2.1. Quasar Lyα Nebulae
Being the first and more luminous high-redshift object discovered above z > 2, quasars have been obvious signposts since the mid 1980s to look for putative galactic and gaseous "companions" in Lyα emission. One of the first attempt reported in the literature is the pioneering narrow-band observation of Djorgovski et al. (1985) on the Lick Observatory 3 meter telescope centred on the radio-loud QSO PKS1614+051 at z ∼ 3.2 that resulted in the discovery of a companion Lyα emitting galaxy (a narrow-line AGN) at about 5" from the quasar. Later observations by Hu & Cowie (1987) using the same technique on the 3.6 meter Canada France Hawaii Telescope (CFHT), showed the presence of a "bridge" of Lyα emission between the quasar and the companion galaxy. In the same year, Schneider et al. (1987) reported the discovery of "companion" Lyα emission to the triply-lensed radio-loud quasar Q2016+112.
These initial discoveries prompted an intense effort to search for Lyα emission around quasars, mostly of which radio-quiet, in subsequent years but despite the large number of quasars observed (about 50), no detectable Lyα candidates were found (these results were mostly unpublished, see discussion in Hu et al. 1991). The situation changed in the early 1990s when observational surveys focused on the much smaller sub-sample of radio-loud quasars (Hu et al. 1991, Heckman et al. (1991) reported a detection rate of compact or extended companion Lyα emission close to 100%. In particular, Heckman et al. (1991) reported the discovery of Lyα Nebulae with sizes of about 100 kpc for 15 of the 18 radio-loud quasars observed. The contemporarily discovery of large Lyα Nebulae around non-QSO radio-sources (e.g., McCarthy et al. 1987 at z ∼ 1.8) as I will discuss in section 2.2, suggested to these authors a possible link between the radio and the Lyα emission. At the same time, searches around radio-loud quasars were also motivated by the possibility to test the hypothesis that radio-loud quasars and radio galaxies were the same class of objects viewed along different angles with respect to the radio axis.
One of the first detection of "companion" Lyα emission to radio-quiet quasars was due to a serendipitous observation by Steidel, Sargent & Dickinson (1991) at the Palomar 5 meter telescope: originally searching for the continuum counterpart of a z ∼ 0.8 MgII absorber in the spectrum of the radio-quiet QSO Q1548+0917, they found instead a narrow and extended Lyα emission line at the same redshift of the quasar in the the spectrum of a faint continuum source ∼ 5" away from the QSO. Also, additional narrow-band imaging suggested the presence of extended Lyα emission around the quasar. In the following year, Bremer et al. (1992) reported the discovery of extended (∼ 5") emission in long-slit spectra of two radio-quiet quasars at z ∼ 3.6. These results showed that "companion" Lyα emission was not restricted to radio-loud quasars only.
Giant Lyα Nebulae with sizes larger than 10" (or larger than about 100 kpc) around radio-quiet quasars remained however elusive for more than two decades after the survey of Heckman et al. (1991) around radio-loud quasars. The only exception was the serendipitous discovery of Bergeron et al. (1999) around the z ∼ 2.2 radio-quiet quasar J2233-606 located in one of the parallel fields of the Hubble Deep Field South. The field was observed during science verification of the VLT-UT1 Test Camera using broadband filters and a narrow-band filter centred on the quasar Lyα emission. Despite some large-scale residual of the flat-fielding procedure, the narrow-band image seemed to show extended emission with a maximum projected size of about 12" (about 100 kpc with current cosmological parameters) around the quasar 1. On the other hand, during the same years a few individual detections of small nebulae extending up to a few arcsec around radio-quiet quasars were reported (Fried 1998; Møller et al. 2000; Bunker et al. 2003; Weidinger et al. 2004) and in some cases associated with intergalactic gas (e.g., Weidinger et al. 2004, 2005). By the beginning of the 2010s, thanks to long-slit spectroscopic surveys and small Integral-Field-Unit (IFU) observations, a common picture emerged that associated only relatively small nebulae (i.e., < 60-70 kpc) to about 50% of radio-quiet quasars between 2 < z < 5 (Christensen et al. 2006; Courbin et al. 2008; North et al. 2012; Hennawi & Prochaska 2013; but see Herenz et al. 2015) However, as I discuss below, these results may have been limited by the small sizes of the IFU Field-of-View (FOV) and by the use of long-slit spectroscopy that cannot capture the full extent of asymmetric nebulae.
2.1.1. Rise of the Giants
The last few years witnessed a complete revolution in our knowledge of giant Lyα nebulae around radio-quiet quasars thanks to serendipitous discoveries by means of NB imaging with custom-built filters (Cantalupo et al. 2014; Martin et al. 2014; Hennawi et al. 2015) and dedicated surveys with VLT/MUSE (e.g., Borisova et al. 2016). Two decades after Hu et al. (1991) and Heckman et al. (1991), a new narrow-band campaign on quasar fields was initiated in order to search for "dark galaxies" and fluorescently illuminated intergalactic gas following the prediction, e.g., of Cantalupo et al. (2005). Two pilot programs using a spectroscopic "multi-slit plus filter" technique (Cantalupo et al. 2007) and deep NB imaging (∼ 20 hours) on FORS/VLT using a custom-built filter for a quasar at z ∼ 2.4 (Cantalupo et al. 2012), revealed a dozen of compact Lyα sources with no detectable continuum and Equivalent Widths (EW) larger than 240Å, the best candidates for "dark galaxies" illuminated by the quasar. Circumgalactic gas was also detected in emission extending by several tens of kpc around a few bright galaxies but the quasar did not show evidence for extended nebulae, in agreement with previous findings.
Stimulated by these initial results, Cantalupo et al. (2014) initiated a campaign using Keck/LRIS and custom-built filters to search for "dark galaxies" around about ten radio-quiet quasars at z ∼ 2. Surprisingly, the first quasar observed at Keck/LRIS, i.e. UM287, showed clear evidences of extended emission over scales larger than 10" after the first 20 minutes exposure was obtained. At the end of the total integration of 10 hours and detailed data reduction, a giant Lyα nebula extending to about 55" (∼ 460 kpc) was found around this quasar (Cantalupo et al. 2014), see Fig. 1, and named "Slug Nebula" (given its morphology and in honor of the mascot of the University of California, Santa Cruz). This discovery was surprising for several reasons: i) despite its association with a radio-quiet quasar, it is at least twice as large as any previously detected Lyα Nebulae including the much more common radio-galaxy halos and Lyα blobs as discussed below; ii) given its size, it extend well beyond the viral halo of the quasar into the intergalactic medium; iii) it shows a very high Surface Brightness over very large scales that cannot be easily explained unless a large gas clumping factor within intergalactic gas is invoked (see section 3.1). During the same night, a second quasar field was observed using a different custom-built filter, selected among the quasar-pair sample of Hennawi & Prochaska (2013) and showing some hints of extended emission, and another giant Lyα nebula with a size of about 300 kpc was discovered (Hennawi et al. 2015). The particularity of this discovery included also the presence of a physically associated quasar quartet and a large overdensity of Lyα emitting galaxies (differently than the Slug Nebula). Named "Jackpot Nebula" given the rarity of such systems, it traces likely a very peculiar region of the Universe and possibly a proto-cluster. In the same year of these discoveries, Martin et al. (2014) presented the detection of another giant nebula originally found around one of the six quasars observed in narrow-band imaging as a part of the Keck Baryonic Structure Survey (e.g., Trainor & Steidel 2012). Subsequent observations of other quasars as a part of the same survey and including results obtained on GMOS/Gemini (Arrigoni-Battaia et al. 2016) showed once again however that such detection of giant Lyα nebulae are apparently rare, i.e. with a frequency less than 10%.
Figure 1. Continuum-subtracted narrow-band image of the Slug Nebula (Cantalupo et al. 2014) discovered around the bright, radio-quiet quasar UM287 (labeled “a” in the image) in a 10-hour deep observation made with a custom filter installed on Keck/LRIS. The nebula shows filamentary emission extending on a total projected length of about 55 arcsec (∼ 460 kpc) and is currently one of the largest and more luminous Lyα nebulae discovered to date. Its surprising properties in terms of extension and high values of Surface Brightness are discussed in details in section 2.1.1. (Figure reproduced with permission from Cantalupo et al. 2014).
The installation of the MUSE Integral-Field-Spectrograph on the Very Large Telescope in 2014 provided new opportunities for the detection and study of Giant Lyα nebulae around quasars thanks to its a large FOV of 1' × 1' (about 450 × 450 kpc2; individual spatial elements have a size of 0.2" × 0.2") and because, by design, it does not suffer from either narrow-band filter losses or spectroscopic slit losses. Also, the large number of spatial and spectral elements allows for better quasar PSF estimation and removal with respect to NB surveys. Because accurate systemic redshifts are not needed for MUSE observations (as for any other spectroscopic survey) any quasar with Lyα redshifted between the blue and red edges of the MUSE wavelength range (2.9 < z < 6.5) can be observed. In one of the first exploratory study as a part of the MUSE Guaranteed Time Observations (GTO), Borisova et al. (2016) observed with short total exposure times (1 hour) 17 of the brightest radio-quiet quasars in the Universe at 3.1 < z < 3.7 and complemented them with two radio-loud quasar at the same redshifts. The picture emerging from these MUSE observations is very different than that based on previous surveys, in that giant nebulae with sizes larger than 100 pkpc are found around essentially every quasar above a surface brightness level of about 10−18 erg s−1 cm−2 arcsec−2. The nebulae detected with MUSE present a large range in sizes and morphologies, ranging from circular nebulae with a projected diameter of about 110 pkpc to filamentary structures with a projected linear size of 320 pkpc (see Fig.2). Despite these differences, the circularly averaged SB profiles show a strong similarity between all the giant Quasar Nebulae (including the Slug Nebula at z ∼ 2 once corrected for redshift-dimming) with very few exceptions, both in terms of slope and normalization, suggesting a similar origin for these systems.
Figure 2. "Optimally extracted" narrow-band images of the sample of luminous quasars at 3 < z < 4 observed with MUSE by Borisova et al. (2016) as a part of a "snapshot" survey using short integration times, i.e. 1 hour per field (see Borisova et al. 2016 for details on the detection and extraction of these images from the datacubes). The quasars are all radio-quiet with the exception of the two fields labeled "R1" and "R2". All nebulae are larger than 100 kpc and extending in some cases up to at least 320 kpc (e.g., the nebula number 3 or "MQN03") with various morphologies including filamentary structure. This survey showed that giant Lyα nebulae are ubiquitous around bright quasars, including radio-quiet ones, in contrast to previous observations at z ∼ 2 as discussed in detail in section 2.1. (Figure reproduced with permission from Borisova et al. 2016).
This 100% detection rate of giant nebulae around radio-quiet quasars obtained with MUSE (see also, Fumagalli et al. 2016) is in stark contrast with previous results in the literature as I have reviewed in this section. While the asymmetric morphology of the MUSE nebulae may explain the discrepancy with spectroscopic surveys using a single slit position, the difference with the detection rate of NB surveys at z ∼ 2 cannot be completely and easily explained by observational limitations such as NB filter losses, uncertainties in quasar systemic redshifts and Quasar Point-Spread-Function (PSF) removal errors. Redshift and quasar luminosities may therefore play a role in the appearance and properties of Lyα nebulae around quasars and future IFU studies extending to lower redshifts (e.g., with the Keck Cosmic Web Imager) and to lower quasar luminosities are necessary to properly address these open questions.
2.2. Radio-galaxy Lyα Halos
As mentioned in the previous section, follow-up observations of radio-loud sources provided already in the second half of the 1980s the first evidences for giant Lyα nebulae. In this section, I will focus on non-QSO radio sources at z > 1, i.e. High-z Radio Galaxies (HzRG).
Although many different classification exists (see e.g. McCarthy 1993 and Antonucci 2012 for a review), the most obvious distinction between the QSO and non-QSO class of radio source traces its roots from the appearance of the optical morphology, i.e. from the presence of a quasi-stellar point source versus a spatially resolved galaxy. Because this distinction may be difficult to be applied at high redshift a more physical definition can be made instead from the properties of the emission-line spectra. Indeed, classical radio-galaxies have spectra with relatively narrow (FWHM < 2000 km/s) permitted lines with respect to the broader lines showed by quasars (FWHM > 5000 km/s). These are sometimes called "Narrow Line Radio Galaxies" or "type 2". Moreover, high-redshift quasars have a much brighter, "thermal" continuum than "type 2" radio-galaxies with a clear non-stellar origin. It is important to notice however that a small fraction of "spatially resolved" radio galaxies show broad lines as well as bright thermal continuum. These are sometimes called "Broad Line Radio Galaxies". To avoid confusion, in this section and in the reminder of this chapter we will always refer to radio-galaxies as "type 2". Like their lower redshift counterparts, HzRG often show extended radio lobes that have been associated with bi-polar jets (e.g., McCarthy et al. 1987) and show a high degree of polarization. The observation of relativistic beaming and smaller, one-sided radio-jets in radio-loud quasars have already suggested in the 1980s that radio-galaxies and radio-loud quasars may be part of the same population of AGN but seen at different orientations (e.g., Barthel 1989; see Antonucci 1993 for a review and, Antonucci 2012 for more recent discussion).
Narrow-band imaging and spectroscopy of HzRGs, soon after their discovery in the 1980s and until very recently, has produced a large literature of detections and studies of large Lyα nebulae, similarly to radio-loud quasars. The first observations by McCarthy et al. (1987) of the radio-galaxy 3C 326.1 at z ∼ 1.8 at the Lick Observatory revealed Lyα emission surrounding the radio lobes and extending by about 70 kpc (with current cosmological parameters). Few years later, McCarthy et al. (1990) reported the discovery of the first giant Lyα nebula extending over 120 kpc around the z ∼ 1.8 radio galaxy 3C 294. This nebula is highly elongated and well aligned with the inner radio source axis. Extended C IV and He II emission was also detected. One of the first detection of extended Lyα emission around a radio-galaxy above redshift of three was reported by Eales et al. (1993) studying the z = 3.22 radio galaxy 6C 1232+39, a system with a "classical" double radio structure oriented along the same direction of the Lyα emission but showing some differences in the morphology and symmetry of the optical emission with respect to the radio.
Larger sample of HzRG observations were started to be obtained in the 1990s as a part of several campaigns that resulted in the discovery of several tens of HzRG at z > 2 (e.g., Röttgering et al. 1994). The most notably discoveries included a giant Lyα halo extending over 140 kpc around a HzRG at z ∼ 3.6 (van Ojik et al. 1995) showing a complex kinematical structure spatially correlated with the radio jet: large velocity widths (FWHM ∼ 15000 km/s) in proximity of the radio jets and a aligned, low SB component with narrow velocities (FWHM ∼ 250 km/s) extending 40 kpc beyond both sides of the radio source. I will discuss and compare in detail in section 4, the kinematical properties of radio-loud and radio-quiet nebulae, including this system.
Subsequent observational campaigns found almost ubiquitous detection of extended Lyα emission around HzRG extending in several cases above 100 kpc in size (e.g., Pentericci et al. 1997, Kurk et al. 2000, Reuland et al. 2003, Villar-Martin et al. 2003, Venemans et al. 2007, Villar-Martin et al. 2007, Humphrey et al. 2008, Sanchez & Humphrey 2009, Roche et al. 2014) and showing a wealth of morphological structures, including filaments, clumpy regions and cone-shaped structures. The common features of all these detections include: i) apparent alignment between Lyα emission and the radio jet axis (e.g. Villar-Martin et al. 2007), ii) broad kinematics (with some exceptions at the far edge of the nebulae), iii) associated extended C IV and He II emission, iv) invariable association with large overdensity as traced by companion Lyα galaxies (e.g., Venemans et al. 2007) or multi-wavelength observations from X-ray to the infrared (Pentericci et al. 1997). In terms of luminosities and surface brightness the HzRG halos show similar values with respect to radio-loud and radio-quiet quasar nebulae, while UV line ratios and kinematics seems to be quite distinct form the radio-quiet quasar Nebulae (see e.g., Borisova et al. 2016) as I will discuss in section 4.
2.3. Lyα blobs
The historically-distinct category of giant Lyα Nebulae called "Lyα blobs" made their appearance with the deep narrow-band observations of Steidel et al. (2000) of a region containing a high overdensity of galaxies both in projected and velocity space in the so called "Small Selected Area 22h" (SSA22) field (Lilly et al. 1991; Steidel et al. 1998). While searching for candidates Lyα emitting galaxies in this field at z = 3.09, Steidel et al. (2000) found to their surprise "two very luminous, very extended regions of line emission which we descriptively call 'blobs'". Extending by about 15 to 17 arcsec (115 to 132 kpc with current cosmology), they were among the largest Lyα Nebulae found at that epoch, with similar extent and luminosities to radio-loud quasar and radio-galaxy Nebulae. However, these "blobs" were apparently lacking any associated, bright continuum or radio source and therefore considered as a possibly different category of objects. In particular, Steidel et al. (2000) considered the possibility of a "cooling flow" origin for the emission, by analogy with Hα emission from observed cooling-flow clusters or photo-ionization by heavily obscured, highly star forming galaxies. However, no firm conclusion was possible with the available data for that time. The "mysterious" nature of these Lyα blobs attracted the attention of several theoretical and numerical studies, right after their discoveries until very recently, that tried to explain the peculiarity of these systems with a variety of physical explanations that did not require a bright photo-ionizing source such as an AGN. These models ranged from galactic superwinds (including e.g., Taniguchi & Shioya 2000) to cooling radiation from the so called "cold-mode accretion" (including, e.g. Haiman et al. 2001; Fardal et al. 2001; Dijkstra & Loeb 2009) as I discuss more in detail in section 3.
From an observational and historical point of view, a Lyα "blob" (LAB) could then be defined as an extended Lyα emission over scales significantly larger than a single galaxy that does not seem to contain an AGN (at the time of their discovery). Extended Lyα nebulae around galaxies that did not contain optically bright or radio-loud AGN in overdense fields were already known before the observations of Steidel et al. (2000), e.g. Francis et al. (1996) and later re-observations published in Francis et al. (2001), and Keel et al. (1999), although in most cases hints of obscured AGN were present in these studies. Steidel "blobs" however were the first one in this category to exceed the "giant Lyα Nebula" size of 100 kpc. Later Subaru observations of the same SSA22 field made by Matsuda et al. (2004) exploring a much larger area than the original survey by Steidel et al. (2000) found several extended Lyα nebulae around galaxies, i.e. 33 nebulae with area exceeding 15 arcsec2 in addition to the Steidel "blobs" (also called LAB1 and LAB2). However, none of those new detections were close to LAB1 and LAB2 in terms of overall sizes. For instance, the largest new detection by Matsuda et al. (2004) had an area that was only half the one of LAB2 and a maximum projected size of about 75 kpc. Nevertheless, this overdense field showed clearly an excess of extended sources with respect to "blank" observation suggesting a possible relation between galaxy (or AGN) overdensities and extended Lyα emission. Similarly, deep narrow band imaging obtained by Palunas et al. (2004) of the overdense field at z ∼ 2.38 found by Francis et al. (2001), revealed a large Lyα blob with sizes compatible with Steidel's LAB1 and several smaller nebulae around galaxies.
LAB1 was the first LAB to be studied in detail in multi wavelength observations and revealed the presence of a strong submillimeter source with a bolometric luminosity in excess of 1013L⊙ (e.g., Chapman et al. 2001, Geach et al. 2005) but no evidence from deep Chandra X-ray observations of a clear X-ray counterpart (Chapman et al. 2004). However, these limits did not exclude the possibility of a luminous AGN but heavily obscured along our line of sight. LAB2 instead showed, in addition to a (fainter) submillimeter source (Chapman et al. 2001), clear evidences for hard X-ray emission (e.g, Basu-Zych & Scharf 2004) and therefore for the presence of a partially unobscured AGN. This link between LABs and luminous star forming galaxies stimulated new observational campaigns to detect LABs by searching, e.g. around luminous infrared sources. During one of such campaigns, Dey et al. (2005) discovered a 160 kpc Lyα emitting nebula around a luminous mid-infrared source first detected with the Spitzer Space Telescope. This nebula shared many similarities with the previously detected LAB1 and LAB2. Although X-ray information was not available at that time, the presence of narrow and centrally concentrated C IV and He II emission within the Nebula suggested again an association with a Type-2 AGN. Differently from radio-galaxy halos, this nebula showed however relatively narrow Lyα emission in velocity space and possibly a ordered velocity shear. As we will discuss in section 4, this is compatible with other nebulae associated with radio-quiet AGN.
Giant LABs with sizes above 100 kpc are mainly discovered by targeting known overdense regions or bright infrared and submillimeter galaxies. Blank-field surveys using narrow-band imaging confirmed that giant LABs were indeed extremely rare (e.g. Saito et al. 2006, Yang et al. 2009), i.e. presenting a comoving number density less than 10−6 Mpc3 (e.g., Yang et al. 2009). Broadband surveys plus spectroscopic follow-ups were slightly more successfully, detecting one LAB with a size of about 100 kpc and three smaller nebulae in a volume of about 108 Mpc3 (Prescott, Dey & Jannuzi 2013). It is instructive to compare these numbers with the comoving number densities at 2 < z < 3 of bright X-ray selected AGN, i.e. nX ∼ 10−6 Mpc−3 for LX > 1044.5 erg s−1 (Ueda et al. 2003), optically-bright quasars at z ∼ 3, i.e. nQSO ∼ 10−7 Mpc−3 for Mi(z = 2) < −26.7 (Shen et al. 2007), and HzRG, i.e. 4 × 10−8 Mpc−3 for L2.7 GHz > 1033 erg s−1 Hz−1 sr−1 (see Venemans et al. 2007).
Detection of smaller nebulae with sizes up to 40-70 kpc are less rare in blind narrow-band surveys and about ten of such discoveries were reported in the past, including Nilsson et al. (2006), Yang et al. (2009), Smith et al. (2009), and Prescott et al. (2009). Deeper and larger narrow-band observations around the SSA22 fields and including some blank fields over a total volume of about 106 Mpc3 as a part of the "Subaru Lyα blob survey" resulted in the discovery of about seven new giant LAB with sizes around 100 kpc (Matsuda et al. 2006). An estimation of the number densities of these systems in this survey however, could be affected by the presence of the very overdense region in SSA22. In almost all cases, evidences for associated submillimeter or AGN sources were found at the time of the discovery or with subsequent multi-wavelength observations (e.g., Geach et al. 2009, Overzier et al. 2013, Hine et al. 2016; see also Scarlata et al. 2009 and Ao et al. 2015).
Despite the different techniques and volume probed, clustering analysis suggests that the sizes of detected LABs could be positively correlated with the environment overdensity, although the statistic is small (e.g., Yang et al. 2009, Matsuda et al. 2006). The very recent detection reported by Cai et al. (2016) of a LAB with a projected size of about ∼ 440 kpc at the center of one of the largest overdensity known at z ∼ 2.3 seem to provide additional support for this suggestion. However, also in this case, there are evidences for at least one associated AGN (see also Valentino et al. 2016 for another example).
As I have review in this section, several decades of observations and discoveries have produced a large literature of extended and giant Lyα nebulae that have been classified in various ways and with different nomenclatures depending on the technique and target of the original surveys, i.e. quasars, radio-galaxies, overdense regions or "apparently blank" fields. Their comparable volume densities and the almost invariable association with AGN or massively star forming galaxies seem to suggest however that most of these distinction may be artificial. There are however some indication that the kinematical properties and line ratios (e.g., considering He II, C IV and Lyα) are distinct among radio-quiet and radio-loud nebulae, as well as for some LAB, as I will discuss in section 4.
1 A very recent and deeper observation with GMOS seems to confirm at least part of this nebula up to a size of about 70 kpc (Arrigoni-Battaia et al, in prep.). Back.