Chandra Observations of Excess Fe Kα Line Emission in Galaxies with High Star Formation Rates: X-Ray Reflection on Galaxy Scales?

Yan, Wei; Hickox, Ryan C.; Chen, Chien-Ting; Ricci, Cláudio; Masini, Alberto; Bauer, F. E.; Alexander, D. M.

doi:10.3847/1538-4357/abfaa0

“…Recently, Yan et al (2021) performed a spectral stacking experiment with Chandra on two samples of massive (M * > 10 10 M ) galaxies at 0.5 < z < 2 in the 7Ms CDFS with different star formation rates. They found a stronger Fe Kα line in the stacked spectrum of highly star-forming galaxies, and interpret this result as possible evidence for enhanced X-ray reflection linked to the cold, star-forming gas on galaxy scales.…”

Section: Comparison With Theorymentioning

confidence: 99%

Supermassive black holes at high redshift are expected to be obscured by their massive host galaxies’ interstellar medium

Gilli¹,

Norman

²

,

Calura³

et al. 2022

A&A

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We combine results from deep ALMA observations of massive (M* > 1010 M⊙) galaxies at different redshifts to show that the column density of their interstellar medium (ISM) rapidly increases toward early cosmic epochs. Our analysis includes objects from the ASPECS and ALPINE large programs, as well as individual observations of z ∼ 6 quasar hosts. When accounting for non-detections and correcting for selection effects, we find that the median surface density of the ISM of the massive galaxy population evolves as ∼(1 + z)3.3. This means that the ISM column density toward the nucleus of a z > 3 galaxy is typically > 100 times larger than locally, and it may reach values as high as Compton-thick at z ≳ 6. Remarkably, the median ISM column density is on the same order of what is measured from X-ray observations of large active galactic nucleus (AGN) samples already at z ≳ 2. We have developed a simple analytic model for the spatial distribution of ISM clouds within galaxies, and estimate the total covering factor toward active nuclei when obscuration by ISM clouds on the host scale is added to that of parsec-scale circumnuclear material (the so-called torus). The model includes clouds with a distribution of sizes, masses, and surface densities, and also allows for an evolution of the characteristic cloud surface density with redshift, Σc, * ∝ (1 + z)γ. We show that, for γ = 2, such a model successfully reproduces the increase in the obscured AGN fraction with redshift that is commonly observed in deep X-ray surveys, both when different absorption thresholds and AGN luminosities are considered. Our results suggest that 80–90% of supermassive black holes in the early Universe (z > 6 − 8) are hidden to our view, primarily by the ISM in their hosts. We finally discuss the implications of our results and how they can be tested observationally with current and forthcoming facilities (e.g., VLT, E-ELT, ALMA, and JWST) and with next-generation X-ray imaging satellites. By extrapolating the observed X-ray nebulae around local AGN to the environments of supermassive black holes at high redshifts, we find ≲1″ nebulae impose stringent design constraints on the spatial resolution of any future X-ray imaging Great Observatory in the coming decades.

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“…Such structures might be absent in more-luminous AGN owing to the enhanced radiation pressure from the central engine. It is also notable that distant reflection clouds have been found in the environment around Sgr A * , and in other nearby galaxies, (e.g., Ponti et al 2010;Fabbiano et al 2017;Marinucci et al 2017;Jones et al 2020;Yan et al 2021). This is a plausible explanation for a high baseline level of distant reflection in NGC 5033.…”

Section: Discussionmentioning

confidence: 82%

“…Recent work suggests that some of these lines are not truly narrow and likely originate within the optical broad line region (BLR), or even closer to the black hole (Shu et al 2010;Miller et al 2018;Zoghbi et al 2019). It is also clear that Fe Kα line flux can originate on kiloparsec scales (e.g., Ponti et al 2010;Fabbiano et al 2017;Marinucci et al 2017;Jones et al 2020;Yan et al 2021).…”

Section: Introductionmentioning

confidence: 99%

Extreme X-Ray Reflection in the Nucleus of the Seyfert Galaxy NGC 5033

Yun

¹

,

Miller

²

,

Barret

³

et al. 2022

ApJ

0

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NGC 5033 is an intriguing Seyfert galaxy because its subclassification may change with time, and because optical and submillimeter observations find that the massive black hole does not sit at the dynamical center of the galaxy, pointing to a past merger. We obtained a new optical spectrum of NGC 5033 using the 200″ Hale telescope at Palomar that clearly reveals a broad Hβ line (FWHM = 5400 ± 300 km s−1). This signals a clear view of the optical broad line region and requires Seyfert-1.5 designation. Some spectra obtained in the past suggest a Seyfert-1.9 classification, potentially signaling a variable or “changing-look” geometry. Our analysis of a 2019 Chandra spectrum of the massive black hole reveals very little obscuration, also suggesting a clean view of the central engine. However, the narrow Fe Kα emission line is measured to have an equivalent width of EW = 460 − 90 + 100 eV. This value is extremely high compared to typical values in unobscured active galactic nuclei . Indeed, the line is persistently strong in NGC 5033: the line equivalent width in a 2002 XMM-Newton snapshot is EW = 250 − 40 + 40 eV, similar to the EW = 290 − 100 + 100 eV equivalent width measured using ASCA in 1999. These results can likely be explained through a combination of an unusually high covering factor for reflection, and fluxes that are seen out of phase owing to light travel times. We examine the possibility that NGC 5033 may strengthen evidence for the X-ray Baldwin effect.

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“…The projected distances range from tens of parsecs (as in the Circinus galaxy: Marinucci et al 2013) up to the kpc scale (as in NGC 1068: Young et al 2001). Furthermore, a role from reflection on the host galaxy scales has been recently discussed in Yan et al (2021). In RBS 1055, if a fraction of the Fe Kα emission line is produced from material located light-years away from the Xray source, this could echo the higher intrinsic flux of the source observed in the past years.…”

Section: Discussionmentioning

confidence: 99%

Breaking the rules at z ≃ 0.45: The rebel case of RBS 1055

Marinucci

¹

,

Vietri

²

,

Piconcelli

³

et al. 2022

A&A

3

0

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Context. Very luminous quasars are unique sources for studying the circumnuclear environment around supermassive black holes. Several components contribute to the overall X-ray spectral shape of active galactic nuclei (AGN). The hot (kTe = 50 − 100 keV) and warm (kTe = 0.1 − 1 keV) coronae are responsible for the hard and soft power-law continua, while the circumnuclear toroidal reflector accounts for the Fe Kα emission line and the associated Compton hump. However, all these spectral features are simultaneously observed only in a handful of sources above z ≃ 0.1. Aims. An ideal astrophysical laboratory for this investigation is the quasar RBS 1055, at z ≃ 0.45. With a luminosity L2 − 10 keV = 2 × 1045 erg s−1, it is the brightest radio-quiet quasar from the ROSAT Bright Survey. Despite the known anti-correlation between the equivalent width (EW) of the narrow neutral Fe Kα line and L2 − 10 keV, an intense Fe Kα was previously detected for this source. Methods. We report findings based on a long (250 ks) NuSTAR observation performed in March 2021 and archival XMM-Newton pointings (185 ks) taken in July 2014. We also analyzed an optical spectrum of the source taken with the Double Spectrograph at the Palomar Observatory quasi-simultaneously to the NuSTAR observations. Results. We find that the two-corona model, in which a warm and hot corona coexist, well reproduces the broad band spectrum of RBS 1055, with temperatures kTe = 0.12−0.03+0.08 keV, kTe = 30−10+40 keV and Thomson optical depths τ = 30−10+15 and τ = 3.0−1.4+1.0 for the former and the latter component, respectively. We confirm the presence of an intense Fe Kα emission line (EW = 55 ± 6 eV) and find, when a toroidal model is considered for reproducing the Compton reflection, a Compton-thin solution with NH = (3.2−0.8+0.9) × 1023 cm−2 for the circumnuclear reflector. A detailed analysis of the optical spectrum reveals a likely peculiar configuration of our line of sight with respect to the nucleus, and the presence of a broad [O III] component tracing outflows in the Narrow Line Region, with a velocity shift v = 1500 ± 100 km s−1, leading to a mass outflow rate Ṁout = 25.4 ±1.5 M⊙ yr−1 and outflow kinetic power normalized by the bolometric luminosity Ēkin/LBol ∼ 0.33%. We estimate the BH mass to be in the range 2.8 × 108–1.2 × 109 M⊙, according to different broad line region emission lines, with an average value of ⟨MBH⟩ = 6.5 × 108 M⊙. Conclusions. With an Fe Kα that is 3σ above the value predicted from the EW–L2 − 10 keV relation and an extreme source brightness at 2 keV (a factor 10−15 higher than the one expected from the optical/UV), we can confirm that RBS 1055 is an outlier in the X-rays compared to other objects in the same luminosity and redshift range.

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Chandra Observations of Excess Fe Kα Line Emission in Galaxies with High Star Formation Rates: X-Ray Reflection on Galaxy Scales?

Cited by 9 publications

References 62 publications

Supermassive black holes at high redshift are expected to be obscured by their massive host galaxies’ interstellar medium

Supermassive black holes at high redshift are expected to be obscured by their massive host galaxies’ interstellar medium

Extreme X-Ray Reflection in the Nucleus of the Seyfert Galaxy NGC 5033

Breaking the rules at z ≃ 0.45: The rebel case of RBS 1055

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