Sample variance, source clustering and their influence on the counts of faint radio sources

The shape of the curves defined by the counts of radio sources per unit area as a function of their flux density was one of the earliest cosmological probes. Radio source counts continue to be an area of astrophysical interest as they can be used to study the relative populations of galaxy types in the Universe (as well as investigate any cosmological evolution in their respective luminosity functions). They are also a vital consideration for determining howsource confusion may limit the depth of a radio interferometer observation, and are essential for characterizing the extragalactic foregrounds in cosmicmicrowave background experiments. There is currently no consensus as to the relative populations of the faintest (sub-mJy) source types, where the counts show a turn-up. Most of the source count data in this regime are gathered from multiple observations that each use a deep, single pointing with an interferometric radio telescope. These independent count measurements exhibit large amounts of scatter (factors of the order of a few) that significantly exceeds their respective stated uncertainties. In this paper, we use a simulation of the extragalactic radio continuum emission to assess the level at which sample variance may be the cause of the scatter. We find that the scatter induced by sample variance in the simulated counts decreases towards lower flux density bins as the raw source counts increase. The field-to-field variations make significant contributions to the scatter in the measurements of counts derived from deep observations that consist of a single pointing, and could even be the sole cause at >100 μJy. We present a method for evaluating the flux density limit that a radio survey must reach in order to reduce the count uncertainty induced by sample variance to a specific value. We also derive a method for correcting Poisson errors on source counts from existing and future deep radio surveys in order to include the uncertainties due to the cosmological clustering of sources. A conclusive empirical constraint on the effect of sample variance at these low luminosities is unlikely to arise until the completion of future large-scale radio surveys with next-generation radio telescopes.Web of Scienc

Mrk 1419 - a new distance determination

Water vapor megamasers from the center of active galaxies provide a powerful tool to trace accretion disks at sub-parsec resolution and, through an entirely geometrical method, measure direct distances to galaxies up to 200 Mpc. The Megamaser Cosmology Project (MCP) is formed by a team of astronomers with the aim of identifying new maser systems, and mapping their emission at high angular resolution to determine their distance. Two types of observations are necessary to measure a distance: single-dish monitoring to measure the acceleration of gas in the disk, and sensitive VLBI imaging to measure the angular size of the disk, measure the rotation curve, and model radial displacement of the maser feature. The ultimate goal of the MCP is to make a precise measurement of H0 by measuring such distances to at least 10 maser galaxies in the Hubble flow. We present here the preliminary results from a new maser system, Mrk 1419. Through a model of the rotation from the systemic masers assuming a narrow ring, and combining these results with the acceleration measurement from the Green Bank Telescope, we determine a distance to Mrk 1419 of 81\pm10 Mpc. Given that the disk shows a significant warp that may not be entirely traced by our current observations, more sensitive observations and more sophisticated disk modeling will be essential to improve our distance estimation to this galaxy.Comment: 5 pages, 3 figures, to appear in the proceedings of IAU Symposium 287 "Cosmic Masers- from OH to Ho", in Stellenbosch, S

The Megamaser Cosmology Project. VII. Investigating disk physics using spectral monitoring observations

We use single-dish radio spectra of known 22 GHz H$_2$O megamasers, primarily gathered from the large dataset observed by the Megamaser Cosmology Project, to identify Keplerian accretion disks and to investigate several aspects of the disk physics. We test a mechanism for maser excitation proposed by Maoz & McKee (1998), whereby population inversion arises in gas behind spiral shocks traveling through the disk. Though the flux of redshifted features is larger on average than that of blueshifted features, in support of the model, the high-velocity features show none of the predicted systematic velocity drifts. We find rapid intra-day variability in the maser spectrum of ESO 558-G009 that is likely the result of interstellar scintillation, for which we favor a nearby ($D \approx 70$ pc) scattering screen. In a search for reverberation in six well-sampled sources, we find that any radially-propagating signal must be contributing $\lesssim$10% of the total variability. We also set limits on the magnetic field strengths in seven sources, using strong flaring events to check for the presence of Zeeman splitting. These limits are typically 200--300 mG ($1\sigma$), but our most stringent limits reach down to 73 mG for the galaxy NGC 1194.Comment: Accepted for publication in Ap

Microwave Continuum Emission and Dense Gas Tracers in NGC 3627: Combining Jansky VLA and ALMA Observations

We present Karl G. Jansky Very Large Array (VLA) Ka band (33 GHz) and Atacama Large Millimeter Array (ALMA) Band 3 (94.5 GHz) continuum images covering the nucleus and two extranuclear star-forming regions within the nearby galaxy NGC 3627 (M 66), observed as part of the Star Formation in Radio Survey (SFRS). Both images achieve an angular resolution of $\lesssim$2\arcsec, allowing us to map the radio spectral indices and estimate thermal radio fractions at a linear resolution of $\lesssim$90 pc at the distance of NGC 3627. The thermal fraction at 33 GHz reaches unity at and around the peaks of each HII region; we additionally observed the spectral index between 33 and 94.5 GHz to become both increasingly negative and positive away from the peaks of the HII regions, indicating an increase of non-thermal extended emission from diffusing cosmic-ray electrons and the possible presence of cold dust, respectively. While the ALMA observations were optimized for collecting continuum data, they also detected line emission from the $J=1\rightarrow0$ transitions of HCN and HCO$^{+}$. The peaks of dense molecular gas traced by these two spectral lines are spatially offset from the peaks of the 33 and 94.5 GHz continuum emission for the case of the extranuclear star-forming regions, indicating that our data reach an angular resolution at which one can spatially distinguish sites of recent star formation from the sites of future star formation. Finally, we find trends of decreasing dense gas fraction and velocity dispersion with increasing star formation efficiency among the three regions observed, indicating that the dynamical state of the dense gas, rather than its abundance, plays a more significant role in the star formation process.Comment: 9 pages, 5 figures, accepted for publication in Ap

The Efficacy of Team-Based Learning in Histology

Team-Based Learning (TBL) is an instructional strategy in which traditional lectures are replaced with in-class activities that promote group discussion and active learning. Students are expected to master the basic facts and concepts of the subject matter prior to coming to class. We sought to determine whether the knowledge obtained using TBL is comparable to that obtained using traditional lectures, and whether students have a preference for either instructional method. From 2006-2008, the students in a graduate histology course were taught the structure and function of the basic tissues using TBL. Other topics in the course were taught using lectures, so the students experienced both instructional methods. Using the same 59 multiple-choice questions, we tested the students’ knowledge about the basic tissues, and compared the results to those obtained in 2005, when the basic tissue material was taught using lectures. In 2006-2008, the mean ± SD exam performance after TBL (87.5 ± 7.5, n = 32; 83.9 ± 11.1, n = 36; 78.9 ± 13.2, n = 24) was similar to that observed in 2005 after lectures (82.7 ± 12.0, n = 39). When asked to respond to the statement, “I prefer TBL sessions rather than traditional lectures”, 40.4% of the students agreed or strongly agreed, 23.4% disagreed or strongly disagreed, and 36.0% had no opinion (89 of 92 students responding). These results suggest that TBL and lectures produce comparable learning outcomes, at least as measured on a multiple-choice exam, and that students have a mild preference for the TBL format.American Association of Clinical Anatomists Annual Meetin

Team-Based Learning (TBL) in Histology: Lessons Learned Through 7 Years of Experience

In 2006, we began the transition from a traditional lecture-based histology course to a TBL course, gradually adding more TBL exercises each year until the course was virtually lecture-free. Our laboratory sessions using microscopes and glass slides remained unchanged. We have previously reported that TBL produces learning outcomes comparable to those of lectures (Clin. Anat. 23: 474, 2010). Based on our trail-and-error experiences of the last 7 years, we now offer 4 key suggestions for successfully implementing TBL in histology: (1) Schedule the laboratory session before the corresponding TBL exercise. This permits the use of histologic images that students already have some familiarity with. (2) Limit Internet access during the TBL exercise, especially for clinically-oriented problems. Students can quickly find the “right” answer via search engines without understanding why it is correct. (3) When discussing the TBL exercise in class, call out the names of individual students to respond using a checklist of the team rosters. This sends a clear message that all team members must fully participate in the process and be prepared to explain and defend the team’s answers. (4) At the conclusion of the TBL exercise, provide a “take-home message” about what the students are expected to understand about the topic. Students often fail to connect what they see in lab or read in the text with the problems presented in the exercise.American Association for Anatomy Spring Meetin

Simple guide to starting a research group

Conducting cutting-edge research and scholarship becomes more complicated with each passing year; forming a collaborative research group offers a way to navigate this increasing complexity. Yet many individuals whose work might benefit from the formation of a collaborative team may feel overwhelmed by the prospect of attempting to build and maintain a research group. We propose this simple guide for starting and maintaining such an enterprise