arXiv subjects

A. Alberdi

Publications and source records attributed to A. Alberdi.

At least 73 records · Page 4Linked to original sources

arXiv2012

e-MERLIN and VLBI observations of the luminous infrared galaxy IC883: a nuclear starburst and an AGN candidate revealed

The high star formation rates of luminous infrared galaxies (LIRGs) make them ideal places for core-collapse supernova (CCSN) searches. At radio frequencies, free from dust extinction, it is possible to detect compact components within the innermost LIRG nuclear regions, such as SNe and SN remnants, as well as AGN buried deep in the LIRG nuclei. We studied the LIRG IC883 aiming at: (i) investigating its (circum-)nuclear regions using the e-EVN at 5GHz, and e-MERLIN at 6.9GHz, complemented by archival VLBI data; (ii) detecting at radio frequencies the two recently reported circumnuclear SNe 2010cu and 2011hi, which were discovered by near-IR (NIR) adaptive optics observations of IC883; and (iii) further investigating the nature of SN2011hi at NIR by means of observations with Gemini-North. The circumnuclear regions traced by e-MERLIN at 6.9GHz have an extension of ~1kpc, and show a striking double-sided structure, which very likely corresponds to a warped rotating ring, in agreement with previous studies. Our e-EVN observations at 5GHz and complementary archival VLBI data at 5GHz and 8.4GHz, reveal the presence of various milliarcsec compact components in the nucleus of IC883. A single compact source, an AGN candidate, dominates the emission at both nuclear and circumnuclear scales, as imaged with the e-EVN and e-MERLIN, respectively. The other milliarcsec components are very suggestive of ongoing nuclear CCSN activity. Our e-EVN observations also resulted in upper limits to the radio luminosity of the two SNe in IC883 recently discovered at NIR. We refine the classification of SN2011hi as a Type IIP SN according to our latest Gemini-North epoch from 2012, in agreement with a low-luminosity radio SN nature. We estimate a CCSN rate lower limit of 1.1_{-0.6}^{+1.3} yr^{-1} for the entire galaxy, based on three nuclear radio SNe and the circumnuclear SNe 2010cu and 2011hi. (abridged)

astro-ph.CO↗

arXiv2012

NACO/SAM observations of sources at the Galactic Center

Sparse aperture masking (SAM) interferometry combined with Adaptive Optics (AO) is a technique that is uniquely suited to investigate structures near the diffraction limit of large telescopes. The strengths of the technique are a robust calibration of the Point Spread Function (PSF) while maintaining a relatively high dynamic range. We used SAM+AO observations to investigate the circumstellar environment of several bright sources with infrared excess in the central parsec of the Galaxy. For our observations, unstable atmospheric conditions as well as significant residuals after the background subtraction presented serious problems for the standard approach of calibrating SAM data via interspersed observations of reference stars. We circumvented these difficulties by constructing a synthesized calibrator directly from sources within the field-of-view. When observing crowded fields, this novel method can boost the efficiency of SAM observations because it renders interspersed calibrator observations unnecessary. Here, we presented the first NaCo/SAM images reconstructed using this method.

astro-ph.IM↗

arXiv2012

Evidence of nuclear disks in starburst galaxies from their radial distribution of supernovae

Galaxy-galaxy interactions are expected to be responsible for triggering massive star formation and possibly accretion onto a supermassive black hole, by providing large amounts of dense molecular gas down to the central kiloparsec region. Several scenarios to drive the gas further down to the central ~100 pc, have been proposed, including the formation of a nuclear disk around the black hole, where massive stars would produce supernovae. Here, we probe the radial distribution of supernovae and supernova remnants in the nuclear regions of the starburst galaxies M82, Arp 299-A, and Arp 220, by using high-angular resolution (< 0."1) radio observations published in the literature (for M82 and Arp 220), or obtained by ourselves from the European VLBI Network (Arp 299-A). Our main goal was to characterize the nuclear starbursts in those galaxies and thus test scenarios that propose that nuclear disks of sizes ~100 pc form in the central regions of starburst galaxies. We obtained the radial distribution of supernovae (SNe) in the nuclear starbursts of M82, Arp 299-A, and Arp 220, and derived scale-length values for the putative nuclear disks powering the bursts in those central regions. The scale lengths for the (exponential) disks range from ~20-30 pc for Arp 299-A and Arp 220, up to ~140 pc for M82. The radial distribution of SNe for the nuclear disks in Arp 299-A and Arp 220 is also consistent with a power-law surface density profile of exponent gamma=1, as expected from detailed hydrodynamical simulations of nuclear disks. Our results support scenarios where a nuclear disk of size ~100 pc is formed in (U)LIRGs, and sustained by gas pressure, in which case the accretion onto the black hole could be lowered by supernova feedback.

astro-ph.CO↗

arXiv2012

The nuclear starburst in Arp 299-A: From the 5.0 GHz VLBI radio light-curves to its core-collapse supernova rate

The nuclear region of the Luminous Infra-red Galaxy Arp 299-A hosts a recent ($\simeq 10$ Myr), intense burst of massive star formation which is expected to lead to numerous core-collapse supernovae (CCSNe). Previous VLBI observations, carried out with the EVN at 5.0 GHz and with the VLBA at 2.3 and 8.4 GHz, resulted in the detection of a large number of compact, bright, non-thermal sources in a region $\lsim$150 pc in size. We aim at establishing the nature of all non-thermal, compact components in Arp 299-A, as well as estimating its core-collapse supernova rate. We use multi-epoch European VLBI Network (EVN) observations taken at 5.0 GHz to image with milliarcsecond resolution the compact radio sources in the nuclear region of Arp 299-A. We also use one single-epoch 5.0 GHz Multi-Element Radio Linked Interferometer Network (MERLIN) observation to image the extended emission in which the compact radio sources --traced by our EVN observations-- are embedded. Twenty-six compact sources are detected, 8 of them are new objects not previously detected. The properties of all detected objects are consistent with them being a mixed population of CCSNe and SNRs. We find clear evidence for at least two new CCSNe, implying a lower limit to the CCSN rate of $ν_{\rm SN}\gsim$0.80 SN/yr indicating that the bulk of the current star formation in Arp 299-A is taking place in the innermost $\sim 150$ pc. Our MERLIN observations trace a region of diffuse, extended emission which is co-spatial to the region where all compact sources are found. From this diffuse, non-thermal radio emission we obtain an independent estimate for the core-collapse supernova rate, which is in the range $ν_{\rm SN}=0.40$ - 0.65 SN/yr, roughly in agreement with previous estimates and our direct estimate of the CCSN rate from the compact radio emission.

astro-ph.CO↗

arXiv2011

Discovery of Two Supernovae in the Nuclear Regions of the Luminous Infrared Galaxy IC 883

We report the discovery of two consecutive supernovae (SNe), 2010cu and 2011hi, located at 0.37" (180 pc) and 0.79" (380 pc) projected distance respectively from the centre of the K-band nucleus of the luminous infrared galaxy IC 883. The SNe were discovered in an ongoing near-infrared K-band search for core-collapse SNe in such galaxies using the ALTAIR/NIRI adaptive optics system with laser guide star at the Gemini-North Telescope. These are thus the closest SNe yet discovered to a LIRG nucleus in optical or near-infrared wavelengths. The near-infrared light curves and colours of both SNe are consistent with core-collapse events. Both SNe seem to suffer from relatively low host galaxy extinction suggesting that regardless of their low projected galactocentric distances, they are not deeply buried in the nuclear regions of the host galaxy.

astro-ph.SR↗

arXiv2011

VLBI observations of SN2011dh: imaging of the youngest radio supernova

We report on the VLBI detection of supernova SN2011dh at 22GHz using a subset of the EVN array. The observations took place 14 days after the discovery of the supernova, thus resulting in a VLBI image of the youngest radio-loud supernova ever. We provide revised coordinates for the supernova with milli-arcsecond precision, linked to the ICRF. The recovered flux density is a factor 2 below the EVLA flux density reported by other authors at the same frequency and epoch of our observations. This discrepancy could be due to extended emission detected with the EVLA or to calibration problems in the VLBI and/or EVLA observations.

astro-ph.HE↗

arXiv2011

Detection of jet precession in the active nucleus of M81

(See the complete and formatted abstract in the paper). We report on VLBI monitoring of the low-luminosity AGN (LLAGN) in M81 at 1.7, 2.3, 5, and 8.4GHz. These observations are phase-referenced to the supernova SN1993J (located in the same galaxy) and cover from late 1993 to late 2005. The source consists at all frequencies of a slightly resolved core and a small jet extension towards the north-east direction (position angle of ~65 degrees) in agreement with previous publications. We find that the position of the intensity peak in the images at 8.4GHz is very stable in the galactic frame of M81 (proper motion upper limit about 0.010 mas per year). We confirm previous reports that the peaks at all frequencies are systematically shifted among them, possibly due to opacity effects in the jet as predicted by the standard relativistic jet model. We use this model to estimate the magnetic field in the jet and the mass of the central black hole. We obtain a black-hole mass of ~2.e+7 solar masses, comparable to estimates previously reported, but the magnetic fields obtained are thousands of times lower than previous estimates. We find that the positions of the cores at 1.7, 2.3, and 5GHz are less stable than that at 8.4GHz and evolve systematically, shifting southward at a rate of several tens of micro-arcsec per year. The evolution in the jet orientation seems to be related to changes in the inclination of the cores at all frequencies. These results can be interpreted as due to a precessing jet. The jet precession also seems to be related to a flare in the flux densities at 5.0 and 8.4GHz, which lasts ~4 years. A continued monitoring of the flux density and the jet structure evolution in this LLAGN will be necessary to further confirm our jet precession model.

astro-ph.HE↗

arXiv2011

The mean infrared emission of SagittariusA*

(abridged) The massive black hole at the center of the Milky Way, SagittariusA* is, in relative terms, the weakest accreting black hole accessible to observations. At the moment, the mean SED of SgrA* is only known reliably in the radio to mm regimes. The goal of this paper is to provide constraints on the mean emission from SgrA* in the near-to-mid infrared. Excellent imaging quality was reached in the MIR by using speckle imaging combined with holographic image reconstruction, a novel technique for this kind of data. No counterpart of SgrA* is detected at 8.6 microns. At this wavelength, SgrA* is located atop a dust ridge, which considerably complicates the search for a potential point source. An observed 3 sigma upper limit of ~10 mJy is estimated for the emission of SgrA* at 8.6 microns, a tighter limit at this wavelength than in previous work. The de-reddened 3 sigma upper limit, including the uncertainty of the extinction correction, is ~84 mJy . Based on the available data, it is argued that, with currently available instruments, SgrA* cannot be detected in the MIR, not even during flares. At 4.8 and 3.8 microns, on the other hand, SgrA* is detected at all times, at least when considering timescales of a few up to 13 min. We derive well-defined time-averaged, de-reddened flux densities of 3.8+-1.3 mJy at 4.8 microns and 5.0+-0.6 mJy at 3.8 microns. Observations with NIRC2/Keck and NaCo/VLT from the literature provide good evidence that SgrA* also has a fairly well-defined de-reddened mean flux of 0.5-2.5 mJy at wavelengths of 2.1-2.2 microns. We present well-constrained anchor points for the SED of SgrA* on the high-frequency side of the Terahertz peak. The new data are in general agreement with published theoretical SEDs of the mean emission from SgrA*, but we expect them to have an appreciable impact on the model parameters in future theoretical work.

astro-ph.GA↗

arXiv2011

A model for the thermal radio-continuum emission from radiative shocks in colliding stellar winds

Aims. The interaction of two isotropic stellar winds is studied in order to calculate the free-free emission from the wind collision region. The effects of the binary separation and the wind momentum ratio on the emission from the wind-wind interaction region are investigated. Methods. We developed a semi-analytical model for calculating the thermal emission from colliding stellar winds. Assuming radiative shocks for the compressed layer, which are expected in close binaries, we obtained the emission measure of the thin shell. Then, we computed the total optical depth along each line of sight to obtain the emission from the whole configuration. Results. Here, we present predictions of the free-free emission at radio frequencies from analytic, radiative shock models in colliding wind binaries. It is shown that the emission from the wind collision region mainly arises from the optically thick region of the compressed layer and scales as ~ D^{4/5}, where D is the binary separation. The predicted flux density from the wind collision region becomes more important as the frequency increases, showing higher spectral indices than the expected 0.6 value from the unshocked winds. We also investigate the emission from short-period WR+O systems calculated with our analytic formulation. In particular, we apply the model to the binary systems WR 98 and WR 113 and compare our results with the observations. Our theoretical results are in good agreement with the observed thermal spectra from these sources.

astro-ph.GA↗

arXiv2011

Molecules as tracers of galaxy evolution: an EMIR survey. I. Presentation of the data and first results

We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers. Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis. We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected transitions are the J=1-0 lines of CO, 13CO, HCN, HNC, HCO+, CN, and C2H. We also detect HC3N J=10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC3N detections are in objects with HCO+/HCN<1 and warm IRAS colours. Galaxies with the highest HC3N/HCN ratios have warm IRAS colours (60/100 μm>0.8). The brightest HC3N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state.We find low HNC/HCN line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical models. Bright HC3N emission in HCO+-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO+ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC3N and faint HCO+ could be signatures of embedded starformation, instead of AGN activity.

astro-ph.CO↗

arXiv2010

Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data

We have performed a complete re-calibration and re-analysis of all the available VLBI observations of supernova SN1993J, following an homogeneous and well-defined methodology. Observations of SN1993J at 69 epochs, spanning 13 years, were performed by two teams, which used different strategies and analysis tools. The results obtained by each group are similar, but their conclusions on the supernova expansion and the shape and evolution of the emitting region differ significantly. From our analysis of the combined set of observations, we have obtained an expansion curve with unprecedented time resolution and coverage. We find that the data from both teams are compatible when analyzed with the same methodology. One expansion index ($m_3 = 0.87 \pm 0.02$) is enough to model the expansion observed at 1.7\,GHz, while two expansion indices ($m_1 = 0.933\pm0.010$ and $m_2 = 0.796\pm0.005$), separated by a break time, $t_{br} = 390\pm30$ days, are needed to model the data, at frequencies higher than 1.7\,GHz, up to day 4000 after explosion. We thus confirm the wavelength dependence of the size of the emitting region reported by one of the groups. We also find that all sizes measured at epochs later than day 4000 after explosion are systematically smaller than our model predictions. We estimate the fractional shell width ($0.31 \pm 0.02$, average of all epochs and frequencies) and the level of opacity to the radio emission by the ejecta. We find evidence of a spectral-index radial gradient in the supernova shell, which is indicative of a frequency-dependent ejecta opacity. Finally, we study the distribution and evolution of the azimuthal anisotropies (hot spots) found around the radio shell during the expansion. These anisotropies have intensities of $\sim 20$% of the mean flux density of the shell, and appear to systematically evolve during the expansion.

astro-ph.HE↗

arXiv2010

Extreme Starbursts in the Local Universe

The "Extreme starbursts in the local universe" workshop was held at the Insituto de Astrofisica de Andalucia in Granada, Spain on 21-25 June 2010. Bearing in mind the advent of a new generation of facilities such as JWST, Herschel, ALMA, eVLA and eMerlin, the aim of the workshop was to bring together observers and theorists to review the latest results. The purpose of the workshop was to address the following issues: what are the main modes of triggering extreme starbursts in the local Universe? How efficiently are stars formed in extreme starbursts? What are the star formation histories of local starburst galaxies? How well do the theoretical simulations model the observations? What can we learn about starbursts in the distant Universe through studies of their local counterparts? How important is the role of extreme starbursts in the hierarchical assembly of galaxies? How are extreme starbursts related to the triggering of AGN in the nuclei of galaxies? Overall, 41 talks and 4 posters with their corresponding 10 minutes short talks were presented during the workshop. In addition, the workshop was designed with emphasis on discussions, and therefore, there were 6 discussion sessions of up to one hour during the workshop. Here is presented a summary of the purposes of the workshop as well as a compilation of the abstracts corresponding to each of the presentations. The summary and conclusions of the workshop along with a description of the future prospects by Sylvain Veilleux can be found in the last section of this document. A photo of the assistants is included.

astro-ph.CO↗

arXiv2010

Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves

We report on a simultaneous modelling of the expansion and radio light curves of SN1993J. We have developed a simulation code capable of generating synthetic expansion and radio light curves of supernovae by taking into consideration the evolution of the expanding shock, magnetic fields, and relativistic electrons, as well as the finite sensitivity of the interferometric arrays used in the observations. Our software successfully fits all the available radio data of SN 1993J with an standard emission model for supernovae extended with some physical considerations, as an evolution in the opacity of the ejecta material, a radial drop of the magnetic fields inside the radiating region, and a changing radial density profile of the circumstellar medium beyond day 3100 after explosion.

astro-ph.HE↗

arXiv2009

An extremely prolific supernova factory in the buried nucleus of the starburst galaxy IC 694

Context. The central kiloparsec of many local Luminous Infra-red Galaxies are known to host intense bursts of massive star formation, leading to numerous explosions of core-collapse supernovae (CCSNe). However, the dust-enshrouded regions where those supernovae explode hamper their detection at optical and near-infrared wavelengths. Aims. We investigate the nuclear region of the starbust galaxy IC 694 (=Arp 299-A) at radio wavelengths, aimed at discovering recently exploded CCSNe, as well as to determine their rate of explosion, which carries crucial information on star formation rates, the initial mass function and starburst scenarios at work. Methods. We use the electronic European VLBI Network to image with milliarcsecond resolution the 5.0 GHz compact radio emission of the innermost nuclear region of IC 694. Results. Our observations reveal the presence of a rich cluster of 26 compact radio emitting sources in the central 150 pc of the nuclear starburst in IC 694. The large brightness temperatures observed for the compact sources indicate a non-thermal origin for the observed radio emission, implying that most, if not all, of those sources are young radio supernovae (RSNe) and supernova remnants (SNRs). We find evidence for at least three relatively young, slowly-evolving, long-lasting RSNe (A0, A12, and A15) which appear to display unusual properties; suggesting that the conditions in the local circumstellar medium (CSM) play a significant role in determining the radio behaviour of expanding SNe. Their radio luminosities are typical of normal RSNe, which result from the explosion of Type IIP/b and Type IIL SNe. All these results yield support for a recent (less than 10-15 Myr) instantaneous starburst in the innermost regions of IC 694.

astro-ph.CO↗

arXiv2009

A Decade of SN1993J: Discovery of Wavelength Effects in the Expansion Rate

We have studied the growth of the shell-like radio structure of supernova SN1993J in M81 from September 1993 through October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18cm. For this purpose, we have developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure like SN1993J. The source structure of SN1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J through approximately day 1500 after explosion with an expansion parameter $m= 0.845\pm0.005$ ($R \propto t^{m}$). However, from that day onwards the expansion is different when observed at 6 and 18cm. Indeed, at 18cm, the expansion can be well characterized by the same $m$ as before day 1500, while at 6cm the expansion appears more decelerated, and is characterized by another expansion parameter, $m_{6}= 0.788\pm0.015$. Therefore, since about day 1500 on, the radio source size has been progressively smaller at 6cm than at 18cm. These findings are in stark contrast to previous reports by other authors on the details of the expansion. In our interpretation the supernova expands with a single expansion parameter, $m= 0.845\pm0.005$, and the 6cm results beyond day 1500 are due to physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6cm radiation, and (b) a radial decrease of the magnetic field in the emitting region. (Long abstract cut. Please, read full abstract in manuscript).

astro-ph.HE↗

arXiv2009

1.6 GHz VLBI Observations of SN 1979C: almost-free expansion

We report on 1.6 GHz Very-Long-Baseline-Interferometry (VLBI) observations of supernova SN 1979C made on 18 November 2002. We derive a model-dependent supernova size. We also present a reanalysis of VLBI observations made by us on June 1999 and by other authors on February 2005. We conclude that, contrary to our earlier claim of strong deceleration in the expansion, SN 1979C has been undergoing almost-free expansion ($m = 0.91\pm0.09$; $R \propto t^m$) for over 25 years.

astro-ph.HE↗

arXiv2009

Radio monitoring of NGC 7469: Late time radio evolution of SN 2000ft and the circumnuclear starburst in NGC 7469

We present the results of an eight-year long monitoring of the radio emission from the Luminous Infrared Galaxy (LIRG) NGC 7469, using 8.4 GHz Very Large Array (VLA) observations at 0.3'' resolution. Our monitoring shows that the late time evolution of the radio supernova SN 2000ft follows a decline very similar to that displayed at earlier times of its optically thin phase. The late time radio emission of SN 2000ft is therefore still being powered by its interaction with the presupernova stellar wind, and not with the interstellar medium (ISM). Indeed, the ram pressure of the presupernova wind is ρ_w v_w^2 \approx 7.6E-9 dyn/cm^2, at a supernova age of approximately 2127 days, which is significantly larger than the expected pressure of the ISM around SN 2000ft. At this age, the SN shock has reached a distance r_{sh \approx 0.06 pc, and our observations are probing the interaction of the SN with dense material that was ejected by the presupernova star about 5820 years prior to its explosion. From our VLA monitoring, we estimate that the swept-up mass by the supernova shock after about six years of expansion is \approx 0.29 M_sun, assuming an average expansion speed of the supernova of 10000 km/s. We also searched for recently exploded core-collapse supernovae in our VLA images. Apart from SN 2000ft (S_ν\approx 1760 microJy at its peak, corresponding to 1.1E28 erg/s/Hz, we found no evidence for any other radio supernova (RSN) more luminous than \approx 6.0E26 erg/s/Hz, which suggests that no other Type IIn SN has exploded since 2000 in the circumnuclear starburst of NGC 7469.

astro-ph.CO↗

arXiv2009

23 GHz VLBI Observations of SN 2008ax

We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8$\pm$0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48$\pm$0.12 mJy, compatible with the flux densities measured with the VLA at epochs close in time to our VLBI observations. The lowest average expansion velocities derived from our observations are $(1.90 \pm 0.30) \times 10^5$ km s$^{-1}$ (case of a double source) and $(5.2 \pm 1.3) \times 10^4$ km s$^{-1}$ (taking the weaker source component as a spurious, close by, noise peak, which is the more likely interpretation). These velocities are 7.3 and 2 times higher, respectively, than the maximum ejecta velocity inferred from optical-line observations.

astro-ph.GA↗