arXiv subjects

M. Villar-Martín

Publications and source records attributed to M. Villar-Martín.

At least 19 recordsLinked to original sources

arXiv2026

High-resolution radio imaging of TGSSJ1530+1049, a radio galaxy in a dense environment at z=4

High-redshift radio galaxies can provide insights into the structure formation and galaxy evolution at earlier cosmological epochs. TGSSJ1530+1049 was selected as a candidate high-redshift radio galaxy. Subsequent observations with the James Webb Space Telescope (JWST) presented in a companion paper (Saxena et al., 2026) have shown that it is located at a redshift z=4.0. The JWST data furthermore showed that the radio source is part of one of the densest structures of galaxies and ionized gas known at these redshifts. The complex system qualitatively resembles a massive (cluster) galaxy forming early through a rapid succession of mergers. TGSSJ1530+1049 is an unresolved source down to ~0.6" scale in multiple radio surveys. To reveal its high-resolution radio structure and allow for a detailed comparison with JWST observations, we studied its morphology at various angular scales with different radio interferometric instruments. We observed TGSSJ1530+1049 at milliarcsecond (mas) scale angular resolution with the European VLBI Network (EVN), and at ~100-mas scale resolution with the enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN). We recovered a complex north--south oriented structure with steep-spectrum radio-emitting features, which are associated with lobes and hot spots of a jetted active galactic nucleus. The centre of the radio galaxy proved to be too faint at cm wavelengths to be unambiguously detected in our observations. Nevertheless, the linear size (~5.5 kpc) and the radio power place it among the so-called medium-sized symmetric objects, a smaller and/or confined version of larger radio galaxies. Comparison between the radio morphology and that of the ionized gas observed by JWST shows that the two are closely aligned. However, the optical emission line gas extends out to ~25 kpc, which is well beyond the detected radio structures. (Abridged)

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arXiv2026

The Galaxy Activity, Torus, and Outflow Survey (GATOS): N. Unveiling physical processes in local active galaxies. Unsupervised hierarchical clustering of JWST MIRI/MRS observations

With the rise of the integral field spectroscopy, we are currently dealing with large amounts of spatially resolved data, whose analysis has become challenging, especially when observing complex objects such as nearby galaxies. We aim to develop a method to automatically separate different physical regions within the central parts (1"~160 pc, on average) of galaxies. This can allow us to better understand the systems, and provide an initial characterisation of the main ionisation sources affecting its evolution. We have developed an unsupervised hierarchical clustering algorithm to analyse data cubes based on spectral similarity. It clusters together spaxels with similar spectra, which is useful to disentangle between different physical processes. We have applied this method to a sample of 15 nearby (distances <100 Mpc) galaxies, 7 from the Galaxy Activity, Torus, and Outflow Survey (GATOS) and 8 archival sources, all observed with the medium resolution spectrometer (MRS) of the Mid-Infrared Instrument (MIRI) on board of the JWST. From the clusters, we computed their median spectrum and measured the line and continuum properties. We used these measurements to train random forest models and create several empirical mid-IR diagnostic diagrams for the MRS channel 3 wavelength range, including among others the bright [Ne II], [Ne III], and [Ne V] lines, several H2 transitions, and PAH features. The clustering technique allows to differentiate emission coming from an AGN, the disc, and star forming regions in galaxies, and other composite regions, potentially ionised by several sources simultaneously. This is supported by the results from the empirical diagnostic diagrams, that are indeed able to separate physically distinct regions. This innovative method serves as a tool to identify regions of interest in any data cube prior to an in-depth analysis of the sources. [abridged]

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arXiv2024

A biconical ionised gas outflow and evidence for positive feedback in NGC 7172 uncovered by MIRI/JWST

We present observations of the type-2 Seyfert NGC7172 obtained with the medium-resolution spectrometer (MRS) of the Mid-Infrared Instrument (MIRI) on board of the JWST. This galaxy hosts one of the lowest ionised gas mass outflow rates (Mout~0.005 M/yr) in a sample of six AGN with similar bolometric luminosities (log Lbol~44erg/s) within the Galactic Activity, Torus and Outflow Survey (GATOS). We aim to understand the properties of the ionised gas outflow, mainly using the emission lines from the neon transitions, that cover a broad range of ionisation potentials (IP) from ~20 eV to ~130 eV. We applied parametric and non-parametric methods to characterise the line emission and kinematics. The low excitation lines (IP<25eV, e.g.[NeII]) trace the rotating disc emission. The high excitation lines (IP>90eV, e.g.[NeV]), which are likely photoionised exclusively by the AGN, are expanding in the direction nearly perpendicular to the disc of the galaxy, with maximum projected velocities of ~350-500 km/s. In particular, [NeV] and [NeVI] lines reveal a biconical ionised gas outflow emerging N-S from the nuclear region, extending at least ~2.5"N and 3.8"S (projected distance of ~450 and 680 pc). Most of the emission arising in the northern part of the cone was not previously detected due to obscuration. Given the almost face-on orientation of the outflow and the almost edge-on orientation of the galaxy, NGC7172 may be a case of weak coupling. Nevertheless, we found evidence for positive feedback in two distinct outflowing clumps at projected distances of 3.1" and 4.3" (i.e. ~560 and 780 pc) SW from the AGN. We estimated a star formation rate in these regions using the [NeII] and [NeIII] luminosities of 0.08 M/yr, that is ~10% of that found in the circumnuclear ring. The star formation activity might have been triggered by the interaction between the ionised gas outflow and the ISM of the galaxy.

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arXiv2023

A radio-jet driven outflow in the Seyfert 2 galaxy NGC 2110?

We present a spatially-resolved study of the ionised gas in the central 2 kpc of the Seyfert 2 galaxy NGC 2110 and investigate the role of its moderate luminosity radio jet (kinetic radio power of $P_\mathrm{jet} = 2.3 \times 10^{43}\mathrm{erg\ s^{-1}}$). We use new optical integral-field observations taken with the MEGARA spectrograph at GTC. We fit the emission lines with a maximum of two Gaussian components, except at the AGN position where we used three. Aided by existing stellar kinematics, we use the observed velocity and velocity dispersion of the emission lines to classify the different kinematic components. The disc component is characterised by lines with $σ\sim 60-200\ \mathrm{km\ s^{-1}}$. The outflow component has typical values of $σ\sim 700\ \mathrm{km\ s^{-1}}$ and is confined to the central 400 pc, which is coincident with linear part of the radio jet detected in NGC 2110. At the AGN position, the [O III]$λ$5007 line shows high velocity components reaching at least $1000\ \mathrm{km\ s^{-1}}$. This and the high velocity dispersions indicate the presence of outflowing gas outside the galaxy plane. Spatially-resolved diagnostic diagrams reveal mostly LI(N)ER-like excitation in the outflow and some regions in the disc, which could be due to the presence of shocks. However, there is also Seyfert-like excitation beyond the bending of the radio jet, probably tracing the edge of the ionisation cone that intercepts with the disc of the galaxy. NGC 2110 follows well the observational trends between the outflow properties and the jet radio power found for a few nearby Seyfert galaxies. All these pieces of information suggest that part of observed ionised outflow in NGC 2110 might be driven by the radio jet. However, the radio jet was bent at radial distances of 200 pc (in projection) from the AGN, and beyond there, most of the gas in the galaxy disc is rotating.

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arXiv2023

Limited impact of jet induced feedback in the multi-phase nuclear interstellar medium of 4C12.50

Although the ultraluminous infrared radio galaxy 4C12.50 at z=0.12 is a promising candidate to reveal how radio induced feedback may regulate star formation in galaxies, we find no solid evidence for current or past impact of this mechanism on the evolution of this system, neither by clearing out the dusty central cocoon efficiently, nor by suppressing star formation. We study in detail for the first time the hot (>~1500 K) molecular gas in this object. The potential impact of the radio jet on this gas phase, as well as on the star formation activity, are investigated. 4C12.50 hosts (2.1+/-0.4)x1e4 Msun of hot molecular gas. An unusually high rotational temperature T =3020+/-160 K is inferred. The molecular gas mass obeys a power law temperature distribution d(M(H2))/dT ~ T^-5 from T~300 K and up to ~3000 K. Both results support that shocks (probably induced by the radio jet) contribute to the heating and excitation of the hot molecular gas. A molecular outflow is not detected. The coupling of the outflowing ionized and neutral outflows with the hot molecular gas is poor. We find no evidence for star formation supression. NIR and MIR integral field spectroscopy at very high spatial resolution (for instance, with the JWST) would be of key value to further investigate these issues.

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arXiv2022

Compact molecular gas emission in local LIRGs among low- and high-z galaxies

We present new CO(2-1) observations of a representative sample of 24 local (z$<$0.02) luminous infrared galaxies (LIRGs) obtained at high spatial resolution ($<$100 pc) from ALMA. We derive the effective radii of the CO(2-1) and the 1.3 mm continuum emissions using the curve-of-growth method. LIRGs show an extremely compact cold molecular gas distribution (median R$_{CO}$ $\sim$0.7 kpc), which is a factor 2 smaller than the ionized gas, and 3.5 times smaller than the stellar size. The molecular size of LIRGs is similar to that of early-type galaxies (R$_{CO}\sim$1 kpc) and about a factor of 6 more compact than local Spirals of similar stellar mass. Only the CO emission in low-z ULIRGs is more compact than these local LIRGs by a factor of 2. Compared to high-z (1$<$z$<$6) systems, the stellar sizes and masses of local LIRGs are similar to those of high-z MS star-forming galaxies (SFG) and about a factor of 2-3 lower than sub-mm galaxies (SMG). The molecular sizes of high-z MS SFGs and SMGs are larger than those derived for LIRGs by a factor of $\sim$3 and $\sim$8, respectively. These results indicate that while low-z LIRGs and high-z MS-SFGs have similar stellar masses and sizes, the regions of current star formation (traced by the ionized gas) and of potential star-formation (traced by the molecular gas) are substantially smaller in LIRGs, and constrained to the central kpc region. High-z galaxies represent a wider population but their star-forming regions are more extended, even covering the overall size of the host galaxy. High-z galaxies have larger fractions of gas than low-z LIRGs, and therefore the formation of stars could be induced by interactions and mergers in extended disks or filaments with large enough molecular gas surface density involving physical mechanisms similar to those identified in the central kpc of LIRGs.

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arXiv2021

Duality in spatially resolved star-formation relations in local LIRGs

We analyse the star formation (SF) relations in a sample of 16 nearby luminous infrared galaxies (LIRGs) with more than 2800 regions defined on scales of 90 to 500 pc. We used ALMA to map the distribution of the cold molecular gas traced by the J = 2-1 line of CO and archival PaαHST/NICMOS imaging to trace the recent SF. In four objects, we find two different branches in the Kennicutt-Schmidt relation at 90 pc scales, suggesting the existence of a duality in this relation. The two branches correspond to two different dynamical environments within each galaxy. One branch, which corresponds to the central region of these galaxies 90% of the regions are located at radii < 0.85 kpc), shows higher gas and star formation rate surface densities with higher velocity dispersion. The other branch, which shows lower molecular gas and SF rate surface densities, corresponds to the more external disk regions (r $\sim$ 1 kpc). Despite the scatter, the SF efficiency of the galaxies with a dual behaviour increases with increasing boundedness as measured by the b parameter (b $\equiv$ $Σ_{H_{2}}$/$σ^{2}$ $\propto$ $α_{vir}^{-1}$). At larger spatial scales (250 and 500 pc), the duality disappears. The rest of the sample does not show evidence of this dual behaviour at any scale.

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arXiv2019

Discovery of a giant and luminous Lya+CIV+HeII nebula at z=3.326 with extreme emission line ratios

We present the discovery of HLock01-LAB, a luminous and large Lya nebula at z=3.326. Medium-band imaging and long-slit spectroscopic observations with the Gran Telescopio Canarias reveal extended emission in the Lya 1215Å, CIV1550Å, and HeII 1640Ålines over ~100kpc, and a total luminosity L(Lya)=(6.4+/-0.1)x10^44 erg s^-1. HLock01-LAB presents an elongated morphology aligned with two faint radio sources contained within the central ~8kpc of the nebula. The radio structures are consistent to be faint radio jets or lobes of a central galaxy, whose spectrum shows nebular emission characteristic of a type-II active galactic nucleus (AGN). The continuum emission of the AGN at short wavelengths is, however, likely dominated by stellar emission of the host galaxy, for which we derive a stellar mass M* = 2.3x10^11 Msun. The detection of extended emission in CIV and CIII] indicates that the gas within the nebula is not primordial. Feedback may have enriched the halo at at least 50 kpc from the nuclear region. Using rest-frame UV emission-line diagnostics, we find that the gas in the nebula is likely heated by the AGN. Nevertheless, at the center of the nebula we find extreme emission line ratios of Lya/CIV~60 and Lya/HeII~80, one of the highest values measured to date, and well above the standard values of photoionization models (Lya/HeII~30 for case B photoionization). Our data suggest that jet-induced shocks are likely responsible for the increase of the electron temperature and, thus, the observed Lya enhancement in the center of the nebula. This scenario is further supported by the presence of radio structures and perturbed kinematics in this region. The large Lya luminosity in HLock01-LAB is likely due to a combination of AGN photoionization and jet-induced shocks, highlighting the diversity of sources of energy powering Lya nebulae. [abridged]

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arXiv2018

Photoionization models for extreme Ly$α$ $λ$1216 and HeII $λ$1640 ratios in quasar halos, and PopIII vs AGN diagnostics

We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or to enhance the observed number of Ly-alpha photons per incident ionizing photon, in extended AGN-photoionized nebulae at high-redshift. Using photoionization models, we explore the impact of ionization parameter, gas metallicity, ionizing spectrum, electron energy distribution, and cloud viewing angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the observed number of Ly-alpha photons per incident ionizing photon. We find that low ionization parameter, a relatively soft or filtered ionizing spectrum, low gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha photons by HI can all result in significantly enhanced Ly-alpha relative to other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing spectrum, the observed number of Ly-alpha photons per incident ionizing photon is itself significantly enhanced above the nominal Case B value of 0.66 due to collisional excitation, reaching values up to 5.3 in our 'extreme case' model. At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is predominantly via collisional excitation rather than recombination. In addition, we find that collisional excitation of Ly-alpha becomes more efficient if the ionizing continuum is pre-filtered through an optically thin screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of the z~3.5 quasars studied by Borisova et al. (2016) are consistent with AGN-photoionization of gas with moderate to low metallicity and/or low ionization parameter, without requiring exotic ionization/excitation mechanisms such as strong line-transfer effects. We also present UV-optical diagnostic diagrams to distinguish between photoionization by Pop III stars and AGN photoionization.

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arXiv2017

The MUSE 3D view of feedback in a high-metallicity radio galaxy at z = 2.9

We present a detailed study of the kinematic, chemical and excitation properties of the giant Ly$α$ emitting nebula and the giant \ion{H}{I} absorber associated with the $z = 2.92$ radio galaxy MRC 0943--242, using spectroscopic observations from VLT/MUSE, VLT/X-SHOOTER and other instruments. Together, these data provide a wide range of rest-frame wavelength (765 Å$\,$ -- 6378 Å$\,$ at $z = 2.92$) and 2D spatial information. We find clear evidence for jet gas interactions affecting the kinematic properties of the nebula, with evidence for both outflows and inflows being induced by radio-mode feedback. We suggest that the regions of relatively lower ionization level, spatially correlated with the radio hotspots, may be due to localised compression of photoionized gas by the expanding radio source, thereby lowering the ionization parameter, or due to a contribution from shock-heating. We find that photoionization of super-solar metallicity gas ($Z/Z_{\odot}$ = 2.1) by an AGN-like continuum ($α$=--1.0) at a moderate ionization parameter ($U$ = 0.018) gives the best overall fit to the complete X-SHOOTER emission line spectrum. We identify a strong degeneracy between column density and Doppler parameter such that it is possible to obtain a reasonable fit to the \ion{H}{I} absorption feature across the range log N(\ion{H}{I}/cm$^{-2}$) = 15.20 and 19.63, with the two best-fitting occurring near the extreme ends of this range. The extended \ion{H}{I} absorber is blueshifted relative to the emission line gas, but shows a systematic decrease in blueshift towards larger radii, consistent with a large scale expanding shell.

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arXiv2017

The spatially resolved stellar population and ionized gas properties in the merger LIRG NGC 2623

We report on a detailed study of the stellar populations and ionized gas properties in the merger LIRG NGC 2623, analysing optical Integral Field Spectroscopy from the CALIFA survey and PMAS LArr, multiwavelength HST imaging, and OSIRIS narrow band H$α$ and [NII]$λ$6584 imaging. The spectra were processed with the STARLIGHT full spectral fitting code, and the results compared with those for two early-stage merger LIRGs (IC 1623 W and NGC 6090), together with CALIFA Sbc/Sc galaxies. We find that NGC 2623 went through two periods of increased star formation (SF), a first and widespread episode, traced by intermediate-age stellar populations ISP (140 Myr-1.4 Gyr), and a second one, traced by young stellar populations YSP ($<$140 Myr), which is concentrated in the central regions ($<$1.4 kpc). Our results are in agreement with the epochs of the first peri-center passage ($\sim$200 Myr ago) and coalescence ($<$100 Myr ago) predicted by dynamical models, and with high resolution merger simulations in the literature, consistent with NGC 2623 representing an evolved version of the early-stage mergers. Most ionized gas is concentrated within $<$2.8 kpc, where LINER-like ionization and high velocity dispersion ($\sim$220 km/s) are found, consistent with the previously reported outflow. As revealed by the highest resolution OSIRIS and HST data, a collection of HII regions is also present in the plane of the galaxy, which explains the mixture of ionization mechanisms in this system. It is unlikely that the outflow in NGC 2623 will escape from the galaxy, given the low SFR intensity ($\sim$0.5 M$_{\odot}$yr$^{-1}$kpc$^{-2}$), the fact that the outflow rate is 3 times lower than the current SFR, and the escape velocity in the central areas higher than the outflow velocity.

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arXiv2016

Ionization and feedback in Ly$α$ halos around two radio galaxies at z$\sim$2.5

We present new spectroscopic observations of two high redshift radio galaxies, TXS 0211-122 (z=2.34) and TXS 0828+193 (z=2.57), known to be associated with large Ly$α$ halos. The observations were taken with the slits placed perpendicularly to the radio axis. With access to pre-existing Keck II observations taken with the slit placed along the radio axis we are able to compare the properties of the gas in different regions of the galaxies. In both objects we detect spatially extended Ly$α$ emission perpendicularly to the radio axis. In TXS 0211-122, the flux and velocity profiles of Ly$α$ are strongly affected by HI absorption/scattering. In line with previous studies, we find evidence for outflowing gas along the radio axis which may be the result of jet-gas interactions. In the slit oriented perpendicularly to the radio axis we find less perturbed gas kinematics, suggesting outflows of ionized gas in this object are focused along the radio jet axis. Additionally, we find evidence for a giant, UV-emitting arc or shell-like structure surrounding the radio galaxy Ly$α$ halo, possibly resulting from feedback activity. In TXS 0828+193 a large Ly$α$ halo ($\sim$56 kpc) is detected perpendicularly to the radio axis. Along both slit position angles we find evidence for outflowing gas, which we argue is part of an approximately spherical, expanding shell or bubble of gas powered by feedback activity in the central regions of the galaxy. Our results suggest a diversity in the spatial distribution of ionized outflows in powerful radio galaxies at z$\sim$2.5.

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arXiv2016

High-velocity extended molecular outflow in the star-formation dominated luminous infrared galaxy ESO 320-G030

We analyze new high spatial resolution (~60 pc) ALMA CO(2-1) observations of the isolated luminous infrared galaxy ESO 320-G030 (d=48 Mpc) in combination with ancillary HST optical and near-IR imaging as well as VLT/SINFONI near-IR integral field spectroscopy. We detect a high-velocity (~450 km/s) spatially resolved (size~2.5 kpc; dynamical time ~3 Myr) massive (~10^7 Msun; mass rate~2-8 Msun/yr) molecular outflow originated in the central ~250 pc. We observe a clumpy structure in the outflowing cold molecular gas with clump sizes between 60 and 150 pc and masses between 10^5.5 and 10^6.4 Msun. The mass of the clumps decreases with increasing distance, while the velocity is approximately constant. Therefore, both the momentum and kinetic energy of the clumps decrease outwards. In the innermost (~100 pc) part of the outflow, we measure a hot-to-cold molecular gas ratio of 7x10^-5, which is similar to that measured in other resolved molecular outflows. We do not find evidence of an ionized phase in this outflow. The nuclear IR and radio properties are compatible with strong and highly obscured star-formation (A_k ~ 4.6 mag; SFR~15 Msun/yr). We do not find any evidence for the presence of an active galactic nucleus. We estimate that supernova explosions in the nuclear starburst (ν(SN) ~ 0.2 yr^-1) can power the observed molecular outflow. The kinetic energy and radial momentum of the cold molecular phase of the outflow correspond to about 2% and 20%, respectively, of the supernovae output. The cold molecular outflow velocity is lower than the escape velocity, so the gas will likely return to the galaxy disk. The mass loading factor is ~0.1-0.5, so the negative feedback due to this star-formation powered molecular outflow is probably limited.

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arXiv2016

Sub-kpc star-formation law in the local luminous infrared galaxy IC 4687 as seen by ALMA

We analyze the spatially resolved (250 pc scales) and integrated star-formation (SF) law in the local luminous infrared galaxy (LIRG) IC4687. This is one of the first studies of the SF law on a starburst LIRG at these small spatial scales. We combined new interferometric ALMA CO(2-1) data with existing HST/NICMOS Pa$α$ narrow-band imaging and VLT/SINFONI near-IR integral field spectroscopy to obtain accurate extinction corrected SF rate (SFR) and cold molecular gas surface densities ($Σ_{gas}$ and $Σ_{SFR}$). We find that IC4687 forms stars very efficiently with an average depletion time ($t_{dep}$) of 160 Myr for the individual 250 pc regions. This is approximately one order of magnitude shorter than the $t_{dep}$ of local normal spirals and also shorter than that of main-sequence high-z objects, even when we use a Galactic $α_{CO}$ conversion factor. This result suggests a bimodal SF law in the $Σ_{SFR} \propto Σ_{gas}^{N}$ representation. A universal SF law is recovered if we normalize the $Σ_{gas}$ by the global dynamical time. However, at the spatial scales studied here, we find that the SF efficiency (or $t_{dep}$) does not depend on the local dynamical time for this object. Therefore, an alternative normalization (e.g., free-fall time) should be found if a universal SF law exists at these scales.

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arXiv2015

ALMA unveils a triple merger and gas exchange in a hyper-luminous radio galaxy at z=2: the Dragonfly Galaxy (II)

The Dragonfly Galaxy (MRC0152-209), at redshift z~2, is one of the most vigorously star-forming radio galaxies in the Universe. What triggered its activity? We present ALMA Cycle 2 observations of cold molecular CO(6-5) gas and dust, which reveal that this is likely a gas-rich triple merger. It consists of a close double nucleus (separation ~4 kpc) and a weak CO-emitter at ~10 kpc distance, all of which have counterparts in HST/NICMOS imagery. The hyper-luminous starburst and powerful radio-AGN were triggered at this precoalescent stage of the merger. The CO(6-5) traces dense molecular gas in the central region, and complements existing CO(1-0) data, which revealed more widespread tidal debris of cold gas. We also find ~10$^{10}$ M(sun) of molecular gas with enhanced excitation at the highest velocities. At least 20-50% of this high-excitation, high-velocity gas shows kinematics that suggests it is being displaced and redistributed within the merger, although with line-of-sight velocities of |v| < 500 km/s, this gas will probably not escape the system. The processes that drive the redistribution of cold gas are likely related to either the gravitational interaction between two kpc-scale discs, or starburst/AGN-driven outflows. We estimate that the rate at which the molecular gas is redistributed is at least ~1200 +- 500 M(sun)/yr, and could perhaps even approach the star formation rate of ~3000 +- 800 M(sun)/yr. The fact that the gas depletion and gas redistribution timescales are similar implies that dynamical processes can be important in the evolution of massive high-z galaxies.

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arXiv2015

VLT-FORS2 optical imaging and spectroscopy of 9 luminous type 2 AGN at 0.3<z<0.6: I. Ionized gas nebulae

We present optical imaging and long slit spectroscopic observations of 9 luminous type 2 AGNs within the redshift range 0.3<z<0.6 based on VLT-FORS2 data. Most objects (6/9) are high luminosity Seyfert 2, and three are type 2 quasars (QSO2), with our sample extending to lower luminosity than previous works. Seven out of nine objects (78%) show morphological evidence for interactions or mergers in the form of disturbed morphologies and/or peculiar features such as tidal tails, amorphous halos, or compact emission line knots. The detection rate of morphological evidence for interaction is consistent with those found during previous studies of QSO2 at similar z, suggesting that the merger rate is independent of AGN power at the high end of the AGN luminosity function. We find the emission line flux spatial profiles are often dominated by the often spatially unresolved central source. In addition, all but one of our sample is associated with much fainter, extended line emission. We find these extended emission line structures have a variety of origins and ionization mechanisms: star forming companions, tidal features, or extended ionized nebulae. AGN related processes dominate the excitation of the nuclear gas. Stellar photoionization sometimes plays a role in extended structures often related to mergers/interactions.

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arXiv2015

GTC optical imaging of extremely red 5C radio galaxies at high redshift

We investigate the nature of seven unusual radio galaxies from the 5C catalogue that were previously known to have extremely red R-K colours, and for which emission lines were previously found to be weak or absent in their optical spectra. We present and discuss u, g, or r images of these radio galaxies, obtained using the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at the Gran Telescopio Canarias (GTC). We have detected all seven targets in our g-band imaging. Their optical emission is extended, and we tentatively detect a radio-optical alignment effect in this sample. A subset of our sample (three sources) shows broad-band spectral energy distributions that flatten out near the wavelength range of the g-band, implying a dominant contribution there due to young stars and/or scattered or reprocessed radiation from the active nucleus.

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arXiv2015

Witnessing the transformation of a quasar host galaxy at z=1.6

A significant minority of high redshift radio galaxy (HzRG) candidates show extremely red broad band colours and remain undetected in emission lines after optical `discovery' spectroscopy. In this paper we present deep GTC optical imaging and spectroscopy of one such radio galaxy, 5C 7.245, with the aim of better understanding the nature of these enigmatic objects. Our g-band image shows no significant emission coincident with the stellar emission of the host galaxy, but does reveal faint emission offset by ~3" (26 kpc) therefrom along a similar position angle to that of the radio jets, reminiscent of the `alignment effect' often seen in the optically luminous HzRGs. This offset g-band source is also detected in several UV emission lines, giving it a redshift of 1.609, with emission line flux ratios inconsistent with photoionization by young stars or an AGN, but consistent with ionization by fast shocks. Based on its unusual gas geometry, we argue that in 5C 7.245 we are witnessing a rare (or rarely observed) phase in the evolution of quasar hosts when stellar mass assembly, accretion onto the back hole, and powerful feedback activity has eradicated its cold gas from the central ~20 kpc, but is still in the process of cleansing cold gas from its extended halo.

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