arXiv subjects

M. A. Prieto

Publications and source records attributed to M. A. Prieto.

At least 37 records · Page 2Linked to original sources

arXiv2017

ALMA polarization observations of the particle accelerators in the hot spot of the radio galaxy 3C 445

We present Atacama Large Millimeter Array (ALMA) polarization observations at 97.5 GHz of the southern hot spot of the radio galaxy 3C 445. The hot spot structure is dominated by two bright components enshrouded by diffuse emission. Both components show fractional polarization between 30 and 40 per cent, suggesting the presence of shocks. The polarized emission of the western component has a displacement of about 0.5 kpc outward with respect to the total intensity emission, and may trace the surface of a front shock. Strong polarization is observed in a thin strip marking the ridge of the hot spot structure visible from radio to optical. No significant polarization is detected in the diffuse emission between the main components, suggesting a highly disordered magnetic field likely produced by turbulence and instabilities in the downstream region that may be at the origin of the extended optical emission observed in this hot spot. The polarization properties support a scenario in which a combination of both multiple and intermittent shock fronts due to jet dithering, and spatially distributed stochastic second-order Fermi acceleration processes are present in the hot spot complex.

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arXiv2016

The central parsecs of M87: jet emission and an elusive accretion disc

We present the first simultaneous spectral energy distribution (SED) of M87 core at a scale of 0.4 arcsec ($\sim 32\, \rm{pc}$) across the electromagnetic spectrum. Two separate, quiescent, and active states are sampled that are characterized by a similar featureless SED of power-law form, and that are thus remarkably different from that of a canonical active galactic nuclei (AGN) or a radiatively inefficient accretion source. We show that the emission from a jet gives an excellent representation of the core of M87 core covering ten orders of magnitude in frequency for both the active and the quiescent phases. The inferred total jet power is, however, one to two orders of magnitude lower than the jet mechanical power reported in the literature. The maximum luminosity of a thin accretion disc allowed by the data yields an accretion rate of $< 6 \times 10^{-5}\, \rm{M_\odot \, yr^{-1}}$, assuming 10% efficiency. This power suffices to explain M87 radiative luminosity at the jet-frame, it is however two to three order of magnitude below that required to account for the jet's kinetic power. The simplest explanation is variability, which requires the core power of M87 to have been two to three orders of magnitude higher in the last 200 yr. Alternatively, an extra source of power may derive from black hole spin. Based on the strict upper limit on the accretion rate, such spin power extraction requires an efficiency an order of magnitude higher than predicted from magnetohydrodynamic simulations, currently in the few hundred per cent range.

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arXiv2016

The nuclear dust lane of Circinus: collimation without a torus

In some AGN, nuclear dust lanes connected to kpc-scale dust structures provide all the extinction required to obscure the nucleus, challenging the role of the dusty torus proposed by the Unified Model. In this letter we show the pc-scale dust and ionized gas maps of Circinus constructed using sub-arcsec-accuracy registration of infrared VLT AO images with optical \textit{Hubble Space Telescope} images. We find that the collimation of the ionized gas does not require a torus but is caused by the distribution of dust lanes of the host galaxy on $\sim$10 pc scales. This finding questions the presumed torus morphology and its role at parsec scales, as one of its main attributes is to collimate the nuclear radiation, and is in line with interferometric observations which show that most of the pc-scale dust is in the polar direction. We estimate that the nuclear dust lane in Circinus provides $1/3$ of the extinction required to obscure the nucleus. This constitutes a conservative lower limit to the obscuration at the central parsecs, where the dust filaments might get optically thicker if they are the channels that transport material from $\sim$100 pc scales to the centre.

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arXiv2015

The warm molecular gas and dust of Seyfert galaxies: two different phases of accretion?

The distribution of warm molecular gas (1000--3000 K), traced by the near-IR H$_2$ 2.12 $μ$m line, has been imaged with a resolution $<0.5$ arcsec in the central 1 kpc of seven nearby Seyfert galaxies. We find that this gas is highly concentrated towards the central 100 pc and that its morphology is often symmetrical. Lanes of warm H$_2$ gas are observed only in three cases (NGC\,1068, NGC\,1386 and Circinus) for which the morphology is much wider and extended than the dust filaments. We conclude that there is no one-to-one correlation between dust and warm gas. This indicates that, if the dust filaments and lanes of warm gas are radial streaming motions of fuelling material, they must represent \textit{two different phases of accretion}: the dust filaments represent a colder phase than the gas close to the nucleus (within $\sim$100 pc). We predict that the morphology of the nuclear dust at these scales should resemble that of the cold molecular gas (e.g. CO at 10--40 K), as we show for CenA and NGC\,1566 by Atacama Large Millimeter/submillimeter Array (ALMA) observations, whereas the inner H$_2$ gas traces a much warmer phase of material identified with warmer (40-500 K) molecular gas such as CO(6-5) or HCN (as shown by ALMA for NGC\,1068 and NGC\,1097). We also find that X-ray heating is the most likely dominant excitation mechanism of the H$_{2}$ gas for most sources.

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arXiv2014

Time-resolved infrared emission from radiation-driven central obscuring structures in Active Galactic Nuclei

The central engines of Seyfert galaxies are thought to be enshrouded by geometrically thick gas and dust structures. In this article, we derive observable properties for a self-consistent model of such toroidal gas and dust distributions, where the geometrical thickness is achieved and maintained with the help of X-ray heating and radiation pressure due to the central engine. Spectral energy distributions (SEDs) and images are obtained with the help of dust continuum radiative transfer calculations with RADMC-3D. For the first time, we are able to present time-resolved SEDs and images for a physical model of the central obscurer. Temporal changes are mostly visible at shorter wavelengths, close to the combined peak of the dust opacity as well as the central source spectrum and are caused by variations in the column densities of the generated outflow. Due to the three-component morphology of the hydrodynamical models -- a thin disc with high density filaments, a surrounding fluffy component (the obscurer) and a low density outflow along the rotation axis -- we find dramatic differences depending on wavelength: whereas the mid-infrared images are dominated by the elongated appearance of the outflow cone, the long wavelength emission is mainly given by the cold and dense disc component. Overall, we find good agreement with observed characteristics, especially for those models, which show clear outflow cones in combination with a geometrically thick distribution of gas and dust, as well as a geometrically thin, but high column density disc in the equatorial plane.

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arXiv2014

The central parsecs of active galactic nuclei: challenges to the torus

Type 2 AGN are by definition nuclei in which the broad-line region and continuum light are hidden at optical/UV wavelengths by dust. Via accurate registration of infrared (IR) Very Large Telescope adaptive optics images with optical \textit{Hubble Space Telescope} images we unambiguously identify the precise location of the nucleus of a sample of nearby, type 2 AGN. Dust extinction maps of the central few kpc of these galaxies are constructed from optical-IR colour images, which allow tracing the dust morphology at scales of few pc. In almost all cases, the IR nucleus is shifted by several tens of pc from the optical peak and its location is behind a dust filament, prompting to this being a major, if not the only, cause of the nucleus obscuration. These nuclear dust lanes have extinctions $A_V \geq 3-6$ mag, sufficient to at least hide the low-luminosity AGN class, and in some cases are observed to connect with kpc-scale dust structures, suggesting that these are the nuclear fueling channels. A precise location of the ionised gas H$α$ and [\textsc{Si\,vii}] 2.48 $μ$m coronal emission lines relative to those of the IR nucleus and dust is determined. The H$α$ peak emission is often shifted from the nucleus location and its sometimes conical morphology appears not to be caused by a nuclear --torus-- collimation but to be strictly defined by the morphology of the nuclear dust lanes. Conversely, [\textsc{Si\,vii}] 2.48 $μ$m emission, less subjected to dust extinction, reflects the truly, rather isotropic, distribution of the ionised gas. All together, the precise location of the dust, ionised gas and nucleus is found compelling enough to cast doubts on the universality of the pc-scale torus and supports its vanishing in low-luminosity AGN. Finally, we provide the most accurate position of the NGC 1068 nucleus, located at the South vertex of cloud B.

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arXiv2014

The innermost globular clusters of M87

We present a comprehensive multiwavelength photometric analysis of the innermost (3x3 square kpc) 110 globular clusters (GCs) of M87. Their spectral energy distributions (SEDs) were built taking advantage of new ground-based high resolution near-IR imaging aided by adaptive optics at the Very Large Telescope (VLT) combined with Hubble Space Telescope (HST) ultraviolet--optical archival data. These GC SEDs are among the best photometrically sampled extragalactic GC SEDs. To compare with our SEDs we constructed equally sampled SEDs of Milky Way GCs. Using both these Milky Way cluster templates and different stellar population models, ages of >10 Gyr and metallicities of [Fe/H] -0.6 dex are consistently inferred for the inner GCs of M87. In addition, the metallicity of these GCs is low (Dif([Fe/H]) 0.8 dex) compared to that of their host galaxy. These results agree with the idea that the GC formation in M87 ceased earlier than that of the bulk of the stars of the central part of the galaxy. The ages of the inner GCs of M87 support the idea that these central parts of the galaxy formed first. Our data do not support evidence of recent wet merging.

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arXiv2014

Evidence of parsec-scale jets in Low-luminosity Active Galactic Nuclei

The nuclear radio emission of low-luminosity active galactic nuclei (LLAGN) is often associated with unresolved cores. In this paper we show that most LLAGN present extended jet radio emission when observed with sufficient angular resolution and sensitivity. They are thus able to power, at least, parsec-scale radio jets. To increase the detection rate of jets in LLAGN, we analyze subarcsecond resolution data of three low-ionization nuclear emission regions. This yields the detection of extended jet-like radio structures in NGC\,1097 and NGC\,2911 and the first resolved parsec-scale jet of NGC\,4594 (Sombrero). The three sources belong to a sample of nearby LLAGN for which high-spatial-resolution spectral energy distribution of their core emission is available. This allows us to study their accretion rate and jet power ($Q_\mathrm{jet}$) without drawing on (most) of the ad hoc assumptions usually considered in large statistical surveys. We find that those LLAGN with large-scale radio jets ($>$100 pc) have $Q_\mathrm{jet} > 10^{42}$ erg s$^{-1}$, while the lowest $Q_\mathrm{jet}$ correspond to those LLAGN with parsec-scale ($\leq$100 pc) jets. The $Q_\mathrm{jet}$ is at least as large as the radiated bolometric luminosity for all LLAGN, in agreement with previous statistical studies. Our detection of parsec-scale jets in individual objects further shows that the kinematic jet contribution is equally important in large- or parsec-scale objects. We also find that the Eddington-scaled accretion rate is still highly sub-Eddington ($<10^{-4}$) when adding the $Q_\mathrm{jet}$ to the total emitted luminosity (radiated plus kinetic). This indicates that LLAGN are not only inefficient radiators but that they also accrete inefficiently or are very efficient advectors.

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arXiv2012

The central molecular gas structure in LINERs with low luminosity AGN: evidence for gradual disappearance of the torus

We present observations of the molecular gas in the nuclear environment of three prototypical low luminosity AGN (LLAGN), based on VLT/SINFONI AO-assisted integral-field spectroscopy of H2 1-0 S(1) emission at angular resolutions of ~0.17". On scales of 50-150 pc the spatial distribution and kinematics of the molecular gas are consistent with a rotating thin disk, where the ratio of rotation (V) to dispersion (sigma) exceeds unity. However, in the central 50 pc, the observations reveal a geometrically and optically thick structure of molecular gas (V/sigma<1 and N_H>10^{23} cm^{-2}) that is likely to be associated with the outer extent of any smaller scale obscuring structure. In contrast to Seyfert galaxies, the molecular gas in LLAGN has a V/sigma<1 over an area that is ~9 times smaller and column densities that are in average ~3 times smaller. We interpret these results as evidence for a gradual disappearance of the nuclear obscuring structure. While a disk wind may not be able to maintain a thick rotating structure at these luminosities, inflow of material into the nuclear region could provide sufficient energy to sustain it. In this context, LLAGN may represent the final phase of accretion in current theories of torus evolution. While the inflow rate is considerable during the Seyfert phase, it is slowly decreasing, and the collisional disk is gradually transitioning to become geometrically thin. Furthermore, the nuclear region of these LLAGN is dominated by intermediate-age/old stellar populations (with little or no on-going star formation), consistent with a late stage of evolution.

astro-ph.CO↗

arXiv2012

The SED of Low-Luminosity AGNs at high-spatial resolution

The inner structure of AGNs is expected to change below a certain luminosity limit. The big blue bump, footprint of the accretion disk, is absent for the majority of low-luminosity AGNs (LLAGNs). Moreover, recent simulations suggest that the torus, a keystone in the Unified Model, vanishes for nuclei with L_bol < 10^42 erg/s. However, the study of LLAGN is a complex task due to the contribution of the host galaxy, which light swamps these faint nuclei. This is specially critical in the IR range, at the maximum of the torus emission, due to the contribution of the old stellar population and/or dust in the nuclear region. Adaptive optics imaging in the NIR (VLT/NaCo) together with diffraction limited imaging in the mid-IR (VLT/VISIR) permit us to isolate the nuclear emission for some of the nearest LLAGNs in the Southern Hemisphere. These data were extended to the optical/UV range (HST), radio (VLA, VLBI) and X-rays (Chandra, XMM-Newton, Integral), in order to build a genuine spectral energy distribution (SED) for each AGN with a consistent spatial resolution (< 0.5") across the whole spectral range. From the individual SEDs, we construct an average SED for LLAGNs sampled in all the wavebands mentioned before. Compared with previous multiwavelength studies of LLAGNs, this work covers the mid-IR and NIR ranges with high-spatial resolution data. The LLAGNs in the sample present a large diversity in terms of SED shapes. Some of them are very well described by a self-absorbed synchrotron (e.g. NGC 1052), while some other present a thermal-like bump at ~1 micron (NGC 4594). All of them are significantly different when compared with bright Seyferts and quasars, suggesting that the inner structure of AGNs (i.e. the torus and the accretion disk) suffers intrinsic changes at low luminosities.

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arXiv2011

Complex particle acceleration processes in the hotspots of 3C105 and 3C445

We investigate the nature of the broad-band emission associated with the low-power radio hotspots 3C105 South and 3C445 South. Both hotspot regions are resolved in multiple radio/optical components. High-sensitivity radio VLA, NIR/optical VLT and HST, and X-ray Chandra data have been used to construct the multi-band spectra of individual hotspot components. The radio-to-optical spectra of both hotspot regions are well fitted by a synchrotron model with steep spectral indices ~0.8 and break frequencies 10^12-10^14 Hz. 3C105 South is resolved in two optical components: a primary one, aligned with the jet direction and possibly marking the first jet impact with the surrounding medium, and a secondary, further out from the jet and extended in a direction perpendicular to it. This secondary region is interpreted as a splatter-spot formed by the deflection of relativistic plasma from the primary hotspot. Radio and optical images of 3C445 South show a spectacular 10-kpc arc-shape structure characterized by two main components, and perpendicular to the jet direction. HST images in I and B bands further resolve the brightest components into thin elongated features. In both 3C105 South and 3C445 South the main hotspot components are enshrouded by diffuse optical emission on scale of several kpcs, indicating that very high energy particles, possibly injected at strong shocks, are continuously re-accelerated in situ by additional acceleration mechanisms. We suggest that stochastic processes, linked to turbulence and instabilities, could provide the required additional re-acceleration.

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arXiv2011

The Near-Infrared Coronal Line Spectrum of 54 Nearby Active Galactic Nuclei

(Abridge) The relationship between coronal line (CL) emission and nuclear activity in active galactic nuclei (AGNs) is analyzed, for the first time, based on NIR spectra. The 8 CLs studied, of Si, S, Fe, Al and Ca elements and corresponding to ionization potentials (IP) in the range 125-450 eV, are detected in 67% (36 AGNs) of the sample. The four most frequent CLs - [SiVI] 19630Å, [SVIII] 9913Å, [SIX] 12520Å and [SiX] 14320Å, - display a narrow range in luminosity, with most lines located in the interval logL 39-40 erg/s. We found that the non-detection is largely associated with either a lost of spatial resolution or increasing object distance. Yet, there are AGNs where the lack of CLs may be genuine and reflect an AGN ionising continuum lacking photons below a few keV. The FWHM of the lines profiles increases with increasing IP up to energies around 300 eV, where a maximum in the FWHM is reached. For higher IP lines, the FWHM remains nearly constant or decreases with increasing IP. We ascribe this effect to an increasing density environment as we approach to the innermost regions of the AGN, where densities above the critical density of the CLs with IP larger than 300 eV are reached. This sets a strict range limit for the density in the boundary region between the narrow and the broad region of 10^8 - 10^9 cm^{-3}. A relationship between the luminosity of the coronal lines and that of the soft and hard X-ray emission and the soft X-ray photon index is observed: the coronal emission becomes stronger with both increasing x-ray emission (soft and hard) and steeper X-ray photon index. Thus, photoionization appears as the dominant excitation mechanism. These trends hold when considering Type 1 sources only; they get weaker or vanish when including Type 2 sources, very likely because the X-ray emission measured in the later is not the intrinsic ionising continuum.

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arXiv2010

The most recent burst of Star Formation in the Massive Elliptical Galaxy NGC 1052

High-spatial resolution near-infrared (NIR) images of the central 24 x 24 arcsec^2 (~ 2 x 2 kpc^2) of the elliptical galaxy NGC 1052 reveal a total of 25 compact sources randomly distributed in the region. Fifteen of them exhibit Halpha luminosities an order of magnitude above the estimate for an evolved population of extreme horizontal branch stars. Their Halpha equivalent widths and optical-to-NIR spectral energy distributions are consistent with them being young stellar clusters aged < 7 Myr. We consider this to be the first direct observation of spatially resolved star-forming regions in the central kiloparsecs of an elliptical galaxy. The sizes of these regions are ~< 11 pc and their median reddening is E(B - V) ~ 1 mag. According to previous works, NGC 1052 may have experienced a merger event about 1 Gyr ago. On the assumption that these clusters are spreaded with similar density over the whole galaxy, the fraction of galaxy mass (5 x 10^{-5}) and rate of star formation (0.01 Msun/yr) involved, suggest the merger event as the possible cause for the star formation we see today.

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arXiv2010

The Stellar Kinematic Center and the True Galactic Nucleus of NGC253

We present the first sub-arcsecond resolution two-dimensional stellar kinematics and X-ray observations of the prototypical starburst galaxy NGC253 which define the position and nature of the galactic nucleus. We get an estimate of the stellar kinematic center location corresponding to an area of r~1.2" centered 0.7" southwest from the radio core (TH2). Newly processed Chandra data reveal a central hard X-ray source (X-1) lying 0.4" southwest from the kinematic center. Very accurate alignment between radio, infrared and X-ray sources shows that TH2, the IR photometric center and X-1 are not associated with each other. As the kinematic center is consistent with TH2 and X-1, we consider the two as possible galactic nucleus candidates. Although TH2 is the strongest radio source in the nuclear region, it does not have any infrared, optical or X-ray counterparts. If the kinematic center is associated with this source, by analogy we suggest that the nucleus of NGC253 resembles our Galactic Center SgrA*. On the other hand, X-1 is a heavily absorbed object only detected at energies >2 keV. If X-1 is instead associated with the kinematic center, the nucleus of NGC253 is compatible with an obscured low luminosity active galactic nucleus (AGN) or a spatially resolved super star cluster (SSC) brightening up in X-rays most probably due to young supernovae or supernova remnants. If no SSC is associated with the kinematic center, we conclude that NGC253 is a galaxy in which a strong starburst and a weak AGN (either TH2 or X-1) coexist. Results from few other high resolution studies of nearby starburst galaxies indicate that the AGN in these systems, if present, is always in the low luminosity regime. This may indicate that the onset of nuclear activity in galaxies is closely related with the occurrence of star formation, and that we are witnessing the emergence or disappearance of an AGN.

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arXiv2009

VLT diffraction-limited imaging at 11 and 18 micron of the nearest active galactic nuclei

Mid-infrared imaging at resolutions of 300 mas of the central kpc region of 13 nearby, well-known active galaxies is presented. The bulk of the mid-IR emission is concentrated on an unresolved central source within a size of less than 5 to 130 pc, depending on the object distance. Further resolved emission is detected in 70% of the sample in the form of circumnuclear star-forming rings or diffuse nuclear extended emission. In the three cases with circumnuclear star formation, the stellar contribution is at least as important as that of the AGN. In those with extended nuclear emission -- a third of the sample -- this emission represents a few per cent of the total measured; however, this contribution may be underestimated because of the chopped nature of these observations. This extended emission is generally collimated in a preferential direction often coinciding with that of the extended ionized gas or the jet. In all cases, the nuclear fluxes measured at 11.8 and 18.7 micron represent a minor contribution of the flux levels measured by large aperture IRAS data at the nearest energy bands of 12 and 25 micron. This contribution ranges from 30% to less than 10%. In only three cases do the AGN fluxes agree with IRAS to within a factor of 2. In the AGNs with strong circumnuclear star formation, this component can well account for most of the IRAS flux measured in these objects. But in all other cases, either a low surface brightness component extending over galactic scales or strong extra-nuclear IR sources -- e.g. HII regions in spiral arms -- have to be the main source of the IRAS emission. In either case, the contribution of these components dwarfs that of the AGN at mid-IR wavelengths.

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arXiv2009

Radio structures of the nuclei of nearby Seyfert galaxies and the nature of the missing diffuse emission

We present archival high spatial resolution VLA and VLBA data of the nuclei of seven of the nearest and brightest Seyfert galaxies in the Southern Hemisphere. At VLA resolution (~0.1 arcsec), the nucleus of the Seyfert galaxies is unresolved, with the exception of MCG-5-23-16 and NGC 7469 showing a core-jet structure. Three Seyfert nuclei are surrounded by diffuse radio emission related to star-forming regions. VLBA observations with parsec-scale resolution pointed out that in MRK 1239 the nucleus is clearly resolved into two components separated by ~30 pc, while the nucleus of NGC 3783 is unresolved. Further comparison between VLA and VLBA data of these two sources shows that the flux density at parsec scales is only 20% of that measured by the VLA. This suggests that the radio emission is not concentrated in a single central component, as in elliptical radio galaxies, and an additional low-surface brightness component must be present. A comparison of Seyfert nuclei with different radio spectra points out that the ``presence'' of undetected flux on milli-arcsecond scale is common in steep-spectrum objects, while in flat-spectrum objects essentially all the radio emission is recovered. In the steep-spectrum objects, the nature of this ``missing'' flux is likely due to non-thermal AGN-related radiation, perhaps from a jet that gets disrupted in Seyfert galaxies because of the denser environment of their spiral hosts.

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arXiv2009

The spectral energy distribution of the central parsecs of the nearest AGN

Spectral energy distributions (SEDs) of the central few tens of parsec region of some of the nearest, most well studied, active galactic nuclei (AGN) are presented. These genuine AGN-core SEDs, mostly from Seyfert galaxies, are characterised by two main features: an IR bump with the maximum in the 2-10 micron range, and an increasing X-ray spectrum in the 1 to ~200 keV region. These dominant features are common to Seyfert type 1 and 2 objects alike. Type 2 AGN exhibit a sharp drop shortward of 2 micron, with the optical to UV region being fully absorbed, while type 1s show instead a gentle 2 micron drop ensued by a secondary, partially-absorbed optical to UV emission bump. Assuming the bulk of optical to UV photons generated in these AGN are reprocessed by dust and re-emitted in the IR in an isotropic manner, the IR bump luminosity represents >70% of the total energy output in these objects while the high energies above 20 keV are the second energetically important contribution. Galaxies selected by their warm IR colours, i.e. presenting a relatively-flat flux distribution in the 12 to 60 micron range have often being classified as AGN. The results from these high spatial resolution SEDs question this criterion as a general rule. It is found that the intrinsic shape of the IR SED of an AGN and inferred bolometric luminosity largely depart from those derived from large aperture data. AGN luminosities can be overestimated by up to two orders of magnitude if relying on IR satellite data. We find these differences to be critical for AGN luminosities below or about 10^{44} erg/s. Above this limit, AGNs tend to dominate the light of their host galaxy regardless of the aperture size used. We tentatively mark this luminosity as a threshold to identify galaxy-light- vs AGN- dominated objects.

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arXiv2008

NIR/Optical Counterparts of Hotspots in Radio Galaxies

We present new high spatial resolution VLT and VLA observations of a sample of nine low-power (P_{1.4 GHz} < 10^{25} W/Hz) radio hotspots. Infrared/optical emission is definitely detected in four of the nine observed objects, resulting in a detection rate of at least 45%. This emission is interpreted as synchrotron radiation from the electrons accelerated in the hot spots. The integrated spectra of these hotspots reveal typical break frequencies between 10^5 and 10^6 GHz, two orders of magnitude higher than typically found in high-power hotspots. This supports the idea that in low-power hotspots with their relatively low magnetic field strengths electrons emit most of their energy at higher frequencies. A simple spectral ageing analysis would imply that the emitting electrons have been injected into the hotspot volume less than ~10^3 years ago. We discuss possible scenarios to explain the lack of older electrons in the hotspot region. In particular, the extended morphology of the NIR/optical emission would suggest that efficient re-acceleration mechanisms rejuvenate the electron populations.

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