THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT 2003-2004

2. Scientific Highlights

Semester 04A saw the completion of Rawlings' program of photometry of candidate z>4 radio galaxies in the 6C survey (Cruz et al. astro-ph 0403366). About 40% of the sample are found to have z>2 and five objects possibly have z>5.

An example of a successful multi-wavelength campaign is the announcement by Stevens et al. (2004, ApJ 604 L17) of a 400 kpc long chain of star forming galaxies in the region around a z = 1.8 X-ray selected QSO, discovered by combining UKIRT K-band imaging data with WHT R-band images and JCMT SCUBA maps (Figure 1). The authors suggest that this star formation is associated with galaxy mergers or encounters within the filament, such as those predicted by the popular hierarchical model of galaxy formation. The observations suggest that strong absorption in the X-ray spectra of QSOs at high redshifts may result from a veil of gas thrown up by a merger or merger-induced activity, rather than an orientation-dependent obscuring torus. It is argued that these systems are the precursors of elliptical galaxies found today in the core regions of all rich galaxy clusters.

Figure 1: 450 µm S/N contours (2, 3, 3.5, 4 and 5 sigma) overlaid on the UFTI K-band image. The QSO which has an absorbed X-ray spectrum and is at z = 1.819 is indicated by the arrow. From Stevens et al. 2004.

Figure 2: Velocity field of N2 850.4 from UIST IFU Hα emission overlaid on the HST NICMOS H-band image. From Swinbank et al. 2005.

CGS4 continues to make significant contributions to UKIRT science, fourteen years after first light. Hatch et al. (2005 MNRAS 358, 765) detect molecular hydrogen in the outer filaments of the Hα nebula surrounding the central galaxy of the Perseus cluster, NGC 1275, which implies the presence of warm molecular gas clouds at distances up to 24 kpc from the nucleus of the host galaxy. The warm gas is embedded in the hot intracluster medium and the emission-line intensity ratios reveal that the molecular hydrogen is predominantly thermally excited. The excitation temperature of 1600-2200K suggests a lack of pressure balance between the molecular component and its surrounding medium. Excitation by stellar UV or by the central AGN is unlikely, whilst thermal excitation by X-rays or conduction from the ICM or shocks are all possible. Evidence for a non-thermal component is found in the spectra of some regions based on a low ortho-to-para ratio.

Figure 3: Left: WIYN image of the structure surrounding NGC1275 (Conselice et al. 2001). Right: details of the Eastern slit region. From Hatch et al. 2005.

Figure 4: CGS4 spectra of the regions identified in Figure 3, line labels have redshift z=0.0172. From Hatch et al. 2005.

2.2. Stellar/Substellar

Figer et al. (2005 ApJ 622, L49) used UKIRT UIST and Keck NIRSPEC spectra in their discovery of hot and massive stars in the cluster surrounding the soft gamma repeater SGR 1806-20. Three of the newly identified stars are Wolf-Rayet stars of types WC8, WN6, and WN7, and a fourth star is an OB supergiant. These stars, along with four previously discovered, and the absence of red supergiants, imply a cluster age of 3.0 to 4.5 Myr. Assuming coevality, this age suggests that the progenitor of SGR 1806-20 had an initial mass greater than 50 M_solar which is consistent with SGRs being post-supernova end states of massive progenitors, and which may suggest that only massive stars evolve into magnetars that produce SGRs. It also suggests that very massive stars can evolve into neutron stars, not just black holes, as recently predicted by theory.

Figure 5: UIST spectra of the newly discovered W-R stars (Figer et al. 2005).

Leggett and collaborators continued their followup of candidate brown dwarfs identified in the Sloan Digital Sky Survey. The work was summarised in two publications Knapp et al. (2004 AJ 127 3553) and Golimowski et al. (2004 AJ 127 3516). Golimowski et al. used UKIRT L' and M' photometry to derive accurate bolometric luminosities and hence effective temperatures for those dwarfs with measured parallaxes; the relationship is shown in Figure 6. Dwarfs with type L7 to T4 have nearly constant T_eff of 1450 K, implying that the observed spectral changes are controlled by changes in the condensate cloud decks.

Figure 6: BC_K and T_eff for L and T dwarfs, from Golimowski et al. 2004.

2.3. Circumstellar Disks and Envelopes

Oliveira et al. (2004, MNRAS 347 1327) have carried out a survey for circumstellar disks around K- and M-type stars in the young σ Ori cluster through K and L' imaging. They have identified objects with (K - L') excesses by analysing colour-colour diagrams and comparing the observations with empirical main-sequence colours. The derived disk frequency is around 50%. They show that the (K - L') excess is a robust and reliable disk indicator. If the age of σ Ori is 3 Myr then these results support the overall disk lifetime of 6 Myr that has been proposed in the literature, however an age >4 Myr would imply a slower disk destruction rate. The team later applied this technique to NGC 6611, where they find that 58% of pre-main sequence objects with masses in the range 0.45 to 2 solar masses have disks. Hence the harsher environment of this cluster does not appear to hasten disk dissipation (Oliveira et al. 2005 MNRAS 358 L21).

2.4. Transient Objects

Semester 03A produced an uncommonly high rate of transient sources, with several Gamma-Ray Bursts and supernovae overriding PATT observers. One paper to which these observations contributed is that by Castro-Tirado et al. (2003 A&A 411, L315) which presents multiwavelength observations of GRB 030227 taken between 5.3 hours and 1.7 days after the event. The power-law decline of the afterglow and the spectral index may be explained by a relativistic expansion of a fireball in the cooling regime.

Figure 7:The broadband spectrum for the GRB 030227 afterglow at t₀+0.87 days. The solid line shows the near-infrared/optical spectrum and the dotted line the absorbed X-ray spectrum from XMM-Newton, which can be unabsorbed and represented by a power law, shown as the dashed line. From Castro-Tirado et al. 2003.

Figure 8: Sum of the observed spectra (gray line) overlaid with a model spectrum of ²⁷AlO (black line) for an assumed rotational temperature of 200 K. A good match is found except for the broad feature marked with an asterisk, which is due to an unidentified species and not AlO. Bottom: Effect on the pure ²⁷AlO spectrum of the top panel by including a contribution from ²⁶Al at different strengths. From Banerjee et al. 2004.

2.5. Solar System

Mars was at a favourable opposition in Semester 03B and Bailey and collaborators had a very successful imaging and spectroscopy campaign as described in the Autumn 2003 issue of the UKIRT Newsletter.

Figure 9: Mars imaged by UKIRT on August 17th 2003, when Mars was at a distance of 57 million km from Earth and had an apparent diameter of 24.6 arc seconds. The observers were Jeremy Bailey and Sarah Chamberlain (Anglo-Australian Observatory and Australian Centre for Astrobiology, Macquarie University).