The H i Column Density Distribution of the Galactic Disk and Halo

Abstract We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm−2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude ∣b∣ ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ–Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = − 1.01 − 0.14 + 0.15 and β HVC = − 1.05 − 0.06 + 0.07 . Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with −0.88 ± 0.40 M ⊙ yr−1 of HVC inflow versus ≈0.20 ± 0.10 M ⊙ yr−1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

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Synthetic catalog of black holes in the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936557 ◽

2020 ◽

Vol 638 ◽

pp. A94 ◽

Cited By ~ 5

Author(s):

A. Olejak ◽

K. Belczynski ◽

T. Bulik ◽

M. Sobolewska

Keyword(s):

Black Holes ◽

Star Formation ◽

Binary Systems ◽

Milky Way ◽

Galactic Halo ◽

Galactic Disk ◽

Compact Objects ◽

Star Formation History ◽

Average Mass ◽

Synthetic Catalog

Aims. We present an open-access database that includes a synthetic catalog of black holes (BHs) in the Milky Way, divided by the components disk, bulge, and halo. Methods. To calculate the evolution of single and binary stars, we used the updated population synthesis code StarTrack. We applied a new model of the star formation history and chemical evolution of Galactic disk, bulge, and halo that was synthesized from observational and theoretical data. This model can be easily employed for other studies of population evolution. Results. We find that at the current Milky Way (disk+bulge+halo) contains about 1.2 × 108 single BHs with an average mass of about 14 M⊙, and 9.3 × 106 BHs in binary systems with an average mass of 19 M⊙. We present basic statistical properties of the BH population in three Galactic components such as the distributions of BH masses, velocities, or the numbers of BH binary systems in different evolutionary configurations. Conclusions. The metallicity of a stellar population has a significant effect on the final BH mass through the stellar winds. The most massive single BH in our simulation of 113 M⊙ originates from a merger of a BH and a helium star in a low-metallicity stellar environment in the Galactic halo. We constrain that only ∼0.006% of the total Galactic halo mass (including dark matter) can be hidden in the form of stellar origin BHs. These BHs cannot be detected by current observational surveys. We calculated the merger rates for current Galactic double compact objects (DCOs) for two considered common-envelope models: ∼3–81 Myr−1 for BH-BH, ∼1–9 Myr−1 for BH-neutron star (NS), and ∼14–59 Myr−1 for NS-NS systems. We show the evolution of the merger rates of DCOs since the formation of the Milky Way until the current moment with the new star formation model of the Galaxy.

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HI Column Density Distribution Function at z = 0: Connection to Damped Lyα Statistics

Publications of the Astronomical Society of Australia ◽

10.1071/as99100 ◽

1999 ◽

Vol 16 (1) ◽

pp. 100-105 ◽

Cited By ~ 18

Author(s):

Martin A. Zwaan ◽

Marc A. W. Verheijen ◽

Frank H. Briggs

Keyword(s):

Distribution Function ◽

Density Distribution ◽

Column Density ◽

Density Distribution Function ◽

Neutral Gas ◽

Low Surface Brightness ◽

A Minor ◽

Low Surface Brightness Galaxies ◽

The Universe ◽

High Column

AbstractWe present a measurement of the HI column density distribution function f(Nhi) at the present epoch for column densities > 1020 cm−2. These high column densities compare to those measured in damped Lyα lines seen in absorption against background quasars. Although observationally rare, it appears that the bulk of the neutral gas in the Universe is associated with these damped Lyα systems. In order to obtain a good anchor point at z = 0 we determine f(Nhi) in the local Universe by using 21 cm synthesis observations of a complete sample of spiral galaxies. We show that f(Nhi) for damped Lyα systems has changed significantly from high z to the present and that change is greatest for the highest column densities. The measurements indicate that low surface brightness galaxies make a minor contribution to the cross section for HI, especially for NHI > 1021 cm−2.

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Hot gas in Milky Way size galaxies at z=0

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316009340 ◽

2016 ◽

Vol 11 (S321) ◽

pp. 72-74

Author(s):

Santi Roca-Fàbrega ◽

Pedro Colin ◽

Octavio Valenzuela ◽

Francesca Figueras ◽

Yair Krongold

Keyword(s):

Dark Matter ◽

Column Density ◽

Milky Way ◽

Galactic Halo ◽

Careful Analysis ◽

Dark Matter Halo ◽

Hot Gas ◽

Satellite Galaxy ◽

Real Distribution ◽

Axisymmetric Structures

AbstractWe present a new set of cosmological Milky Way size galaxy simulations using ART. In our simulations the main system has been evolved inside a 28 Mpc cosmological box with a spatial resolution of 109 pc. At z=0 our systems have an Mvir = 6 − 8 × 1011 M⊙. In several of out models we have observed how a well defined disk is formed inside the dark matter halo and the overall amount of gas and stars is comparable with MW observations. Several non-axisymmetric structures arise out of the disk: spirals, bars and also a warp. We have also observed that a huge reservoir of hot gas is present at large distances from the disk, embedded in the dark matter halo region, accounting for only a fraction of the ”missing baryons”. Gas column density, emission (EM) and dispersion (DM) measure have been computed from inside the simulated disk at a position of 8 kpc from the center and in several directions. Our preliminary results reveal that the distribution of hot gas is non-isotropic according with observations (Gupta et al. 2012, Gupta et al. 2014). Also its metallic content presents a clear bimodality what is a consequence of a recent accretion of a satellite galaxy among others. After a careful analysis we confirm that due to the anisotropy in the gas distribution a new observational parameter needs to be defined to recover the real distribution of hot gas in the galactic halo (Roca-Fàbrega et al. 2016).

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NICER Study of Pulsed Thermal X-Rays from Calvera: A Neutron Star Born in the Galactic Halo?

The Astrophysical Journal ◽

10.3847/1538-4357/ac34f2 ◽

2021 ◽

Vol 922 (2) ◽

pp. 253

Author(s):

S. Mereghetti ◽

M. Rigoselli ◽

R. Taverna ◽

L. Baldeschi ◽

S. Crestan ◽

...

Keyword(s):

Neutron Star ◽

Galactic Halo ◽

Galactic Disk ◽

Good Description ◽

Hydrogen Atmosphere ◽

Galactic Latitude ◽

X Rays ◽

X Ray ◽

Polar Caps ◽

Star Surface

Abstract Calvera (1RXS J141256.0+792204) is an isolated neutron star detected only through its thermal X-ray emission. Its location at high Galactic latitude (b = +37°) is unusual if Calvera is a relatively young pulsar, as suggested by its spin period (59 ms) and period derivative (3.2 × 10−15 s s−1). Using the Neutron Star Interior Composition Explorer, we obtained a phase-connected timing solution spanning four years, which allowed us to measure the second derivative of the frequency ν ̈ = − 2.5 × 10 − 23 Hz s−2 and to reveal timing noise consistent with that of normal radio pulsars. A magnetized hydrogen atmosphere model, covering the entire star surface, provides a good description of the phase-resolved spectra and energy-dependent pulsed fraction. However, we found that a temperature map more anisotropic than that produced by a dipole field is required, with a hotter zone concentrated toward the poles. By adding two small polar caps, we found that the surface effective temperature and that of the caps are ∼0.1 and ∼0.36 keV, respectively. The inferred distance is ∼3.3 kpc. We confirmed the presence of an absorption line at 0.7 keV associated with the emission from the whole star surface, difficult to interpret as a cyclotron feature and more likely originating from atomic transitions. We searched for pulsed γ-ray emission by folding seven years of Fermi-LAT data using the X-ray ephemeris, but no evidence for pulsations was found. Our results favor the hypothesis that Calvera is a normal rotation-powered pulsar, with the only peculiarity of being born at a large height above the Galactic disk.

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Contributions to the Galactic halo from in-situ, kicked-out, and accreted stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921315007140 ◽

2015 ◽

Vol 11 (S317) ◽

pp. 241-246

Author(s):

Allyson A. Sheffield ◽

Kathryn V. Johnston ◽

Katia Cunha ◽

Verne V. Smith ◽

Steven R. Majewski

Keyword(s):

Neutron Capture ◽

Milky Way ◽

Galactic Halo ◽

Galactic Disk ◽

High Resolution Spectroscopy ◽

Chemical Abundances ◽

Giant Stars ◽

Halo Stars ◽

Tentative Evidence

AbstractWe report chemical abundances for a sample of 66 M giants with high S/N high-resolution spectroscopy in the inner halo of the Milky Way. The program giant stars have radial velocities that vary significantly from those expected for stars moving on uniform circular orbits in the Galactic disk. Thus, based on kinematics, we expect a sample dominated by halo stars. Abundances are derived for α-elements and neutron capture elements. By analyzing the multi-dimensional abundance space, the formation site of the halo giants – in-situ or accreted – can be assessed. Of particular interest are a class of stars that form in-situ, deep in the Milky Way's gravitational potential well, but are “kicked out” of the disk into the halo due to a perturbation event. We find: (1) our sample is dominated by accreted stars and (2) tentative evidence of a small kicked-out population in our Milky Way halo sample.

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The Vertical Structure of Warm Ionised Gas in the Milky Way

Publications of the Astronomical Society of Australia ◽

10.1071/as08004 ◽

2008 ◽

Vol 25 (4) ◽

pp. 184-200 ◽

Cited By ~ 202

Author(s):

B. M. Gaensler ◽

G. J. Madsen ◽

S. Chatterjee ◽

S. A. Mao

Keyword(s):

Electron Density ◽

Filling Factor ◽

Milky Way ◽

Galactic Disk ◽

Emission Measure ◽

Scale Height ◽

Thick Disk ◽

Joint Analysis ◽

Dispersion Measure ◽

Galactic Latitude

AbstractWe present a new joint analysis of pulsar dispersion measures and diffuse Hα emission in the Milky Way, which we use to derive the density, pressure and filling factor of the thick disk component of the warm ionised medium (WIM) as a function of height above the Galactic disk. By excluding sightlines at low Galactic latitude that are contaminated by Hii regions and spiral arms, we find that the exponential scale-height of free electrons in the diffuse WIM is 1830–250+120 pc, a factor of two larger than has been derived in previous studies. The corresponding inconsistent scale heights for dispersion measure and emission measure imply that the vertical profiles of mass and pressure in the WIM are decoupled, and that the filling factor of WIM clouds is a geometric response to the competing environmental influences of thermal and non-thermal processes. Extrapolating the properties of the thick-disk WIM to mid-plane, we infer a volume-averaged electron density 0.014 ± 0.001 cm−3, produced by clouds of typical electron density 0.34 ± 0.06 cm−3 with a volume filling factor 0.04 ± 0.01. As one moves off the plane, the filling factor increases to a maximum of ∼30% at a height of ≈1–1.5 kpc, before then declining to accommodate the increasing presence of hot, coronal gas. Since models for the WIM with a ≈1 kpc scale-height have been widely used to estimate distances to radio pulsars, our revised parameters suggest that the distances to many high-latitude pulsars have been substantially underestimated.

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Inflow of Neutral Gas toward the Galactic Disk

International Astronomical Union Colloquium ◽

10.1017/s0252921100024167 ◽

1989 ◽

Vol 120 ◽

pp. 396-407 ◽

Cited By ~ 1

Author(s):

I.F. Mirabel

Keyword(s):

Atomic Hydrogen ◽

Large Scale ◽

Milky Way ◽

Accretion Rate ◽

Galactic Disk ◽

Solar Neighborhood ◽

Milky Way Galaxy ◽

Neutral Gas ◽

The Galaxy ◽

Very High

Summary:I highlight the evidence for inflow of neutral gas toward the disk of the Galaxy. The Milky Way is accreting 0.2-0.5 M⊙/yr of extragalactic atomic hydrogen at very high velocities. The interaction of infalling clouds with galactic material produces large-scale disturbances in the interstellar medium. Although the injection of energy into the galactic disk by infalling neutral gas is only 1% of the energy from supernovae, the impinging of high velocity neutral gas may be a relatively important source of energy in localized regions of the outer Galaxy.In the solar neighborhood the downfall rate of HI at intermediate velocities is 2.72 x 10-8 z-1(kpc) M⊙ pc-2 yr-1, which if representative of the whole galactic disk, is at least 10 times more massive than the estimated accretion rate of extragalactic HI at very high velocities. This implies that most of the neutral gas that is infalling in the solar vicinity has originated in the galactic disk. It is concluded that in the Milky Way galaxy there is a moderate inflow of extragalactic neutral gas on top of a more intensive disk-halo circulation.

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The Galactic Halo and CDM

Symposium - International Astronomical Union ◽

10.1017/s0074180900183780 ◽

2004 ◽

Vol 220 ◽

pp. 431-438 ◽

Cited By ~ 2

Author(s):

Michael R. Merrifield

Keyword(s):

Dark Matter ◽

Density Distribution ◽

Power Law ◽

Strong Evidence ◽

Cold Dark Matter ◽

Milky Way ◽

Galactic Halo ◽

Central Density ◽

Available Information

This paper reviews the available information on the central density distribution and shape of the Milky Way's halo. At present, there is no strong evidence that the Milky Way's halo properties conflict with the predictions of cold dark matter (CDM): a primordial central power law cusp can be accommodated by the observations, and the current constraints on flattening are also consistent with the predictions of the theory. If you want to pick a fight with CDM, then the Milky Way is probably not the place to do it.

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The ORFEUS Far Ultraviolet Spectrum of the LMC Binary Star HDE 269546

International Astronomical Union Colloquium ◽

10.1017/s0252921100071530 ◽

1997 ◽

Vol 166 ◽

pp. 517-520

Author(s):

H. Widmann ◽

G. Krämer ◽

I. Appenzeller ◽

J. Barnstedt ◽

A. Fromm ◽

...

Keyword(s):

Molecular Hydrogen ◽

Milky Way ◽

Galactic Halo ◽

Binary Star ◽

Ultraviolet Spectrum ◽

Echelle Spectrograph ◽

Hot Gas ◽

First Results ◽

Far Ultraviolet ◽

First Time

AbstractFar UV high resolution spectra of 3 LMC and SMC stars were obtained with the Echelle spectrograph during the second ORFEUS mission in Dec. 1996. We present the first results from observations of the LMC star HDE 269546. We find definitely components of very hot gas identified as OVI and SVI absorption in the galactic halo of the Milky Way and in the LMC. Additionally, more than 30 ions of the most abundant elements in different stages of ionization can be identified in both our galaxy and the LMC. For the first time we can identify a significant absorption component of molecular hydrogen in the ORFEUS II Echelle spectrum with a redshift of 200 km s−1, doubtlessly to be attributed to the LMC.

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