Mapping of interstellar clouds with infrared light scattered from dust: TMC-1N

Mapping of near-infrared (NIR) scattered light is a recent method for the study of interstellar clouds, complementing other, more commonly used methods, like dust emission and extinction. Our goal is to study the usability of this method on larger scale, and compare the properties of a filament using NIR scattering and other methods. We also study the radiation field and differences in grain emissivity between diffuse and dense areas. We have used scattered J, H, and K band surface brightness WFCAM-observations to map filament TMC-1N in Taurus, covering an area of 1dx1d corresponding to ~(2.44 pc)^2. We have converted the data into optical depth and compared the results with NIR extinction and Herschel observations of submm dust emission. We see the filament in scattered light in all three NIR bands. We note that our WFCAM observations in TMC-1N show notably lower intensity than previous results in Corona Australis using the same method. We show that 3D radiative transfer simulations predict similar scattered surface brightness levels as seen in the observations. However, changing the assumptions about the background can change the results of simulations notably. We derive emissivity by using optical depth in the J band as an independent tracer of column density. We obtain opacity sigma(250um) values 1.7-2.4x10^-25 cm^2/H, depending on assumptions of the extinction curve, which can change the results by over 40%. These values are twice as high as obtained for diffuse areas, at the lower limit of earlier results for denser areas. We show that NIR scattering can be a valuable tool in making high resolution maps. We conclude, however, that NIR scattering observations can be complicated, as the data can show relatively low-level artefacts. This suggests caution when planning and interpreting the observations.Comment: abstract shortened and figures reduced for astrop

Cosmology with redshift surveys of radio sources

We use the K-z relation for radio galaxies to illustrate why it has proved difficult to obtain definitive cosmological results from studies based entirely on catalogues of the brightest radio sources, e.g. 3C. To improve on this situation we have been undertaking redshift surveys of complete samples drawn from the fainter 6C and 7C radio catalogues. We describe these surveys, and illustrate the new studies they are allowing. We also discuss our `filtered' 6C redshift surveys: these have led to the discovery of a radio galaxy at z=4.4, and are sensitive to similar objects at higher redshift provided the space density of these objects is not declining too rapidly with z. There is currently no direct evidence for a sharp decline in the space density of radio galaxies for z > 4, a result only barely consistent with the observed decline of flat-spectrum radio quasars.Comment: 8 pages Latex, To appear in the "Cosmology with the New Radio Surveys" Conference - Tenerife 13-15 January 199

Mid-infrared selection of quasar-2s in Spitzer's First Look Survey

We present early results from the spectroscopic follow-up of a sample of candidate obscured AGN selected in the mid-infrared from the Spitzer First Look Survey. Our selection allows a direct comparison of the numbers of obscured and unobscured AGN at a given luminosity for the first time, and shows that the ratio of obscured to unobscured AGN at infrared luminosities corresponding to low luminosity quasars is ~1:1 at z~0.5. Most of our optically-faint candidate obscured AGN have the high-ionization, narrow-line spectra expected from type-2 AGN. A composite spectrum shows evidence for Balmer absorption lines, indicating recent star-formation activity in the host galaxies. There is tentative evidence for a decrease in the obscured AGN fraction with increasing AGN luminosity.Comment: To appear in the proceedings of the workshop "Multiband approach to AGN" Bonn October 2004 in Memorie della Societa Astronomica Italian

Deep spectroscopy of z~1 6C radio galaxies - II. Breaking the redshift-radio power degeneracy

The results of a spectroscopic analysis of 3CR and 6C radio galaxies at redshift z~1 are contrasted with the properties of lower redshift radio galaxies, chosen to be matched in radio luminosity to the 6C sources studied at z~1, thus enabling the P-z degeneracy to be broken. Partial rank correlations and principal component analysis have been used to determine which of z and P are the critical parameters underlying the observed variation of the ionization state andd kinematics of the emission line gas. [OII]/H-beta is shown to be a useful ionization mechanism diagnostic. Statistical analysis of the data shows that the ionization state of the emission line gas is strongly correlated with radio power, once the effects of other parameters are removed. No dependence of ionization state on z is observed, implying that the ionization state of the emission line gas is solely a function of the AGN properties rather than the hostt galaxy and/or environment. Statistical analysis of the kinematic properties of the emission line gas shows that these are strongly correlated independently withh both P and z. The correlation with redshift is the stronger of the two, suggesting that host galaxy composition or environment may play a role in producing the less extreme gas kinematics observed in the emission line regions of low redshift galaxies. For both the ionization and kinematic properties of thee galaxies, the independent correlations observed with radio size are strongest. Radio source age is a determining factor for the extended emission line regions.Comment: 10 pages, 5 figures, accepted for publication in MNRA

Gravitational instabilities in a protosolar-like disc - I. Dynamics and chemistry

MGE gratefully acknowledges a studentship from the European Research Council (ERC; project PALs 320620). JDI gratefully acknowledges funding from the European Union FP7-2011 under grant agreement no. 284405. ACB's contribution was supported, in part, by The University of British Columbia and the Canada Research Chairs program. PC and TWH acknowledge the financial support of the European Research Council (ERC; project PALs 320620).To date, most simulations of the chemistry in protoplanetary discs have used 1 + 1D or 2D axisymmetric α-disc models to determine chemical compositions within young systems. This assumption is inappropriate for non-axisymmetric, gravitationally unstable discs, which may be a significant stage in early protoplanetary disc evolution. Using 3D radiative hydrodynamics, we have modelled the physical and chemical evolution of a 0.17 M⊙ self-gravitating disc over a period of 2000 yr. The 0.8 M⊙ central protostar is likely to evolve into a solar-like star, and hence this Class 0 or early Class I young stellar object may be analogous to our early Solar system. Shocks driven by gravitational instabilities enhance the desorption rates, which dominate the changes in gas-phase fractional abundances for most species. We find that at the end of the simulation, a number of species distinctly trace the spiral structure of our relatively low-mass disc, particularly CN. We compare our simulation to that of a more massive disc, and conclude that mass differences between gravitationally unstable discs may not have a strong impact on the chemical composition. We find that over the duration of our simulation, successive shock heating has a permanent effect on the abundances of HNO, CN and NH3, which may have significant implications for both simulations and observations. We also find that HCO+ may be a useful tracer of disc mass. We conclude that gravitational instabilities induced in lower mass discs can significantly, and permanently, affect the chemical evolution, and that observations with high-resolution instruments such as Atacama Large Millimeter/submillimeter Array (ALMA) offer a promising means of characterizing gravitational instabilities in protosolar discs.Publisher PDFPeer reviewe

Molecular line profiles as diagnostics of protostellar collapse: modelling the `blue asymmetry' in inside-out infall

The evolution of star-forming core analogues undergoing inside-out collapse is studied with a multi-point chemodynamical model which self-consistently computes the abundance distribution of chemical species in the core. For several collapse periods the output chemistry of infall tracer species such as HCO+, CS, and N2H+, is then coupled to an accelerated Lambda-iteration radiative transfer code, which predicts the emerging molecular line profiles using two different input gas/dust temperature distributions. We investigate the sensitivity of the predicted spectral line profiles and line asymmetry ratios to the core temperature distribution, the time-dependent model chemistry, as well as to ad hoc abundance distributions. The line asymmetry is found to be strongly dependent on the adopted chemical abundance distribution. In general, models with a warm central region show higher values of blue asymmetry in optically thick HCO+ and CS lines than models with a starless core temperature profile. We find that in the formal context of Shu-type inside-out infall, and in the absence of rotation or outflows, the relative blue asymmetry of certain HCO+ and CS transitions is a function of time and, subject to the foregoing caveats, can act as a collapse chronometer. The sensitivity of simulated HCO+ line profiles to linear radial variations, subsonic or supersonic, of the internal turbulence field is investigated in the separate case of static cores.Comment: Accepted to MNRAS; 20 pages, 13 fig

What Powers the Compact Radio Emission in Nearby Elliptical and S0 Galaxies?

Many nearby early-type (elliptical and S0) galaxies contain weak (milli-Jansky level) nuclear radio sources on scales a few hundred parsecs or less. The origin of the radio emission, however, has remained unclear, especially in volume-limited samples that select intrinsically less luminous galaxies. Both active galactic nuclei and nuclear star formation have been suggested as possible mechanisms for producing the radio emission. This paper utilizes optical spectroscopic information to address this issue. A substantial fraction of the early-type galaxies surveyed with the Very Large Array by Wrobel & Heeschen (1991) exhibits detectable optical emission lines in their nuclei down to very sensitive limits. Comparison of the observed radio continuum power with that expected from the thermal gas traced by the optical emission lines implies that the bulk of the radio emission is nonthermal. Both the incidence and the strength of optical line emission correlate with the radio power. At a fixed line luminosity, ellipticals have stronger radio cores than S0s. The relation between radio power and line emission observed in this sample is consistent with the low-luminosity extension of similar relations seen in classical radio galaxies and luminous Seyfert nuclei. A plausible interpretation of this result is that the weak nuclear sources in nearby early-type galaxies are the low-luminosity counterparts of more powerful AGNs. The spectroscopic evidence supports this picture. Most of the emission-line objects are optically classified as Seyfert nuclei or low-ionization nuclear emission-line regions (LINERs), the majority of which are likely to be accretion-powered sources.Comment: LaTex, 16 pages including embedded figures. Accepted for publication in the Astrophysical Journa