Harvard Astronomy 201b

Key (Open) Questions in the Study of the ISM of Other Galaxies, and of the Intergalactic Medium

In Uncategorized on April 14, 2011 at 1:42 pm
  1. How similar is the ISM in nearby galaxies to that of the Milky Way?  In what ways is it different?  (see SAGE Survey paper Journal Club discussion; also see Fukui & Kawmura review).

    Color image of H I 21 cm emission from Deul & van der Hulst (1987) with CO clouds overlaid as green dots. All molecular clouds lie in regions of H I overdensity. The area of the molecular cloud has been scaled to represent the relative masses of the clouds. The coincidence of molecular clouds with H I overdensity is evidence that clouds form out of the atomic gas. From Engargiola, Plambeck, Rosolowsky and Blitz 2003.

  2. How do ISM properties depend on galaxy type (and vice-versa!)?  (e.g. elliptical galaxies are well-known to have virtually no gas–why is that?)

    Spitzer data for elliptical and S0 galaxies. Circles represent observations, triangles upper limits. Filled symbols refer to elliptical galaxies with de Vaucouleurs classification parameter T < –3; open symbols refer to S0 galaxies with T> – 3. Red (blue) symbols have optical colors U-V < 1.1 (U-V>1.1); galaxies with unknown colors are plotted with green symbols. (from Temi Brighenti & Mathews 2009)

  3. How efficient is the ISM at forming stars in other galaxies?  Does this depend only on surface density (Kennicutt-Schmidt relation) or on more (e.g. galaxy mass, metallicity, size, etc.)? Link to nice PDF of PPT on K-S within galaxies.
  4. Is the IMF really “Universal,” and if so, what does that mean? (and how to measure it?…)
  5. How do ISM properties depend on redshift?  What affects “galaxy evolution” over various redshift ranges? (e.g. metallicity of galaxies increases over time as stars “pollute” the ISM with elements heavier than helium…when does that start?  What have big surveys like SDSS told us about this? What can we learn from “deep” observations like the HDF and HUDF?)

    Cosmic star formation rate (per unit comoving volume, h = 0.6, q0 = 0.5) as a function of redshift (the ‘Madau’ plot, Madau et al 1996). The black symbols (with error bars) denote the star formation history deduced from (non-extinction corrected) UV data (Steidel et al 1999 and references therein). Upward pointing dotted green arrows with open boxes mark where these points move when a reddening correction is applied. The green, four arrow symbol is derived from (non-extinction corrected) Hα NICMOS observations (Yan et al 1999). The red, three arrow symbol denotes the lower limit to dusty star formation obtained from SCUBA observations of HDF (N) (Hughes et al 1998). The continuous line marks the total star formation rate deduced from the COBE background and an ‘inversion’ with a starburt SED (Lagache et al 1999b). The filled hatched blue and yellow boxes denotes star formation rate deduced from ISOCAM (CFRS field, Flores et al 1999b) and ISOPHOT-FIRBACK (Puget et al 1999, Dole et al 1999). The light blue, dashed curve is model ‘A’ (no ULIRGSs) and the red dotted curve model ‘E’ (with ULIRGs) of Guuiderdoni et al (1998). Reproduced from Genzel & Cesarsky, ARA&A, 2000.

  6. What is the nature of the Intergalactic Medium under different conditions?  (e.g. in galaxy clusters, where hot X-ray haloes & cooling flows are important vs. in “empty” voids)
  7. What can be learned from long line-of-sight observations, e.g. of distant quasars? (Overview)
    1. Lyman-alpha forest
    2. metallicity variations with redshift (how long did it take the first stars to pollute the ISM?)
    3. Gunn-Peterson effect (ADS link to paper)

      Redshift distributions of galaxies and C IV absorbers in the field of Q1422+2309. The top panel shows the number of objects observed at each redshift. The bottom panel shows the implied overdensity as a function of redshift after smoothing our raw redshifts by a Gaussian of width z = 0.008. The good correspondence of features in the bottom panel shows that C IV systems are preferentially found within galaxy overdensities. (from Adelberger, Steidel, Shapley & Pettini 2003)

  8. What can be learned from direct observations of neutral hydrogen in the “intergalactic” medium before there were even many galaxies?  (New telescopes may be able to detect neutral hydrogen structures).
    1. Probes of the Epoch of Reionization
    2. Tomography” of HI in the Early Universe

      The transition from the neutral IGM left after the Universe recombined, at z ≈ 1,100, to the fully ionized IGM observed today is termed cosmic reionization. After recombination, when the CMB radiation was released, hydrogen in the IGM remained neutral until the first stars and galaxies2, 4 formed, at z ≈ 15–30. These primordial systems released energetic ultraviolet photons capable of ionizing local bubbles of hydrogen gas. As the abundance of these early galaxies increased, the bubbles increasingly overlapped and progressively larger volumes became ionized. This reionization process ended at z ≈ 6–8, ~1 Gyr after the Big Bang. At lower redshifts, the IGM remains highly ionized by radiation provided by star-forming galaxies and the gas accretion onto supermassive black holes that powers quasars. (from Robertson et al., Nature, 2010)

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