J/A+A/607/A48 AGN data and absorption-line measurements (Richter+, 2017)
An HST/COS legacy survey of high-velocity ultraviolet absorption
in the Milky Way's circumgalactic medium and the Local Group.
Richter P., Nuza S.E., Fox A.J., Wakker B.P., Ben Bekhti N., Fechner C.,
Wendt M., Howk J.C., Muzahid S., Ganguly R., Charlton J.
<Astron. Astrophys. 607, A48 (2017)>
=2017A&A...607A..48R 2017A&A...607A..48R (SIMBAD/NED BibCode)
ADC_Keywords: QSOs; Positional data; Spectroscopy; Equivalent widths
Keywords: Galaxy: halo - Galaxy: evolution - ISM: kinematics and dynamics-
techniques: spectroscopic - ultraviolet: ISM
Abstract:
The Milky Way is surrounded by large amounts of diffuse gaseous matter
that connects the stellar body of our Galaxy with its large-scale
Local Group (LG) environment.
To characterize the absorption properties of this circumgalactic
medium (CGM) and its relation to the LG we present the so-far largest
survey of metal absorption in Galactic high-velocity clouds (HVCs)
using archival ultraviolet (UV) spectra of extragalactic background
sources. The UV data are obtained with the Cosmic Origins Spectrograph
(COS) onboard the Hubble Space Telescope (HST) and are supplemented by
21cm radio observations of neutral hydrogen. Along 270 sightlines we
measure metal absorption in the lines of SiII, SiIII, CII, and CIV and
associated HI 21cm emission in HVCs in the velocity range
|vLSR|=100-500km/s. With this unprecedented large HVC sample we
were able to improve the statistics on HVC covering fractions,
ionization conditions, small-scale structure, CGM mass, and inflow
rate. For the first time, we determine robustly the angular two point
correlation function of the high-velocity absorbers, systematically
analyze antipodal sightlines on the celestial sphere, and compare the
HVC absorption characteristics with that of Damped Lyman alpha
absorbers (DLAs) and constrained cosmological simulations of the LG
(CLUES project).
The overall sky-covering fraction of high-velocity absorption is
77±6 percent for the most sensitive ion in our survey, SiIII, and
for column densities log N(SiIII)>12.1. This value is 4-5 times higher
than the covering fraction of 21 cm neutral hydrogen emission at log
N(HI)>18.7 along the same lines of sight, demonstrating that the Milky
Way's CGM is multi-phase and predominantly ionized. The measured
equivalent-width ratios of SiII, SiIII, CII, and CIV are
inhomogeneously distributed on large and small angular scales,
suggesting a complex spatial distribution of multi-phase gas that
surrounds the neutral 21cm HVCs. We estimate that the total mass and
accretion rate of the neutral and ionized CGM traced by HVCs is
MHVC>3.0x109M☉ and dMHVC/dt>6.1M☉/yr, where the
Magellanic Stream (MS) contributes with more than 90 percent to this
mass/mass-flow. If seen from an external vantage point, the Milky Way
disk plus CGM would appear as a DLA that would exhibit for most
viewing angles an extraordinary large velocity spread of
dv=400-800km/s, a result of the complex kinematics of the Milky Way
CGM that is dominated by the presence of the MS. We detect a velocity
dipole of high-velocity absorption at low/high galactic latitudes that
we associate with LG gas that streams to the LG barycenter. This
scenario is supported by the gas kinematics predicted from the LG
simulations.
Our study confirms previous results, indicating that the Milky Way CGM
contains sufficient gaseous material to feed the Milky Way disk over
the next Gyr at a rate of a few solar masses per year, if the CGM gas
can actually reach the MW disk. We demonstrate that the CGM is
composed of discrete gaseous structures that exhibit a large-scale
kinematics together with small-scale variations in physical
conditions. The MS clearly dominates both the cross section and mass
flow of high-velocity gas in the Milky Way's CGM. The possible
presence of high-velocity LG gas underlines the important role of the
local cosmological environment in the large-scale gas-circulation
processes in and around the Milky Way.
Description:
Names and positions for 270 QSOs are provided in Table A.1.
Table A.2 summarizes the absorption-line measurements for
high-velocity absorption along 187 sightlines including absorption
velocities, equivalent-widths, and column densities for various
different ions.
Table A.4 lists the hydrogen column densities for the high-velocity
absorbers.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 54 270 QSO names and Galactic coordinates
tablea2.dat 114 1049 Absorption-line measurements in HVC
tablea4.dat 68 187 Hydrogen column densities in HVCs
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
3- 5 I3 --- No Row number
7- 33 A27 --- Name QSO name
41- 45 F5.1 deg GLON Galactic longitude
50- 54 F5.1 deg GLAT Galactic latitude
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Byte-by-byte Description of file: tablea2.dat
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Bytes Format Units Label Explanations
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2- 5 I4 --- No Row number
7- 33 A27 --- Name QSO name (COS)
38- 42 A5 --- Ion Ion
48- 53 F6.1 0.1nm lambda0 Laboratory wavelength
58- 60 I3 --- S/N Signal-to-noise per resolution element
66- 69 I4 km/s vmin Lower velocity limit of HVC absorption
75- 78 I4 km/s vmax Upper velocity limit of HVC absorption
83 A1 --- Flag [t] Flag for tentative HVC detection
84- 87 I4 0.1pm W ? Equivalent width
94- 96 I3 0.1pm e_W ? Sigma equivalent width
100 A1 --- l_logN Limit flag on logN
101-105 F5.2 [cm/2] logN ? Logarithmic column density (or limit)
111-114 F4.2 [cm/2] e_logN ? Sigma logarithmic column density
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Byte-by-byte Description of file: tablea4.dat
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Bytes Format Units Label Explanations
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2- 4 I3 --- No Row number
6- 32 A27 --- Name QSO name (COS)
38- 41 I4 km/s vHVC Center velocity of HVC absorption/emission
45 A1 --- l_logN(HI) Limit flag on logN(HI)
46- 50 F5.2 [cm/s2] logN(HI) Logarithmic column density (or limit) of HI
54 A1 --- l_logN(H) Limit flag on logN(H)
55- 59 F5.2 [cm/s2] logN(H) Logarithmic column density (or limit) of H
63 A1 --- l_logf(HI) Limit flag on logf(HI)
64- 68 F5.2 [cm/s2] logf(HI) Logarithmic neutral hydrogen fraction
(or limit)
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Acknowledgements:
Philipp Richter, prichter(at)astro.physik.uni-potsdam.de
(End) P. Richter [Univ. Potsdam, Germany], P. Vannier [CDS] 20-Jun-2017