ASTRONOMY 3303   HW #7     due Wed Nov 1, 2017


In answering the questions to Parts 1 and 2, please give short answers to the questions, but be sure to write your explanations in your own words not just copying from the textbook or a website.     Hint: review class notes, the textbook and PEs first.


Part 1:   Clusters and groups of galaxies

a.   What do we mean by the "crossing time" of a cluster? Compare the crossing times of the Local Group, the Virgo cluster and the Persus cluster. Be sure to state your assumptions. Hint: you may find the table in P.E. #16 useful.

b.   When we say that the temperature of the intracluster gas observed in X-rays has a temperature of 5 keV, what temperature in Kelvins does that correspond to, and how do we get the equivalent units of keV?

d.   Suppose you observe a sample of spiral galaxies in a cluster of galaxies which itself is a moderately bright, extended X-ray source. The "HI deficiency" of a galaxy is the difference between a galaxy's HI mass and that expected for an isolated galaxy of the same size and morphological type. For each galaxy, you measure the HI deficiency (relative to a sample of isolated galaxies), and its redshift (therefore allowing an estimate of each galaxy's velocity with respect to the cluster overall). Under the assumption that ram pressure stripping is the underlying cause of the HI deficiency, what relationship would you expect to see between HI deficiency and the relative velocity of the galaxy with respect to the cluster? Explain your answer.



Part 2:   Galaxy collisions and interactions

a.   Suppose you are interested in studying tidal tails in interacting systems in the local universe. Why do you expect to find more of them in loose groups of galaxies than in rich clusters of galaxies?

b.   Suppose you observe a pair of equal-mass galaxies which are clearly involved in a tidal interaction with each other. One of the galaxies shows a long tail and a bridge but the other one appears to be only mildly perturbed. What can you conclude about the geometry of the encounter?



Part 3:   Continued development of the final project
For next week, continue on your investigation done as part of HW #6 and add focus on the scientific project and its motivation for your project.

For Wednesday, starting with the summary you handed in for HW #6, produce and expand on an outline of two sections of your final paper: one dealing with the facility that you are reporting on (which you should have explored last week) and a second that answers the questions mentioned in HW #6:
  • What is the science objective? Here, mention the main science goals; focus in detail on the one we highlight. What is the current state of the science (i.e., what are today's results, conclusions and questions?). What will be observed? What is the scientific significance of the measurement?
    Feel free to add additional questions of your own.

    For HW #7, develop the outline in some detail (bulleted points still ok). Focus on the astrophysics associated with th observational evidence that will address the science goals and why the observational program is scientifically exciting. For example, if spectroscopy is involved, what lines(s) will be observed, what causes the line emission and how will its characteristics be interpreted to give us new scientific insight. If it is not line emission, what is the radiation mechanism, what causes it and what can we learn about the responsible physical processes? In some cases, the new facility offers multiple insights into a scientific program;focus on the most important and transformational ones. What about complementary observations made with different telescopes and/or at different wavelengths? Why and how will the future observations be scientifically transformative?

    With a diverse set of projects, there is no one set of questions we can suggest you answer, but in this next installment of your outline, you should include some details of the astrophysics involved. Include information on the current status of observations and how this new facility will improve physical constraints on the science problem; be quantitative where you can be. If you are not sure how to proceed, please ask.

    We expect the next version (HW #7) to be considerably longer than what you turned in as HW #6; the more work you do now, the further down the path to completing the final project you'll get. You can start to include figures if you wish, but be sure you give your own figure caption, including the citation, explaining what is displayed (on each axis, as contours, etc, including units) and what each figure shows in support of the science case.

    For Wed Nov 1st
                    (1) Print your expanded outline to hand in during class on Nov 1st.
                    (2) Email your expanded outline to BOTH Dominik (riechers_aT_astro.cornell.edu) and Martha (haynes_aT_astro.cornell.edu) by class time.
                    (3) Be prepared to give a 3 minute oral report (1-2 slides; not 3 - we will only let you show 2!) to the class explaining the astrophysics of the observations that will be required. Email your slides (preferably in PDF format) to BOTH Dominik (riechers_aT_astro.cornell.edu) and Martha (haynes_aT_astro.cornell.edu) by noon on Wed.
                    (4) Be sure to save a copy of your expanded outline because it will be useful as the framework for the written version of your final project (which is due on Wed Dec 6th).

    As always, if you have questions about your individual project, feel free to talk with either of us, but for consistency, we've assigned one of us as the "mentor" for each project.

    Who Mentor Facility Science objective
    Tim Martha JWST Properties of the First Stars with the James Webb Space Telescope
    Clare Martha LSST Studies of Weak Gravitational Lensing with the Large Synoptic Survey Telescope
    Ruixin Dominik HERA Probing the Cosmic Dark Ages with the Hydrogen Epoch of Reionization Array
    Sam Dominik ALMA/ngVLA The Interstellar Medium in Distant Galaxies with ALMA and the ngVLA
    Julia Dominik E-ELT/TMT/GMT Adaptive Optics Guided Studies of the Galactic center with 30m class telescopes
    Peter Martha EHT Imaging the Event Horizon of Black Holes with the Event Horizon Telescope
    Hortense Dominik OST Probing the AGN-Galaxy connection with the Origins Space Telescope
    David Martha CCAT-prime Studies of the Sunyaev-Zel'dovich Effect in Galaxy Clusters with CCAT-prime