Andrew Bunker

Abstract:

We present deep Keck spectroscopy, using the Deep Imaging Multi-Object Spectrograph and the Low-Resolution Imaging Spectrometer spectrographs, of a large and representative sample of 67 extremely red objects (EROs) to H = 20.5 in three fields (SSA22, Chandra Deep Field South and NTT Deep Field) drawn from the Las Campanas Infrared Survey (LCIRS). Using the colour cut (I - H) > 3.0 (Vega magnitudes) adopted in earlier papers in this series, we verify the efficiency of this selection for locating and studying distant old sources. Spectroscopic redshifts are determined for 44 sources, of which only two are contaminating low-mass stars. When allowance is made for incompleteness, the spectroscopic redshift distribution closely matches that predicted earlier on the basis of photometric data. Our spectra are of sufficient quality that we can address the important question of the nature and homogeneity of the z > 0.8 ERO population. A dominant old stellar population is inferred for 75 per cent of our spectroscopic sample, a higher fraction than that seen in smaller, less complete samples with broader photometric selection criteria (e.g. R - K). However, only 28 per cent have spectra with no evidence of recent star formation activity, such as would be expected for a strictly passively evolving population. More than ∼30 per cent of our absorption-line spectra are of the 'E+A' type with prominent Balmer absorption consistent, on average, with mass growth of 5-15 per cent in the past gigayear. We use our spectroscopic redshifts to improve earlier estimates of the spatial clustering of this population as well as to understand the significant field-to-field variation. Our spectroscopy enables us to pinpoint a filamentary structure at z = 1.22 in the Chandra Deep Field South. Overall, our study suggests that the bulk of the ERO population is an established population of clustered massive galaxies undergoing intermittent activity consistent with continued growth over the redshift interval 0.8 < z < 1.6. © 2005 RAS.

Abstract:

We analyse near-infrared Hubble Space Telescope (HST)/Near-Infrared Camera and Multi-Object Spectrometer F110W (J) and F160W (H) band photometry of a sample of 27 i′-drop candidate z ≃ 6 galaxies in the central region of the HST/Advanced Camera for Surveys Ultra Deep Field. The infrared colours of the 20 objects not affected by near neighbours are consistent with a high-redshift interpretation. This suggests that the low-redshift contamination of this i′-drop sample is smaller than that observed at brighter magnitudes, where values of 10-40 per cent have been reported. The J-H colours are consistent with a slope flat in fv(fλ ∝ λ-2), as would be expected for an unreddened starburst. However, there is evidence for a marginally bluer spectral slope (f λ ∝ λ-2.2), which is perhaps indicative of an extremely young starburst (∼10 Myr old) or a top heavy initial mass function and little dust. The low levels of contamination, median photometric redshift of z ∼ 6.0 and blue spectral slope, inferred using the near-infrared data, support the validity of the assumptions in our earlier work in estimating the star formation rates, and that the majority of the i-drop candidates galaxies lie at z ∼ 6. © 2005 RAS.

Abstract:

We present optical and near-infrared integral field spectroscopy of the SCUBA galaxy SMM J163650.43+405734.5 (ELAIS N2 850.4) at z = 2.385. We combine Lyα and Hα emission line maps and velocity structure with high-resolution HST ACS and NICMOS imaging to probe the complex dynamics of this vigorous starburst galaxy. The imaging data show a complex morphology, consisting of at least three components separated by ∼1 arcsec (8 kpc) in projection. When combined with the Hα velocity field from UKIRT UIST IFU observations we identify two components whose redshifts are coincident with the systemic redshift, measured from previous CO observations, one of which shows signs of AGN activity. A third component is offset by 220 ± 50 km s -1 from the systemic velocity. The total star-formation rate of the whole system (estimated from the narrow-line Hα and uncorrected for reddening) is 340 ± 50 M⊙ yr-1. The Lyα emission mapped by the GMOS IFU covers the complete galaxy and is offset by +270 ± 40 km s-1 from the systemic velocity. This velocity offset is comparable to that seen in rest-frame UV-selected galaxies at similar redshifts and usually interpreted as a starburst-driven wind. The extended structure of the Lyα emission suggests that this wind is not a nuclear phenomenon, but is instead a galactic-scale outflow. Our observations suggest that the vigorous activity in N2 850.4 is arising as a result of an interaction between at least two dynamically-distinct components, resulting in a strong starburst, a starburst-driven wind and actively-fuelled AGN activity. Whilst these observations are based on a single object, our results clearly show the power of combining optical and near-infrared integral field spectroscopy to probe the power sources, masses and metallicities of far-infrared luminous galaxies, as well as understanding the role of AGN- and starburst-driven feedback processes in these high-redshift systems. © 2005 RAS.