Deprojecting galaxy-cluster cold fronts: evidence for bulk, magnetised spiral flows. (arXiv:2002.00971v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Naor_Y/0/1/0/all/0/1">Yossi Naor</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Keshet_U/0/1/0/all/0/1">Uri Keshet</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_Q/0/1/0/all/0/1">Qian Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reiss_I/0/1/0/all/0/1">Ido Reiss</a>
Tangential discontinuities known as cold fronts (CFs) are abundant in groups
and clusters of galaxies (GCs). The relaxed, spiral-type CFs were initially
thought to be isobaric, but a significant, $10%$–$20%$ jump in the thermal
pressure $P_t$ was reported when deprojected CFs were stacked, interpreted as
missing $P_t$ below the CFs (i.e. at smaller radii $r$) due to a
locally-enhanced nonthermal pressure $P_{nt}$. We report a significant
($sim4.3sigma$) deprojected jump in $P_t$ across a single sharp CF in the
Centaurus cluster. Additional seven CFs are deprojected in the GCs A2029,
A2142, A2204, and Centaurus, all found to be consistent (stacked:
$sim1.9sigma$) with similar pressure jumps. Combining our sample with high
quality deprojected CFs from the literature indicates pressure jumps at
significance levels ranging between $2.7sigma$ and $5.0sigma$, depending on
assumptions. Our nominal results are consistent with $P_{nt}simeq
(0.1mbox{–}0.3)P_t$ just below the CF. We test different deprojection and
analysis methods to confirm that our results are robust, and show that without
careful deprojection, an opposite pressure trend may incorrectly be inferred.
Analysing all available deprojected data, we also find: (i) a nearly constant
CF contrast $q$ of density and temperature within each GC, monotonically
increasing with the GC mass $M_{200}$ as $qpropto M_{200}^{0.23pm0.04}$; (ii)
hydrostatic mass discontinuities indicating fast bulk tangential flows below
all deprojected CFs, with a mean Mach number $sim0.76$; and (iii) the newly
deprojected CFs are consistent (stacked: $sim2.9sigma$) with a
$1.25^{+0.09}_{-0.08}$ metallicity drop across the CF. These findings suggest
that GCs quite generally harbor extended spiral flows.
Tangential discontinuities known as cold fronts (CFs) are abundant in groups
and clusters of galaxies (GCs). The relaxed, spiral-type CFs were initially
thought to be isobaric, but a significant, $10%$–$20%$ jump in the thermal
pressure $P_t$ was reported when deprojected CFs were stacked, interpreted as
missing $P_t$ below the CFs (i.e. at smaller radii $r$) due to a
locally-enhanced nonthermal pressure $P_{nt}$. We report a significant
($sim4.3sigma$) deprojected jump in $P_t$ across a single sharp CF in the
Centaurus cluster. Additional seven CFs are deprojected in the GCs A2029,
A2142, A2204, and Centaurus, all found to be consistent (stacked:
$sim1.9sigma$) with similar pressure jumps. Combining our sample with high
quality deprojected CFs from the literature indicates pressure jumps at
significance levels ranging between $2.7sigma$ and $5.0sigma$, depending on
assumptions. Our nominal results are consistent with $P_{nt}simeq
(0.1mbox{–}0.3)P_t$ just below the CF. We test different deprojection and
analysis methods to confirm that our results are robust, and show that without
careful deprojection, an opposite pressure trend may incorrectly be inferred.
Analysing all available deprojected data, we also find: (i) a nearly constant
CF contrast $q$ of density and temperature within each GC, monotonically
increasing with the GC mass $M_{200}$ as $qpropto M_{200}^{0.23pm0.04}$; (ii)
hydrostatic mass discontinuities indicating fast bulk tangential flows below
all deprojected CFs, with a mean Mach number $sim0.76$; and (iii) the newly
deprojected CFs are consistent (stacked: $sim2.9sigma$) with a
$1.25^{+0.09}_{-0.08}$ metallicity drop across the CF. These findings suggest
that GCs quite generally harbor extended spiral flows.
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