The Hot Gas Exhaust of Starburst Engines in Mergers: Testing Models of Stellar Feedback and Star Formation Regulation

Beverly J. Smith; Brianne Dunn; Curtis Struck; Douglas Swartz; Mark L. Giroux; Peter Wagstaff; Roberto Soria

REVIEW 2 major objections 5 minor 148 references

The Hot Gas Exhaust of Starburst Engines in Mergers: Testing Models of Stellar Feedback and Star Formation Regulation

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read In merging galaxies, hot gas mass tracks star formation rate linearly.

desk verdict New hot-gas volume and mass measurements for 49 mergers; the M_X-SFR slope is real but only conditional because the mass estimate implicitly sets the filling factor to unity. read the letter →

arxiv 1908.09402 v1 pith:FU4NNFFG submitted 2019-08-25 astro-ph.GA

Beverly J. Smith , Peter Wagstaff , Curtis Struck , Roberto Soria , Brianne Dunn , Douglas Swartz , Mark L. Giroux This is my paper · ORCID

classification astro-ph.GA

keywords hotgasX-rayemissionstarformationrategalaxymergersstellarfeedbackinterstellarmediumChandramolecular

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Using Chandra archival imaging of 49 nearby interacting galaxies, pairs, mergers, and remnants, this paper measures the spatial extent of diffuse soft X-ray emission and derives volumes, electron densities, and masses of the hot interstellar gas. It claims that for systems forming stars faster than about one solar mass per year, both the volume and the mass of hot gas are nearly proportional to the star formation rate, with log-log slopes of 0.97 ± 0.15 for volume and 0.88 ± 0.10 for mass. That linearity is the signature the authors read as stellar winds and supernovae, rather than older stars, setting the hot-gas budget during a starburst, and it agrees with recent merger simulations in which the ratio of hot to cold gas rises with star formation. The paper also finds that the hot-to-cold gas mass ratio increases with star formation rate and with dust temperature, while the hot gas mass per unit star formation falls as the 60-to-100 micron flux ratio rises.

What carries the argument

The load-bearing identity is the X-ray emission relation $L_X(\mathrm{gas}) = \Lambda n_e^2 f V$, relating the measured thermal luminosity to the cooling function $\Lambda$, the electron density $n_e$, the volume filling factor $f$, and the hot-gas volume $V$. Since only the product $n_e\sqrt{f}$ is determined, the paper sets the hot-gas mass to $M_X(\mathrm{gas}) = m_p n_e V$, effectively assuming $f=1$ and overestimating the mass by $1/\sqrt{f}$ if the gas is clumpy. Volumes come from ellipses fitted to 0.3–1.0 keV maps at a common surface-brightness cutoff of $3\times10^{-9}$ photons s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$, chosen so that the enclosed counts match those inside the optical isophote within about 10 percent. This machinery converts the X-ray images plus spectra into the correlated quantities — hot gas volume, hot gas mass, and the hot-to-cold gas ratio — that are then compared with SFR, the stellar mass proxy $L_K$, dust temperature, and star-formation efficiency.

What would settle it

Measure electron densities independently of the filling factor for a subsample of these systems — for example, from X-ray line ratios, absorption measurements, or dispersion toward background sources — and recompute M_X(gas) using the measured f values; if f varies systematically with SFR, the claimed unity-slope M_X-SFR relation should either survive with f included or flatten, which would show that the linear relation was an artifact of the constant-f assumption.

Watch

Extended reading notes

Core claim

The central discovery claim is that in these 49 merging systems the hot X-ray-emitting gas behaves like a direct exhaust product of the current starburst: for SFR $> 1\,M_\odot\,\mathrm{yr}^{-1}$, $\log M_X(\mathrm{gas})$ versus $\log\,\mathrm{SFR}$ has slope $0.88 \pm 0.10$, and the hot gas volume versus SFR has slope $0.97 \pm 0.15$, both consistent with proportionality. The mass is computed from $L_X = \Lambda n_e^2 f V$ under an assumed temperature of 0.3 keV where no spectral temperature is available, and the hot-to-cold ratio $M_X(\mathrm{gas})/(M_{\mathrm{H}_2}+M_{\mathrm{HI}})$ increases with SFR, with a slope consistent with unity when a variable CO-to-H$_2$ ratio is used and low-SFR systems are excluded. This is presented as support for stellar and supernova feedback as the dominant hot-gas source in starbursting mergers, and as an observational match to hydrodynamic simulation predictions. Additional correlations, including an excess of hot gas in low-SFR, high-stellar-mass remnants and a possible deficit in low-mass systems, are flagged as uncertain because the sample contains few such galaxies.

Load-bearing premise

The hot-gas mass is calculated as if the emitting gas fills the entire fitted volume; only the product of the electron density and the square root of the filling factor is actually measured, so the quoted masses and all correlations built on them assume that the filling factor is either unity or does not vary systematically with star formation rate.

Editorial extensions

If this is right

For high-SFR systems, the near-unity slopes imply that every unit of star formation produces roughly a fixed amount and volume of hot gas, so a merger boosts the hot-gas reservoir simply by boosting the star formation rate.
The rising hot-to-cold gas mass ratio with SFR, consistent with hydrodynamic merger simulations, means the starburst redistributes interstellar mass toward the hot phase without destroying the cold reservoir that sustains the burst.
The constancy of $M_X(\mathrm{gas})/\mathrm{SFR}$ at high SFR, with scatter near the measurement uncertainty of about 0.34–0.37 dex, argues for a near-steady-state feedback loop operating on a timescale comparable to the radiative cooling time and the roughly 100 Myr averaging time of the SFR indicator.
Low-SFR, high-stellar-mass merger remnants deviate upward from the $M_X$ – SFR relation, identifying a separate hot-gas channel — likely virialized mass loss from old stars — that dominates when star formation is weak.
The mild anti-correlation of $M_X(\mathrm{gas})/\mathrm{SFR}$ with the 60/100 micron flux ratio and with the 3.6–24 micron color points to the spatial concentration of young stars or the efficiency of early feedback, not the total SFR, as a second driver of hot-gas yield per unit star formation.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the linear $M_X$ – SFR relation is real, it supplies a direct conversion from an easily measured SFR to the size of the hot reservoir that regulates future star formation; one consequence is that the hot-gas mass could serve as a feedback calibrator for galaxy simulations without needing to resolve individual supernovae.
The filling-factor degeneracy means the published masses are upper limits; if $f$ varies systematically with SFR, as simulations suggest for higher-density star formation, the reported slope could steepen or flatten once real filling factors are included, so an independent electron-density measurement is the decisive test.
Applying the same surface-brightness-cutoff method to isolated starbursts and post-starbursts, not just mergers, would separate merger-specific effects such as tidal compression and triggered bursts from the universal feedback relation claimed here.
If the low-SFR excess is indeed old-stellar mass loss, then the hot-gas mass of a post-merger elliptical may act as a clock: comparing $M_X/\mathrm{SFR}$ across remnants of different ages could map how the virialized hot halo accumulates after the starburst fades.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Colleague—

The short version: this is a careful archival paper with genuinely new measurements, and its main qualitative claims probably survive the one real flaw in the mass estimate. The stress-test note is right, but the paper is not broken.

What's new: Paper I gave L_X-SFR. This one measures the spatial extent of hot gas in 49 mergers, derives volumes and electron densities, and then compares M_X(gas) to cold gas masses from published CO and HI. The relation between hot gas volume and SFR is strong and independent of filling factor. The trend of increasing M_X(gas)/(M_H2+M_HI) with SFR and the comparison to Moreno et al. (2019) are new. The authors test alternate temperatures and two CO conversion factors, and they flag their own weak statistics.

The soft spot is exactly where the stress-test points. In Section 5 they define L_X = Lambda n_e^2 f V, correctly note that only n_e sqrt(f) is measured, then compute M_X = m_p n_e V as if f=1. The absolute masses are overestimated by 1/sqrt(f), and if f increases with SFR, the derived slope 0.88 ± 0.10 is not the true slope. Since Section 7.2 itself argues f may vary with SFR, the linear M_X-SFR claim is not quantitatively nailed down. The hot-to-cold ratios have the same bias. I don't think this kills the paper, because the volume-SFR correlation is f-free and the qualitative M_X-SFR trend is very unlikely to vanish for any plausible f(SFR), but the specific slope and the mass scale should be treated as conditional.

Minor complaints: no error bars on the plots (they state factor-of-two uncertainties); CO data incomplete; sample is archive-selected. None of these are hidden; the authors are candid about them.

Who should read it: observers and simulators working on feedback in mergers. It's a useful data point for the subfield, not a paradigm change. Yes, it deserves a serious referee. I'd accept it with moderate revision, mainly asking them to present the f-dependence of their slopes explicitly, and to add error bars where feasible.

Referee Report

2 major / 5 minor

Summary. The paper uses archival Chandra observations of 49 nearby interacting galaxy pairs, mergers, and merger remnants to measure the spatial extent of diffuse hot gas. From the X-ray extent and the thermal luminosity taken from Paper I, the authors derive hot-gas volumes, electron-density estimates, and hot-gas masses, and correlate these with star formation rates, molecular and atomic gas masses, and other galaxy properties. The main claims are that for systems with SFR > 1 Msun/yr the hot-gas volume and mass correlate strongly and near-linearly with SFR, that the ratio of hot gas mass to cold gas mass increases with SFR, and that these trends are consistent with the Moreno et al. (2019) merger simulations. The paper also reports weak anti-correlations between M_X(gas)/SFR and dust temperature tracers and identifies possible excess hot gas in low-SFR, high-stellar-mass systems such as NGC 1700.

Significance. If the derived masses are reliable, the paper provides a valuable observational constraint on stellar feedback in merging systems using independent X-ray, CO, and HI data. Its strengths include a well-documented procedure for measuring X-ray extents, explicit checks against an alternative extraction method, tests of the assumed gas temperature, and comparisons using two CO-to-H2 conversion schemes. The comparison to an external simulation (Moreno et al. 2019) is a genuine test rather than a fit. The central correlation between hot-gas volume and SFR does not depend on the filling factor and appears robust. However, the hot-gas mass scale and all mass-based slopes depend on the unknown filling factor, as discussed below, so the headline mass-SFR slope and the hot-to-cold mass ratio comparison should be interpreted with caution until that degeneracy is addressed.

major comments (2)

[Section 5 and Section 7.1] The derivation of M_X(gas) implicitly sets the filling factor f = 1. In Section 5 the paper correctly states that L_X(gas) = Lambda n_e^2 f V and that only the product n_e sqrt(f) can be determined, but then computes M_X(gas) = m_p n_e V by substituting the measured quantity n_e sqrt(f) for n_e. The resulting quantity is m_p V sqrt(L_X/(Lambda V)) = M_true/sqrt(f). For the range f ~ 0.1-0.9 quoted in Section 7.2, the masses are overestimated by factors of 1.05-3.2. More importantly, if f varies with SFR, then the derived slope d log M_X(derived)/d log SFR equals d log M_true/d log SFR - 0.5 d log f/d log SFR. Since Section 7.2 explicitly notes that simulations predict f to increase with the density of star formation and hence potentially with SFR, the headline slope of 0.88 +/- 0.10 in Section 7.1 is not a measurement of the true M_X-SFR relation unless f is constant or its SFR dependence is known. The authors should re-run the correlation analysis with a parameterized f(SFR) (e.g., using the Li et al. 2015 range) or otherwise quantify how the slope shifts.
[Sections 6.3 and 7.1] The same filling-factor degeneracy propagates into the hot-to-cold gas mass ratio M_X(gas)/(M_H2 + M_HI) and the comparison with the Moreno et al. (2019) simulations. If f increases with SFR while the true M_X-SFR slope is shallower than the derived slope, the reported increase of M_X(gas)/(M_H2 + M_HI) with SFR (slope 0.82 +/- 0.16 for the variable CO/H2 ratio and SFR > 1 Msun/yr, Table 5) could be partly an artifact of the 1/sqrt(f) factor. The paper acknowledges the degeneracy in Section 7.2 but does not propagate it into the correlation analysis or the simulation comparison. A quantitative test with an assumed filling-factor model is needed before the agreement with Moreno et al. can be considered established.

minor comments (5)

[Figures 9-19] The figures showing the key correlations omit error bars entirely, despite the paper quoting a factor-of-two uncertainty in M_X(gas) and comparable uncertainties in volume. At minimum, representative error bars should be shown on the most load-bearing plots (Figures 13 and 15) so readers can judge the scatter against the uncertainties.
[Table 5] Several rows in Table 5 refer to 'LOG n_e' (e.g., in the comparisons with volume and with M_X(gas)/SFR), but the paper only measures n_e sqrt(f), not n_e alone. The column headers and entries should be relabeled as log(n_e sqrt(f)) to avoid implying an independent determination of n_e.
[Section 6.2] In the paragraph beginning 'As with NGC 1700', the text reads 'separating out this additional component to the hot gas is is uncertain'; the duplicated 'is' should be removed.
[Section 8] The summary states 'we see a possible deﬁcient of hot gas in low mass systems'; 'deﬁcient' should be 'deficiency'.
[Section 7.2] In the list of parameters that are not correlated, the sentence 'M X(gas)/SFR is not correlated with for a variable CO/H 2 ratio' is missing its subject; it should presumably read 'not correlated with SFE for a variable CO/H2 ratio'.

Circularity Check

1 steps flagged · score 6.0 of 10

The claimed M_X(gas) ∝ SFR relation is built into the definition of M_X(gas) from L_X(gas) and V, both of which already scale linearly with SFR.

self definitional [Section 5 (L_X = Λ n_e^2 f V; M_X = m_p n_e V) combined with Section 6.1 (V–SFR slope) and Section 7.1 (M_X–SFR slope)]
"we used the relation L X(gas) = Λn e2fV ... From the X-ray luminosity, the volume, and the temperature we derive n e √ f ... We then calculated the mass of the hot X-ray-emitting gas M X(gas) = m pneV."

By the paper's own equations, log M_X = const + 0.5 log L_X + 0.5 log V (for fixed filling factor and cooling function). The paper states in the Introduction that L_X(gas) is proportional to SFR for star-forming galaxies (citing Paper I), and in Section 7.1 it reports that the V–SFR slope is 0.97 ± 0.15 for SFR > 1 M_sun/yr. Therefore log M_X = 0.5 log SFR + 0.5 log SFR = log SFR by construction. The headline slope 0.88 ± 0.10 in Section 7.1 is not an independent empirical confirmation of stellar feedback; it is the arithmetic mean of two input proportionalities already known to hold. The M_X/(M_H2+M_HI) vs. SFR correlation inherits the same constructed numerator.

full rationale

The paper is largely a legitimate observational study: it measures new X-ray spatial extents, uses external published CO and HI data, and compares with the independent Moreno et al. (2019) simulations. Paper I is a legitimate prior measurement of L_X(gas), not a fit to the target relation, so self-citation is not itself the problem. The central circularity is narrower: the derived quantity M_X(gas) is defined via M_X = m_p V sqrt(L_X/(Λ V)), so M_X ∝ sqrt(L_X V). Once the paper adopts L_X ∝ SFR (from Paper I) and measures V ∝ SFR, the M_X ∝ SFR result is forced by the definition rather than being a new physical constraint. The paper presents this as supporting feedback models, but the correlation is a derived identity from its inputs. The filling factor degeneracy (only n_e sqrt(f) is measured) is disclosed and discussed, and the temperature assumptions are tested; these are robustness caveats, not circularity. Because the central M_X–SFR 'prediction' reduces by construction, a score of 6 is appropriate; the rest of the analysis retains independent content.

Assumptions & free parameters 6 free parameters · 6 assumptions · 0 invented entities

The paper's physical inferences rest on standard astrophysical assumptions (X-ray plasma emission, SFR tracers, distances) plus several ad hoc choices: an assumed temperature for most galaxies, an implicit filling factor of one, a fixed surface brightness cutoff, and a CO-to-H2 conversion prescription. No new entities are introduced.

free parameters (6)

Implicit filling factor f = 1 (assumed, not stated)
Section 5 derives n_e sqrt(f) but then computes M_X = m_p n_e V without f; mass values are m_p V sqrt(L_X/(Lambda V)) and overestimate true mass by 1/sqrt(f).
Assumed gas temperature kT = 0.3 keV for 34 galaxies without spectral fits
Section 5 and 6.5; M_X depends on temperature through the cooling function. Authors test 0.3 and 0.6 keV and T-SFR or T-L_X relations, and report conclusions unchanged.
CO-to-H2 conversion factor = 2e20, 4e19, 5e20 cm^-2/(K km/s)
Section 3: constant Galactic value vs a variable three-tier prescription based on L_FIR and L_K thresholds; both are used throughout the correlation analysis.
Surface brightness cutoff for X-ray radius = 3e-9 photons s^-1 cm^-2 arcsec^-2
Section 4: chosen to enclose about 90% of the 0.3-1.0 keV flux and to match the optical isophote counts within 10%; this cutoff sets V and affects all derived densities and masses.
Line-of-sight depth of hot gas ellipsoid = average of major and minor axes
Section 5: the third dimension is assumed equal to the average of the other two, giving a factor 1.5 uncertainty in volume and a factor 1.8 uncertainty in n_e sqrt(f).
Power-law photon index in spectral decomposition = 1.8
Paper I; fixed in the xspec fits. If the true photon index differs, the thermal LX (hot gas) component changes, contributing roughly a factor of two uncertainty in LX.

assumptions (6)

domain assumption The thermal MEKAL component of the diffuse X-ray spectrum is entirely hot gas in the target galaxy, with the power-law component fully attributed to unresolved point sources.
Section 1 and Paper I; if AGN or unresolved binaries contaminate the thermal component, LX and hence M_X are biased.
domain assumption The X-ray plasma is in collisional ionization equilibrium and emits according to a standard cooling function Lambda(T).
Section 5 uses the McKee and Cowie 1977 / McCray 1987 cooling function; deviations would alter the density and mass estimates.
domain assumption SFR estimates from UV and IR photometry trace star formation over about 100 Myr and are not strongly affected by AGN activity.
Section 2; SFRs from Spitzer and GALEX data via Kennicutt and Evans 2012 calibrations.
standard math Distances assume H0 = 73 km/s/Mpc with peculiar velocity corrections.
Table 1; all physical sizes, luminosities, and masses scale with distance squared.
domain assumption K-band luminosity is a reliable stellar mass proxy.
Section 2; used to trace stellar mass and to define low-mass and high-mass subsets.
domain assumption The archival sample selection biases described by the authors do not drive the main correlations.
Section 2; mid-merger galaxies are more distant and have higher SFR, creating an entanglement between merger stage, distance, and SFR.

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Cite this review

Pith. "Pith review of The Hot Gas Exhaust of Starburst Engines in Mergers: Testing Models of Stellar Feedback and Star Formation Regulation." pith.science (2026). https://pith.science/paper/FU4NNFFG

@misc{pith2026190809402,
  author       = {Pith},
  title        = {Pith review of: The Hot Gas Exhaust of Starburst Engines in Mergers: Testing Models of Stellar Feedback and Star Formation Regulation},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FU4NNFFG}},
  note         = {Machine review of arXiv:1908.09402}
}

read the original abstract

Using archival data from the Chandra X-ray telescope, we have measured the spatial extent of the hot interstellar gas in a sample of 49 nearby interacting galaxy pairs, mergers, and merger remnants. For systems with SFR > 1 M(sun)/yr, the volume and mass of hot gas are strongly and linearly correlated with the star formation rate (SFR). This supports the idea that stellar/supernovae feedback dominates the production of hot gas in these galaxies. We compared the mass of X-ray-emitting hot gas Mx(gas) with the molecular and atomic hydrogen interstellar gas masses in these galaxies (M(H2) and M(HI), respectively), using published carbon monoxide and 21 cm HI measurements. Systems with higher SFRs have larger Mx(gas)/(M(H2) + M(HI)) ratios on average, in agreement with recent numerical simulations of star formation and feedback in merging galaxies. The Mx(gas)/(M(H2) + M(HI)) ratio also increases with dust temperature on average. The ratio Mx(gas)/SFR is anti-correlated with the IRAS 60 micron to 100 micron flux ratio and with the Spitzer 3.6 micron to 24 micron. These trends may be due to variations in the spatial density of young stars, the stellar age, the ratio of young to old stars, the initial mass function, and/or the efficiency of stellar feedback. Galaxies with low SFR (<1 M(sun)/yr) and high K band luminosities may have an excess of hot gas relative to the relation for higher SFR galaxies, while galaxies with low K band luminosities (and therefore low stellar masses) may have a deficiency in hot gas, but our sample is not large enough for strong statistical significance.

Figures

Figures reproduced from arXiv: 1908.09402 by the authors.

**Figure 4.** — Comparisons between SFE and F60/F100 (top panels) and SFE and SFR (bottom panels). The left panels use a constant CO/H2 ratio, while the right use a variable CO/H2 ratio. The best-fit line for the full sample is plotted as a solid black line, while the best-fit line for systems with SFR > 1.0 M⊙ yr−1 is given as a dotted line. The best-fit slope and the Spearman rank correlation coefficient for the full set is sh… view at source ↗

**Figure 5.** — Montage of major axis radial profiles from elliptical annuli, plotted against the distance along the major axis. These were obtained using the dmextract software. The blue horizontal line (short dashes) mark the nominal surface brightness cutoff of 3 × 10−9 photons s−1 cm−2 arcsec−2 . The red vertical line (long dashes) mark the ‘best’ estimate of the radial extent of the X-ray emission, used for the volume deter… view at source ↗

Figures from the paper (12 more)

**Figure 7.** Figure 7: — Montage of major axis radial profiles from elliptical annuli, plotted against the distance along the major axis. These were obtained using the dmextract software. The blue horizontal line (short dashes) mark the nominal surface brightness cutoff of 3 × 10−9 photons …

**Figure 9.** Figure 9: — Plots of X-ray volume (top row) and X-ray/optical size ratio (bottom row) vs. merger stage (first column), LX/SFR (second column), and SFR (third column). The best-fit line is plotted in the top right panel. Black open squares mark the sample galaxies, with those ci…

**Figure 12.** Figure 12: — Upper left: SFR plotted against ne √ f. Upper right: F60/F100 vs. ne √ f. The middle row compares SFE calculated with the two methods with ne √ f. Bottom left: [3.6] − [24] vs. ne √ f. Bottom right: MX(gas)/LX(gas) vs. ne √ f for the 15 systems with measured tempera…

**Figure 14.** Figure 14: — MX(gas)/SFR vs. the two estimates of SFE (first and second panels, top row), MX(gas)/SFR vs. [3.6] − [24] (top right), MX(gas)/SFR vs. LFIR/FK (bottom left), MX(gas)/SFR vs. F60/F100 (bottom middle), and MX(gas)/SFR vs. ne √ f (bottom right). Merger stages 1 and 2 …

**Figure 15.** Figure 15: — Plots of MX(gas)/(MHI+MH2 ) vs. SFR (top panels) and SFE (bottom panels). The left column uses a constant CO/H2 ratio, while the right column uses a variable ratio. Merger stages 1 and 2 systems are marked as open green triangles. Merger stages 3, 4, and 5 are open …

**Figure 17.** Figure 17: — Comparisons between MX(gas)/(MHI+MH2 ) and [3.6] − [24] (top panels), and LFIR/LK (bottom panels). The best-fit line for the full sample is plotted as a solid black line, while the best-fit line for systems with SFR > 1.0 M⊙ yr−1 is given as a dotted line. The best-…

**Figure 20.** Figure 20: — Montage of images of the galaxies. The left panel is either the SDSS g image or the GALEX NUV image (if no SDSS images exist). The right panel is the unsmoothed exposurecorrected Chandra 0.3 − 1.0 keV low energy map. Logarithmic contours are overlaid in white on t…

**Figure 21.** Figure 21: — Montage of images of the galaxies. The left panel is either the SDSS g image or the GALEX NUV image (if no SDSS images exist). The right panel is the unsmoothed exposurecorrected Chandra 0.3 − 1.0 keV low energy map. Logarithmic contours are overlaid in white on t…

**Figure 23.** Figure 23: — Montage of images of the galaxies. The left panel is either the SDSS g image or the GALEX NUV image (if no SDSS images exist). The right panel is the unsmoothed exposurecorrected Chandra 0.3 − 1.0 keV low energy map. Logarithmic contours are overlaid in white on t…

**Figure 25.** Figure 25: — Montage of images of the galaxies. The left panel is either the SDSS g image or the GALEX NUV image (if no SDSS images exist). The right panel is the unsmoothed exposurecorrected Chandra 0.3 − 1.0 keV low energy map. Logarithmic contours are overlaid in white on t…

**Figure 26.** Figure 26: — Montage of images of the galaxies. The left panel is either the SDSS g image or the GALEX NUV image (if no SDSS images exist). The right panel is the unsmoothed exposurecorrected Chandra 0.3 − 1.0 keV low energy map. Logarithmic contours are overlaid in white on t…

**Figure 27.** Figure 27: — Montage of images of the galaxies. The left panel is either the SDSS g image or the GALEX NUV image (if no SDSS images exist). The right panel is the unsmoothed exposurecorrected Chandra 0.3 − 1.0 keV low energy map. Logarithmic contours are overlaid in white on t…

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