Brendan Mullan

NEOWISE Observations of Near-Earth Objects: Preliminary Results

With the NEOWISE portion of the Wide-field Infrared Survey Explorer (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 μm, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The surveys uniform sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981 ± 19 NEAs larger than 1 km and 20,500 ± 3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32 ± 0.14 below 1.5 km. This power-law slope produces ~13,200 ± 1900 NEAs with D > 140 m. Although previous studies predict another break in the cumulative size distribution below D ~ 50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100 and 1000 m is lower than previous estimates. The numbers of near-Earth comets and potentially hazardous NEOs will be the subject of future work.

Smooth and Starburst Tidal Tails in the GEMS and GOODS Fields

GEMS and GOODS fields were examined to z ~ 1.4 for galaxy interactions and mergers. The basic morphologies are familiar: antennae with long tidal tails, tidal dwarfs, and merged cores; M51-type galaxies with disk spirals and tidal arm companions; early-type galaxies with diffuse plumes; equal-mass grazing collisions; and thick J-shaped tails beaded with star formation and double cores. One type is not common locally and is apparently a loose assemblage of smaller galaxies. Photometric measurements were made of the tails and clumps, and physical sizes were determined assuming photometric redshifts. Antennae tails are a factor of ~3 smaller in GEMS and GOODS systems compared to local antennae; their disks are a factor of ~2 smaller than locally. Collisions among early type galaxies generally show no fine structure in their tails, indicating that stellar debris is usually not unstable. One exception has a 5 × 109 M☉ smooth red clump that could be a pure stellar condensation. Most tidal dwarfs are blue and probably form by gravitational instabilities in the gas. One tidal dwarf looks like it existed previously and was incorporated into the arm tip by tidal forces. The star-forming regions in tidal arms are 10-1000 times more massive than star complexes in local galaxies, although their separations are about the same. If they all form by gravitational instabilities, then the gaseous velocity dispersions in interacting galaxies have to be larger than in local galaxies by a factor of ~5 or more; the gas column densities have to be larger by the square of this factor.

UNDER PRESSURE: STAR CLUSTERS AND THE NEUTRAL HYDROGEN MEDIUM OF TIDAL TAILS

Using archival data from ATCA, WHISP, and the Very Large Array, we have analyzed the H I emission of 22 tidal tail regions of the Mullan et al. sample of pairwise interacting galaxies. We have measured the column densities, line-of-sight velocity dispersions, and kinetic energy densities on ~kpc scales. We also constructed a tracer of the line-of-sight velocity gradient over ~10 kpc scales. We compared the distributions of these properties between regions that do and do not contain massive star cluster candidates (MV 46 erg pc–2). Thus, high H I densities and pressures, partly determined by the tail dynamical age and other interaction characteristics, are connected to large-scale cluster formation in tidal tails overall. Last, we find that the high mechanical energy densities of the gas are likely not generally due to feedback from star formation. Rather, these properties are more likely to be a cause of star formation than a result.

TIDAL TAILS OF MINOR MERGERS. II. COMPARING STAR FORMATION IN THE TIDAL TAILS OF NGC 2782

The peculiar spiral NGC?2782 is the result of a minor merger with a mass ratio ~4: 1 occurring ~200?Myr ago. This merger produced a molecular and H I-rich, optically bright eastern tail and an H I-rich, optically faint western tail. Non-detection of CO in the western tail by Braine et al. suggested that star formation had not yet begun. However, deep UBVR and H? narrowband images show evidence of recent star formation in the western tail, though it lacks massive star clusters and cluster complexes. Using Herschel PACS spectroscopy, we discover 158 ?m [C II] emission at the location of the three most luminous H? sources in the eastern tail, but not at the location of the even brighter H? source in the western tail. The western tail is found to have a normal star formation efficiency (SFE), but the eastern tail has a low SFE. The lack of CO and [C II] emission suggests that the western tail H II region may have a low carbon abundance and be undergoing its first star formation. The western tail is more efficient at forming stars, but lacks massive clusters. We propose that the low SFE in the eastern tail may be due to its formation as a splash region where gas heating is important even though it has sufficient molecular and neutral gas to make massive star clusters. The western tail, which has lower gas surface density and does not form high-mass star clusters, is a tidally formed region where gravitational compression likely enhances star formation.

A tale of two tails: exploring stellar populations in the tidal tails of NGC 3256

We have developed an observing program using deep, multiband imaging to probe the chaotic regions of tidal tails in search of an underlying stellar population, using NGC 3256s 400 Myr twin tidal tails as a case study. These tails have different colours of

Population growth, energy use, and the implications for the search for extraterrestrial intelligence

u - g = 1.05 \pm 0.07

THE Ĝ INFRARED SEARCH FOR EXTRATERRESTRIAL CIVILIZATIONS WITH LARGE ENERGY SUPPLIES. I. BACKGROUND AND JUSTIFICATION

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STAR CLUSTERS IN THE TIDAL TAILS OF INTERACTING GALAXIES: CLUSTER POPULATIONS ACROSS A VARIETY OF TAIL ENVIRONMENTS

r - i = 0.13 \pm 0.07

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. IV. Stephan's Quintet

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Under pressure: Star clusters in the tidal debris of interacting galaxies

u - g = 1.26 \pm 0.07