Swayamtrupta Panda1,2*†- 1International Gemini Observatory/NSF NOIRLab, La Serena, Chile
- 2Laboratório Nacional de Astrofísica - MCTI, Itajubá, Brazil
The Eigenvector 1 schema, or the main sequence of quasars, was introduced as an analogous scheme to the HR diagram that would allow us to understand the more complex, extended sources - active galactic nuclei (AGNs) that harbor accreting supermassive black holes. The study has spanned more than three decades and has advanced our knowledge of the diversity of Type-1 AGNs from both observational and theoretical aspects. The quasar main sequence, in its simplest form, is the plane between the FWHM of the broad H
1 What is the quasar main sequence? - a brief history
More than three decades ago, Boroson and Green (1992) put forward the idea of a main sequence of quasars - an analogous schema to the Hertzsprung-Russel (HR) diagram (Hertzsprung, 1911; Russell, 1914) that has allowed us to track the evolution of stars of varied ages and diverse properties utilizing the classification based on their color and magnitude. Akin to the HR diagram, the quasar main sequence (QMS) was envisioned to help put together the diverse population of Type-1, unobscured active galactic nuclei (AGNs) through the compilation of spectral properties from the broad- and narrow line-emitting regions of a sample of nearby, bright AGNs.
Before diving into the recent advances, we would like to reflect on the importance of Boroson and Green’s work with a brief account of the procedure carried out to realize the first results that laid the foundations of the Quasar Main Sequence.
1.1 The inception of the main sequence of quasars
Boroson and Green conducted their study within the low-redshift range (z
Figure 1. Left: Spectral decomposition (optical region) of a Type-1 Narrow-line Seyfert (NLS1) galaxy, SDSS J134704.91 + 144137.6. The original spectrum is shown in light gray, the H
To interpret the PCA results, the authors identified several key parameters that could influence the observed properties: (1) the mass accretion rate, (2) the black hole (BH) mass, (3) the covering factor of the BLR clouds, (4) the degree of anisotropy in the emitted radiation from the continuum source, (5) the orientation of the source to the observer, (6) the velocity distribution of the BLR clouds, and (7) the ionization parameter.
To summarize, the paper by Boroson and Green (1992) is fundamental for two main reasons:
1. It is one of the first publications in AGN research to use principal component analysis (PCA) to explore the connections between the observed properties of quasars, particularly in the study of the Quasar Main Sequence (see right panel of Figure 1 for a recent rendition). This sequence unifies the diverse group of AGNs through Eigenvectors, specifically, Eigenvector 1, which shows an anti-correlation between the width of the optical Fe ii blend (4434-4684 Å) and the peak intensity of the forbidden [OIII]
2. For the first time, the paper constructed the Fe ii pseudo-continuum template from the optical spectrum of I Zw 1. This template has become widely used in analyzing the optical spectra of AGNs, facilitating the study of the Fe ii complex (Phillips, 1978a) both theoretically and observationally. It helped understand the excitation mechanisms (Phillips, 1978b; Verner et al., 1999) behind the thousands of spectral transitions from the UV to the NIR, transforming Fe ii from being considered a spectral contaminant to an evolution tracer and fundamental component of the BLR in AGNs (Marinello et al., 2016; Marziani et al., 2018; Panda et al., 2019c; Martínez-Aldama et al., 2021b; Panda, 2022).
1.2 The broad contextualization in the form of 4D Eigenvector 1
An advanced version of the Eigenvector 1 (EV1) schema was introduced by Sulentic et al. (2000), incorporating additional parameters beyond the initial (1) FWHM of the broad component of H
Accumulating evidence from subsequent studies, following Boroson and Green (1992), indicates that EV1 correlations involve at least two principal independent parameters: (1) the source’s bolometric luminosity (
1.3 Getting the “bigger” picture
The onset of the new century saw the rise of large spectroscopic surveys, such as the Sloan Digital Sky Survey (SDSS, York et al., 2000; Shen et al., 2011). These surveys revitalized EV1 studies and extended their applicability to much larger samples. Shen and Ho (2014) made significant strides with their seminal paper, utilizing data from over 20,000 spectroscopically observed SDSS quasars, analyzed using an automated spectral fitting pipeline (Shen et al., 2011). This study provided spectral parameters for a wide range of emission lines, as well as estimates for black hole masses and Eddington ratios. Leveraging this comprehensive dataset, Shen and Ho redefined the main sequence of quasars and concluded that (1) the average Eddington ratio increases from left to right on the sequence, and (2) the dispersion in FWHM(H
More recently, the exploration of large samples has been extended to include sources from the Southern Hemisphere (Chen et al., 2018, and references therein), and the number of Type-1 AGNs, including strong Fe ii-emitting ones, has grown many folds with deeper surveys extending to fainter magnitudes (Rakshit et al., 2020; Wu and Shen, 2022; Paliya et al., 2024; Panda et al., 2024a). We are now at a stage where AGNs are frequently revisited and thus we also have a wealth of multi-epoch, multi-wavelength data for samples of AGNs. This has greatly helped to build samples of AGNs that demonstrate changes in their continuum and emission line properties - the Changing-Look AGNs (see recent compilations in Panda and Śniegowska, 2024; Guo et al., 2024; Zeltyn et al., 2024, and references therein), especially investigating the changes in the Fe ii emission in the context of the quasar main sequence (Panda and Śniegowska, 2024).
The paper is organized as follows: In Section 2, we highlight some recent advances in the last decade on the studies with the quasar main sequence - Fe ii template creation and improvements, theoretical predictions, and advancements in photoionization modeling including some direct confirmations of long-standing hypotheses. We discuss the connection of the main sequence with one of the fundamental properties demonstrated by AGNs - Variability in Section 3, especially in advancing our knowledge through techniques like reverberation mapping (RM), and the renewed interest in Changing-look/Changing-state AGNs. We then give a brief account of the present-day scenario of incorporating AGNs (and quasars) as standard(izable) candles and touch upon some relevant studies that have progressed in this direction. Finally, we conclude this mini-review with some closing remarks and perspective for the future in Section 4 with up-and-coming massive, multiplex surveys that will make things more intriguing.
2 Quasar main sequence - current state and advances
In this section, we touch upon a few of the ongoing, interesting lines of research to improve our understanding of the main sequence of quasars.
2.1 Generating Fe II templates
Owing to its complexity and uncertainties in transition probabilities and excitation mechanisms, the most successful approach to model the Fe ii emission in AGNs consists of deriving empirical templates from observations and supplementing the missing transitions with state-of-the-art radiative transfer models, e.g., CLOUDY (Ferland et al., 2017; Chatzikos et al., 2023). The templates thus derived using this methodology are referred to as semi-empirical. The work of Kovačević et al. (2010) is seminal in this regard who provided the AGN community with an interface1 to create Fe ii templates by combining the Fe ii transitions from theoretical expectations and those revealed in the spectrum of the prototypical Fe ii-emitter, I Zw 1. Their methodology involves the knowledge of the temperature of the ionized cloud responsible for the Fe ii emission, information on the dynamics of the Fe ii profile in the observed spectrum, and intensities of the strongest Fe ii multiplets collected in three groups (
In recent years, there has been noteworthy development to improve the atomic datasets available for iron emission, with updated radiative and electron collisional rates, and include higher levels (up to 716) and energies as high as 26.4 eV. We refer the readers to Sarkar et al. (2021) for an overview of these datasets and their performance within the spectral synthesis code, CLOUDY.
2.2 Fe ii spectral synthesis and inferring the BLR cloud properties
Over the years after Boroson and Green put forward their findings from the Eigenvector 1, the expected parameters that should influence the observed correlation in the quasar main sequence have been looked at, albeit separately. Notable among them are (i) the Fe ii emission model developed in Verner et al. (1999) with 371 atomic levels producing 13,157 (permitted) emission lines with the highest energy level of
In more recent years, a clearer picture of the Fe ii emission, especially in the optical region, linking to the quasar main sequence has been achieved. There is a growing consensus that the main sequence of quasars, earlier thought to be primarily driven by the Eddington ratio, is in reality, dependent on a combination of parameters of the underlying accretion disk and the BELR clouds Panda et al. (2018), 2019a,c. These parameters are: (1) Eddington ratio, (2) BH mass; (3) shape of the ionizing continuum (SED); (4) BLR density; (5) BLR metallicity; (6) velocity distribution of the BLR clouds (including microturbulence); (7) source’s orientation; and (8) BLR cloud sizes (see Panda, 2021a). The 8-dimensional parameter space was first presented by Panda et al. (2019c), and extended by Panda et al. (2020b) wherein through large grids of photoionization models with CLOUDY and massive observational spectroscopic catalogs (Shen et al., 2011; Rakshit et al., 2020) the inherent trends along the main sequence have been confirmed. This almost completes the circle initiated with the hypotheses in Boroson and Green (1992) although more progress is needed, from observational and theoretical aspects. This multi-dimensional parameterization includes the viewing angle to the source (or orientation), which is constrained for a small fraction of the AGNs, especially those that show strong radio “jetted” emissions (see, e.g., Padovani et al., 2017, for an overview) or strong water masers (Neufeld et al., 1994; Greenhill et al., 2003). For the remaining sources, the viewing angle is estimated indirectly - through dynamical modeling (Pancoast et al., 2011; Li et al., 2013; Williams et al., 2018; Li et al., 2024), through polarization studies of the emission lines (Savić et al., 2018; Jiang et al., 2021; Śniegowska et al., 2023; Jose et al., 2024), or broad-band SED modeling (Yang et al., 2020; Martínez-Ramírez et al., 2024). The knowledge of the viewing angle is crucial since it can be combined with the spatial and velocity distribution of BELR clouds and their location from the central ionizing source, to estimate the black hole mass of the source. The methodology presented by Panda et al. (2020b) is powerful and acts in dual-purpose - for sources with known orientation and spectroscopically measured Fe ii emission, it can allow to constrain the BLR density and metallicity. On the other hand, through observed UV diagnostics if the BLR density and metallicity can be inferred (in addition to the Fe ii emission), one can recover the orientation angle of the source. The methodology, at present, includes the state-of-the-art broad-band SEDs presented in Panda et al. (2019c) and Ferland et al. (2020), while the BH mass, Eddington ratio, velocity distributions and line intensities are from the SDSS QSO catalogs (Shen et al., 2011; Rakshit et al., 2020) for observed AGNs, and can be refined with future multi-wavelength campaigns. Recent works by Pandey et al. (2023, 2024) have extended these results with new, and up-to-date Fe ii atomic datasets and accounting for dust within the BLR. Additionally, using these new datasets, Dias dos Santos et al. (2023, 2024) have probed into the Fe ii emission in the NIR regime with the added advantage of transitions being isolated and less in number relative to the optical and UV.
In another recent work (Floris et al., 2024), we performed a multi-component analysis on the strongest UV and optical emission lines and using
There are multiple studies predating the aforementioned papers that have paved the way to our current understanding of the Fe ii emission and we recommend the readers to the detailed accounts by, e.g., Sulentic et al. (2000); Marziani et al. (2001); Zamfir et al. (2010); D’Onofrio et al. (2012); Shen and Ho (2014); Sulentic and Marziani (2015); Marziani et al. (2018); Gaskell et al. (2022); Panda and Marziani (2023a).
2.3 Developing AGN SEDs along the main sequence
The shape of the ionizing continuum has been an integral part of the main sequence of quasars studies. Around the same time as Boroson and Green, researchers were already developing mean AGN SEDs (Vanden Berk et al., 2001; Richards et al., 2006), be it to distinguish the sources based on radio dichotomy (Laor et al., 1997) and more recently in Marziani et al. (2021a) or to reveal the prominence of the big blue bump feature in typical Type-1 AGNs (Mathews and Ferland, 1987; Korista et al., 1997). With the advent of large spectroscopic surveys spearheaded by SDSS (York et al., 2000; Shen et al., 2011) AGNs exhibiting stronger Fe ii emission alike I Zw 1 were being consistently discovered and led to the creation of a mean SED representing Narrow-line Seyfert 1 galaxies (Marziani and Sulentic, 2014). We now have broad-band mean SEDs grouped in Eddington ratios ranging from sub-to super-Eddington limits (Jin et al., 2012; 2017; Ferland et al., 2020). Although these mean SEDs have helped provide statistical inferences on the role of AGN SED in the main sequence trends, having broad-band SED for individual AGNs is a much more recent endeavor that has seen growth. With the increase in simultaneous observations across multiple spectral regimes and the development of self-consistent AGN SED models (Done et al., 2012; Kubota and Done, 2018; 2019; Hagen and Done, 2023), the number of individual sources with broad-band SEDs is growing at a rapid pace, especially for sources demonstrating the most intense Fe ii emission (Marinello et al., 2020; Jin et al., 2023). Another important extension in the area of SED building is the slim disk AGN SED models (Abramowicz et al., 1988; Wang et al., 2014; Panda and Marziani, 2023b) applicable to those sources accreting at or above the Eddington limit, that show signatures of strong outflows even in the low-ionization emitting regions (e.g., Rodríguez-Ardila et al., 2024).
3 QMS and AGN variability
Another equally important finding was the discovery of the variation in the intensities of emission lines over timescales of weeks to months, suggesting very small emitting regions of the order of a few thousand Schwarzschild radii (Greenstein and Schmidt, 1964). This region is now well-known as the broad-line region (BLR). This crucial discovery opened up a new sub-field called reverberation mapping (RM), which has led to the estimation of black hole masses in hundreds of low-to high-luminosity Seyferts and quasars (Blandford and McKee, 1982; Peterson, 1988; 1993; Peterson et al., 2004), supplemented by single/multi-epoch spectroscopy (Kaspi et al., 2000; Bentz et al., 2013; Du et al., 2016). The BLR’s location (
On the other hand, the complexity in the modeling and extracting Fe ii emission from the spectra has led many to search for viable alternatives. Most prominent among the proxy is the Ca ii triplet (or CaT) in the NIR given the similarity of the physical conditions required to produce the two ionic species in the BLR (Panda et al., 2020a; Panda, 2021b). In an ongoing series of works (Martínez-Aldama et al., 2015; Marinello et al., 2016; Panda et al., 2020a; Martínez-Aldama et al., 2021b), we have compiled optical Fe ii and NIR CaT emission strengths and weighed them against each other. We find a robust correlation between the two (Martínez-Aldama et al., 2015; Panda et al., 2020a) primarily driven by the Eddington ratio and in parts to the BH mass (Martínez-Aldama et al., 2021b). This led us to investigate whether CaT can be a viable replacement for the strength of the Fe ii emission (or
3.1 Changing-look AGNs and our renewed interest in them
Changing-look AGNs have been known for almost as long as the main sequence existed (see recent review by Komossa et al., 2024; Ricci and Trakhtenbrot, 2023). The spectral changes over multiple epochs have now been detected in numerous AGNs - be it extreme variability with the changes in the continuum and emission lines so strong that can be associated with external interference such as obscuration or tidal disruption events (LaMassa et al., 2015; Dodd et al., 2023; Trakhtenbrot et al., 2019), but could very well be associated with intrinsic effects such as disk transition/disk instabilities (Noda and Done, 2018; Ross et al., 2018; Sniegowska et al., 2020) although, the timescales of such events can be widely different (Czerny, 2006).
With the growing interest in finding new changing-look AGNs, the focus has been also to look for AGNs showing variations in their Fe ii emission (see, e.g., Gaskell et al., 2022; Petrushevska et al., 2023). The regular variable nature of AGNs has helped to gain insights into their emitting regions, with some sources where we have estimates of their Fe ii-emitting locations (see, e.g., Hu et al., 2015; Barth et al., 2013) although there are now instances of exceptional changes in the Fe ii intensities. Panda and Śniegowska (2024) made a compilation of such sources and tracked their transition along the Eigenvector 1 schema and categorized sources that either stay within the same population (A or B, see right panel of Figure 1) or make an inter-population movement as a function of spectral epoch.
3.2 New avenues in reverberation mapping: BLR saturation and Fe ii-based R-L relations
In addition to Changing-look AGNs, dedicated spectro-photometric monitoring campaigns on individual sources (e.g., Mrk 6, NGC 5548, NGC 4151, NGC 4051), have allowed us to re-affirm the Pronik-Chuvaev effect, i.e., the increase, albeit with a gradual saturation, in the H
Another interesting revelation has been the construction of the first Fe ii UV R-L relation in Zajaček et al. (2024b). Here, in addition to improving the existing Mg ii-based R-L with 194 sources [more recent compilation in Shen et al. (2024)], we have been able to constrain the R-L behavior in UV-emitting Fe ii for 5 AGNs. The results are motivating as the slope of the R-L is in close agreement with one expected from the standard photoionization theory (i.e., = 0.5). Although it is interesting to note that this relation appears steeper than the Mg ii R-L relations such that for low-luminosity regimes, the Fe ii emitting region is closer than the Mg ii-emitting region, whereas, at higher luminosities, both relations converge and intersect. This intriguing behaviour needs more explanation which the upcoming RM campaigns may have an answer to. We note that the optical Fe ii-based R-L relation has been around for some time (see Gaskell et al., 2022, for a recent review). A recent compilation of 17 AGNs (including multiple epoch Fe ii time lag measurements) from Prince et al. (2023) reveals an R-L for the optical Fe ii with a slope close to 0.5, and the comparison with the aforementioned UV-based R-L reveals an offset by a factor of 1.8, i.e., the optical Fe ii emitting regions are located 1.8 times further out relative to the UV Fe ii regions.
3.3 Quasars for cosmology: role of the main sequence
Quasars, with their extragalactic origin and persistent bright nature, have long been proposed as “standardizable candles”. With the knowledge of their luminosities (with the aid of the RM and R-L relation) and independently of their fluxes from spectroscopic monitoring, we can determine the luminosity distances of these sources. With a growing number of AGNs (now
However, the recent detection of shorter lags in R-L linked to high-accreting sources (Du et al., 2015; Grier et al., 2017; Du et al., 2016) has put the use of R-L relation into uncertainty. In Martínez-Aldama et al. (2019), we looked into the dispersion in the R-L and upon further investigation found the extent of offset of the source’s time-lag is proportional to the Eddington ratio (or more specifically its mass accretion rate). While this helped “standardize” the R-L relation, there remained a circularity problem - the mass accretion rate needs the knowledge of luminosity apriori, and the latter can be estimated assuming a cosmological model. This defeats the purpose of using quasars for cosmology and thus, requires us to find a direct observable parameter that can replace the mass accretion rate. What can be that? In Du and Wang (2019), the authors found that the Fe ii strength (or
In a parallel direction, efforts to reconcile the use of quasars along with other distance indicators, e.g., SNIa, Gamma-ray bursts, Baryon Acoustic Oscillations, and temperature anisotropy across the microwave background, have been made including the
4 Closing remarks and future perspective
Fe ii emission has been long perceived as a contaminant in the AGN spectra and ways to remove this contamination were sought to enable study and reliable extraction of other emission line properties. The emission turned out to be so useful that a niche of studies linking to the Fe ii emission was proposed and expanded. To date, the studies stemming from the Fe ii analysis have crucial contributions in developing our understanding of the line-emitting regions in the BLR leading up to the standardization of quasar-based scaling relations. This mini-review cannot do justice to the enormous literature about the study of Fe ii emission and its link to the quasar main sequence. Yet, we have tried to touch upon some key aspects in this short overview. We are already in the data-driven astronomy era with multiple facilities working in cohesion, to reveal more connections to the quasar main sequence.
Finally, we glance upon some recent avenues that will have a direct impact on the ongoing studies:
Author contributions
SP: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing–original draft, Writing–review and editing.
Funding
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. SP acknowledges the financial support of the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fellowships 300936/2023-0 and 301628/2024-6. SP is supported by the international Gemini Observatory, a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation, on behalf of the Gemini partnership of Argentina, Brazil, Canada, Chile, the Republic of Korea, and the United States of America.
Acknowledgments
This mini-review has been made possible thanks to many past and ongoing collaborations; I would like to thank Bożena Czerny, Paola Marziani, Alberto Rodríguez Ardila, Mary Loli Martínez-Aldama, Murilo Marinello, Marzena Śniegowska, Francisco Pozo-Nuñez, Michal Zajaček, Edi and Nataša Bon, Szymon Kozłowski and many others for their invaluable support, constant motivation and fruitful discussions. I am grateful to the organizers of the “Frontiers in Astronomy and Space Sciences: A Decade of Discovery and Advancement - 10th Anniversary Conference” for the invitation to write this mini-review.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
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Footnotes
1http://servo.aob.rs/FeII_AGN/
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Keywords: active galactic nuclei, quasars, seyfert galaxies, emission lines, AGN variability, changing-look AGNs, accretion disks
Citation: Panda S (2024) Unveiling the quasar main sequence: illuminating the complexity of active galactic nuclei and their evolution. Front. Astron. Space Sci. 11:1479874. doi: 10.3389/fspas.2024.1479874
Received: 12 August 2024; Accepted: 09 September 2024;
Published: 19 September 2024.
Edited by:
Didier Fraix-Burnet, UMR5274 Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), FranceReviewed by:
Pu Du, Chinese Academy of Sciences (CAS), ChinaCopyright © 2024 Panda. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Swayamtrupta Panda, swayamtrupta.panda@noirlab.edu
†Gemini Science Fellow