arXiv subjects

L. M. Young

Publications and source records attributed to L. M. Young.

29 records · Page 2Linked to original sources

arXiv2002

Molecular Gas in Elliptical Galaxies: Distribution and Kinematics

I present interferometric images (approx. 7" resolution) of CO emission in five elliptical galaxies and nondetections in two others. These data double the number of elliptical galaxies whose CO emission has been fully mapped. The sample galaxies have 10^8 to 5x10^9 solar masses of molecular gas distributed in mostly symmetric rotating disks with diameters of 2 to 12 kpc. Four out of the five molecular disks show remarkable alignment with the optical major axes of their host galaxies. The molecular masses are a few percent of the total dynamical masses which are implied if the gas is on circular orbits. If the molecular gas forms stars, it will make rotationally supported stellar disks which will be very similar in character to the stellar disks now known to be present in many ellipticals. Comparison of stellar kinematics to gas kinematics in NGC 4476 implies that the molecular gas did not come from internal stellar mass loss because the specific angular momentum of the gas is about three times larger than that of the stars.

astro-ph↗

arXiv2001

Properties of a Molecular Cloud in NGC 185

The dwarf elliptical galaxy NGC 185- the closest early-type galaxy with detectable molecular gas- offers a unique opportunity for high angular resolution studies of the interstellar medium in early-type galaxies. I present interferometric images (17 pc $\times$ 14 pc resolution) of CO emission from NGC 185. The majority of the molecular gas in NGC 185 is in one resolved cloud, closely associated with dust and an HI structure which may be a photodissociated envelope. The high spatial resolution also reveals a velocity gradient across the cloud, which allows a dynamical mass estimate independent of the H2/CO conversion factor or the virial theorem. The linear sizes, velocity gradients, and dynamical masses of the molecular clouds in NGC 185 and NGC 205, a similar dwarf elliptical, are comparable to those of the largest Galactic giant molecular clouds (GMCs). If the dynamical masses of the clouds are assumed to be good estimates of their true masses, the inferred H2/CO conversion factor in NGC 185 is at least eight times larger than a standard Galactic conversion factor, and the reasons for this difference are not yet understood. In any case, it seems clear that structures similar to Galactic GMCs can form in small elliptical or early type galaxies, even in the absence of spiral density waves.

astro-ph↗

arXiv2000

Star Formation and the ISM in Dwarf Galaxies

High spatial and spectral resolution observations of the atomic interstellar medium in nearby dwarf galaxies reveal evidence for warm and cold neutral gas, just like the phases in our own Galaxy. The cold or quiescent phase (about 20% of the HI in the galaxies studied, except for LGS 3) seems to be associated with star formation activity--- it may mark the regions where the conditions are right for star formation. These results help to explain the patterns of star formation activity which are seen in color-magnitude data for the dwarf irregulars.

astro-ph↗

arXiv2000

Molecular Gas in Elliptical Galaxies

The distribution and kinematics of the molecular gas in elliptical galaxies give information on the origin and history of the gas and the rate of star formation activity in ellipticals. I describe some preliminary results of a survey which will more than double the number of elliptical galaxies with resolved molecular distributions.

astro-ph↗

arXiv2000

Determining Phase-Space Properties of the LEDA RFQ Output Beam

Quadrupole scans were used to characterize the LEDA RFQ beam. Experimental data were fit to computer simulation models for the rms beam size. The codes were found to be inadequate in accurately reproducing details of the wire scanner data. When this discrepancy is resolved, we plan to fit using all the data in wire scanner profiles, not just the rms values, using a 3-D nonlinear code.

physics.acc-ph↗

arXiv2000

High Power Operations of LEDA

The LEDA RFQ, a 350-MHz continuous wave (CW) radio-frequency quadrupole (RFQ), successfully accelerated a 100-mA CW proton beam from 75 keV to 6.7 MeV. We have accumulated 111 hr of beam on time with at least 90 mA of CW output beam current. The 8-m-long RFQ accelerates a dc, 75-keV, ~106-mA H+ beam from the LEDA injector with ~94% transmission. When operating the RFQ at the RF power level for which it was designed, the peak electrical field on the vane tips is 33 MV/m. However, to maintain the high transmission quoted above with the CW beam, it was necessary to operate the RFQ with field levels ~10% higher than design. The RFQ dissipates 1.5 MW of RF power when operating with this field. Three klystrons provide the 2.2 MW of RF power required by the RFQ to accelerate the 100-mA beam. The beam power is 670 kW. Some of the challenges that were met in accelerating a 100-mA CW proton beam to 6.7 MeV, will be discussed.

physics.acc-ph↗

arXiv2000

LEDA Beam Operations Milestone and Observed Beam Transmission Characteristics

Recently, the Low-Energy Demonstration Accelerator (LEDA) portion of the Accelerator Production of Tritium (APT) project reached its 100-mA, 8-hr CW beam operation milestone. LEDA consists of a 75-keV proton injector, 6.7-MeV, 350-MHz CW radio-frequency quadrupole (RFQ) with associated high-power and low-level rf systems, a short high-energy beam transport (HEBT) and high-power (670-kW CW) beam dump. During the commissioning phase it was discovered that the RFQ field level must to be approximately 5-10% higher than design in order to accelerate the full 100-mA beam with low losses. Measurements of a low-duty-factor, 100-mA beam show the beam transmission is unexpectedly low for RFQ field levels between ~90 and 105% of design. This paper will describe some aspects of LEDA operations critical to achieving the above milestone. Measurement and simulation results for reduced RFQ beam transmission near design operating conditions are also presented.

physics.acc-ph↗

arXiv2000

Properties of the Molecular Clouds in NGC 205

The nearby dwarf elliptical galaxy NGC 205 offers a unique opportunity for high resolution studies of the interstellar medium in an elliptical galaxy. This paper investigates the distribution of molecular gas, molecular line ratios, and the relationships between atomic gas, molecular gas, and dust in NGC 205. The line ratios 12CO(2-1)/(1-0) and 12CO(1-0)/13CO(1-0) in one of the molecular clouds in NGC 205 are consistent with the ratios found in other elliptical galaxies and in Galactic giant molecular clouds; they suggest that the CO in this cloud is probably subthermally excited. Atomic gas, molecular gas, and dust are very closely associated on scales of 100 pc; the atomic gas can be understood as photodissociated envelopes around the molecular clouds. The atomic column densities in this galaxy are quite low (10^20 cm^-2) because the interstellar UV field is relatively low. The total gas-to-dust column density ratios are consistent with Galactic gas-to-dust ratios. In short, the molecular gas in NGC 205 seems to have very similar properties to the familiar molecular clouds in our own Galaxy, except for the low atomic column densities.

astro-ph↗

arXiv1999

Searches for HI in the Outer Parts of Four Dwarf Spheroidal Galaxies

Previous searches for atomic gas in our Galaxy's dwarf spheroidal companions have not been complete enough to settle the question of whether or not these galaxies have HI, especially in their outer parts. We present new observations of the dwarf spheroidals Sextans, Leo I, Ursa Minor, and Draco, using the NRAO 140-foot telescope to search much farther in radius than has been done before. The new data go out to at least 2.5 times the core radius in all cases, and well beyond even the tidal radius in two cases. These observations give HI column density limits of 2-6 x 10^17 atoms cm^-2. Unless HI is quite far from the galaxies' centers, we conclude that these galaxies don't contain significant amounts of atomic gas at the present time. We discuss whether the observations could have missed some atomic gas.

astro-ph↗

arXiv1999

Improved Searches for HI in Three Dwarf Spheroidal Galaxies

Previous searches for HI in our Galaxy's dwarf spheroidal companions have not been complete enough to settle the question of whether or not these galaxies have HI, especially in their outer parts. We present VLA observations of three dwarf spheroidals: Fornax, Leo II, and Draco, all of which have known stellar velocities. The new data show no HI emission or absorption. Column density limits in emission are 4--7 x 10^18 atoms/cm^2 in the centers of the galaxies. The importance of the new observations is that they cover larger areas than previous searches and they are less plagued by confusion with foreground (Galactic) HI. The apparent absence of neutral gas in the Fornax dwarf spheroidal is especially puzzling because recent photometry shows evidence of stars only 10^8 years old. We discuss whether the VLA observations could have missed significant amounts of HI.

astro-ph↗

arXiv1998

Properties of the Neutral Interstellar Medium in NGC 185 and NGC 205

We describe the properties and kinematics of the neutral gas in the two dwarf elliptical galaxies NGC 185 and NGC 205, companions of M31. The kinematics of the HI suggest that an internal origin for the gas (i.e. stellar mass loss) should be considered, at least in the case of NGC 185. The molecular clouds in these dwarf ellipticals are similar to Galactic giant molecular clouds in many ways. However, the HI column density in the dwarf ellipticals is surprisingly low. A lower interstellar UV field may make it easier to form and retain molecular gas in the dwarf ellipticals than in the solar neighborhood.

astro-ph↗