Constance E. Walker

NGC 253 and a proposed sequence for nuclear starbursts

The modest-luminosity, isolated starburst galaxy NGC 253 shows strong shocked molecular hydrogen emission from a 100 pc region centered on its nucleus. This emission can arise in an early stage of a nuclear starburst where supernova explosions lead to large amounts of shocked molecular gas through collisions of dense molecular clouds and cloud implosion. Other properties of NGC 253 also indicate it is at a relatively early stage in its starburst. A sequence of starburst development is suggested; examples along this sequence from young to old would include NGC 5253, NGC 253, M82, NGC 4736, and M31. This sequence is based on the expected evolution of a broad variety of observed parameters, and it is consistent with the observed relation between nonthermal radio and thermal infrared luminosities. 48 references.

The Density and Temperature of Molecular Clouds in M33

We have observed the 12CO J = 2-1,13CO J = 2-1, and 12CO J = 3-2 lines in a sample of seven giant molecular clouds in the Local Group spiral galaxy M33 using the James Clerk Maxwell Telescope. The 12CO/13CO J = 2-1 line ratio is constant across the entire sample, while the observed 12CO J = 3-2/J = 2-1 line ratio has a weak dependence on the star formation environment of the cloud, with large changes in the line ratio seen only for clouds in the immediate vicinity of an extremely luminous H II region. A large velocity gradient analysis indicates that clouds without H II regions have temperatures of 10-20 K, clouds with H II regions have temperatures of 15-100 K, and the cloud in the giant H II region has a temperature of at least 100 K. Interestingly, the giant H II region appears capable of raising the kinetic temperature of the molecular gas only for clouds that are quite nearby (<100 pc). The continuous change of physical conditions across the observed range of star formation environments suggests that the unusual physical conditions in the cloud in the giant H II region are due to post-star formation changes in the molecular gas, rather than intrinsic properties of the gas related to the formation of the giant H II region. The results from this study of M33 suggest that similar observations of ensembles of giant molecular clouds in more distant normal spiral galaxies are likely to give meaningful measurements of the average physical conditions inside the molecular clouds. These results also imply that clouds with a factor of 3 difference in metallicity have similar density and temperature, which in turn implies that the differences in the CO-to-H2 conversion factor seen in these clouds can be attributed to metallicity effects entirely.

Observations of 1. 3 millimeter continuum emission from the centers of galaxies

A search for 1.3 mm continuum emission from the centers of eight galaxies has been conducted, and three sources of emission, all apparently arising from cold dust, have been detected. The gas mass associated with this dust is calculated. A rough correlation found between the J = 1-0 CO emission and the millimeter continuum flux, as well as a fair agreement between the masses determined from the two types of observation, support the view that the cool dust is mixed with the molecular material in the program objects. Atomic hydrogen masses estimated for the same cources from 21 cm line observations show that there is little H I in the objects. Most of the objects form stars at about the same efficiency as does the Galaxy. 54 references.

Diffuse molecular clouds and the molecular interstellar medium from (13)CO observations of M33

We have obtained 12CO and 13CO J=1-0 observations of the nearby spiral galaxy M33 to try to resolve the long-standing discrepancy between 12CO/13CO line ratios measured in Galactic giant molecular clouds and external galaxies. Interferometer maps of the molecular cloud MC20 give a 12CO/13CO line ratio of 7.5+/-2.1, which agrees reasonably well with the line ratio measured in Galactic giant molecular clouds. In contrast, the 12CO/13CO line ratio obtained from single dish data is 10.0+/-0.9, significantly higher than Galactic values but in good agreement with line ratios measured in other galaxies. The interferometer map of MC20 reveals that the cloud has similar spatial and velocity extents in the two lines, and thus the high single dish line ratio cannot be due to different filling factors in the two lines. In addition, the single dish data show no evidence for significant variations in the line ratio with metallicity, which eliminates abundance changes as the explanation for the high single dish line ratio. We conclude that the high 12CO/13CO line ratio s observed in M33, and in the disks of spiral galaxies in general, are due to the presence of a population of diffuse molecular clouds. The lower limit to the fraction of the total 12CO emission from M33 that originates in the diffuse clouds is 30+/-30%, while the upper limit is ~60%.

Using misconceptions research in the design of optics instructional materials and teacher professional development programs

To create the Hands-On Optics program and its associated instructional materials, we needed to understand a number of basic optics misconceptions held by children (and adults) and how to address them through a proper educational approach. The activities have been built with an understanding of the naïve concepts many people have about light, color, and optical phenomena in general. Our own experience is that the concepts that children and adults have of light are often not that different from each other. This paper explores the most common misconceptions about light and color, according to educational research, and describes how they can be addressed in optics education programs. This understanding of misconceptions was useful as well in the professional development component of the program where educators were trained on the Hands-On Optics modules. The professional development work for the optics industry volunteers who worked with the educators was also based on research on how an optics professional can work more effectively in multi-cultural settings–an area with great applicability to industry volunteers working in the very different culture of science centers or after-school programs.

Hands-On Optics: an educational initiative for exploring light and color in after-school programs, museums, and hands-on science centers

Hands-On Optics (HOO) is a collaborative four-year National Science Foundation fundedprogram (Principal Investigator A. Johnson) designed to create a sustainable science education program to excite students about science by actively engaging them in optics activities. It will reach underrepresented middle school students in after-school programs and at hands-on science centers across the United States. The project creates and distributes educational modules and provides professional development for educators and optics resource volunteers.

Optics Education in the International Year of Astronomy

The International Year of Astronomy (IYA) will be celebrated in 2009 to commemorate the 400th anniversary of Galileo’s first use of the telescope for astronomical observation. The National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, USA, is participating in a variety of international education activities to build awareness of the role of astronomy and optics in our modern technological society. We will outline our education plans specifically related to optics for the International Year of Astronomy. These plans include outreach activities that appeal to professional museum and classroom educators as well as the general public.

Observations of Cold and Warm CO in the Irregular Galaxy NGC 4449

We present observations of cold 12CO (1–0) in H I concentrations located in and around the peculiar irregular galaxy NGC 4449. The observed positions range from the center out to 18 kpc, or nearly 4 times the Holmberg radius, but most are in the large H I complexes that encircle the optical galaxy. We have detected CO (1–0) in two positions outside the center of the galaxy, including one that has no detectable sign of star formation. No CO was detected in the H I concentrations located far from the galaxy center. In general, CO emission was detected at positions where approximately σHI,max ≥ 2 × 1021 atoms cm-2, a factor of 3 higher than expected for self-shielding for the metallicity of the galaxy, and R ≤ 5 kpc from the center of the galaxy. In the H I complexes surrounding the galaxy, M/MHI,max is approximately constant. In the center of the galaxy, there is more Hα emission relative to M than in the outer H I complexes. Generally, there is no relationship between the presence or amount of Hα emission and M. We have also presented observations of warm molecular 12CO (2–1) gas in the center of the galaxy where CO (1–0) had already been detected. In the center of the galaxy, the ratio ICO (2–1)/ICO (1–0) is unusually high, indicating the presence of a significant amount of hot, optically thin gas. This is generally consistent with the unusual activity that has taken place in the central regions of the galaxy, but the ratio is higher than is observed in other galaxies with high star formation rates.

Spectroscopy of PKS 0528–250: New Limits on CO Absorption and Emission*

We have obtained a moderate-resolution spectrum of the quasar PKS 0528-250 with the Red Channel Spectrograph on the Multiple Mirror Telescope in order to study a damped Lyα absorption-line system at z = 2.8115. We obtain a new upper limit for the CO column density for the z = 2.8108 velocity component in the z = 2.8115 damped Lyα system. The ionization of different species in this component rules out a quasar spectral energy distribution as the ionization field and implies an ultraviolet radiation field intensity a few times that of the Milky Way value. The estimated total number density is n(H) ~ 20 cm-3. The physical size for the z = 2.8108 component implied by these models is about 40 pc. The ionization of different species also suggests a structure with a hot intercloud medium associated with a H I cloud in this component, that is, most low ionized ions are from the cold medium where photoionization and photodissociation dominates. The highly ionized species may be from the intercloud medium where collisional ionization dominates. We also present newly identified Ni II absorption lines in the z = 2.1408 and z = 2.8115 damped Lyα systems. The derived depletion of nickel by dust confirms previous results that the dust-to-gas ratio in these two damped Lyα systems is about 10% of the Milky Way ratio. Millimeter-wavelength observations obtained at the NRAO 12 m telescope provide new upper limits on CO (3-2) emission in the z = 2.8115 damped Lyα system.

A study of quasar absorption-line systems with IRAS

A survey of quasar absorbers was conducted using the Infrared Astronomical Satellite (IRAS) database. Quasars with known intervening absorption-line systems and broad absorption line (BAL) QSOs were selected primarily from the Junkkarinen, Hewitt & Burbidge catalog (1991). Of the 570 quasars with IRAS data, 52 showed 3 or better detections in at least one band in SCANPI analysis. The origin of the IRAS ux could be from the absorption line systems, other galaxies or the quasar itself. The spectral energy distributions for quasars detected in the absorption-line sample and BAL QSOs were found to be redder than those of two control samples which suggests that some of the IRAS ux may arise in dust associated with the intervening galaxies. IRAS SUPERSCANPI processing was carried out for 77 quasars with known Mg II absorption at z abs < 1 to investigate the ensemble far-infrared properties of these objects. SUPERSCANPI processing evaluates the median ux for many diierent positions on the sky, resulting in an improvement in the eeective sensitivity. A control sample consisting of objects with no Mg II absorption known at z abs < 1 but with the same distribution of absolute V-magnitude, z em and radio-loud fraction for the background quasars was also processed. The Mg II sample was detected at 3 or better in all four IRAS bands with a signiicantly larger ux than the control sample at 60m and 100m. If this far-infrared emission is from the absorber galaxies, then the far-infrared luminosity of the composite Mg II absorber was found to be comparable to that of a starbursting galaxy, although such a high star-formation rate is inconsistent with the optical and near infrared colors of low-redshift Mg II systems. Four of the quasars with individual IRAS detections have intervening galaxies identiied with the Mg II absorption-lines. The spectral energy distributions of these galaxies imply far-infrared luminosities in excess of what Arp 220 would give at their redshifts. While all the external evidence suggests that the detection of far-infrared emission from the absorber sample may not be connected to the { 2 { presence of the Mg II absorber, we discuss future observations which may help explain our results.