Comment on the paper "Calorimetric Dark Matter Detection with Galactic Center Gas Clouds"

TL;DR

This paper critiques a previous study on dark matter detection via galactic gas clouds, highlighting issues with cloud stability assumptions and parameter choices that affect the derived limits on dark matter interactions.

Contribution

It challenges prior work by pointing out the instability of clouds in extreme environments and incorrect parameter usage, questioning the validity of the original limits on dark matter interactions.

Findings

Galactic center gas clouds in extreme environments are likely unstable over relevant timescales.

Previous limits on dark matter interactions based on these clouds may be unreliable due to stability issues.

Incorrect cloud parameters were used in the original analysis, affecting the results.

Abstract

The paper "Calorimetric Dark Matter Detection with Galactic Center Gas Clouds" (Bhoonah et al. 2018) aims to derive limits on dark matter interactions by demanding that heat transfer due to DM interactions is less than that by astrophysical cooling, using clouds in the hot, high-velocity nuclear outflow wind of the Milky Way ($T_{wind} \sim 10^{6-7}$ K, $V_{wind} \sim$ 330 km/s). We argue that clouds in such an extreme environment cannot be assumed to be stable over the long timescales associated with their radiative cooling rates. Furthermore, Bhoonah et al. (2018) uses incorrect parameters for their clouds.

Full text

Comment on the paper

“Calorimetric Dark Matter Detection with Galactic Center Gas Clouds”

Glennys R. Farrar

gf25@nyu.edu

Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003, USA

  

Felix J. Lockman

jlockman@nrao.edu

Green Bank Observatory, Green Bank, WV 24944, USA

  

N. M. McClure-Griffiths

naomi.mcclure-griffiths@anu.edu.au

Research School of Astronomy & Astrophysics, Australian National University, Canberra ACT 2611, Australia.

  

Digvijay Wadekar

jay.wadekar@nyu.edu

Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003, USA

(January 14, 2020)

In a recent Letter, Bhoonah et al. Bhoonah et al. (2018) (hereafter B18) attempted to derive limits on dark matter interactions with ordinary matter by demanding that DM heating of gas clouds not exceed the known astrophysical cooling rate based on the temperature, density and metallicity of the observed clouds. In B18, the cloud G1.4$-$1.8+87 from McClure-Griffiths et al. (2013) (hereafter McG13) was singled out as most suitable by virtue of its apparently exceptionally low temperature and relatively low density. In this Comment, we point out a fundamental conceptual error in B18, namely their use of clouds in the high-velocity nuclear outflow (HVNO) of the Galaxy for the analysis. This, along with additional detailed errors, invalidates the limits reported in B18.

The conceptual error with B18 is their use of complex, poorly understood and likely-short-lived clouds for placing limits. The HVNO clouds are in the hot, high-velocity wind ($10^{6-7}$K, 330 km/s) emanating from the Galactic Center. This extreme environment causes shocks and other destructive effects, likely making the clouds transient objects Cooper et al. (2008); Scannapieco and Brüggen (2015); Schneider and Robertson (2017); Armillotta et al. (2017); Melioli et al. (2013); McCourt et al. (2018); Gronke and Oh (2018); Sparre et al. (2019). However deriving DM bounds based on heat transport requires the system to be in a steady state at the current temperature over the long timescales associated with its purported radiative cooling rate, invalidating the use of a system for which the required stability is doubtful. The subsequent more detailed analysis in Bhoonah et al. Bhoonah et al. (2019) also ignores the effect of the extreme environment on the HVNO clouds and hence suffers from the same fundamental problem.

A further problem is that B18 calculated the temperature of G1.4$-$1.8+87 to be $T_{g}\lessapprox 22$ K by taking the velocity dispersion to be 1 km/s. Fig. 1 shows the Hi   spectrum at the location of the cloud G1.4$-$1.8+87, from the public online data McClure-Griffiths et al. (2012). As seen in Fig. 1, most of the Hi   emission for this cloud is characterized by a line with a FWHM of 26.6 km/s (red line), with the narrow 1 km/s spike being a single-channel fluctuation (see new ). For comparison, the spectrum of a robust cloud G33.4-8.0 Pidopryhora et al. (2015), used in Wadekar and Farrar (2019), is also shown.

Using the correct width 26.6 km/s gives $T_{g}$ above 15,000 K. Some other parameters given in B18 for the cloud G1.4$-$1.8+87 are also in error: B18 quotes the mass and radius of cloud to be $311M_{\odot}$ and 12 pc, whereas the correct values in McG13 are $17$ $M_{\odot}$ and 8.2 pc. The incorrect values of B18 appear to have been read from adjacent lines of the table in McG13. The cooling function drops drastically for $T<100$ K, so the net effect of correcting the temperature and the density is that the radiative cooling rate of G1.4$-$1.8+87 increases by a factor $\approx 10^{6}$ and thus the conclusions drawn by B18 from G1.4$-$1.8+87 are incorrect, even if using HVNO clouds were legitimate.

Two other errors in B18 need mentioning to avoid others follow their example. First, B18 confuses the velocity of the cloud relative to the local standard of rest V${}_{\textup{LSR}}=87$ km/s, reported in McClure-Griffiths et al. (2013), with the velocity of the cloud relative to the Galaxy’s center of mass. V${}_{\textup{LSR}}$ is defined to be an object’s line-of-sight velocity relative to a frame of reference in a circular orbit around the Galactic Center at the position of the solar system. Instead, the velocity of the cloud relative to the Galaxy is to a good approximation the outflow velocity of the Hi  clouds entrained in the nuclear wind, $\sim 330$ km/s from Di Teodoro et al. (2018).

Second, a conservative bound requires adopting the smallest local DM density consistent with observations, which near the Galactic Center is generally given by the Burkert profile Burkert (1995). B18 takes incorrect parameter values which exaggerate the Burkert density by a factor of 9 (without citing a source), $\rho_{b}=14$ GeV/cm3 and $r_{b}=3$ kpc, instead of $\rho_{b}=1.57$ GeV/cm3 and $r_{b}=9.26$ kpc from the latest fitNesti and Salucci (2013); the expression quoted in B18 for the form of the Burkert profile is also incorrect.

Limits on DM scattering from the cooling of suitable Milky Way clouds, and new and complementary constraints on DM from the Leo T dwarf galaxy, are reported in Wadekar and Farrar (2019); a more detailed discussion of HVNO clouds is given in its Supplemental Material. The true limits from Galactic clouds are $10^{2}$ and $10^{3}$ times less stringent for the millicharge parameter $\epsilon$ and the DM-nucleon scattering cross section, respectively, than claimed in B18 (see Wadekar and Farrar (2019)).

We thank C. McKee for helpful comments. GRF acknowledges support of NSF-1517319. The Green Bank Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc.