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2 Report of the Panel on the Galactic Neighborhood
Pages 53-94

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From page 53...
... Indeed, almost every field of astrophysics -- from the evolution of stars to the structure of dark matter halos, from the formation of supermassive black holes to the flows of gas in and out of galaxies -- benefits from the detailed physical constraints that are 53
From page 54...
... To appreciate the impact of the galactic neighborhood, first consider studies of the universe on the largest scales. The interpretation of observations of the most distant galaxies is built on a foundation of knowledge established in the galactic neighborhood, including knowledge about the evolution of stellar populations, the existence of dark matter, the scaling relations of supermassive black holes, the effects of feedback from supernovae, the importance of accretion, the relationship between star formation and gas density, and the stellar initial mass function, among many others.
From page 55...
... This question addresses the use of the galactic neighborhood as a laboratory of fundamental physics. The local universe offers the opportunity to isolate the nearest and smallest dark matter halos and to study astrophysically "dark" systems at high spatial resolution.
From page 56...
... The major feedback processes include the deposition and transport of mass, momentum, energy, and heavy elements into the ISM, CGM, and IGM by stars and supermassive black holes in galactic nuclei. The bulk of the energy and metals from these feedback channels is still not accounted for observationally.
From page 57...
... as well as the heavy elements detected throughout much of the IGM. Indirect evidence for gas flows comes from the observation that many galaxies contain lower fractions of baryons to dark matter than the primordial ratio, Wb/Wm.
From page 58...
... The high spatial resolution and panchromatic approach possible for nearby galaxies make the galactic neighborhood the best location for establishing the content and circulation of the circumgalactic gas. Indeed, observations from radio to X-ray have established the presence of galactic winds in starburst galaxies and feedback from active galactic nuclei (AGN)
From page 59...
... , and their relation to the feedback process in more normal galaxies remain controversial. The coupling of the radiation, momentum, and kinetic energy from stars and supermassive black holes with the surrounding interstellar and circumgalactic gas is extremely difficult to model from first principles.
From page 60...
... . Tomography of the hot gas in and around the Milky Way will allow the decom position of hot gaseous components of the Milky Way (galactic disk, bulge, halo)
From page 61...
... around many nearby galaxies, which are especially important for studying the interplay between hot and cool components of the CGM. Absorption studies can also probe the relationship between the hot CGM and infalling material, such as cool high-velocity gas clouds.
From page 62...
... The energy-carrying phase of a galactic wind emits primarily at X-ray frequen cies, where observations have been limited to a small number of galaxies. Current facilities lack the velocity resolution required to measure gas kinematics in the hot phase, so only the thermal energy content (and not the kinetic energy)
From page 63...
... Studies of the galactic halo suggest that warm gas constitutes a major component of the infall feeding the galaxy, but they are limited by distance uncertainties, which will not apply to observations of nearby galaxies. Emission-line imaging in the ultraviolet (e.g., O VI, H I Ly-a, C IV, C III, N V, O III, Mg II)
From page 64...
... Important strides have been made in the past two decades in determining how galaxies grow from their dark matter seeds to the complex systems of dark matter, stars, and gas that are observed at z = 0. Simulations of the formation of structure in a universe dominated by cold dark matter (ΛCDM)
From page 65...
... , to parsecs (formation of giant molecular clouds, starbursts, and star clusters) , and down to the sub-parsec level where star formation occurs.
From page 66...
... This image shows the importance of large-scale multiwavelength surveys for elucidating structures in the Milky Way interstellar medium. X-rays arise from hot gas heated by supernova explosions that sweep up cool H I
From page 67...
... This is especially true for low-mass low-metallicity galaxies that are presumably the analogs of star-forming galaxies in the early universe. The discovery of ultrafaint dwarf galaxies in the Local Group challenges conventional wisdom about the formation of molecular clouds and star formation in such low-mass systems.
From page 68...
... The Milky Way and nearby galaxies are devouring their molecular star-forming gas at rates that are unsustainable for more than approximately 2 Gyr on average. Either the end of the star-forming era in the universe is near, which is unlikely, or the molecular gas lost to star formation is being replaced from reservoirs of H I in the outer parts of galaxies, or H II from the halo or IGM.
From page 69...
... satellite have shown that star formation occurs, even in the most distant regions of H I disks, challenging present understanding of how and where molecules form and of how star formation takes place on global scales. Older issues such as how outer H I disks maintain nearly constant velocity dispersion with radius and what keeps gas disks thin in the apparent absence of stellar disks remain unanswered.
From page 70...
... Zauderer, et al., Dynamically driven evolution of the interstellar medium in M51, Astrophysical Journal Letters 700(2) :L132, 2009, reproduced by permission of the AAS.
From page 71...
... Spitzer has surveyed star formation throughout the Milky Way and many external galaxies. Herschel (3.5-m aperture)
From page 72...
... What Is the Structure of the Magnetic Field in the Interstellar Medium? Magnetic fields are dynamically important in the interstellar medium: strong enough to control gas motions in H I clouds and star-forming molecular clouds and to affect outflows from the disk into the CGM.
From page 73...
... Theoretical work must focus on these fundamental processes. Key questions are these: What processes are responsible for generating galactic magnetic fields?
From page 74...
... Modern cosmology provides a theoretical paradigm for galaxy formation and assembly; astronomers can test these ideas with observations of galaxy properties such as morphology, luminosity, and color, and of the distribution of such properties in populations from nearby galaxies to redshifts that possibly overlap the epoch of reionization. As valuable and insightful as those observations will be, the study of ensembles necessarily obscures the physical processes affecting individual galaxies by providing only average properties.
From page 75...
... In sum, these tools and techniques promise to reveal the full history of galaxy formation in galaxies of many different shapes and sizes, going all the way back to their origins in the first stars. Fossil Record of Galaxies from Resolved Stellar Populations The fossil record of whole external galaxies can be read in their stellar colormagnitude diagrams (CMDs)
From page 76...
... Majewski, et al., The extended star formation history of the Andromeda spheroid at 35 kpc on the minor axis, Astrophysical Journal Letters 685:L121, 2008, reproduced by permission of the AAS.
From page 77...
... The formation history of fair samples of both elliptical and spiral galaxies out to tens of Mpc can also be probed by other stellar population measures, such as globular clusters and spectroscopy of the integrated light of galaxies. These approaches utilize high-quality wide-field imaging and highly multiplexed deep spectroscopy, and they provide both stellar populations and kinematic tests of how individual galaxies in the local universe came to be.
From page 78...
... , some of which have a total luminosity comparable to that of a single giant star. These streams and small galaxies offer the opportunity to gain an understanding of the internal and external influences on galaxy formation at the smallest mass scale and with the most sensitive indicators of such processes as reionization, chemical enrichment, supernova feedback, and tidal disruption.
From page 79...
... These objects hold our interest as the ultimate beginning of galaxy formation and chemical enrichment in the universe. Theorists have reached the robust conclusion that the truly first stars formed in small dark matter halos at z = 10 to 30 and were likely massive (tens to hundreds of solar masses)
From page 80...
... The first step in the process will be realized by the same large photometric and spectroscopic surveys that map the Milky Way. New data-mining algorithms must be developed to select efficiently the most metal-poor stars from these vast samples, and efficient multiplexed spectrographs on large telescopes will be needed to obtain abundances of the elements that tell with statistical confidence the full story of primordial star formation and chemical enrichment.
From page 81...
... Here, the panel outlines the importance of the local universe with respect to making progress in one of the greatest unsolved mysteries of modern astrophysics. Using the Local Universe as a Dark Matter Laboratory The lambda cold dark matter (ΛCDM)
From page 82...
... As is discussed below, likely approaches include the following: identifying systems where baryons are negligible, constraining the amount of low-mass substructure, improving the understanding of inner halo kinematics, and directly detecting signals from dark matter interac tions at high densities. These experiments will inevitably focus on the galactic neighborhood, which is the only environment in which astronomers have sufficient sensitivity and spatial resolution to conduct such tests, including those related to the following questions: • What is the distribution of dwarf satellite galaxies in the Milky Way?
From page 83...
... Their shallow gravitational potential wells make them susceptible to disruption by feedback from galactic winds, suppression by reionization, and stripping by outflows from nearby massive galaxies. Hints of this fragility can be seen in the variation in mass-to-light ratio of the faintest dwarf galaxies, which span four orders of magnitude in luminosity but have comparable velocity dispersions.
From page 84...
... The galactic center is probably the brightest source in the sky for such annihilations, but the astrophysical backgrounds in this region make it difficult to disentangle photons produced by the candidate dark matter particle from those produced by supernova remnants, pulsars, and binaries. The best place to look for the signature of weakly interacting dark matter may be in the heart of the ultrafaint dwarf galaxies.
From page 85...
... Although dwarf galaxies are superb laboratories for probing dark matter physics, they are only one extreme of the halo mass distribution. It is therefore critical to push the constraints on dark matter structure to higher masses.
From page 86...
... This progress will come primarily from studies of nearby galaxies and the Milky Way, the only systems that can be observed with the necessary spatial resolution and sensitivity. What Controls the Masses of Black Holes?
From page 87...
... Although some evidence suggests that intermediate-mass black hole seeds grow to become supermassive black holes, detections remain sparse and controversial. The local universe offers the most promising avenues for identifying potential seed black holes, by direct detection through dynamical studies of nearby systems, by indirect studies of low-luminosity AGNs, and through the possibility of measuring gravitational waves of black hole inspiral events.
From page 88...
... If true, this result will have significant consequences both for the transfer of angular momentum between the accreting material and the black hole and for the merger histories of black holes. Significant improvements in measures of black hole spin over a wide mass range are possible over the next decade, coming primarily from improved X-ray observations with large collecting areas and good spectral resolution (DE < 3 eV at 6 keV)
From page 89...
... Transient events observed in nearby galaxies allow one to characterize the luminosities of the events, their rates, and connections with underlying stellar populations and galactic structure with an ease that is not possible in the galaxy where distances are hard to measure, or over cosmological distances, at which highresolution imaging and spectroscopic follow-up is difficult. Obtaining follow-up spectra is important to enable time-domain studies to reach their full scientific potential.
From page 90...
... Even well-understood, time-variable stellar phenomena such as RR Ly rae, Cepheids, and long-period variables offer unambiguous tracers of the galactic neighborhood's ancient stellar populations, thereby giving a three-dimensional structure of stellar streams and their orbits, and thus constraining our galaxy's dark matter halo. In summary, time-domain astronomy within z < 0.1 will allow scientists to map the content and evolution of stellar structure in galaxies at a level of detail and precision not easily obtained outside the local universe.
From page 91...
... , believed to coincide with the black hole at the galactic center, now yields the most accurate measurement of the angular speed of the Sun in its galactic orbit, as well as strong constraints on the mass of the black hole and the mass and orbit of any possible companion black hole(s)
From page 92...
... 0.3-8 keV at R ≈ 300 X-ray imaging Mapping hot gas outflows, supernova remnants, FOV ~ 20′ and Dq ~ 5″ spectroscopy superbubbles Ilim ~ 10−14 ergs s−1 cm−2 arcmin−2 0.5-7 keV at 1-3 eV resolution Mapping hot plasma in SNRs, stellar winds, superbubbles Line detection sensitivity ~ 10× Chandra Black hole mass measurements 0.5-7 keV Dark matter indirect detection X-ray absorption 0.1-1 keV Abs-line spectroscopy of hot CGM R ≈ 3,000 spectroscopy Abs-line spectroscopy of hot ISM Aeff ~ 1,000 cm2 Dust grain mineralogy Inventory of abundant elements in ISM; obtaining kinematics of gas UV imaging 1,000-3,000 Å Mapping warm ISM and CGM R ≈ 1,000-2,000 Mapping galaxy outflows FOV ~ 20′ and Dq ~ 1″ Mapping hot plasma in the ISM and at the disk-halo Ilim ~ 100 LU (ph s−1 cm−2 sr−1) interface Now: STIS, GALEX Mapping star-forming regions Needed: Dq ~ 1″ 900-3,000 Å at R ≈ 30,000 UV spectroscopy Abs-line spectra of warm ISM, CGM Aeff ~ 104 cm2 Diagnose hot plasma in ISM and at disk-halo interface Now: STIS and COS (1,150-3,000 Å)
From page 93...
... V ~ 18-28 Multiband photometry Locating and identifying faint Milky Way halo Dq = 0.04″ substructure (Q3.2, Q4) including streams mlim ~ 25 and dwarf galaxies FOV ~ 10′ Dq = 0.04″ Probing Milky Way black hole mlim ~ 21 Proper motions of Milky Way stars Optical/near-IR R = 10,000-40,000, V ~ 18-19 Kinematics and abundances in Milky Way spectroscopy 500-1,000 multiobject spectrograph halo, dwarfs, and streams (Q3.2)
From page 94...
... for high High-resolution line and continuum images of starimaging and resolution forming regions' high-excitation molecules, dust spectroscopy Needed: ~25-m telescope at high site Mapping star-forming clouds at galactic center Dq < 3″ at 350 mm Dust maps for nearby galaxies 350 mm-1 mm Potential for imaging black holes Polarimetry (linear) would be valuable Millimeter-wave Approved: ALMA High-resolution images of dust in star-forming imaging regions Needed: CARMA array receivers (1 mm)


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