Sky Subtraction Research Articles

The reanalysis of spectra of GRB 080913 to estimate the neutral fraction of the IGM at a redshift of 6.7

Aims: We reanalyse optical spectra of the z=6.7 gamma-ray burst GRB 080913, adding hitherto unpublished spectra, in order to reassess the measurement of the neutral fraction of the IGM at high redshifts. Methods: In the data reduction we take particular care to minimise systematic errors in the sky subtraction, which are evident in the published spectrum, and compromise the analysis. The final combined spectrum has a higher S/N than the previously published spectrum by a factor of 1.3. Results: We find a single significant absorption line redward of the Ly-alpha continuum break, which we identify with the SII+SiII blend (rest wavelength of 0.1260 micron), at z=6.733. The sharp spectral break at Ly-alpha implies a comparatively low total column density of neutral hydrogen along the line of sight, log [N(HI)/cm^-2] < 20. We model the absorption with a host-galaxy DLA, surrounded by an ionised region of unknown size r, within the IGM of neutral fraction, x(HI). Despite knowing the source redshift, and the improved S/N of the spectrum, when fitting only over wavelengths redward of Ly-alpha, no useful constraints on x(HI) can be obtained. We consider the possibility of including the ionised region, blueward of Ly-alpha, in constraining the fit. For the optimistic assumption that the ionised region is transparent, tau_{GP} << 1, we find that the region is of small size r < 2 proper Mpc, and we obtain an upper limit to the neutral fraction of the IGM at z=6.7 of x(HI) < 0.73.

MASE: A New Data-Reduction Pipeline for the Magellan Echellette Spectrograph

ABSTRACTWe introduce a data-reduction package written in Interactive Data Language (IDL) for the Magellan Echellete Spectrograph (MAGE). MAGE is a medium-resolution (R ∼ 4100), cross-dispersed, optical spectrograph, with coverage from . The MAGE Spectral Extractor (MASE) incorporates the entire image reduction and calibration process, including bias subtraction, flat fielding, wavelength calibration, sky subtraction, object extraction, and flux calibration of point sources. We include examples of the user interface and reduced spectra. We show that the wavelength calibration is sufficient to achieve ∼5 km s-1 rms accuracy and relative flux calibrations better than 10%. A lightweight version of the full reduction pipeline has been included for real-time source extraction and signal-to-noise estimation at the telescope.

Metallicity gradients: Mass dependency in dwarf elliptical galaxies

AbstractThe formation and evolution of galaxies is imprinted on their stellar population radial gradients. Two recent articles present conflicting results concerning the mass dependence of the metallicity gradients for early‐type dwarf galaxies. On one side, Spolaor et al. (2009) show a tight positive correlation between the total metallicity [Z /H] and the mass. On the other side, in a distinct sample, we do not find any trend involving [Fe/H] (Koleva et al. 2009). In order to investigate the origin of the discrepancy, we examine various factors that may affect the determination of the gradients: namely the sky subtraction and the signal‐to‐noise ratio. We conclude that our detection of gradients are well above the possible analysis biases. Then, we measured the [Mg/Fe] relative abundance profile and found moderate gradients. The derived [Z /H] gradients scatter around –0.4 dex/re. The two samples contain the same types of objects and the reason of the disagreement is still not understood (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The Sixth Data Release of the Sloan Digital Sky Survey

With the Sixth Data Release of the Sloan Digital Sky Survey, the imaging of the Northern Galactic Cap is now complete. The survey contains images and parameters of roughly 287 million objects over 9583 deg^2, and 1.27 million spectra of stars, galaxies, quasars and blank sky (for sky subtraction) selected over 7425 deg^2. This release includes much more extensive stellar spectroscopy than previously, and also includes detailed estimates of stellar temperatures, gravities, and metallicities. The results of improved photometric calibration are now available, with uncertainties of roughly 1% in g, r, i, and z, and 2% in u, substantially better than the uncertainties in previous data releases. The spectra in this data release have improved wavelength and flux calibration, especially in the extreme blue and extreme red, leading to the qualitatively better determination of stellar types and radial velocities. The spectrophotometric fluxes are now tied to point spread function magnitudes of stars rather than fiber magnitudes, giving a 0.35 mag change in the spectrophotometric flux scale. Systematic errors in the velocity dispersions of galaxies have been fixed, and the results of two independent codes for determining spectral classifications and redshifts are made available. (Abridged)

Spectroscopy of clusters in the ESO distant cluster survey (EDisCS). II.

Aims. We present spectroscopic observations of galaxies in 15 survey fields as part of the ESO Distant Cluster Survey (EDisCS). We determine the redshifts and velocity dispersions of the galaxy clusters located in these fields, and we test for possible substructure in the clusters. Methods: We obtained multi-object mask spectroscopy using the FORS2 instrument at the VLT. We reduced the data with particular attention to the sky subtraction. We implemented the method of Kelson for performing sky subtraction prior to any rebinning/interpolation of the data. From the measured galaxy redshifts, we determine cluster velocity dispersions using the biweight estimator and test for possible substructure in the clusters using the Dressler-Shectman test. Results: The method of subtracting the sky prior to any rebinning/interpolation of the data delivers photon-noise-limited results, whereas the traditional method of subtracting the sky after the data have been rebinned/interpolated results in substantially larger noise for spectra from tilted slits. Redshifts for individual galaxies are presented and redshifts and velocity dispersions are presented for 21 galaxy clusters. For the 9 clusters with at least 20 spectroscopically confirmed members, we present the statistical significance of the presence of substructure obtained from the Dressler-Shectman test, and substructure is detected in two of the clusters. Conclusions: Together with data from our previous paper, spectroscopy and spectroscopic velocity dispersions are now available for 26 EDisCS clusters with redshifts in the range 0.40-0.96 and velocity dispersions in the range 166 km s-1-1080 km s-1. Based on observations collected at the European Southern Observatory, Chile, as part of large programme 166.A-0162 (the ESO Distant Cluster Survey). Full Table 4 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/482/419

An integral-field spectroscopic strong lens survey

We present the observational results of a survey for strong gravitational lens systems consisting of extended emission-line galaxies lensed by intervening early-type galaxies, conducted using integral field units (IFUs) of the Magellan IMACS and Gemini GMOS-N spectrographs. These data are highly valuable for corroborating the lensing interpretation of Hubble Space Telescope imaging data. We show that in many cases, ground-based IFU spectroscopy is in fact competitive with space-based imaging for the measurement of the mass model parameters of the lensing galaxy. We demonstrate a novel technique of three-dimensional gravitational lens modeling for a single lens system with a resolved lensed rotation curve. We also describe the details of our custom IFU data analysis software, which performs optimal multi-fiber extraction, relative and absolute wavelength calibration to a few hundredths of a pixel RMS and nearly Poisson-limited sky subtraction.

The correlation between star formation and 21-cm emission during the reionization epoch

Reionization is thought to be dominated by low mass galaxies, while direct observations of resolved galaxies probe only the most massive, rarest objects. The cross-correlation between fluctuations in the surface brightness of the cumulative Ly-alpha emission (which serves as a proxy for the star formation rate) and the redshifted 21cm signal from neutral hydrogen in the intergalactic medium (IGM), will directly probe the causal link between the production of ionizing photons in galaxies and the reionization of the IGM. We discuss the prospects for detecting this cross-correlation for unresolved galaxies. We find that on angular scales <10 arc-minutes, detection will be practical using widefield near-IR imaging from space in combination with the forthcoming Mileura Widefield Array - Low Frequency Demonstrator. When redshifted 21cm observations of the neutral IGM are combined with space-based near-IR imaging of Ly-alpha emission, the detection on angular scales <3 arc-minutes will be limited by the sensitivity of the 21cm signal, even when a small aperture optical telescope (~2m) and a moderate field of view (~10 square degrees) are used. On scales >3 arc-minutes, the measurement of cross-correlation will be limited by the accuracy of the foreground sky subtraction.

The Wide Field Spectrograph (WiFeS)

This paper describes the Wide Field Spectrograph (WiFeS) under construction at the Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) for the ANU 2.3 m telescope at the Siding Spring Observatory. WiFeS is a powerful integral field, double-beam, concentric, image-slicing spectrograph designed to deliver excellent throughput, wavelength stability, spectrophotometric performance and superb image quality along with wide spectral coverage throughout the 320–950 nm wavelength region. It provides a 25×38 arcsec field with 0.5 arcsec sampling along each of twenty five 38×1 arcsec slitlets. The output format is optimized to match the 4096×4096 pixel CCD detectors in each of two cameras individually optimized for the blue and the red ends of the spectrum, respectively. A process of “interleaved nod-and-shuffle” will be applied to permit quantum noise-limited sky subtraction. Using VPH gratings, spectral resolutions of 3000 and 7000 are provided. The full spectral range is covered in a single exposure at R=3000, and in two exposures in the R=7000 mode. The use of transmissive coated optics, VPH gratings and optimized mirror coatings ensures a throughput (including telescope atmosphere and detector) >30% over a wide spectral range. The concentric image-slicer design ensures an excellent and uniform image quality across the full field. To maximize scientific return, the whole instrument is configured for remote observing, pipeline data reduction, and the accumulation of calibration image libraries.

Telescope search for decaying relic axions

A search for optical line emission from the two-photon decay of relic axions was conducted in the galaxy clusters Abell 2667 and 2390, using spectra from the VIMOS (Visible MultiObject Spectrograph) integral field unit at the Very Large Telescope. New upper limits to the two-photon coupling of the axion are derived, and are at least a factor of 3 more stringent than previous upper limits in this mass window. The improvement follows from a larger collecting area, integration time, and spatial resolution, as well as from improvements in signal to noise and sky subtraction made possible by strong-lensing mass models of these clusters. The new limits either require that the two-photon coupling of the axion be extremely weak or that the axion mass window between 4.5 eV and 7.7 eV be closed. Implications for sterile-neutrino dark matter are discussed briefly also.

Luminosity dependence in the Fundamental Plane projections of elliptical galaxies

We analyse the Fundamental Plane projections of elliptical galaxies as a function of luminosity, using a sample of ≈80 000 galaxies drawn from Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). We separate brightest cluster galaxies (BCGs) from our main sample and reanalyse their photometry due to a problem with the default pipeline sky subtraction for BCGs. The observables we consider are effective radius (Re), velocity dispersion (σ), dynamical mass (Mdyn∝Reσ2), effective density (σ2/R2e) and effective surface brightness (μe). With the exception of the L–Mdyn correlation, we find evidence of variations in the slope (i.e. the power-law index) of the Fundamental Plane projections with luminosity for our normal elliptical galaxy population. In particular, the radius–luminosity and Faber–Jackson relations are steeper at high luminosity relative to low luminosity, and the more luminous ellipticals become progressively less dense and have lower surface brightnesses than lower luminosity ellipticals. These variations can be understood as arising from differing formation histories, with more luminous galaxies having less dissipation. Data from the literature and our reanalysis of BCGs show that BCGs have radius–luminosity and Faber–Jackson relations steeper than the brightest non-BCG ellipticals in our sample, consistent with significant growth of BCGs via dissipationless mergers. The variations in slope we find in the Faber–Jackson relation of non-BCGs are qualitatively similar to that reported in the black hole mass–velocity dispersion (MBH–σ) correlation. This similarity is consistent with a roughly constant value of MBH/M★ over a wide range of early-type galaxies, where M★ is the stellar mass.

The Lyα Forest Power Spectrum from the Sloan Digital Sky Survey

We measure the power spectrum, PF(k, z), of the transmitted flux in the Ly? forest using 3035 high-redshift quasar spectra from the Sloan Digital Sky Survey. This sample is almost 2 orders of magnitude larger than any previously available data set, yielding statistical errors of ~0.6% and ~0.005 on, respectively, the overall amplitude and logarithmic slope of PF(k, z). This unprecedented statistical power requires a correspondingly careful analysis of the data and of possible systematic contaminations in it. For this purpose we reanalyze the raw spectra to make use of information not preserved by the standard pipeline. We investigate the details of the noise in the data, resolution of the spectrograph, sky subtraction, quasar continuum, and metal absorption. We find that background sources such as metals contribute significantly to the total power and have to be subtracted properly. We also find clear evidence for Si III correlations with the Ly? forest and suggest a simple model to account for this contribution to the power. Our tests indicate that any residual systematics in the analysis are unlikely to affect the inference of cosmological parameters from PF(k, z). These results should provide an essential ingredient for all future attempts to constrain modeling of structure formation, cosmological parameters, and theories for the origin of primordial fluctuations.

TMT novel technologies

The ongoing conceptual design activities for the Thirty Meter Telescope (TMT) illustrate many (if not virtually all) of the advanced instrumentation technologies under consideration for future extremely large telescopes. First light capabilities must be based upon credible extrapolations of existing systems and components, while potential upgrades and follow-on systems should explore the full range of advanced and innovative technologies currently proposed for scientific instrumentation and adaptive optics (AO). An affordable technology development program must then be implemented which balances these conflicting objectives.In this paper, we outline the range of innovative AO component technologies now under discussion for TMT, and describe some of the contracts and studies comprising our AO development program. Components that require advances include piezostack and MEMS deformable mirrors; adaptive secondary mirrors; fast, low noise detectors for both laser- and natural guide star wavefront sensing; guidestar laser sources; and processing electronics for the implementation of AO control algorithms. Instrumentation studies are also underway that investigate issues related to the huge size of seeing-limited instruments; large gratings; integral field spectroscopy; detectors; and advanced techniques for sky subtraction.

The VIMOS Integral Field Unit: Data‐Reduction Methods and Quality Assessment

ABSTRACTWith the new generation of spectrographs, integral field spectroscopy is becoming a widely used observational technique. The Integral Field Unit (IFU) of the Visible Multi–Object Spectrograph (VIMOS) on the ESO VLT allows sampling of a field as large as 54′′ × 54′′, covered by 6400 fibers coupled with microlenses. We present here the methods of the data‐processing software that has been developed to extract the astrophysical signal of faint sources from the VIMOS IFU observations. We focus on the treatment of the fiber‐to‐fiber relative transmission and the sky subtraction, and the dedicated tasks we have built to address the peculiarities and unprecedented complexity of the data set. We review the automated process we have developed under the VIPGI data organization and reduction environment (Scodeggio et al. ), along with the quality control performed to validate the process. The VIPGI IFU data‐processing environment has been available to the scientific community to process VIMOS IFU data since 2003 November.

Fourier transform spectroscopy of the submillimetre continuum emission from hot molecular cores

We have used a Fourier Transform Spectrometer (FTS) on the James Clerk Maxwell Telescope (JCMT) to study the submillimetre continuum emission from dust in three hot molecular cores (HMCs). The spectral index β of the dust emission for these sources has been determined solely within the 30-GHz wide 350-GHz (850 μm) passband to an accuracy comparable to those determined through multiwavelength observations. We find an average β≃ 1.6, in agreement with spectral indices determined from previous submillimetre observations of these sources and with those determined for HMC in general. The largest single source of uncertainty in these results is the variability of the atmosphere at 350 GHz, and with better sky subtraction techniques we show that the dust spectral index can clearly be determined within one passband to high accuracy with a submillimetre FTS. Using an imaging FTS on the Submillimetre Common User Bolometer Array (SCUBA)-2, the next generation wide-field submillimetre camera currently under development to replace SCUBA at the JCMT in 2006, we calculate that at 350 GHz it will be possible to determine β to ±0.1 for sources as faint as 400 mJy beam−1 and to ±0.3 for sources as faint as 140 mJy beam−1.

New insights to the photometric structure of Blue Compact Dwarf galaxies from deep Near-Infrared studies

We have analyzed deep Near Infrared (NIR) broad band images for a sample of Blue Compact Dwarf Galaxies (BCDs), observed with the ESO NTT and Calar Alto 3.6 m telescopes. The data presented here allows for the detection and quantitative study of the extended stellar low-surface brightness (LSB) host galaxy in all sample BCDs. NIR surface brightness profiles (SBPs) of the LSB host galaxies agree at large galactocentric radii with those from optical studies, showing also an exponential intensity decrease and compatible scale lengths. At small to intermediate radii (within 1–3 exponential scale lengths), however, the NIR data reveals for more than one half of our sample BCDs evidence for a significant flattening of the exponential profile of the LSB component. Such profiles (type V SBPs, Binggeli & Cameron [CITE]) have rarely been detected in the LSB component of BCDs at optical wavelengths, where the relative flux contribution of the starburst, being stronger than in the NIR, can readily hide a possible central intensity depression in the underlying LSB host. The structural properties, frequency and physical origin of type V LSB profiles in BCDs and dwarf galaxies in general have not yet been subject to systematic studies. Nevertheless, the occurrence of such profiles in an appreciable fraction of BCDs would impose important new observational constraints to the radial mass distribution of the stellar LSB component, as well as to the photometric fading of these systems after the termination of star-forming activities. We test the suitability of two empirical fitting functions, a modified exponential distribution (Papaderos et al. [CITE]) and the Sérsic law, for the systematization of the structural properties of BCD host galaxies which show a type V intensity distribution. Either function has been found to satisfactorily fit a type V distribution. However, it is argued that the practical applicability of Sérsic fits to the LSB emission of BCDs is limited by the extreme sensitivity of the achieved solutions to, e.g., small uncertainties in the sky subtraction and SBP derivation. We find that most of the sample BCDs show in their stellar LSB host galaxy optical-NIR colors indicative of an evolved stellar population with subsolar metallicity. Unsharp-masked NIR maps reveal numerous morphological details and indicate in some cases, in combination with optical data, appreciable non-uniform dust absorption on a spatial scale as large as ~1 kpc.

A Technique for Separating the Gravitational Torques of Bars and Spirals in Disk Galaxies

We describe a Fourier-based method of separating bars from spirals in near-infrared images. The method takes advantage of the fact that a bar is typically a feature with a relatively fixed position angle and uses the simple assumption that the relative Fourier amplitudes due to the bar decline with radius past a maximum in the same or a similar manner as they rose to that maximum. With such an assumption, the bar can be extrapolated into the spiral region and removed from an image, leaving just the spiral and the axisymmetric background disk light. We refer to such a bar-subtracted image as the "spiral plus disk" image. The axisymmetric background (Fourier index m = 0 image) can then be added back to the bar image to give the "bar plus disk" image. The procedure allows us to estimate the maximum gravitational torque per unit mass per unit square of the circular speed for the bar and spiral forcing separately, parameters that quantitatively define the bar strength Qb and the spiral strength Qs following the recent study of Buta & Block. For the first time, we are able to measure the torques generated by spiral arms alone, and we can now define spiral torque classes, in the same manner as bar torque classes are delineated. We outline the complete procedure here using a 2.1 μm image of NGC 6951, a prototypical SAB(rs)bc spiral having an absolute blue magnitude of -21 and a maximum rotation velocity of 230 km s-1. Comparison between a rotation curve predicted from the m = 0 near-infrared light distribution and an observed rotation curve suggests that NGC 6951 is maximum disk in its bar and main spiral region, implying that our assumption of a constant mass-to-light ratio in our analysis is probably reliable. We justify our assumption on how to make the bar extrapolation using an analysis of NGC 4394, a barred spiral with only weak near-infrared spiral structure, and we justify the number of needed Fourier terms using NGC 1530, one of the most strongly barred galaxies (bar class 7) known. We also evaluate the main uncertainties in the technique. Allowing for uncertainties in vertical scale height, bar extrapolation, sky subtraction, orientation parameters, and the asymmetry in the spiral arms themselves, we estimate Qb = 0.28 ± 0.04 and Qs = 0.21 ± 0.06 for NGC 6951.

Deep Near‐Infrared Mapping of Young and Old Stars in Blue Compact Dwarf Galaxies

We analyze J, H and Ks near-infrared data for 9 Blue Compact Dwarf (BCD) galaxies, selected from a larger sample that we have already studied in the optical. We present contour maps, surface brightness and color profiles, as well as color maps of the sample galaxies. The morphology of the BCDs in the NIR has been found to be basically the same as in the optical. The inner regions of these systems are dominated by the starburst component. At low surface brightness levels the emission is due to the underlying host galaxy; the latter is characterized by red, radially constant colors and isophotes well fit by ellipses. We derive accurate optical near--infrared host galaxy colors for eight of the sample galaxies; these colors are typical of an evolved stellar population. Interestingly, optical near--infrared color maps reveal the presence of a complex, large-scale absorption pattern in three of the sample galaxies. We study the applicability of the Sersic law to describe the surface brightness profiles of the underlying host galaxy, and find that, because of the limited surface brightness interval over which the fit can be made, the derived Sersic parameters are very sensitive to the selected radial interval and to errors in the sky subtraction. Fitting an exponential model gives generally more stable results, and can provide a useful tool to quantify the structural properties of the host galaxy and compare them with those of other dwarf classes as well as with those of star-forming dwarfs at higher redshifts.

INTERNATIONAL DATABASE PROJECT FOR SPECTRAL OBSERVATIONS ON ASTEROIDS

32 bit database application for Windows has been created using a client-server model. Object-oriented design of a multiple-document interface allows us to accumulate spectra in the database and to operate with different kinds of spectral information. All stored spectra are wavelength calibrated and passed through instrumental reductions such as flat-fielding and sky background subtraction. Stream socket interface facilities allows us to exchange by spectral information between different computers on the Internet and to organize a uniform system of international banks of spectral information. The application is developed as a dynamic data exchange server. It also uses Microsoft exchange communication facilities.

Mission: impossible (escape from the Lyman limit)

We investigate the intrinsic opacity of high-redshift galaxies to outgoing ionizing photons using high-quality photometry of a sample of 27 spectroscopically identified galaxies of redshift 1.9 > z > 3.5 in the Hubble Deep Field. Our measurement is based on maximum-likelihood fitting of model galaxy spectral energy distributions – including the effects of intrinsic Lyman-limit absorption and random realizations of intervening Lyman-series and Lyman-limit absorption – to photometry of galaxies from space- and ground-based broad-band images. Our method provides several important advantages over the methods used by previous groups, including most importantly that two-dimensional sky subtraction of faint-galaxy images is more robust than one-dimensional sky subtraction of faint-galaxy spectra. We find at the 3σstatistical confidence level that on average no more than 4 per cent of the ionizing photons escape galaxies of redshift 1.9 > z > 3.5. This result is consistent with observations of low- and moderate-redshift galaxies but is in direct contradiction to a recent result based on medium-resolution spectroscopy of high-redshift (z≈ 3) galaxies. Dividing our sample into subsamples according to luminosity, intrinsic ultraviolet colour and redshift, we find no evidence for selection effects that could explain such a discrepancy. Even when all systematic effects are included, the data could not realistically accommodate any escape fraction value larger than ≈15 per cent.

The DDO IVC Distance Project: Survey Description and the Distance to G139.6+47.6

We present a detailed analysis of the distance determination for one intermediate-velocity cloud (IVC, G139.6+47.6) from the ongoing DDO IVC Distance Project. Stars along the line of sight to G139.6+47.6 are examined for the presence of sodium absorption attributable to the cloud, and the distance bracket is established by astrometric and spectroscopic parallax measurements of demonstrated foreground and background stars. We detail our strategy regarding target selection, observational setup, and analysis of the data, including a discussion of wavelength calibration and sky subtraction uncertainties. We find a distance estimate of 129 ± 10 pc for the lower limit and 257 pc for the upper limit. Given the high number of stars showing absorption due to this IVC, we also discuss the small-scale covering factor of the cloud and the likely significance of nondetections for subsequent observations of this and other similar IVCs. Distance measurements of the remaining targets in the DDO IVC project will be detailed in a companion paper.