The first near-infrared, high-resolution échelle spectroscopy of the outflow in NGC 4151 - A study of the clouds covering the Eye of Sauron

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Volume 711 (July 2026)

A&A, 711 (2026) A212

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Table 2.

Best-fit parameters of the NGC 4151 NIR spectral features.

Parameter	Continuum emission			Units

	2020	2021	2022
β_PL	−4.9 ± 1.5	−3.6 ± 0.1	−3.3 ± 0.8	–
$T_{BB}^{(1)}$ $Mathematical equation: $ T_{\mathrm{BB}}^{(1)} $$	2170 ± 230	1880 ± 20	1630 ± 10	K
$T_{BB}^{(2)}$ $Mathematical equation: $ T_{\mathrm{BB}}^{(2)} $$	970 ± 50	870 ± 10	–	K

χ²/ν_d.o.f.	1513.1/1505	2073.2/2050	390.1/385	–

	He I emission (FWHM_NLR = 220 ± 20 km s⁻¹)

	2020	2021	2022

FWHM_int	1150 ± 10	1410 ± 20	820 ± 10	km s⁻¹
FWHM_BLR	4330 ± 50	4430 ± 120	3200 ± 50	km s⁻¹
\|Δv_int\|	50 ± 10	≲30 ^*	30 ± 10	km s⁻¹
\|Δv_BLR\|	840 ± 40	1030 ± 100	910 ± 20	km s⁻¹

χ²/ν_d.o.f.	23569.6/23553	23506.0/23164	5528.5/5508	–

	He I absorption

	2020–2021		2022

EW₁	110 ± 30		20 ± 10	km s⁻¹
\|Δv₁\|	160 ± 30		50 ± 10	km s⁻¹
FWHM₁	190 ± 40		120 ± 20	km s⁻¹
EW₂	110 ± 20		20 ± 10	km s⁻¹
\|Δv₂\|	420 ± 30		180 ± 10	km s⁻¹
FWHM₂	260 ± 20		130 ± 30	km s⁻¹

EW₃	20 ± 10		90 ± 10	km s⁻¹
\|Δv₃\|	710 ± 70		380 ± 10	km s⁻¹
FWHM₃	150 ± 80		310 ± 20	km s⁻¹
$N_{HeI}^{(1)} (2^{3} S)$ $Mathematical equation: $ N^{(1)}_{\mathrm{HeI}}(2^3S) $$	(0.61 ± 0.12)×10¹³		(0.12 ± 0.02)×10¹³	cm⁻²
$N_{HeI}^{(2)} (2^{3} S)$ $Mathematical equation: $ N^{(2)}_{\mathrm{HeI}}(2^3S) $$	(0.60 ± 0.11)×10¹³		(0.10 ± 0.04)×10¹³	cm⁻²
$N_{HeI}^{(3)} (2^{3} S)$ $Mathematical equation: $ N^{(3)}_{\mathrm{HeI}}(2^3S) $$	≲0.20 × 10¹³^*		(0.50 ± 0.04)×10¹³	cm⁻²
$N_{HeI}^{(tot)} (2^{3} S)$ $Mathematical equation: $ N^{\mathrm{(tot)}}_{\mathrm{HeI}}(2^3S) $$	(1.27 ± 0.30)×10¹³		(0.72 ± 0.10)×10¹³	cm⁻²

Notes. The top section reports the best-fit parameters of the NGC 4151 continuum emission components for the analysed epochs of observation, along with the values of the corresponding goodness-of-fit statistical analysis. The central section reports the best-fit velocity parameters of the He Iλ10 380 emission features. The bottom section reports the best-fit velocity parameters of the He Iλ10 380 absorption features.

Upper limit at 95% confidence level.

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