Probing the Outflowing Multiphase Gas ∼1 kpc below the Galactic Center

“Using the notation [Fe II/S II]=log(N(Fe II)/N(S II))–log(Fe/S)e with log(Fe/S)e= 0.38 dex from Asplund et al. (2009), we give the values of [Fe II/ S II] in column (6) of Table 11.”

“The Galaxy is a bared spiral (Bland-Hawthorn & Gerhard 2016).”

“In diffuse ISM cloud absorbers, [Fe/H] is found to range from −2.4 to −1.0 dex, with the smaller abundances found in denser disk gas and the larger abundances found in halo clouds (Savage & Sembach 1996).”

“The Galactic center (GC) is the host to the Galactic black hole, Sgr A* (Schodel et al. 2002; Ghez et al. 2003), along with three extraordinary clusters of stars, the Central cluster (Krabbe et al. 1991), the Arches cluster (Nagata et al. 1995), and the Quintuplet cluster (Glass et al. 1990; Okuda…”

“Using vGSR=vLSR + (254 km s −1) sin(l) cos(b), where 254 km s−1 is the rotation velocity of the LSR at the Sunʼs distance from the GC (Reid et al. 2009), we find that the very large outflow velocity for the direction to LS 4825 traced by C II of vLSR=−290 km s −1 corresponds to vGSR= −283 km s−1.”

“The spectacular γ-ray emission bubbles (Fermi bubbles), discovered by the Fermi Gamma-ray Space Telescope, extend±50° in Galactic latitude (±10 kpc) and ±20° in Galactic longitude (Dobler et al. 2010; Su et al. 2010; Crocker & Aharonian 2011; Ackermann et al. 2014).”

“We note that VPFIT uses the Morton (2003) C I, C I*, and C I** oscillator strengths rather than those from Jenkins & Tripp (2001).”

“The Galactic center (GC) is the host to the Galactic black hole, Sgr A* (Schodel et al. 2002; Ghez et al. 2003), along with three extraordinary clusters of stars, the Central cluster (Krabbe et al. 1991), the Arches cluster (Nagata et al. 1995), and the Quintuplet cluster (Glass et al. 1990; Okuda…”

“The difference in the values of log Na(w) for the weak and log Na(s) strong line of a high absorption doublet, Δ(w−s), can be used to determine a correction to the saturation by following the procedures recommended by Savage & Sembach (1991) and listed in their Table 4.”

“The three clusters are massive (M∼104–105Me) and contain hundreds of O and WN stars (Figer et al. 1999, 2002; Paumard et al. 2006).”

“Figure 1 displays the 1.5 keV X-ray image of the GC adapted from Snowden et al. (1997) by Wang (2002), along with the and γ-ray image adapted from Ackermann et al. (2014).”

“Figure12 in Tripp et al. (1993) shows the expected velocities of absorbing gas along the line of sight to each star as a function of distance assuming co-rotation of disk and halo gas and the galactic rotation curve of Clemens (1985).”

“The three clusters are massive (M∼104–105Me) and contain hundreds of O and WN stars (Figer et al. 1999, 2002; Paumard et al. 2006).”

“Using the Walborn (1972) absolute visual magnitude (MV) calibration of −5.0 for a B1 II star and −6.0 for a B1 Ib star, they found MV=−5.5±0.5 for LS 4825.”

“The GC shows a dramatic excess of nonthermal radio, microwave, mid-IR, X-ray, and γ-ray emission extending several kiloparsecs in z-distance above and below the GC midplane (Sofue & Handa 1984; Snowden et al. 1997; BlandHawthorn & Cohen 2003; Finkbeiner 2004; Su et al. 2010).”

“Figure 1 displays the 1.5 keV X-ray image of the GC adapted from Snowden et al. (1997) by Wang (2002), along with the and γ-ray image adapted from Ackermann et al. (2014).”

“…low z that the sightline to LS 4825 passes under include the Sagittarius spiral arm at d∼1.5 kpc; the Scutum arm at d∼4 kpc; the expanding 3 kpc arm (Oort 1977) at d∼6 kpc, which has v∼−53 km s−1 in the direction l=−2 and b=0 (Bania 1980); and the galactic worm candidate GW 0.5−5.9 (see Figure2…”

“The three clusters are massive (M∼104–105Me) and contain hundreds of O and WN stars (Figer et al. 1999, 2002; Paumard et al. 2006).”

“The TML concept of the mixing of cool, warm, and hot gas through turbulence was first introduced by Begelman & Fabian (1990) and subsequently applied to the ionization of the transition temperature (T∼105 to 106 K) ions by Slavin et al. (1993), Kwak & Shelton (2010), and Kwak et al. (2015).”

“…exception of HD 44743 and BD +48 3437, which are from Morgan et al. (1955), LS 4825, which is from Ryans et al. (1997), and HD 99857, which is from Diplas & Savage (1994). c The color excess, E(B−V ), listed used the values of B−V and the intrinsic colors from Wegner (1994). d The LSR stellar…”

“The spectacular γ-ray emission bubbles (Fermi bubbles), discovered by the Fermi Gamma-ray Space Telescope, extend±50° in Galactic latitude (±10 kpc) and ±20° in Galactic longitude (Dobler et al. 2010; Su et al. 2010; Crocker & Aharonian 2011; Ackermann et al. 2014).”

“See Kimble et al. (1998) for a discussion of the properties of STIS and its in-flight performance.”

“…(1997), and HD 99857, which is from Diplas & Savage (1994). c The color excess, E(B−V ), listed used the values of B−V and the intrinsic colors from Wegner (1994). d The LSR stellar radial velocity was determined from the Si III λ1417 line using the AOD method (Savage & Sembach 1991) with vLSR=…”

“…center (GC) is the host to the Galactic black hole, Sgr A* (Schodel et al. 2002; Ghez et al. 2003), along with three extraordinary clusters of stars, the Central cluster (Krabbe et al. 1991), the Arches cluster (Nagata et al. 1995), and the Quintuplet cluster (Glass et al. 1990; Okuda et al. 1990).”

“The spectacular γ-ray emission bubbles (Fermi bubbles), discovered by the Fermi Gamma-ray Space Telescope, extend±50° in Galactic latitude (±10 kpc) and ±20° in Galactic longitude (Dobler et al. 2010; Su et al. 2010; Crocker & Aharonian 2011; Ackermann et al. 2014).”

“…BD +48 3437. b The spectral types, V magnitude, and B−V are from Bowen et al. (2008), with the exception of HD 44743 and BD +48 3437, which are from Morgan et al. (1955), LS 4825, which is from Ryans et al. (1997), and HD 99857, which is from Diplas & Savage (1994). c The color excess, E(B−V ),…”

“Other starburst explanations are given by Crocker et al. (2015) and Carretti et al. (2013).”

“The origin of the outer structure is unclear because it requires a huge explosion with E∼3×1058 erg (Sanders & Prendergast 1974).”

“This value is similar to that found for many clouds in the diffuse ISM of the Galactic disk, where Jenkins & Tripp (2011) report for a sample of 89 sightlines a mean of log (p/k)=3.6±0.18 (rms) dex.”

“Sembach et al. (2000) have studied the impact of the existence of the WIM on elemental abundance studies and have developed a model for correcting for the contamination assuming that the WIM is ionized by the radiation from hot stars.”

“One might question whether or not a small cloud could survive without being disrupted by instabilities arising from interactions with the surrounding flow (Cooper et al. 2009).”

“That distance is consistent with the 3D maps of the distribution of Na I absorption toward stars with d<300 pc of Welsh et al. (2010), which reveal for l∼2° and b∼−5° that strong absorption by Na I occurs at d∼120 pc and extends to ∼180 pc.”

“STIS E140M observations were also obtained of the foreground comparison star HD 167402 as part of HST GO proposal 13488 (“The Closest Galactic Wind: UV Properties of the Milky Wayʼs Nuclear Outflow,” PI, A. Fox).”

“The GC shows a dramatic excess of nonthermal radio, microwave, mid-IR, X-ray, and γ-ray emission extending several kiloparsecs in z-distance above and below the GC midplane (Sofue & Handa 1984; Snowden et al. 1997; BlandHawthorn & Cohen 2003; Finkbeiner 2004; Su et al. 2010).”

“…center (GC) is the host to the Galactic black hole, Sgr A* (Schodel et al. 2002; Ghez et al. 2003), along with three extraordinary clusters of stars, the Central cluster (Krabbe et al. 1991), the Arches cluster (Nagata et al. 1995), and the Quintuplet cluster (Glass et al. 1990; Okuda et al. 1990).”

“Other active galactic nucleus (AGN) related explanations are given by Guo & Matthews (2012), Yang et al. (2012), and Zubovas et al. (2011).”

“When preparing the Fox et al. (2015) manuscript and considering additional HST observations, we became aware of the extremely interesting HST archival spectroscopic observations for the sightline to the star LS 4825 (CD −30 15464), a luminous B1 Ib−II star lying 21±5 kpc away in the direction…”

“As long as the external mediumʼs temperature is of order 107 K, the flow past the cloud will be supersonic, which reduces the influence of Kelvin–Helmholtz instabilities in breaking up the cloud (Bruggen & Scannapieco 2016).”

“Other starburst explanations are given by Crocker et al. (2015) and Carretti et al. (2013).”

“…center (GC) is the host to the Galactic black hole, Sgr A* (Schodel et al. 2002; Ghez et al. 2003), along with three extraordinary clusters of stars, the Central cluster (Krabbe et al. 1991), the Arches cluster (Nagata et al. 1995), and the Quintuplet cluster (Glass et al. 1990; Okuda et al. 1990).”

“We follow an argument similar to one presented earlier by Jenkins et al. (1983) for the C I excitation observed in the slightly different context of gas clumps immersed in a stellar wind.”

“An additional complication includes the presence of the 1.5 kpc radius tilted H I disk structure in the inner Galaxy observed by Burton & Liszt (1978) and discussed in Section 5.”

“The extracted and combined observations of LS 4825 and six of the comparison stars were included in the Catalog of STIS Ultraviolet Echelle Spectra of Stars of Ayres (2010).”

“…suggesting that the origin occurred at a time when Sgr A* was 103–104 times more active than today, with its accretion flow launching a strong wind (Mou et al. 2014), to a suggestion that the Fermi bubble arises in the termination shock of a nuclear-starburst-driven wind powered by the GC hot…”

“The Galactic exponential scale heights of C IV and Si IV have been estimated to be ∼3 kpc (Savage & Wakker 2009).”

“A study of the energetics and timescales of the interaction of the Fermi bubble with the Milky Wayʼs hot halo by Miller & Bregman (2016) suggests that the Fermi bubble likely formed by a Sgr A* accretion event rather than sustained nuclear star formation activity.”

“With logN(H I)=18.75, the photoionization modeling of Bordoloi et al. (2017) implies that the ionization correction for O I should be small so [O I/H I] should be equal to [O/H].”

“…Scutum arm at d∼4 kpc; the expanding 3 kpc arm (Oort 1977) at d∼6 kpc, which has v∼−53 km s−1 in the direction l=−2 and b=0 (Bania 1980); and the galactic worm candidate GW 0.5−5.9 (see Figure2 in Koo et al. 1992), with an estimated distance of 7 kpc and an H I velocity range from −1 to 26 km s−1.”

“In addition, there are numerous isolated WR and O supergiants in the field of the GC (Mauerhan et al. 2010).”

“The TML concept of the mixing of cool, warm, and hot gas through turbulence was first introduced by Begelman & Fabian (1990) and subsequently applied to the ionization of the transition temperature (T∼105 to 106 K) ions by Slavin et al. (1993), Kwak & Shelton (2010), and Kwak et al. (2015).”

“In order to obtain measurements aimed toward determining the kinematical and the physical state of the gas in the observed ISM absorption line systems, we have employed the VPFIT code Version 10 discussed in Carswell et al. (2002) and Kim et al. (2016).”

“…the origin occurred at a time when Sgr A* was 103–104 times more active than today, with its accretion flow launching a strong wind (Mou et al. 2014), to a suggestion that the Fermi bubble arises in the termination shock of a nuclear-starburst-driven wind powered by the GC hot stars (Lacki 2014).”

“Wakker et al. (2012) provide a comprehensive overview of the many different ionization mechanisms in their Section 5.”

“…and logN(H I)= 19.43±0.01±0.15 from 21 cm emission at −70 km s−1 we obtain [S II/H I]=+0.02±0.05±0.15, [Fe II/H I]=−1.05± 0.04±0.15, and [Al II/H I)]=−0.90±0.13±0.15, where the second errors allow approximately for radio antenna beamsmearing uncertainty as estimated by Wakker et al. (2001).”

“The spectra were Hanning smoothed, calibrated, and corrected for stay radiation using the procedures discussed by Boothroyd et al. (2011).”

“The Na I and Ca II observations of HD 167402, HD 164019, HD 165582, and HD 168941 are from Sembach et al. (1997), while the values for LS 4784 are inferred from Figure4 of Ryans et al. (1997).”

“Early B supergiants with high-ionization P Cygni line profiles exhibit relatively narrow discrete absorption components (DACs) that originate in the stellar wind (Prinja et al. 2002).”

“…spiral arm at d∼1.5 kpc; the Scutum arm at d∼4 kpc; the expanding 3 kpc arm (Oort 1977) at d∼6 kpc, which has v∼−53 km s−1 in the direction l=−2 and b=0 (Bania 1980); and the galactic worm candidate GW 0.5−5.9 (see Figure2 in Koo et al. 1992), with an estimated distance of 7 kpc and an H I…”

“C II * to C II 157.7 m emission is a major source of cooling of the diffuse ISM (Lehner et al. 2004).”

“…center (GC) is the host to the Galactic black hole, Sgr A* (Schodel et al. 2002; Ghez et al. 2003), along with three extraordinary clusters of stars, the Central cluster (Krabbe et al. 1991), the Arches cluster (Nagata et al. 1995), and the Quintuplet cluster (Glass et al. 1990; Okuda et al. 1990).”

“…features appearing in the spectra of stars within or behind the Vela supernova remnant that small clouds pressurized by a blast wave can survive over a time interval at least comparable to the age of the remnant of about 104 yr (Jenkins et al. 1981; Jenkins & Wallerstein 1995; Jenkins et al. 1998).”

“In a related study, Danly et al. (1992) compared the relative velocity behavior of H I 21 cm emission and UV absorption toward many stars in the galactic halo including stars toward the GC.”

“The figure shows log [N(Si IV)/N(C IV)] versus log [N(C IV)/N(N V)] for many ionization mechanism models, along with observational results for the Milky Way halo (Wakker et al. 2012) and the Milky Way disk (Lehner et al. 2011).”

“…suggesting that the origin occurred at a time when Sgr A* was 103–104 times more active than today, with its accretion flow launching a strong wind (Mou et al. 2014), to a suggestion that the Fermi bubble arises in the termination shock of a nuclear-starburst-driven wind powered by the GC hot…”

“132, 1317.217, which are from Jenkins & Tripp (2006), and Ni II λλ1703.”

“HST images of de Marco et al. (2001) reveal a very small diameter for the nebular Hβ emission of (1.3×0.9)±0.2 arcsec, with nebular [O III] λλ4959, 5007 being slightly smaller.”

“Note that the suggested TML process is related to that required to explain the self-irradiated cooling condensations expected to occur in the hot cooling flows discussed by Voit & Donahue (1990) in order to explain the optical line emission from the centers of clusters of galaxies.”

“…features appearing in the spectra of stars within or behind the Vela supernova remnant that small clouds pressurized by a blast wave can survive over a time interval at least comparable to the age of the remnant of about 104 yr (Jenkins et al. 1981; Jenkins & Wallerstein 1995; Jenkins et al. 1998).”

“Over this time interval, the clump would have traveled over a distance of only 170 pc, which means that the original location must still have been of order 1 kpc below the plane, much like the considerably larger cold clouds observed by Ford et al. (2008).”

“The outermost structure at about 3 kpc from the GC is associated with the H I hole in the inner Galaxy extending from l=340 to 20 observed by Lockman & McClure-Griffiths (2016).”

“The TML concept of the mixing of cool, warm, and hot gas through turbulence was first introduced by Begelman & Fabian (1990) and subsequently applied to the ionization of the transition temperature (T∼105 to 106 K) ions by Slavin et al. (1993), Kwak & Shelton (2010), and Kwak et al. (2015).”

“The HST STIS E140M and FUSE spectrum of the central star analyzed by Sterling et al. (2005) and shown in their Figure 2 reveals interstellar absorption near vLSR=6 and 38 km s −1 in S II, Fe II, and Si II.”

“There have been many subsequent studies of the distance and physical conditions in this cloud using optical and UV absorption line observations of hot stars (Bates et al. 1995; Montgomery et al. 1995; Kemp et al. 1996), which places the cloud at d<0.2 kpc.”

“…features appearing in the spectra of stars within or behind the Vela supernova remnant that small clouds pressurized by a blast wave can survive over a time interval at least comparable to the age of the remnant of about 104 yr (Jenkins et al. 1981; Jenkins & Wallerstein 1995; Jenkins et al. 1998).”

“411, 1502.148, where the oscillator strengths are from Berengut et al. (2011).”

“Figure12 in Tripp et al. (1993) shows the expected velocities of absorbing gas along the line of sight to each star as a function of distance assuming co-rotation of disk and halo gas and the galactic rotation curve of Clemens (1985).”

“With the new UV measurements, we can check the validity of the classification and distance estimate for LS 4825 obtained by Ryans et al. (1997).”

“In order to obtain measurements aimed toward determining the kinematical and the physical state of the gas in the observed ISM absorption line systems, we have employed the VPFIT code Version 10 discussed in Carswell et al. (2002) and Kim et al. (2016).”

“At sufficiently high spectral resolution (R & 30, 000), it is possible to distinguish between the three fine-structure levels of the ground electronic state of C I, and use the C I / C I∗ / C I∗∗ ratios to derive the thermal pressure in the absorbing gas (Jenkins and Tripp 2001; Savage et al. 2017).”

“Figure 8 shows that all absorption lines observed by both Fox et al. (2015) and Savage et al. (2017) fit well into our wind model with Vw = 330 km s −1 (bottom panels), while lower velocity winds fail in reproducing these features (e.g., top panels).”

“Savage et al. (2017) note that the interstellar absorption toward a foreground star nearly aligned with LS 4825, but at a distance of only < 7 ± 1.7 kpc, is completely consistent with normal Galactic rotation, confirming that the high non-circular velocities arise in the Fermi Bubble and are not a…”

“The only other published UV analysis of a complete sightline through the southern Fermi Bubble is the recent study of Savage et al. (2017), who examined the absorption properties of the sightline to the distant blue supergiant LS4825.”

“These UV absorbers trace outflowing gas that spans an extremely large range in physical conditions (Keeney et al. 2006; Zech et al. 2008; Fox et al. 2015; Bordoloi et al. 2017; Savage et al. 2017; Karim et al. 2018; Ashley et al. 2020).”

“…(Jenkins & Tripp 2001; Burgh et al. 2010; Jenkins & Tripp 2011), while the C ii 158 µm line allows the characterization of the cold atomic clouds in transition from atomic to molecular form (Pineda et al. 2013; Langer et al. 2014; Pineda et al. 2014, 2017; Richter et al. 2017; Savage et al. 2017).”

“Therefore, it is crucial to fully analyze the few spectra available, including LS 4825 (Savage et al. 2017; Cashman et al. 2021) and HD 156359 (this paper).”

“…2003), X-ray (e.g., Predehl et al. 2020), γ-ray (i.e., the Fermi bubbles in Su et al. 2010) emission and UV absorption (e.g., Zech et al. 2008; Fox et al. 2015; Savage et al. 2017; Karim et al. 2018; Ashley et al. 2020), showing the bipolar outflows on scales of a few degrees to tens of degrees.”

“…of hot stars in Orion (Richardson et al. 2015) and globular cluster M3 (Chayer et al. 2015); outflowing multiphase gas in the Galactic halo (Savage et al. 2017); ISM thermal pressures in the SMC and LMC (Welty et al. 2016), as well as abundances and depletions (Jenkins & Wallerstein 2017;…”

“…CO emission (Di Teodoro et al. 2020; Noon et al. 2023; Heyer et al. 2025), molecular H 2 absorption (Cashman et al. 2021), and UV metal-line absorption (Keeney et al. 2006; Fox et al. 2015; Savage et al. 2017; Karim et al. 2018; Cashman et al. 2023) in sightlines passing through the Fermi Bubbles.”

“This sight line and its intermediate and high-velocity clouds are discussed at length in Savage et al. (2017) and Cashman et al. (2021).”

“Additionally, a substantial fraction of ionized gas coexists with neutral hydrogen gas (B. D. Savage et al. 2017; F. H. Cashman et al. 2021; D. Krishnarao et al. 2020a,b), indicating the warped H i structure acts as an important reservoir for the surrounding ionized gas.”