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We describe the design, construction, and commissioning of FIRE, a 0.82-2.51 μm echelle spectrograph for the 6.5 m Magellan Baade telescope. FIRE may be operated in two modes. Its primary mode is a prism cross-dispersed echelle, which delivers R ¼ 6000 spectra for an 0.6″ slit, with continuous wavelength coverage over the full instrument bandpass in a single setup. Alternatively, the echelle grating may be replaced with a flat mirror to obtain high-throughput R ¼ 400 longslit spectra through the prisms alone-again with full Y =J=H=K coverage. This contribution outlines the details of the optical design and execution, mechanical and thermal design, detector systems, and data analysis software. We also present performance metrics from commissioning observations. These have established that the instrument is achieving its design goals, particularly with regard to throughput, as is required for observations of faint, high-redshift QSOs and the lowest mass brown dwarfs.

Condensate clouds are a salient feature of L dwarf atmospheres, but have been assumed to play little role in shaping the spectra of the coldest T-type brown dwarfs. Here we report evidence of condensate opacity in the near-infrared spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data verify the low-temperature nature of this source, indicating a T8 spectral classification, log 10 L bol /L ⊙ = −5.62±0.03, T ef f = 650±25 K, and a mass at or below the deuterium burning limit. The data also reveal enhanced emission at K-band associated with youth (low surface gravity) and supersolar metallicity, reflecting the properties of the Ross 458 system (age = 150-800 Myr, [Fe/H] = +0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C we confirm a low surface gravity and supersolar metallicity, while the temperature differs depending on the presence (635 +25 −35 K) or absence (760 +70 −45 K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar temperatures (595 +25 −45 K), but distinct surface gravities (log g = 4.0-4.5 cgs versus 5.0-5.5 cgs) and metallicities ([M/H] ≈ +0.2 versus -0.2). In all three cases, cloudy models provide better fits to the spectral data, significantly so for Ross 458C.These results indicate that clouds are an important opacity source in the spectra of young cold T dwarfs, and should be considered when characterizing the spectra of planetary-mass objects in young clusters and directly-imaged exoplanets. The characteristics of Ross 458C suggest it could itself be regarded as a planet, albeit one whose cosmogony does not conform with current planet formation theories.

We present a new determination of the intergalactic C iv mass density at 4.3 < z < 6.3. Our constraints are derived from high signal-to-noise spectra of seven quasars at z > 5.8 obtained with the newly commissioned Folded-Port Infrared Echellette (FIRE) spectrograph on the Magellan Baade telescope, coupled with six observations of northern objects taken from the literature. We confirm the presence of a downturn in the C iv abundance at z = 5.66 by a factor of 4.1 relative to its value at z = 4.96, as measured in the same sight lines. In the FIRE sample, a strong system previously reported in the literature as C iv at z = 5.82 is re-identified as Mg ii at z = 2.78, leading to a substantial downward revision in Ω C iv for these prior studies. Additionally, we confirm the presence of at least two systems with low-ionization C ii, Si ii, and O i absorption but relatively weak signal from C iv. The latter systems may be of interest if the downward trend in Ω C iv at high redshift is driven in part by ionization effects.