Chris Simpson

The detection and photometric redshift determination of distant galaxies using sirtf's infrared array camera

We investigate the ability of the Space Infrared Telescope Facilitys Infrared Array Camera to detect distant (z ~ 3) galaxies and measure their photometric redshifts. Our analysis shows that changing the original long-wavelength filter specifications provides significant improvements in performance in this and other areas.

The Nonstellar Infrared Continuum of Seyfert Galaxies

JHKLM (1–5 μm) imaging of a sample of Seyfert 2 galaxies is presented. We have performed an accurate estimate of the near-infrared nonstellar nuclear fluxes. We confirm that the near-infrared nuclear continuum between 1 and 2.2 μm of some Seyfert 2s is dominated by stellar emission, whereas the continuum emission at longer wavelengths (λ = 3–5 μm) is almost entirely nonstellar in origin. The nonstellar spectral energy distributions (SED) in the infrared (up to 15 μm) of Seyfert galaxies show a variety of shapes, and they are well reproduced with the tapered disk models of Efstathiou & Rowan-Robinson. We have used two models, one including an optically thin cone component found to fit the SED of NGC 1068 and a coneless model. Although our modeling of the SEDs does not allow us to favor either model to account for all the observed SEDs, we find that the viewing angle toward the central source is well constrained by both models. The galaxies in our sample have fitted values of the viewing angle in the range θv = 0°–64°, for the assumed model parameters. We have also investigated nonstellar color-color diagrams (L-M vs. H-M and L-M vs. H-L). The colors of the Seyfert galaxies with viewing angles θv < 30° are better reproduced with the cone model. These diagrams provide a good means to separate Seyfert 2s with moderate obscuration (AV 20 mag from hard X-ray observations) from those with high obscuration. The ground-based 4.8 μm and ISO 9.6 μm luminosities are well correlated with the hard X-ray luminosities of Seyfert 1s and 2s. These continuum emissions appear as a good indicator of the AGN luminosity, at least in the cases of hard X-ray Compton-thin Seyfert galaxies (NH ≤ 1024 cm-2). We finally stress the finding that some Compton thick galaxies show bright nonstellar emission at 5 μm. This suggests that the near-infrared emission in Seyfert galaxies is produced in an extended component illuminated by the central source, that is more visible from all viewing angles, providing a good explanation for the differing NH/AV ratios found in some Seyfert 2s. We discuss possible implications of mid-infrared surveys for the search of counterparts of highly obscured hard X-ray sources.

Thermal-infrared imaging of 3C radio galaxies at z~1

We present the results of a programme of thermal-infrared imaging of 19 z∼ 1 radio galaxies from the 3CR and 3CRR (LRL) samples. We detect emission at L′ (3.8 μm) from four objects; in each case the emission is unresolved at 1-arcsec resolution. 15 radio galaxies remain undetected to sensitive limits of L′≈ 15.5. Using these data in tandem with archived HST data and near-infrared spectroscopy, we show that three of the detected ‘radio galaxies’ (3C 22, 3C 41 and 3C 65) harbour quasars reddened by AV≲ 5 mag. Correcting for this reddening, 3C 22 and 3C 41 are very similar to coeval 3C quasars, whilst 3C 65 seems unusually underluminous. The fourth radio galaxy detection (3C 265) is a more highly obscured (AV∼ 15) but otherwise typical quasar, which previously has been evident only in scattered light. We determine the fraction of dust-reddened quasars at z∼ 1 to be 28−13+25 per cent at 90 per cent confidence. On the assumption that the undetected radio galaxies harbour quasars similar to those in 3C 22, 3C 41 and 3C 265 (as seems reasonable, given their similar narrow emission line luminosities) we deduce extinctions of AV≳: 15 towards their nuclei. The contributions of reddened quasar nuclei to the total K-band light range from ∼0 per cent for the non-detections, through ∼10 per cent for 3C 265 to ∼80 per cent for 3C 22 and 3C 41. Correcting for these effects does not remove the previously reported differences between the K magnitudes of 3C and 6C radio galaxies, so contamination by reddened quasar nuclei is not a serious problem for drawing cosmological conclusions from the K–z relation for radio galaxies. We discuss these results in the context of the ‘receding torus’ model which predicts a small fraction of lightly reddened quasars in samples of high-radio-luminosity sources. We also examine the likely future importance of thermal-infrared imaging in the study of distant powerful radio sources.

Mid-infrared spectroscopy of candidate active galactic nuclei-dominated submillimeter galaxies

Spitzer spectroscopy has revealed that ≃80% of submm galaxies (SMGs) are starburst (SB)-dominated in the mid-infrared. Here we focus on the remaining ≃20% that show signs of harboring powerful active galactic nuclei (AGNs). We have obtained Spitzer-InfraRed Spectrograph spectroscopy of a sample of eight SMGs that are candidates for harboring powerful AGNs on the basis of IRAC color selection (S_(8 μm)/S_(4.5 μm) > 2, i.e., likely power-law mid-infrared spectral energy distributions). SMGs with an AGN dominating (≳50%) their mid-infrared emission could represent the missing link sources in an evolutionary sequence involving a major merger. First of all, we detect polycyclic aromatic hydrocarbon (PAH) features in all of the SMGs, indicating redshifts from 2.5 to 3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Second, we see signs of both star formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with 1.65 works well at selecting mid-infrared energetically dominant AGNs in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.

Hubble Space Telescope Imaging of the Circinus Galaxy

We present a Hubble Space Telescope imaging study of the nearby (4 Mpc distant) Circinus galaxy, which contains the nearest type 2 Seyfert nucleus and prominent circumnuclear star formation. Images have been obtained in the [O III] ?5007, H?, and H2 v = 1?0 S(1) emission lines, and in the green (5470 ?), red (8140 ?), and near-infrared (2.04 and 2.15 ?m) continua. An image in the [Fe II] ?1.644 ?m line has been taken with a ground-based telescope. The [O III] and H? images reveal the detailed structure of the complex of streamers and knots of high-excitation gas, which extends out of the galaxy disk. The morphology some 250 pc from the nucleus strongly suggests that the high-excitation gas is concentrated on the surface of a hollow cone with apex close to the nucleus. Such a structure may result through entrainment of dense gas from a circumnuclear torus in the galaxy disk by a low-density, outflowing wind or jet. Within 40 pc of the nucleus, the high-excitation gas takes the form of a striking, filled V-shaped structure extending in the same direction as the larger scale high-excitation emission. This V can be described as an ionization cone, though a matter-bounded structure is also possible. The implied collimation of the ionizing photons or gaseous outflow must occur within 2 pc of the apex of the cone, presumed to be the location of the nucleus. The H? image shows a complex structure of H II regions, including the well-known starburst ring of radius 150?270 pc. In addition, there is a more compact (40 pc radius), elliptical ring of H II regions around the ionization cone. We argue that this latter ring, which we call the nuclear ring, is intrinsically circular and located in the plane of the galaxy disk. Much of the [Fe II] emission is associated with this nuclear, star-forming ring and is presumably powered by supernova remnants. Hot molecular hydrogen extends to within 10 pc of the nucleus, and possibly closer. The intrinsic infrared?optical continuum colors in the inner regions of the Circinus galaxy are, in many locations, bluer than is typical of bulges, indicating a relatively young stellar population is present. We confirm the presence of a compact (<2 pc), very red nuclear source in the K band. Its properties are consistent with a type 1 Seyfert nucleus viewed through an obscuration of AV = 28 ? 7 mag.

Infrared photometry of z ∼ 1 3C quasars

We present JHKL′ photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65z<1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1xa0μm. About 75xa0per cent of the quasars are tightly grouped in the plane of optical spectral index, αopt, versus near-infrared spectral index, αIR, with the median value of αopt close to the canonical value, and the median αIR slightly flatter. We conclude that the fraction of moderately obscured, red quasars decreases with increasing radio power, in accordance with the ‘receding torus’ model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light.

PDS 456: an extreme accretion rate quasar?

We present quasi-simultaneous ASCA and RXTE observations of the most luminous known active galactic nucleus in the local (z<0.3) Universe, the recently discovered quasar PDS 456. Multiwavelength observations have been conducted that show that PDS 456 has a bolometric luminosity of ∼1047xa0ergxa0s−1xa0peaking in the ultraviolet part of the spectrum. In the X-ray band the 2–10xa0keV (rest-frame) luminosity is 1045xa0ergxa0s−1. The broad-band X-ray spectrum obtained with ASCA and RXTE contains considerable complexity. The most striking feature observed is a very deep, ionized iron K edge, observed at 8.7xa0keV in the quasar rest-frame. We find that these features are consistent with reprocessing from highly ionized matter, probably the inner accretion disc. PDS 456 appeared to show a strong (factor of ∼2.1) outburst in just ∼17xa0ks, although non-intrinsic sources cannot be completely ruled out. If confirmed, this would be an unusual event for such a high-luminosity source, with a light-crossing-time corresponding to ∼2RS. The implication would be that flaring occurs within the very central regions, or else that PDS 456 is a ‘super-Eddington’ or relativistically beamed system. Overall we conclude on the basis of the extreme blue/UV luminosity, the rapid X-ray variability and from the imprint of highly ionized material on the X-ray spectrum, that PDS 456 is a quasar with an unusually high accretion rate.

The optical counterpart of the accreting millisecond pulsar SAX J1808.4-3658 in outburst: constraints on the binary inclination

We present multiband optical/IR photometry of V4580 Sgr, the optical counterpart of the accretion-powered millisecond pulsar SAX J1808.4-3658, taken during the 1998 X-ray outburst of the system. The optical flux is consistent with emission from an X-rayheated accretion disk. Self-consistent modeling of the X-ray and optical emission during the outburst yields a best-fit extinction of AV=0.68+0.37-0.15 and an inclination of cosi=0.65+0.23-0.33 (90% confidence), assuming a distance of 2.5 kpc. This inclination range requires that the stellar companion of the pulsar has extremely low mass, Mc=0.050.10 M. Some of the IR observations are inconsistent with disk emission and are too bright to be from either the disk or the companion, even in the presence of X-ray heating.

Obscured active galactic nuclei from the ELAIS Deep X-ray Survey

The sources discovered in deep hard X-ray surveys with 2-8 keV fluxes of S2 8 � 10 14 ergcm 2 s 1 make up the bulk of the X-ray background at these energies. We present here detailed multi-wavelength observations of three such sources from the ELAIS Deep X-ray Survey. The observations include sensitive near-infrared spectroscopy with the Subaru Telescope and X-ray spectral information from the Chandra X-ray Observatory. The sources observed all have optical-to-near-IR colours redder than an unobscured quasar and comprise a reddened quasar, a radio galaxy and an optically-obscured AGN. The reddened quasar is at a redshift z = 2.61 and shows a very large X-ray absorbing column of NH � 3 × 10 23 cm 2 . This contrasts with the relatively small amount of dust reddening, implying a gas-to-dust ratio along the line-of-sight a hundred times greater than that of the Milky Way. The radio galaxy at z = 1.57 shows only narrow emission lines, but has a surprisingly soft X-ray spectrum. The softness of this spectrum either indicates an unusually low gas-to-dust ratio for the absorbing medium or X-ray emission related to the young radio source. The host galaxy is extremely red (R K = 6.4) and its optical/near-IR spectrum is best fit by a strongly reddened (AV � 2) starburst. The third X-ray source discussed is also extremely red (R K = 6.1) and lies in a close grouping of three other R K > 6 galaxies. No emission or absorption lines were detected from this object, but its redshift (and that of one of the nearby galaxies) are constrained by SED-fitting to be just greater than z = 1. The extremely red colours of these two galaxies can be accounted for by old stellar populations. These observations illustrate the diverse properties of hard X-ray selected AGN at high redshift in terms of obscuration at optical and X-ray wavelengths and the evolutionary states of their host galaxies.

Near-Infrared and X-Ray Obscuration to the Nucleus of the Seyfert 2 Galaxy NGC 3281

We present the results of a near-infrared and X-ray study of the Seyfert 2 galaxy NGC 3281. Emission from the Seyfert nucleus is detected in both regions of the electromagnetic spectrum, allowing us to infer both the equivalent line of sight hydrogen column density, N=71.0+ 11.3−12.3×1026 m-2, and the extinction due to dust, AV = 22 ± 11 mag (90% confidence intervals). We infer a ratio of NH/AV that is an order of magnitude larger than that determined along lines of sight in the Milky Way and discuss possible interpretations. We consider the most plausible explanation to be a dense cloud in the foreground of both the X-ray and infrared-emitting regions that obscures the entire X-ray source but only a fraction of the much larger infrared source.