Where Do Galaxies End?

Abstract

Our current view of galaxies considers them as systems of stars and gas embedded in extended halos of dark matter, much of it formed by the infall of smaller systems at earlier times. The true extent of a galaxy remains poorly determined, with the virial radius (R_vir) providing a characteristic separation between collapsed structures in dynamical equilibrium and external infalling matter. Other physical estimates of the extent of gravitational influence include the gravitational radius, gas accretion radius, and "galactopause" arising from outflows that stall at 100-200 kpc over a range of outflow parameters and confining gas pressures. Physical criteria are proposed to define bound structures, including a more realistic definition of R_vir (M*, M_h, z_a) for stellar mass M* and halo mass M_h, half of which formed at "assembly redshifts" z_a = 0.7-1.3. We estimate the extent of bound gas and dark matter around L* galaxies to be ~200 kpc. The new virial radii, with mean R_vir ~ 200 kpc, are 40-50% smaller than values estimated in recent HST/COS detections of H I and O VI absorbers around galaxies. In the new formalism, the Milky Way stellar mass, log M* = 10.7 +/- 0.1, would correspond to R_vir = 153 (+25,-16) kpc for half-mass halo assembly at z_a = 1.06 +/- 0.03. The frequency per unit redshift of low-redshift O VI absorption lines in QSO spectra suggests absorber sizes ~150 kpc when related to intervening 0.1L* galaxies. This formalism is intended to clarify semantic differences arising from observations of extended gas in galactic halos, circumgalactic medium (CGM), and filaments of the intergalactic medium (IGM). Astronomers should refer to bound gas in the galactic halo or CGM, and unbound gas at the CGM-IGM interface, on its way into the IGM.

26 pages, 1 figure, final version, accepted to Astrophysical Journal

References in corpus (13)

• The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
• The RAVE Survey: Constraining the Local Galactic Escape Speed
• Mass, Metal, and Energy Feedback in Cosmological Simulations
• The large-scale structure of the halo of the Andromeda Galaxy Part I: global stellar density, morphology and metallicity properties
• Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-Line Spectroscopy
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• The Low-z Intergalactic Medium. III. HI and Metal Absorbers at z<0.4
• The long bar as seen by the VVV survey: I. Colour-magnitude diagrams
• Do Hot Haloes Around Galaxies Contain the Missing Baryons?
• The Mass Profile of the Galaxy to 80 kpc
• Transverse Sizes of CIV Absorption Systems Measured from Multiple QSO Sightlines
• Warm-hot intergalactic medium contribution to baryonic matter
• The virialization density of peaks with general density profiles under spherical collapse

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• On the connection between the metal-enriched intergalactic medium and galaxies: an OVI-galaxy cross-correlation study at $z<1$
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• Cold Mode Gas Accretion on Two Galaxy Groups at z$\sim$2
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