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: http://www.naic.edu/~astro/sdss5/handsonprojects
Дата изменения: Unknown Дата индексирования: Tue Oct 2 06:27:01 2012 Кодировка: Поисковые слова: arp 220 |
Nearly all discrete radio sources are extragalactic. Cosmological evolution is so strong that most are at cosmological redshifts z ~ 1 and are distributed quite isotropically over the sky. One or more sources stronger than the receiver noise will appear in every GBT beam at 1.4 GHz, observations at this frequency will therefore reveal the amplitude distribution of this source "confusion."
Confusion is both a curse and a useful tool for single-dish continuum observers. The curse is that confusion limits the faintest sources that can be detected reliably. The amplitude distribution of confusion statistically constrains the sky density of sources too faint to be detected individually, so confusion is a useful tool for studying the nature of faint sources and their cosmological evolution.
Galactic neutral hydrogen (HI) will be observed in absorption towards a strong extragalactic continuum source in the hope of determining the velocity, opacity, and spin temperature of the intervening HI. Students can compare their predicted thermal line width with the measured line width and derive an estimate of the gas density and the distance to the galactic gas. To derive many of these physical quantities, the students will map the HI emission surrounding the continuum source, so as to model the HI emission spectrum that cannot be directly measured toward the continuum source.
We will observe several newly discovered pulsars from the GBT 350 MHz driftscan survey in order to measure their polarization properties. We will use the new GBT pulsar backend known as GUPPI and record data folded modulo the known pulsar spin periods. We will also observe a flux calibrator and a highly polarized pulsar with known polarization in order to characterize our system. The data will be analyzed with the PSRCHIVE suite of pulsar analysis software.
Information on GUPPI and observing with it can be found at https://safe.nrao.edu/wiki/bin/view/CICADA/GUPPiUsersGuide. Information on PSRCHIVE can be found at http://psrchive.sourceforge.net/.
We will observe the radio recombination lines of Hydrogen, Helium and Carbon from the planetary nebula NGC 7027 and the HII region DR 21 at ~9 GHz with the GBT. With the GBT spectrometer, up to 8 different sets of recombination lines can be observed simultaneously with 50 MHz bandwidth. The observations will be done using position switching. Calibration will be done using the "vector calibration" technique developed for the GBT. A cross-scan of each source's continuum emission will also be performed.
Comparing the linew-widths of the Hydrogen, Helium and Carbon recombination lines will allow the determination of the thermal temperature of the emitting gas as well as the turbulent velocity of the gas. The line-to-continuum ratio can be used to determine the electron temperature in the gas. The relative abundances of Helium and Carbon can also be estimated from the data.
The students will learn how to observe spectral lines. They will also learn about standard and advanced calibration methods used with the GBT. From the "vector calibrated" data, the continuum flux can be compared to the result obtained from the cross-scans to estimate the accuracy of the calibration.
Note: this project will be run together with AO-2
Students will formulate a question or hypothesis regarding the neutral hydrogen (HI) properties of galaxies as they relate to other characteristics. They will select a sample of galaxies to observe for testing the hypothesis. Possible hypotheses are listed below.
Note that there will be time to observe perhaps at most 10-12 galaxies. You will need to select galaxies that are observable within the allotted observing time. You will prepare a list of galaxies and calibration sources to be observed, devise an observing strategy, and prepare observing scripts to submit to the GBT system.
HI properties that can be derived from the observations include hydrogen content, total hydrogen mass, redshift, line width, rotational velocity, and line profile morphology.
Possible questions that might be considered are:
Students are not constrained to these questions only; they are encouraged to formulate their own ideas. But note that there is limited time for the project, so best to keep it simple!
A good resource to consult is the NED (NASA Extragalactic Database), for searching for galaxies that have certain selected properties.
Students will learn how to configure the GBT system for observing HI. They will need to prepare a list of galaxies to be observed, giving:
They will also need to decide:
Resources for using the GBT:
Note: this project will be run together with AO-5
This project is designed to demonstrate the fundamentals of spectral line observing. Two separate "hands-on" groups will observe the 21-cm HI emission of the same galaxies with either the GBT or Arecibo radio telescopes. At Arecibo, we will simultaneously search for the 18-cm mainlines of the OH molecule in these galaxies. This project (GBT-7) is to carry out the GBT observing. All chosen targets are known to have HI distributions that extend to distances comparable to, or beyond, the angular size of the Arecibo beam. After data reduction, the Arecibo and GBT spectra, and parameters derived from these, will be compared for agreement/disagreement. Beam maps will also be made with the Arecibo telescope (and hopefully the GBT telescope) in order to provide both beam patterns and sizes.
This exercise will illustrate the fundamentals of spectral line observing, including restrictions due to beam sizes, side lobes, etc. All the spectral-line observations will be made with the position-switching technique. In this, the target position is observed first, followed by a blank-sky (off-source) position. The blank-sky observation is used to remove atmospheric and system noise, and to bandpass calibrate the on-source spectrum. With single dish telescopes, position switching provides the simplest means to eliminate standing waves and other instrumental effects from spectra. The Arecibo beam maps will be produced via continuum "spider scans".
In order to measure the continuum flux of a source, the Arecibo Telescope is driven in a cross pattern across the source. As the telescope crosses the source, this appears as an increase in the flux measured by the receiver. From the position at which the source appears in each arm of the cross, the precise position can be calculated to better than the normal telescope pointing accuracy, and the measured flux can then be corrected for the offset in order to find the true flux at the peak. From measurements made at a number of frequencies, the spectral index of the source can be found, which can be used to identify the physical nature of the source.
Note: this project will be run together with GBT-6
Students will formulate a question or hypothesis regarding the neutral hydrogen (HI) properties of galaxies as they relate to other characteristics. They will select a sample of galaxies to observe for testing the hypothesis. Possible hypotheses are listed below.
Note that there will be time to observe perhaps at most 10-12 galaxies. You will need to select galaxies that are observable within the allotted observing time. You will prepare a list of galaxies and calibration sources to be observed, devise an observing strategy, and prepare observing scripts to submit to the GBT system.
HI properties that can be derived from the observations include hydrogen content, total hydrogen mass, redshift, line width, rotational velocity, and line profile morphology.
Possible questions that might be considered are:
Students are not constrained to these questions only; they are encouraged to formulate their own ideas. But note that there is limited time for the project, so best to keep it simple!
A good resource to consult is the NED (NASA Extragalactic Database), for searching for galaxies that have certain selected properties.
Students will learn how to configure the Arecibo system for observing HI. They will need to prepare a list of galaxies to be observed, giving:
They will also need to decide:
Resources for using Arecibo:
We will observe the planet Venus on Sunday, July 12 from 7am to 11am (local time). We transmit the S-band (2380 MHz) radar signal to the planet surface, and receive the echo from it, and create a delay-doppler map of the terrain. Students will learn how to set up the experiment, carry out the experiment, and reduce and analyze the data. Due to the timing of this experiment, students must be available on Sunday morning, before the start of the summer school lectures.
An Ultra Luminous Infrared Galaxy (ULIRG) is a galaxy in which gas is turning into stars extremely rapidly within its circumnuclear volume. Such starbursts are often triggered by external dynamical disturbances such as galaxy mergers. The dust heating associated with these intense bursts of star formation within giant molecular clouds can produce hugely increased IR luminosity and conditions favorable for radio maser emission. The strong 18-cm OH megamaser emission in some of these galaxies is many orders of magnitude more luminous than its counterpart in our Galaxy (Darling & Giovanelli, 2002, AJ, 124, 100.)
The prototype ULIRG, Arp 220, has recently been found to possess a rich molecular-line spectrum between 1.1 and 10 GHz (Salter et al., 2008, AJ, 136, 389). Some of the molecular species detected are considered to be prebiotic. To see how unusual Arp 220's spectrum is for this class of galaxy in general, a sample of 20 other ULIRGs have now been observed between 4.3 and 5.3 GHz. Preliminary analysis has shown two of the twenty, IC860 and Zw049.057, to have remarkably similar line spectra to Arp 220.
In this Hands-on-Project, the observing team will take spectra between 1.1 and 1.75 GHz for IC860 and Zw049.057 to see to what extent their spectra over this lower frequency range, (usually referred to as the "L-Band") resemble that of Arp 220. L-Band contains potential detections for neutral atomic hydrogen, HI (1420 MHz rest frequency), main-line OH molecules (1665 & 1667 MHz), satellite-line OH (1612 & 1720 MHz) isotopic 18OH (1637 & 1639 MHz), Formic Acid (HCOOH; 1638 MHz), HCN (1346 MHz) and HCO+ (1270 MHz). Detectability of some of these transitions is likely to be affected by the presence of radio frequency interference (RFI) which the team needs to distinguish from celestial emissions. Despite RFI, a number of line detections are anticipated.
Note: this project will be run together with GBT-7
This project is designed to demonstrate the fundamentals of spectral line observing. Two separate "hands-on" groups will observe the 21-cm HI emission of the same galaxies with either the GBT or Arecibo radio telescopes. At Arecibo, we will simultaneously search for the 18-cm mainlines of the OH molecule in these galaxies. This project (AO-5) is to carry out the Arecibo observing. All chosen targets are known to have HI distributions that extend to distances comparable to, or beyond, the angular size of the Arecibo beam. After data reduction, the Arecibo and GBT spectra, and parameters derived from these, will be compared for agreement/disagreement. Beam maps will also be made with the Arecibo telescope (and hopefully the GBT telescope) in order to provide both beam patterns and sizes.
This exercise will illustrate the fundamentals of spectral line observing, including restrictions due to beam sizes, side lobes, etc. All the spectral-line observations will be made with the position-switching technique. In this, the target position is observed first, followed by a blank-sky (off-source) position. The blank-sky observation is used to remove atmospheric and system noise, and to bandpass calibrate the on-source spectrum. With single dish telescopes, position switching provides the simplest means to eliminate standing waves and other instrumental effects from spectra. The Arecibo beam maps will be produced via continuum "spider scans".
Pulsars, neutron stars with rotation periods ranging from a few seconds down to little more than a millisecond, are known for behaving like accurate natural clocks. They emit a radio pulse on each rotation, with the intervals between successive pulses being very precise. But in 2006 a new class of radio-loud neutron stars was discovered: Rotating Radio Transients (RRATs). RRATs emit a pulse only occasionally, from a few times a minute to once per many hours. Since then, the 1.4GHz Pulsar-ALFA (PALFA) survey at Arecibo has found several intermittent objects similar to the RRATs. We want to understand pulsar intermittency as a vehicle to clarifying what drives the coherent radio emission mechanism of pulsars. As a first step, we want to distinguish genuinely intermittent objects from those that only appear to be intermittent because of observational biases.
Pulsar flux densities tend to decline steeply with increasing frequency. For pulsars with steep spectra, only the brightest pulses may be detectable at 1.4GHz, while the underlying periodic emission may be uncovered if the pulsars are observed at lower frequencies. PSR J0627+16, a pulsar with a period of 2 seconds detected as an intermittent source at 1.4GHz by PALFA, was found to be a normal pulsar when observed at 0.33GHz. We will observe at 0.33GHz the rest of the PALFA sources classified as intermittent at 1.4GHz and attempt to detect normal periodic emission. We will go through the stages of pulsar data reduction, searching for periodic signals and single pulses. This project is science in action and our findings may be included in a future publication: these objects have never been observed at a frequency lower than 1.4GHz and we do not know the result in advance.
High-Velocity Clouds (HVCs) consist of neutral hydrogen (HI) at velocities that are incompatible with a simple model of differential galactic rotation. They have been interpreted as the residuals of Local Group building blocks. Compact High-Velocity Clouds (CHVCs) are one variety of the regular HVC population, but are intrinsically compact, isolated objects with angular sizes of about 1 degree. There are different opinions concerning the nature of the CHVCs. Some studies conclude that they are material at substantial distances infalling within the Local Group of galaxies, while others claim that they are associated with the halo of our own Galaxy.
In this project, the participants will carry out the total power mapping of a CHVC in the 21-cm wavelength HI spectral line. Mapping a CHVC will allow them to study the morphology and kinematics of the object. As well as reducing and analyzing the data, they will also derive its physical parameters, e.g. the distributions of central velocity, linewidth, and kinetic temperature for the HVC. In the course of the project, the participants will learn how to select a target from a catalog, the preparation of an observing set-up file, reduction of spectral-line mapping data, and interpretion of observational data.
OH/IR stars generally have strong 1612 MHz masers, and often masers at 1665 & 1667 MHz as well. These are readily detected from simple ON source observations, and are thus excellent targets for setting up appropriate velocity and intensity scales. Recently a C-band excited maser was detected from a proto planetary nebula (PPN), the stellar phase that evolves at the cessation of copious mass-loss into the circum-stellar shell of an OH/IR star.
This project will use the easy to observe 18 cm masers to organize the observing procedure, which includes simultaneous observations of the four 18 cm transitions. It will then search for excited state OH masers at C-band. The observing time allocated to this project allows for this search to include a strong OH/IR star, an evolved PPN, and a very young PPN.
In this project, students will get to see the interior of a microwave cryogenic receiver under construction, including a polarizer that splits the incoming signals into two orthogonal linear components. They will then experimentally verify the concept of changing a receiver's native polarization from linear to circular. This will involve transmitting orthogonally polarised signals from the bowl of the dish into a receiver in the Gregorian dome and digitally controlling from the control room the phase shift through the signal path to achieve the change over. The students will learn about various observing scenarios where this mode is required.