Kim K. Nilsson

UltraVISTA: a new ultra-deep near-infrared survey in COSMOS

In this paper we describe the first data release of the UltraVISTA near-infrared imaging survey of the COSMOS field. We summarise the key goals and design of the survey and provide a detailed description of our data reduction techniques. We provide stacked, sky-subtracted images in YJHK_s and narrow-band filters constructed from data collected during the first year of UltraVISTA observations. Our stacked images reach 5σAB depths in an aperture of 2″ diameter of ~25 in Y and ~24 in JHK_s bands and all have sub-arcsecond seeing. To this 5σ limit, our K_s catalogue contains 216 268 sources. We carry out a series of quality assessment tests on our images and catalogues, comparing our stacks with existing catalogues. The 1σ astrometric rms in both directions for stars selected with 17.0 < K_s(AB) < 19.5 is ~0.08″ in comparison to the publicly-available COSMOS ACS catalogues. Our images are resampled to the same pixel scale and tangent point as the publicly available COSMOS data and so may be easily used to generate multi-colour catalogues using this data. All images and catalogues presented in this paper are publicly available through ESO’s “phase 3” archiving and distribution system and from the UltraVISTA web site.

Optical and radio variability of the BL Lacertae object AO 0235+16: A possible 5-6 year periodicity

The BL Lacertae object AO 0235+16 is well known for its extreme optical and radio variability. New optical and radio data have been collected in the last four years by a wide international collaboration, which conrm the intense activity of this source: on the long term, overall variations of 5 mag in the R band and up to a factor 18 in the radio fluxes were detected, while short-term variability up to 0:5 mag in a few hours and 1: 3m ag in one day was observed in the optical band. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) rst-light campaign organized in November 1997, involving a dozen optical observatories. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection and Discrete Correlation Function (DCF) analysis. On the long term, the autocorrelation function of the optical data shows a double-peaked maximum at 4100{4200 days (11:2{11:5 years), while a double-peaked maximum at 3900{4200 days (10:7{11:5 years) is visible in the radio autocorrelation functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio correlation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts repeat quasi-regularly with a periodicity of5:7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspection and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, but in other cases they lead the radio events. Moreover, a deep inspection of the radio light curves suggests that in at least two occasions (the 1992{1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.

A multi-wavelength study of z = 3.15 Lyman-α emitters in the GOODS south field

Context. Ly�-emitters have proven to be excellent probes of faint, star- forming galaxies in the high redshift universe. However, although the sample of known emitters is increasingly growing, their nature (e.g. stellar masses, ages, metallicities , star-formation rates) is still poorly constrained. Aims. We aim to study the nature of Ly�-emitters, to find the properties of a typical Ly �-emitting galaxy and to compare these properties with the properties of other galaxies at similar redshift, in particular Lyman-break galaxies.

The GRB 030329 host: a blue low metallicity subluminous galaxy with intense star formation

We present broad band photometry and spectroscopic observations of the host galaxy of GRB 030329. Analysis of the spectral emission lines shows that the host is likely a low metallicity galaxy (Z ∼ 0.004). The spectral energy distribution (SED) constructed with the photometric points has been fitted using synthetic and observational templates. The best SED fit is obtained with a starburst template with an age of ∼150 Myr and an extinction Av ∼ 0.6. We find that the GRB 030329 host galaxy is a subluminous galaxy (L ∼ 0.016 L � ) with a stellar mass of 10 8 M� . Three independent diagnostics, based on the restframe UV continuum, the [O ii], and the Balmer emission lines, provide a consistent )

The radio delay of the exceptional 3C 454.3 outburst. Follow-up WEBT observations in 2005-2006

Context. In spring 2005 the blazar 3C 454.3 was observed in an unprecedented bright state from the near-IR to the hard X-ray frequencies. A mm outburst peaked in June-July 2005, and it was followed by a flux increase at high radio frequencies. Aims. In this paper we report on multifrequency monitoring by the WEBT aimed at following the further evolution of the outburst in detail. In particular, we investigate the expected correlation and time delays between the optical and radio emissions in order to derive information on the variability mechanisms and jet structure. Methods. A comparison among the light curves at different frequencies is performed by means of visual inspection and discrete correlation function, and the results are interpreted with a simple model taking into account Doppler factor variations of geometric origin. Results. The high-frequency radio light curves show a huge outburst starting during the dimming phase of the optical one and lasting more than 1 year. The first phase is characterized by a slow flux increase, while in early 2006 a major flare is observed. The lower-frequency radio light curves show a progressively delayed and fainter event, which disappears below 8 GHz. We suggest that the radio major peak is not physically connected with the spring 2005 optical one, but it is actually correlated with a minor optical flare observed in October-November 2005. This interpretation | involves both an intrinsic and a geometric mechanism. The former is represented by disturbances travelling down the emitting jet, the latter being due to the curved-jet motion, with the consequent differential changes of viewing angles of the different emitting regions.

On the redshift-apparent size diagram of double radio sources

We review the data on the angular sizes of double radio sources. A list of 540 Fanaroff-Riley type II double sources is collected from literature, and it is used for a critical discussion of previous work based on smaller samples. We find that results from all previous samples agree well with each other and with our new sample, if the discussion is limited only to well-defined two-sided (classical) doubles. Different results which have appeared in literature are traced to different sample definitions of double radio sources. In addition, we find that the behavior of high radio power quasars and radio galaxies is different from low radio power radio galaxies which, if unnoticed, leads to contradicting conclusions

The Faint Afterglow and Host Galaxy of the Short Hard GRB 060121

We present optical and X-ray observations of the afterglow and host galaxy of the short-hard GRB 060121. The faint R-band afterglow is seen to decline as t-0.66±0.09 while the X-ray falls as t-1.18±0.04, indicating the presence of the cooling break between the two frequencies. However, the R-band afterglow is very faint compared to the predicted extrapolation of the X-ray afterglow to the optical regime (specifically, βOX ~ 0.2), while the K-band is consistent with this extrapolation (βKX ~ 0.6), demonstrating suppression of the optical flux. Late-time HST observations place stringent limits on the afterglow R-band flux, implying a break in the R-band light curve. They also show that the burst occurred at the edge of a faint red galaxy, presumably the host, which most likely lies at a significantly higher redshift than the previous optically identified short-duration bursts. Several neighboring galaxies also have very red colors that are similarly suggestive of higher redshift. The least extreme explanation for the faintness and color of the burst is that it occurred at moderately high redshift and was significantly obscured; however, it is also possible that it lies at z > 4.5, in which case the faintness of the R-band afterglow could be attributed to the Lyman break. We discuss the implications that either scenario would have for the nature of the progenitors of short bursts.

The 2005 November Outburst in OJ 287 and the Binary Black Hole Model

We report observations of the largest optical outburst in 20 years in the quasar OJ 287. In some ways it was expected, due to the well-known quasi-periodic 12 yr outburst cycle of OJ 287. In other ways the timing of the outburst was surprising, since calculations based on the periodicity were predicting such an outburst in late 2006. Here we point out that, in the precessing binary black hole model, first proposed by Sillanp?? et al., and later refined by Lehto & Valtonen and Sundelius et al., the precession shifts the first outburst of each outburst season progressively to earlier times relative to the mean period. Thus, in this model, the timing of the outburst is quite acceptable, even if it was not predicted. The next test of the model comes in 2007 September when the second brightness peak is due. It may then be possible to detect the shortening of the binary period due to emission of gravitational waves from the system.

Near infrared monitoring of a sample of blazars with the Carlos Sanchez Telescope. I

We present results of a program of near infrared monitoring of a sample of blazars and miniblazars observed during 1990 and 1991 with the Carlos Sanchez Telescope, in Teide Observatory, Tenerife. All of the objects in the sample show blazar activity during the course of the observations. Several of the objects in the sample have been monitored for rapid variations on timescales of a few hours; only one of them has not, to date, shown such activity.

A Lyα blob and zabs ≈ zem damped Lyα absorber in the dark matter halo of the binary quasar Q 0151+048

Context. Q 0151+048 is a physical quasar (QSO) pair at z ∼ 1.929 with a separation of 3.3 arcsec on the sky. In the spectrum of the brighter member of this pair, Q 0151+048A, a damped Lyα absorber (DLA) is observed at a higher redshift. We have previously detected the host galaxies of both QSOs, as well as a Lyα blob whose emission surrounding Q 0151+048A extends over 5×3.3 arcsec. Aims. We seek to constrain the geometry of the system and understand the possible relations between the DLA, the Lyα blob, and the two QSOs. We also aim at characterizing the former two objects in more detail. Methods. To study the nature of the Lyα blob, we performed low-resolution, long-slit spectroscopy with the slit aligned with the extended emission. We also observed the whole system using the medium-resolution VLT/X-shooter spectrograph and the slit aligned with the two QSOs. The systemic redshift of both QSOs was determined from rest-frame optical emission lines redshifted into the NIR. We employed line-profile fitting technique, to measure metallicities and the velocity width of low-ionization metal absorption lines associated to the DLA and photo-ionization modeling to characterize the DLA further. Results. We measure systemic redshifts of zem(A) = 1.92924± 0.00036 and zem(B) = 1.92863± 0.00042 from the H β and Hα emission lines, respectively. In other words, the two QSOs have identical redshifts within 2σ. From the width of Balmer emission lines and the strength of the rest-frame optical continuum, we estimate the masses of the black holes of the two QSOs to be 109.33 M and 108.38 M for Q 0151+048A and Q 0151+048B, respectively. We then use the correlation between black hole mass and dark matter halo mass to infer the mass of the dark matter halos hosting the two QSOs: 1013.74 M and 1013.13 M for Q 0151+048A and Q 0151+048B, respectively. We observe a velocity gradient along the major axis of the Lyα blob consistent with the rotation curve of a large disk galaxy, but it may also be caused by gas inflow or outflow. We detect residual continuum in the DLA trough, which we interpret as emission from the host galaxy of Q 0151+048A. The derived H0 column density of the DLA is log NH0 = 20.34 ± 0.02 cm−2. Metal column densities are also determined for a number of low-ionization species resulting in an overall metallicity of 0.01 Z . We detect C ii∗, which allows us to make a physical model of the DLA cloud. Conclusions. From the systemic redshifts of the QSOs, we conclude that the Lyα blob is associated with Q 0151+048A rather than with the DLA. The DLA must be located in front of both the Lyα blob and Q 0151+048A at a distance greater than 30 kpc and has a velocity relative to the blob of 640 ± 70 km s−1. The two quasars accrete at normal Eddington ratios. The DM halo of this double quasar will grow to the mass of our local supercluster at z = 0. We point out that those objects therefore form an ideal laboratory to study the physical interactions in a z = 2 precursor of our local supercluster.