Dynamical Modeling of NGC 7252 and the Return of Tidal Material

Motivated by recent neutral hydrogen observations with the VLA, we have undertaken an investigation into the interaction that produced the well known merger remnant NGC 7252. Through fully self-consistent N-body simulations, we are able to reproduce the kinematic character of the HI observations quite well, including the velocity reversals observed along each tidal tail. In the simulation these reversals arise from particles which have turned around in their orbit and are moving to smaller radii. The bases of the tails fall back quickly to small pericentric distances, while the more distant regions fall back more slowly to ever increasing pericentric distances. The delayed return of tidally ejected material may extend over many Gyr. The evolution of this merger is followed numerically for 800 h^-1 Myr beyond the best fit time. We find that nearly half of the present tail material, or of order 10^9 h^-2 of neutral hydrogen and 2x10^9 h^-2 of starlight, will return to within 13 h^-1 kpc of the nucleus within this time span. While the collisionless stars of the tails will continue orbiting between their inner and outer turning points, the observations show the HI gas of the tails disappearing upon its return. We discuss this result in light of the lack of central HI in the main body of this merger remnant.Comment: 28 pages of uuencoded, compressed postscript. Accepted to AJ. 9 Postscript figures available upon request from [email protected]; postscript text also available from http://ucowww.ucsc.edu/~hos/home.htm

Alien Registration- Hibbard, John (Bangor, Penobscot County)

https://digitalmaine.com/alien_docs/15876/thumbnail.jp

An HI Threshold for Star Cluster Formation in Tidal Debris

Super star clusters are young, compact star clusters found in the central regions of interacting galaxies. Recently, they have also been reported to preferentially form in certain tidal tails, but not in others. In this paper, we have used 21 cm HI maps and the Hubble Space Telescope Wide Field Planetary Camera 2 images of eight tidal tail regions of four merging galaxy pairs to compare the kiloparsec scale HI distribution with the location of super star clusters found from the optical images. For most of the tails, we find that there is an increase in super star cluster density with increasing projected HI column density, such that the star cluster density is highest when log N(HI) >= 20.6 cm^{-2}, but equal to the background count rate at lower HI column density. However, for two tails (NGC 4038/39 Pos A and NGC 3921), there is no significant star cluster population despite the presence of gas at high column density. This implies that the N(HI) threshold is a necessary but not sufficient condition for cluster formation. Gas volume density is likely to provide a more direct criterion for cluster formation, and other factors such as gas pressure or strength of encounter may also have an influence. Comparison of HI thresholds needed for formation of different types of stellar structures await higher resolution HI and optical observations of larger numbers of interacting galaxies.Comment: 19 pages, 6 figures, 3 tables, accepted for publication in MNRA

Gas in merging galaxies

We present observations of the neutral hydrogen, ionized hydrogen, and starlight of galaxies chosen from the 'Toomre Sequence' of merging galaxies. This sequence is meant to represent the progressive stages of the merger of two disk galaxies into a single elliptical-like remnant. The galaxies in this study span the full range of this sequence. The stars and atomic gas are very differently distributed, with the stars more widely distributed at early stages, and the gas much more widely distributed at later stages. Large quantities of neutral gas are sent to large radii (greater than or approximately equal to 100 h(exp -1) kpc), and still persist even after the central remnant has relaxed to an r(sup 1/4) light profile. There are a few times 10(exp 9) solar masses h(exp -2) of both molecular and atomic gas in each of these systems. Throughout the different stages, about half of the total gas mass lies within the galaxies' optical bodies. The fraction of this mass that is in neutral hydrogen drops rapidly in the later stage mergers, suggesting that atomic gas is processed into molecular gas, stars, and hot gas during the merger and resulting starburst. Star formation occurs at all stages of the interaction, both within the tails and in the central bodies. In the early stages, the H(alpha) shows many arcs and plumes. In the late stages, there are large H 2 regions in the tails which are associated with large quantities of neutral hydrogen. There is always a very good correlation between optical, H(alpha), and H1 peaks, with N(sub H1) greater than or approximately equal 3 x 10(exp 20) cm(exp -2) at the location of the H2 regions in the tails

A new red giant-based distance modulus of 13.3 Mpc to the Antennae galaxies and its consequences

The Antennae galaxies are the closest example of an ongoing major galaxy merger, and thereby represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultra-luminous X-ray sources, super-stellar clusters, etc). In a previous paper HST/WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST/ACS imaging that resolves the composite stellar populations, and most importantly, reveals a well-defined red giant branch. The tip of this red giant branch (TRGB) is unambiguously detected at Io(TRGB)=26.65 +/- 0.09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of (m-M)o= 30.62 +/- 0.17 corresponding to a distance of 13.3 +/- 1.0 Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well determined shorter distance.Comment: 11 pages, 3 figures, accepted for publication in the Ap

Blending Learning: The Evolution of Online and Face-to-Face Education from 20082015

In 2008, iNACOL produced a series of papers documenting promising practices identified throughout the field of K–12 online learning. Since then, we have witnessed a tremendous acceleration of transformative policy and practice driving personalized learning in the K–12 education space. State, district, school, and classroom leaders recognize that the ultimate potential for blended and online learning lies in the opportunity to transform the education system and enable higher levels of learning through competency-based approaches.iNACOL's core work adds significant value to the field by providing a powerful practitioner voice in policy advocacy, communications, and in the creation of resources and best practices to enable transformational change in K–12 education.We worked with leaders throughout the field to update these resources for a new generation of pioneers working towards the creation of student-centered learning environments.This refreshed series, Promising Practices in Blended and Online Learning, explores some of the approaches developed by practitioners and policymakers in response to key issues in K–12 education, including:Blended Learning: The Evolution of Online and Face-to-Face Education from 2008-2015;Using Blended and Online Learning for Credit Recovery and At-Risk Students;Oversight and Management of Blended and Online Programs: Ensuring Quality and Accountability; andFunding and Legislation for Blended and Online Education.Personalized learning environments provide the very best educational opportunities and personalized pathways for all students, with highly qualified teachers delivering world-class instruction using innovative digital resources and content. Through this series of white papers, we are pleased to share the promising practices in K–12 blended, online, and competency education transforming teaching and learning today

Hubble Space Telescope Imaging of the Ultracompact Blue Dwarf Galaxy HS 0822+3542: An Assembling Galaxy in a Local Void?

We present deep U, narrow-V, and I-band images of the ultracompact blue dwarf galaxy HS 0822+3542, obtained with the Advanced Camera for Surveys / High Resolution Channel of the Hubble Space Telescope. This object is extremely metal-poor (12 + log(O/H) = 7.45) and resides in a nearby void. The images resolve it into two physically separate components that were previously described as star clusters in a single galaxy. The primary component is only \~100 pc in maximum extent, and consists of starburst region surrounded by a ring-like structure of relatively redder stars. The secondary component is ~50 pc in size and lies at a projected distance of ~80 pc away from the primary, and is also actively star-forming. We estimate masses ~10^7 M(sol) and ~10^6 M(sol) for the two components, based on their luminosities, with an associated dynamical timescale for the system of a few Myr. This timescale and the structure of the components suggests that a collision between them triggered their starbursts. The spectral energy distributions of both components can be fitted by the combination of recent (few Myr old) starburst and an evolved (several Gyr old) underlying stellar population, similar to larger blue compact dwarf galaxies. This indicates that despite its metal deficiency the object is not forming its first generation of stars. However, the small sizes and masses of the two components suggests that HS 0822+3542 represents a dwarf galaxy in the process of assembling from clumps of stars intermediate in size between globular clusters and objects previously classified as galaxies. Its relatively high ratio of neutral gas mass to stellar mass (~1) and high specific star formation rate, log(SFR/M(sol) = -9.2, suggests that it is still converting much of its gas to stars.Comment: 11 pages, 2 figures, accepted for publication in Astrophysical Journal Letter

The Infrared Properties of Hickson Compact Groups

Compact groups of galaxies provide a unique environment to study the mechanisms by which star formation occurs amid continuous gravitational encounters. We present 2MASS (JHK), Spitzer IRAC (3.5-8 micron) and MIPS (24 micron) observations of a sample of twelve Hickson Compact Groups (HCGs 2, 7, 16, 19, 22, 31, 42, 48, 59, 61, 62, and 90) that includes a total of 45 galaxies. The near-infrared colors of the sample galaxies are largely consistent with being dominated by slightly reddened normal stellar populations. Galaxies that have the most significant PAH and/or hot dust emission (as inferred from excess 8 micron flux) also tend to have larger amounts of extinction and/or K-band excess and stronger 24 micron emission, all of which suggest ongoing star formation activity. We separate the twelve HCGs in our sample into three types based on the ratio of the group HI mass to dynamical mass. We find evidence that galaxies in the most gas-rich groups tend to be the most actively star forming. Galaxies in the most gas-poor groups tend to be tightly clustered around a narrow range in colors consistent with the integrated light from a normal stellar population. We interpret these trends as indicating that galaxies in gas-rich groups experience star formation and/or nuclear actively until their neutral gas consumed, stripped, or ionized. The galaxies in this sample exhibit a ``gap'' between gas-rich and gas-poor groups in infrared color space that is sparsely populated and not seen in the Spitzer First Look Survey sample. This gap may suggest a rapid evolution of galaxy properties in response to dynamical effects. These results suggest that the global properties of the groups and the local properties of the galaxies are connected.Comment: 34 pages, 26 figures, accepted for publication in AJ, higher quality images available in publicatio