A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission

By D. Strickland, T. Heckman, E. Colbert, C. Hoopes, K. Weaver. 2003

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We present arcsecond resolution Chandra X-ray and ground-based optical Hα imaging of a sample of 10 edge-on star-forming disk galaxies (seven starburst and three "normal" spiral galaxies), a sample that covers the full range of star formation intensity found in disk galaxies. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extraplanar diffuse X-ray emission is very similar to the extraplanar Hα filaments and arcs, on both small and large scales (scales of tens of parsecs and kiloparsecs, respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse X-ray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extraplanar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range Heff ∼ 2-4 kpc. The presence of extraplanar X-ray emission is always associated with the presence of extraplanar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low-mass normal spiral galaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present, appear to play no role in powering or shaping the outflows from the starburst galaxies in this sample. The Chandra ACIS X-ray spectra of extraplanar emission from all these galaxies can be fitted with a common two-temperature spectral model with an enhanced α-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow-out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height Hg ∼ 4-8 kpc. The soft thermal X-ray emission observed in the halos of the starburst galaxies is either this preexisting halo medium, which has been swept up and shock-heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case, the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

X-ray emission from star-forming galaxies - II. Hot interstellar medium (2012, background): “…by guest on 27 July 2018 characteristics are typical of the diffuse emission from normal starburst galaxies and consistent with measurements published by other authors for several galaxies from our sample (e.g. Strickland et al. 2004; Grimes et al. 2005).”
DO HOT HALOS AROUND GALAXIES CONTAIN THE MISSING BARYONS? (2010, background): “The failure to detect extended hot gaseous halos from X-ray emission (e.g., Strickland et al. 2004; Li et al. 2006, 2007) places particularly strong constraints on the fraction of missing baryons present around galaxies.”
Three-Dimensional Simulations of a Starburst-driven Galactic Wind (2008, background): “…provided clues to the nature of the soft X-ray emission seen in galactic winds, such as a close spatial correlation between the soft X-ray and H emission in the wind (e.g., Strickland et al. 2004a), which is suggestive of a physical relationship between filaments and the production of soft X-rays.”
An origin for multiphase gas in galactic winds and haloes (2015, background): “The additional complications made clear by the beautiful X-ray image of the frustum from Strickland et al. (2004b) is first the importance of the opening angle Ω — a naive estimate indicates Ω ∼ 0.01 − 0.05 — and second the potential importance of nonspherical areal divergence as the flow expands…”
THE BIMODAL METALLICITY DISTRIBUTION OF THE COOL CIRCUMGALACTIC MEDIUM AT z ≲ 1 (2013, background): “…galaxy disks (e.g., Dekel & Birnboim 2006; Stewart et al. 2011b; Rubin et al. 2012; Shen et al. 2013), while on the other hand, stellar feedback in galaxies tends to be bi-conical, near perpendicular to the galaxy major axis (e.g., Strickland et al. 2004; Bouché et al. 2012; Brook et al. 2011).”
Energy spectra of the soft X-ray diffuse emission in fourteen fields observed with Suzaku (2009, methodology): “The spectra of the halo emission of nearby star-forming galaxies and of some of normal galaxies are described by two-temperature models with kT ’s in the range of 0.1 to 0.8 keV (Strickland et al. 2004; Tüllman et al. 2006; Yamasaki et al. 2009).”
X-ray coronae in simulations of disc galaxy formation (2010, methodology): “…a correlation between X-ray luminosity and star formation rate has been found in previous studies that focussed exclusively on galaxies with high X-ray luminosities (e.g. Strickland et al. 2004), it is interesting to note that the addition of measurements for low X-ray luminosity systems in Fig.”
THREE-DIMENSIONAL INTEGRAL FIELD OBSERVATIONS OF 10 GALACTIC WINDS. I. EXTENDED PHASE (≳10 Myr) OF MASS/ENERGY INJECTION BEFORE THE WIND BLOWS (2010, background): “The line ratio maps indicate harder line ratios on the western side of the southern lobe, coincident with the x-ray emitting gas presented by Strickland et al. (2004b).”
THE STRUCTURE OF THE MILKY WAY'S HOT GAS HALO (2013, background): “In addition to detailed analyses of individual galaxies, there have been numerous studies of the global properties of diffuse X-ray emission around galaxies (O’Sullivan et al. 2003; Strickland et al. 2004; Tüllmann et al. 2006).”
The Cold Gaseous Halo of NGC 891 (2007, background): “Halo gas is also detected in H (e.g., Hoopes et al. 1999; Rossa & Dettmar 2003), with kinematics similar to that of the neutral gas (Rand 2000; Tüllmann et al. 2000; Fraternali et al. 2004; Heald et al. 2006b, 2007; Kamphuis et al. 2007), and in X-rays (e.g., Strickland et al. 2004).”
Accretion of gas on to nearby spiral galaxies (2008, methodology): “In this section we concentrate on the edge-on galaxy NGC891, for which parameters of the corona can be obtained from X-ray observations (Bregman & Houck 1997; Strickland et al. 2004).”
Gaseous Galaxy Halos (2012, methodology): “The scale heights are calculated to be 1-2 kpc for many spiral galaxies, and the x-ray features are similar to the extraplanar features detected in Hα emission (Strickland et al., 2004; Bregman & Houck, 1997; Tyler et al., 2004).”
GHASP: an Hα kinematic survey of spiral and irregular galaxies - VI. New Hα data cubes for 108 galaxies (2008, background): “This faint extension cannot be seen on the Hα map from Strickland et al. (2004).”
TRACING OUTFLOWS AND ACCRETION: A BIMODAL AZIMUTHAL DEPENDENCE OF Mg ii ABSORPTION (2012, unknown): “Outflows are expected to extend along the galaxy minor axis (e.g., Strickland et al. 2004), whereas accretion would progress along filaments more planer to the galaxy (e.g., Stewart et al. 2011).”