The Spiderweb Galaxy Field
Project Rationale
Protoclusters are defined as overdense regions in the high-z Universe
which are expected to evolve into massive, virialized clusters of
galaxies at the present epoch. Finding and characterizing
protoclusters is key to study the large scale structure of the
Universe and the transformational processes that affect star formation
and nuclear activity in the member galaxies, such as powerful mergers,
gas cooling, and feedback effects. At present, there are no standard
methods to systematically search for protoclusters, and this is the
reason why biased-tracer techniques are often used to identify
interesting targets. One of these consists in searching around high
redshift, powerful radio galaxies, that are often found to be the
beacon of overdense
regions (Miley et al. 2009). Independently of the selection
method, in order to confirm their nature and to trace the activity of
several processes occurring during their rapid evolution, protocluster
candidates must be followed-up intensively with multiwavelength
campaigns.
In this respect, X-ray observations play a key role, particularly for studying: (1) unresolved emission from active galactic nuclei (AGN) indicative of accretion onto nuclear supermassive black holes (SMBH), (2) the less intense emission from strongly star forming galaxies, and (3) diffuse emission from hot gas (the forming intracluster medium, ICM) and/or relativistic plasma. Despite the scanty X-ray data currently available on protoclusters, a clear aspect is that to search and characterize the diffuse emission in high-z protocluster and, at the same time, the unresolved emission from AGN members, high sensitivity and angular resolution (~1 arcsec) are both required. As of today, the available data are providing tantalizing evidence for the complex and intricate phenomena that occur in such high-density, rapidly evolving regions at cosmic noon. Here we present the results of a new multiwavelength campaign on the archetypal Spiderweb Galaxy and the associated protocluster, one of the most intensively studied region of the extragalactic sky since its discovery in 1994.
Figure 1. VLT Lya contours (blue, resolution ∼1 FWHM) delineating the gaseous
nebula and the VLA 8 GHz contours (red, resolution ~0.3) delineating
the nonthermal radio emission are superimposed on the composite ACS
image. The image shows a 33"X23" region rotated 10 deg from north.
The gaseous nebula extends for more than 200 kpc and is comparable in
size with the envelopes of cD galaxies in the local universe (from
Miley et al. 2006).
In this respect, X-ray observations play a key role, particularly for studying: (1) unresolved emission from active galactic nuclei (AGN) indicative of accretion onto nuclear supermassive black holes (SMBH), (2) the less intense emission from strongly star forming galaxies, and (3) diffuse emission from hot gas (the forming intracluster medium, ICM) and/or relativistic plasma. Despite the scanty X-ray data currently available on protoclusters, a clear aspect is that to search and characterize the diffuse emission in high-z protocluster and, at the same time, the unresolved emission from AGN members, high sensitivity and angular resolution (~1 arcsec) are both required. As of today, the available data are providing tantalizing evidence for the complex and intricate phenomena that occur in such high-density, rapidly evolving regions at cosmic noon. Here we present the results of a new multiwavelength campaign on the archetypal Spiderweb Galaxy and the associated protocluster, one of the most intensively studied region of the extragalactic sky since its discovery in 1994.
The central, powerful radio galaxy, is embedded in a giant Lyalpha halo, surrounded by a >2 Mpc-sized overdensity (corresponding to a radius of 5 arcmin) of star forming galaxies, dusty starbursts, red galaxies and active galactic nuclei. The complex around the Spiderweb Galaxy is considered to be a typical protocluster region which is expected to evolve into a massive cluster in a few Gyr, with the radio galaxy itself showing the properties of a cD progenitor. These properties makes the Spiderweb Complex an ideal place to investigate for environmental effects on galaxy evolution, and to study the formation of the local large scale structure of the Universe.
In the last 25 years, 54 papers have been published on observations of this specific field, including the discovery papers. The complete collection of papers published on the Spiderweb Field before 2021 can be found in the literature page.
In 2021, our Team started a series of works based on new, multiwavelength data set collected in this field including a Chandra Large Program (700 ks on ACIS-S), SZ observations with ALMA, JVLA, GMRT and MeerKAT observations in the radio, and VIRCAM observation in the NIR. This page presents new results on the Spiderweb Galaxy by our Team, the corresponding data products, and the project papers. Below here we briefly introduce the main results obtained so far by our Team.
Nuclear activity in the Spiderweb Protocluster
Our analysis of the 700 ks Chandra ACIS-S observation of the field
around the Spiderweb Galaxy
(Tozzi et al. 2022), allowed us to identify
unresolved X-ray sources down to flux limits of
1.3×10-16 and 3.9×10-16
erg/s/cm2 in the soft (0.5–2.0 keV) and hard (2–10 keV)
band, respectively. We detect 107 X-ray unresolved sources within 5
arcmin (corresponding to 2.5 Mpc) of the Spiderweb Galaxy, among which
13 have optical, near-infrared, or submillimeter counterparts with
spectroscopic redshift 2.11<z<2.20, and 1 source has a
photometric redshift consistent with this range.
We find that the X-ray-emitting protocluster members are distributed
approximately over a 3.2×1.3 Mpc2 rectangular
region. An X-ray spectral analysis for all the sources within the
protocluster shows that their intrinsic spectral slope is consistent
with an average Γ=1.84±0.04. Excluding the Spiderweb Galaxy,
the best-fit intrinsic absorption for five protocluster X-ray members
is NH1023 cm2, while another six have
upper limits of the order of a few times 1022
cm2. Two sources have an unusually flat spectrum, and are
therefore considered Compton-thick candidates. The 0.5–10 keV
rest-frame luminosities of the 11 Compton-thin protocluster members
corrected for intrinsic absorption are greater than
2×1043 erg/s. These values are typical for the bright
end of a Seyfert-like distribution and significantly greater than
X-ray luminosities expected from star formation activity. The X-ray
luminosity function of the AGN in the volume associated to the
Spiderweb protocluster in the range
1043<LX<1044.5 erg/s is at
least ten times higher than that in the field at the same redshift and
significantly flatter, implying an increasing excess at the bright
end. The X-ray AGN fraction is measured to be 25.5±4.5% of the
spectroscopically confirmed members in the stellar mass range
log(M>M*)>10.5. This value corresponds to an enhancement
factor of 6.0+9.0-3.0 for the nuclear activity
with L[0.5-10 keV] > 4×1043 erg/s with respect to the
COSMOS field at comparable redshifts and stellar mass range. Our main
conclusion is that the galaxy population in the Spiderweb protocluster
is characterized by enhanced X-ray nuclear activity triggered by
environmental effects on Megaparsec scales.
Figure 2. Composite image of the Spiderweb protocluster shows X-rays
detected by Chandra (in purple) that have been combined with
optical data from the Subaru telescope on Mauna Kea in Hawaii (red,
green, and white). The large image is 11.3 million light years across.
Most of the "blobs" in the optical image are galaxies in the
protocluster, including 14 that have been detected in the new, deep
Chandra image. These X-ray sources reveal the presence of material
falling towards supermassive black holes containing hundreds of
millions of times more mass than the Sun. The Spiderweb protocluster
exists at an epoch in the Universe that astronomers refer to as
"cosmic noon". Scientists have found that during this time — about 3
billion years after the big bang — black holes and galaxies were
undergoing extreme growth. Blue circles labeled 14 sources belonging to the protocluster (2.11<z<2.20) and detected
by Chandra (Credit: X-ray: NASA/CXC/INAF/P. Tozzi et al; Optical
(Subaru): NAOJ/NINS; Optical (HST): NASA/STScI). From the Chandra Press Release of March 31, 2022.
X-Ray Emission from the Jets and Lobes of the Spiderweb
Deep Chandra and Very Large Array imaging of the Spiderweb Galaxy
reveals a clear correlation between X-ray and radio emission on scales
~100 kpc
(Carilli et al. 2022). The X-ray emission associated
with the extended radio source is dominated by inverse Compton
upscattering of cosmic microwave background photons by the
radio-emitting relativistic electrons. For regions dominated by high
surface brightness emission, such as hot spots and jet knots, the
implied magnetic fields are ~50-70 μG. The nonthermal pressure in
these brighter regions is then ~9×10-10
dynes/cm2, or three times larger than the nonthermal
pressure derived assuming minimum energy conditions, and an order of
magnitude larger than the thermal pressure in the ambient cluster
medium. Assuming ram pressure confinement implies an average advance
speed for the radio source of ~2400 km/s and a source age of
~3×107 yr. In the diffuse radio-emitting regions at
lower surface brightness, we identify an evacuated cavity in the Lyα
emission coincident with the tail of the eastern radio lobe. Making
reasonable assumptions for the radio spectrum, we find that the
relativistic electrons and fields in the lobe are plausibly in
pressure equilibrium with the thermal gas and close to a minimum
energy configuration. The radio morphology suggests that the Spiderweb
is a high-z example of the rare class of hybrid morphology radio
sources (or HyMoRS), which we attribute to interaction with the
asymmetric gaseous environment indicated by the Lyα emission.
Figure 3. Contour image of the 2 GHz to 4 GHz VLA image of the Spiderweb galaxy, at a resolution of 1.3×0.6 arcsec2, major axis north–south. The contour levels are a geometric progression in a factor of two, starting at 20 μJy/beam. Negative contours are dashed. The color scale is the Chandra Observatory total X-ray emission in the 0.5–7 keV band (from Carilli et al. 2022).
Figure 3. Contour image of the 2 GHz to 4 GHz VLA image of the Spiderweb galaxy, at a resolution of 1.3×0.6 arcsec2, major axis north–south. The contour levels are a geometric progression in a factor of two, starting at 20 μJy/beam. Negative contours are dashed. The color scale is the Chandra Observatory total X-ray emission in the 0.5–7 keV band (from Carilli et al. 2022).
The Spiderweb proto-cluster is being magnetized by its central radio jet
Deep broadband radio polarization observations of the Spiderweb Galaxy
(Anderson et al. 2022) yield the most detailed
polarimetric maps yet made of a high redshift radio galaxy. The
intrinsic polarization angles and Faraday Rotation Measures (RMs)
reveal coherent magnetic fields spanning the ∼60 kpc length of the
jets, while ∼50% fractional polarizations indicate these fields are
well-ordered. Source-frame absolute RM values of ∼1,000
rad/m2 are typical, and values up to ∼11,100
rad/m2 are observed. The Faraday-rotating gas cannot be
well-mixed with the synchrotron-emitting gas, or
stronger-than-observed depolarization would occur. Nevertheless, an
observed spatial coincidence between a localized absolute RM
enhancement of ∼1,100 rad/m2, a bright knot of Lyα
emission, and a deviation of the radio jet provide direct evidence for
vigorous jet-gas interaction. We detect a large-scale RM gradient
totaling ∼1,000s rad/m2 across the width of the jet,
suggesting a net clockwise (as viewed from the AGN) toroidal magnetic
field component exists at 10s-of-kpc-scales, which we speculate may be
associated with the operation of a Poynting-Robertson cosmic
battery. We conclude the RMs are mainly generated in a sheath of hot
gas around the radio jet, rather than the ambient foreground
proto-cluster gas. The estimated magnetic field strength decreases
from the jet hotspots (∼90 μG) to the jet sheath (∼10 μG) to the
ambient intracluster medium (∼1 μG). Synthesizing our results, we
propose that the Spiderweb radio galaxy is actively magnetizing its
surrounding proto-cluster environment, with possible implications for
theories of the origin and evolution of cosmic magnetic fields.
Figure 4. Maps of peak-P (X- plus Ka-band), source-frame RM (X- plus Ka-band; note that values saturate at RM = 2000 rad/m^2 to better reveal low RM structure throughout the jet), intrinsic sky-projected magnetic field orientation (Ka-band only), and fractional polarization (Ka-band only) across the Spiderweb radio jet (rows a–d respectively). The eastern and western jet components are shown in the left-hand and right-hand columns respectively. The effective synthesized beam areas are shown in the lower-left corner in white. The location of the X-ray AGN core is indicated with blue concentric circles. The map of fractional polarization has been masked at a full-Ka-band signal-to-noise of 10 in the total intensity (from Anderson et al. 2022).
Figure 4. Maps of peak-P (X- plus Ka-band), source-frame RM (X- plus Ka-band; note that values saturate at RM = 2000 rad/m^2 to better reveal low RM structure throughout the jet), intrinsic sky-projected magnetic field orientation (Ka-band only), and fractional polarization (Ka-band only) across the Spiderweb radio jet (rows a–d respectively). The eastern and western jet components are shown in the left-hand and right-hand columns respectively. The effective synthesized beam areas are shown in the lower-left corner in white. The location of the X-ray AGN core is indicated with blue concentric circles. The map of fractional polarization has been masked at a full-Ka-band signal-to-noise of 10 in the total intensity (from Anderson et al. 2022).
Evidence for Inverse Compton and thermal diffuse emission in the Spiderweb Galaxy
The exquisite angular resolution of Chandra is key to investigate
the nature of the diffuse emission around the Spiderweb galaxies
within a ~12 arcsec radius (correspoding to ~100 kpc). After
obtaining a robust characterization of the unresolved nuclear
emission, we carefully compute the contamination in the
surrounding regions due to the wings of the instrument point
spread function and, eventually, quantify the truly extended
emission. Then, we use the JVLA radio image to identify the
regions overlapping the jets, and perform X-ray spectral analysis
separately in the jet regions and in the complementary area.
The emission in the jet regions is well described by a power law with
spectral index Γ~2-2.5, and it is consistent with Inverse-Compton
upscattering of the CMB photons by the relativistic electrons. The
isotropic emission identified outside the jet regions is significantly
softer, and consistent with thermal bremsstrahlung from hot
intracluster medium (ICM) with a temperature
kT=2.0+0.7-0.4 keV, and metallicity Z<1.6 Z
solar at 1 σ c.l. The average electron density within 100 kpc is
ne=(1.51±0.24±0.14)×10-2 cm-3,
corresponding an upper limit to the total ICM mass of (1.76±0.30
±0.17)×1012 M☉ (where error bars are
1 σ statistical and systematic, respectively). The rest-frame
luminosity L[0.5-10]keV = (2.0±0.5)×1044 erg/s is
about a factor of 2 higher than the extrapolated L-T relation for
massive clusters, but still consistent within the scatter. If we apply
hydrostatic equilibrium to the ICM, we measure a total gravitational
mass M(< 100
kpc)=(1.5+0.5-0.3)×1013 M☉
and, extrapolating at larger radii, we estimate a total mass
M500 =
(3.2+1.1-0.6)×1013 M☉
within a radius r500 = (220±30) kpc. Summarizing, our
analysis is consistent with the presence of hot, diffuse baryons that
may represent the embryonic virialized halo of the forming cluster.
Figure 5. Left: background-subtracted soft band image of the Spiderweb Galaxy after AGN subtraction. The image has been smoothed with a Gaussian kernel with a sigma of 1 pixel. The green and magenta boxes correspond to the regions used for the East and West jet spectral analysis, respectively. Two AGN circular extraction regions (in blue) are removed (the central AGN, with a radius of 2 arcsec, and a nearby AGN with a radius of 1.5 arcsec). The large green circle is the region used for the spectral analysis of the isotropic diffuse emission, excluding both jet regions and the AGNs. Red contours show radio emission observed in the 10 GHz band with the JVLA (Carilli et al. 2022) at levels of 0.03, 0.2, 2 and 20 mJy/beam.
Figure 6. Left: the spectrum (folded with the instrument spectral response) of the isotropic diffuse emission (after removing the central, AGN-dominated region, and the jet regions) fitted with a thermal mekal model plus the AGN contamination. Right: the unfolded spectrum with the two model components shown with dotted lines. The AGN contamination (absorbed power law) is dominant at energies larger than 1.5 keV, while the thermal component is dominant below 1 keV.
Figure 5. Left: background-subtracted soft band image of the Spiderweb Galaxy after AGN subtraction. The image has been smoothed with a Gaussian kernel with a sigma of 1 pixel. The green and magenta boxes correspond to the regions used for the East and West jet spectral analysis, respectively. Two AGN circular extraction regions (in blue) are removed (the central AGN, with a radius of 2 arcsec, and a nearby AGN with a radius of 1.5 arcsec). The large green circle is the region used for the spectral analysis of the isotropic diffuse emission, excluding both jet regions and the AGNs. Red contours show radio emission observed in the 10 GHz band with the JVLA (Carilli et al. 2022) at levels of 0.03, 0.2, 2 and 20 mJy/beam.
Figure 6. Left: the spectrum (folded with the instrument spectral response) of the isotropic diffuse emission (after removing the central, AGN-dominated region, and the jet regions) fitted with a thermal mekal model plus the AGN contamination. Right: the unfolded spectrum with the two model components shown with dotted lines. The AGN contamination (absorbed power law) is dominant at energies larger than 1.5 keV, while the thermal component is dominant below 1 keV.
First SZ detection of the Spiderweb ICM
Deep Band 3 (94.5−110.5 GHz) ALMA and ACA observations reveal for the first time
the detection (about ~6σ) of the thermal SZ effect in the direction of the Spiderweb protocluster.
This result indicates the presence of a nascent ICM within a halo at redshift z>2, around 10 billion years ago.
Considering the assumption that the SZ signal is generated by a spherically symmetric ICM distribution and that
the extended central radio source can be described by a collection of point-like components, the derived halo properties
include M500=3.46−0.43+0.38✕1013M☉ and r500=228.9−9.5+8.4 kpc. The SZ centroid is offset by 6.2±1.3 arcsec (53 kpc) from the central Spiderweb
galaxy and associated AGN, consistent with an inhomogeneous and not yet centrally concentrated ICM. Also, the SZ footprint of intracluster gas should
be expected to extend over characteristic scales >10''.
The SZ signal’s volume-integrated flux YSZ(<5r500) is 1.68−0.32+0.35✕10
−6 Mpc2, lower than expected from dynamical mass estimates.
This discrepancy suggests dynamical disturbances in the ICM, due to interacting subhalos.
Comparisons with hydrodynamical simulations indicate a mass range of M500∼(2−5)✕1013 M☉, consistent with protocluster progenitors of massive clusters.
These results are providing a statistically meaningful confirmation of long-standing predictions from cosmological simulations
for the existence of an extended halo of thermalizing ICM within the Spiderweb protocluster, as well as of observational works,
so far limited just to indirect evidence or tentative detections. Also, this suggests that the ICM is still forming while
turbulent assembly processes take place in the Spiderweb protocluster.
Figure 7. Multiwavelength view of the Spiderweb complex. Composite Hubble Space Telescope image based on the ACS/WFC F475W and F814W data of the Spiderweb field. Overlaid are the emission from the Spiderweb galaxy and associated extended radio jet as measured by the JVLA in X-band (Carilli et al. 2022, Anderson et al. 2022) (8 − 10 GHz; red), the image of the extended LyΑ nebula (Miley et al. 2006, Kurk et al. 2000) observed with the FORS1 instrument on the Very Large Telescope (VLT; pink), and the SZ signal from a combined ALMA+ACA image (light blue). When imaging the ALMA+ACA data, we applied to the visibility weights a uv-taper with σtaper = 20 kλ to both suppress any noise structures on small scales and to emphasise the bulk distribution of the SZ signal from the ICM of the Spiderweb protocluster. The SZ effect offset with respect to the Spiderweb galaxy suggests that the protocluster core is undergoing a dynamically active phase. The white diamonds denote all the spectroscopically confirmed member galaxies summarized in Tozzi et al. 2022a and references therein.
Figure 7. Multiwavelength view of the Spiderweb complex. Composite Hubble Space Telescope image based on the ACS/WFC F475W and F814W data of the Spiderweb field. Overlaid are the emission from the Spiderweb galaxy and associated extended radio jet as measured by the JVLA in X-band (Carilli et al. 2022, Anderson et al. 2022) (8 − 10 GHz; red), the image of the extended LyΑ nebula (Miley et al. 2006, Kurk et al. 2000) observed with the FORS1 instrument on the Very Large Telescope (VLT; pink), and the SZ signal from a combined ALMA+ACA image (light blue). When imaging the ALMA+ACA data, we applied to the visibility weights a uv-taper with σtaper = 20 kλ to both suppress any noise structures on small scales and to emphasise the bulk distribution of the SZ signal from the ICM of the Spiderweb protocluster. The SZ effect offset with respect to the Spiderweb galaxy suggests that the protocluster core is undergoing a dynamically active phase. The white diamonds denote all the spectroscopically confirmed member galaxies summarized in Tozzi et al. 2022a and references therein.
Structure and thermodynamical properties of the Spiderweb ICM
When X-ray and SZ data are available for the same object, thanks to their different
dependence on the electron density, the joint analysis of both observables
allows us to achieve an improved description of the thermodynamic properties
of the ICM. Combining deep X-ray Chandra data and SZ effect ALMA data is crucial to analyze the structure
and properties of the thermal, diffuse emission detected in the halo of the Spiderweb
Galaxy within a radius of ~150 kpc.
Thanks to independent measurements of the pressure profile from the ALMA SZ observation and the electron
density profile from the available X-ray data, Lepore et al. 2024 derived, for the first time,
the temperature profile in the ICM of a z>2 protocluster. It reveals the presence of a strong
cool core (comparable to local ones) that may host a significant mass deposition flow, consistent
with the measured local star formation values.
They also find mild evidence of an asymmetry in the X-ray surface brightness distribution,
which may be tentatively associated with a cavity carved into the proto-ICM by the radio jets.
In this case, the estimated average feedback power would be in excess of ~1043 erg/s.
The cooling time of baryons in the core of the Spiderweb Protocluster is estimated to be
~0.1 Gyr, implying that the baryon cycle in the first stages of protocluster formation
is characterized by a high-duty cycle and a very active environment. In the case of the Spiderweb
protocluster, the results suggest the presence of a strongly peaked core that is possibly
hosting a cooling flow with a mass deposition rate up to 250-1000 M☉/year,
responsible for feeding both the central SMBH and the high star formation rate observed
in the Spiderweb Galaxy. This phase is expected to be rapidly followed by AGN feedback events,
whose onset may have already left an imprint in the radio and X-ray appearance of the
Spiderweb protocluster, eventually driving the ICM into a self-regulated, long-term evolution
in less than one Gyr.
Figure 8. AGN-subtracted soft band image of the Spiderweb protocluster (left). The blue concentric circles, which correspond to 2, 4, 7, 10, 13, and 17 arcsec, and the blue lines separate the quadrants within which we derived the surface brightness as a function of the angle. Magenta squares represent the eastern and western jet excluded regions, while the green circles represent the excluded unresolved sources in the field of view. Surface-brightness profiles as a function of the distance from the central radio source (right). The grey-shaded area represents the surface brightness values parametrized with nd.
Figure 9. Electron density as a function of the radius for different nd values (left). The profiles differ significantly only in the central ~2 arcsec in which we apply the parametrization with nd. Temperature profile as a function of the distance from the central radio source for different nd values (right). The solid black line represents the constant temperature value of the ICM found by Tozzi et al. 2022b, assuming an isothermal profile, while the shaded region mark the 1σ errors associated with the temperature.
Figure 8. AGN-subtracted soft band image of the Spiderweb protocluster (left). The blue concentric circles, which correspond to 2, 4, 7, 10, 13, and 17 arcsec, and the blue lines separate the quadrants within which we derived the surface brightness as a function of the angle. Magenta squares represent the eastern and western jet excluded regions, while the green circles represent the excluded unresolved sources in the field of view. Surface-brightness profiles as a function of the distance from the central radio source (right). The grey-shaded area represents the surface brightness values parametrized with nd.
Figure 9. Electron density as a function of the radius for different nd values (left). The profiles differ significantly only in the central ~2 arcsec in which we apply the parametrization with nd. Temperature profile as a function of the distance from the central radio source for different nd values (right). The solid black line represents the constant temperature value of the ICM found by Tozzi et al. 2022b, assuming an isothermal profile, while the shaded region mark the 1σ errors associated with the temperature.
Last modified: November 27, 2024