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Abstract:

We present near-infrared (near-IR) spectroscopy of three extremely red objects (EROs) using the OHS/CISCO spectrograph at the Subaru Telescope. One target exhibits a strong emission line, which we identify as Ha at z = 1.34. Using new and existing ground-based optical and near-IR imaging, and archival Hubble Space Telescope imaging, we argue that this target is essentially an elliptical galaxy, with an old stellar population of around 4 × 10 ¹¹ M⊙, but having a dust-enshrouded star-forming component with a star formation rate (SFR) of some 50-100 M⊙ yr^-1. There is no evidence that the galaxy contains an active galactic nucleus. Analysis of a further two targets, which do not exhibit any features in our near-IR spectra, suggests that one is a quiescent galaxy in the redshift range 1.2 < z < 1.6, but that the other cannot be conclusively categorized as either star-forming or quiescent. Even though our first target has many of the properties of an old elliptical, the ongoing star formation means that it cannot have formed all of its stellar population at high redshift. While we cannot infer any robust values for the SFR in ellipticals at z > 1 from this one object, we argue that the presence of an object with such a high SFR in such a small sample suggests that a non-negligible fraction of the elliptical galaxy population may have formed a component of their stellar population at redshifts z ∼ 1-2. We suggest that this is evidence for ongoing star formation in the history of elliptical galaxies. © 2005 RAS.

Abstract:

We present BVRIK images and spectroscopic observations of the z = 0.17 host galaxy of the compact steep-spectrum (CSS) radio source MRC B1221 -423. This is a young (∼10⁵ yr) radio source with double lobes lying well within the visible galaxy. The host galaxy is undergoing tidal interaction with a nearby companion, with shells, tidal tails and knotty star-forming regions all visible. We analyse the images of the galaxy and its companion pixel-by-pixel, first using colour-magnitude diagrams and then fitting stellar population models to the spectral energy distributions (SEDs) of each pixel. We also present medium-resolution spectroscopy of the system. The pixels separate cleanly in colour-magnitude diagrams, with pixels of different colours occupying distinct regions of the host galaxy and its companion. Fitting stellar population models to these colours, we have estimated the age of each population. We find three distinct groups of ages: an old population (Ï„ ∼ 15 Gyr) in the outskirts of the host galaxy; an intermediate-age population (Ï„ ∼ 300 Myr) around the nucleus and tidal tail, and a young population (Ï„ 10 Myr) in the nucleus and blue knots. The spectrum of the nucleus shows numerous strong emission lines, including [O I] λ6300, [O II] λ3727, [S II] λλ6716, 6731, Hα and [N II] λλ6548, 6583, characteristic of a low-ionization nuclear emission-line region (LINER) spectrum. The companion galaxy shows much narrower emission lines with very different line ratios, characteristic of a starburst galaxy. We have evidence for three distinct episodes of star formation in B1221-423. The correlation of age with position suggests the two most recent episodes were triggered by tidal interactions with the companion galaxy. The evidence points to the active galactic nucleus (AGN) in the centre of B1221-423 having been caught in the act of ignition. However, none of the components we have identified is as young as the radio source, implying that the delay between the interaction and the triggering of the AGN is at least 3 × 10⁸ yr.