated and inspired many of us on Earth. The GMT project team dreams of passing on a similar gift for future generations. Buddy Martin, Project Scientist at the Steward Observatory and Associate Research Professor of Optical Sciences, University of Arizona and Dae Wook Kim, Assistant Professor of Optical Sciences, University of Arizona. This article was originally published on The Conversation. Read the original article.
Schwarz K.R.,Steward Observatory |
Shirley Y.L.,Steward Observatory |
Dunham M.M.,Yale University
Astronomical Journal | Year: 2012
We present a systematic single-dish search for molecular outflows toward a sample of nine candidate low-luminosity protostars and 30 candidate very low luminosity objects (VeLLOs; L int ≤ 0.1 L ⊙). The sources are identified using data from the Spitzer Space Telescope cataloged by Dunham etal. toward nearby (D < 400 pc) star-forming regions. Each object was observed in 12CO and 13CO J = 2 → 1 simultaneously using the sideband separating ALMA Band-6 prototype receiver on the Heinrich Hertz Telescope at 30″ resolution. Using five-point grid maps, we identify five new potential outflow candidates and make on-the-fly maps of the regions surrounding sources in the dense cores B59, L1148, L1228, and L1165. Of these new outflow candidates, only the map of B59 shows a candidate blue outflow lobe associated with a source in our survey. We also present larger and more sensitive maps of the previously detected L673-7 and the L1251-A-IRS4 outflows and analyze their properties in comparison to other outflows from VeLLOs. The accretion luminosities derived from the outflow properties of the VeLLOs with detected CO outflows are higher than the observed internal luminosity of the protostars, indicating that these sources likely had higher accretion rates in the past. The known L1251-A-IRS3 outflow is detected but not re-mapped. We do not detect clear, unconfused signatures of red and blue molecular wings toward the other 31 sources in the survey indicating that large-scale, distinct outflows are rare toward this sample of candidate protostars. Several potential outflows are confused with the kinematic structure in the surrounding core and cloud. Interferometric imaging is needed to disentangle large-scale molecular cloud kinematics from these potentially weak protostellar outflows. © © 2012. The American Astronomical Society. All rights reserved..