Dr Aprajita Verma

Abstract:

We present 7-180 μm photometry of a sample of hyperluminous infrared galaxies (HyLIGs) obtained with the photometer and camera mounted on the Infrared Space Observatory. We have used radiative transfer models of obscured starbursts and dusty torii to model their spectral energy distributions (SEDs). We find that IRAS F00235+1024, IRAS F14218+3845 and IRAS F15307+3252 require a combination of starburst and active galactic nuclei (AGN) components to explain their mid-to far-infrared (FIR) emission, while for TXS 0052+471 a dust torus AGN model alone is sufficient. For IRAS F00235+1024 and IRAS F14218+3845 the starburst component is the predominant contributor, whereas for IRAS F15307+3252 the dust torus component dominates. The implied star formation rates (SFRs) for these three sources estimated from their infrared luminosities are M*,all > 3000 M⊙ yr-1 h-250 and are amongst the highest SFRs estimated to date. We also demonstrate that the well-known radio-FIR correlation extends into both higher radio and infrared power than previously investigated. The relation for HyLIGs has a mean q value of 1.94. The results of this study imply that better sampling of the infrared spectral energy distributions of HyLIGs may reveal that both AGN and starburst components are required to explain all the emission from the near-infrared to the submillimetre.

Abstract:

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory (ISO). The final map in each band was constructed by the co-addition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 6.7 and 15 μm. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints that they impose on models of galaxy evolution, given the volume of space sampled by this galaxy population.

Abstract:

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory (ISO). We find reliable optical/near-infrared associations for 32 of the 35 sources detected in this field by Oliver et al. (Paper I): eight of them are identified as stars, one is definitely an active galactic nucleus (AGN), a second seems likely to be an AGN too, while the remaining 22 appear to be normal spiral or starburst galaxies. Using model spectral energy distributions (SEDs) of similar galaxies, we compare methods for estimating the star formation rates (SFRs) in these objects, finding that an estimator based on integrated (3-1000 μm) infrared luminosity reproduces the model SFRs best. Applying this estimator to model fits to the SEDs of our 22 spiral and starburst galaxies, we find that they are forming stars at rates of ∼ 1-100M⊙yr-1, with a median value of ∼40M⊙yr-1, assuming an Einstein-de Sitter universe with a Hubble constant of 50km s-1 Mpc-1, and star formation taking place according to a Salpeter initial mass function (IMF) across the mass range 0.1-100 M⊙. We split the redshift range 0.0 ≤ z ≤ 0.6 into two equal-volume bins to compute raw estimates of the star formation rate density, ṗ*, contributed by these sources, assuming the same cosmology and IMF as above and computing errors based on estimated uncertainties in the SFRs of individual galaxies. We compare these results with other estimates of ṗ* made with the same assumptions, showing them to be consistent with the results of Flores et al. from their ISO survey of the CFRS 1415+52 field. However, the relatively small volume of our survey means that our ṗ* estimates suffer from a large sampling variance, implying that our results, by themselves, do not place tight constraints on the global mean star formation rate density.

Abstract:

We present Hubble Space Telescope Wide Field Planetary Camera 2 I-band imaging for a sample of nine hyperluminous infrared galaxies (HLIRGs) spanning a redshift range 0.45 < z < 1.34. Three of the sample have morphologies showing evidence for interactions and six are quasi-stellar objects (QSOs). Host galaxies in the QSOs are detected reliably out to z ∼ 0.8. The detected QSO host galaxies have an elliptical morphology with scalelengths spanning 6.5 < re (kpc) < 88 and absolute k-corrected magnitudes spanning −24.5 < MI < −25.2. There is no clear correlation between the infrared (IR) power source and the optical morphology. None of the sources in the sample, including F15307+3252, shows any evidence for gravitational lensing. We infer that the IR luminosities are thus real. Based on these results, and previous studies of HLIRGs, we conclude that this class of object is broadly consistent with being a simple extrapolation of the ULIRG population to higher luminosities; ULIRGs being mainly violently interacting systems powered by starbursts and/or active galactic nuclei. Only a small number of sources, the infrared luminosities of which exceed 1013 L⊙, are intrinsically less luminous objects that have been boosted by gravitational lensing.

Abstract:

We present deep optical and near infrared imaging over half square degree of sky in the centres of the ELAIS regions N1 and N2 and coincident with deep XMM/Chandra observations. The data have been obtained with the Wide Field Camera (WFC) and the Cambridge InfraRed Survey Instrument (CIRSI) both at the Isaac Newton Telescope (Observatorio del Roque de los Muchachos, Canary Islands). Limiting magnitudes achieved are g'=26.7, r'=26.2, i'=25.0 and H=20.2 (3σ). These data have been used to identify the faint optical counterparts of the Infrared Space Observatory (ISO), radio and X-ray sources in these areas.