Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userLINEAR: A Novel Algorithm for Reconstructing Slitless Spectroscopy from HST/WFC3
63
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present a grism extraction package (LINEAR) designed to reconstruct one-dimensional spectra from a collection of slitless spectroscopic images, ideally taken at a variety of orientations, dispersion directions, and/or dither positions. Our approach is to enumerate every transformation between all direct image positions (ie. a potential source) and the collection of grism images at all relevant wavelengths. This leads to solving a large, sparse system of linear equations, which we invert using the standard LSQR algorithm. We implement a number of color and geometric corrections (such as flat field, pixel-area map, source morphology, and spectral bandwidth), but assume many effects have been calibrated out (such as basic reductions, background subtraction, and astrometric refinement). We demonstrate the power of our approach with several Monte Carlo simulations and the analysis of archival data. The simulations include astrometric and photometric uncertainties, sky-background estimation, and signal-to-noise calculations. The data are G141 observations obtained with the Wide-Field Camera 3 of the Hubble Ultra-Deep Field, and show the power of our formalism by improving the spectral resolution without sacrificing the signal-to-noise (a tradeoff that is often made by current approaches). Additionally, our approach naturally accounts for source contamination, which is only handled heuristically by present softwares. We conclude with a discussion of various observations where our approach will provide much improved spectral one-dimensional spectra, such as crowded fields (star or galaxy clusters), spatially resolved spectroscopy, or surveys with strict completeness requirements. At present our software is heavily geared for Wide-Field Camera 3 IR, however we plan extend the codebase for additional instruments.
rate research
Read More
HST is commonly thought of as an optical-IR imaging or UV-spectroscopy observatory. However, the advent of WFC3-IR made it possible to do slitless infrared spectroscopic surveys over an area significant for galaxy evolution studies (~0.15 deg^2). Slitless infrared spectroscopy is uniquely possible from space due to the reduced background. Redshift surveys with WFC3-IR offer probes of the astrophysics of the galaxy population at z=1-3 from line features, and the true redshift and spatial distribution of galaxies, that cannot be done with photometric surveys alone. While HST slitless spectroscopy is low spectral resolution, its high multiplex advantage makes it competitive with future ground based IR spectrographs, its flux calibration is stable, and its high spatial resolution allows measuring the spatial extent of emission lines, which only HST can do currently for large numbers of objects. A deeper slitless IR spectroscopic survey over hundreds of arcmin^2 (eg one or more GOODS fields) is one of the remaining niches for large galaxy evolution studies with HST, and would produce a sample of thousands of spectroscopically confirmed galaxies at 1<z<3 to H=25 and beyond, of great interest to a large community of investigators. Finally, although JWST multislit spectroscopy will outstrip HST in resolution and sensitivity, I believe it is critical to have a spectroscopic sample in hand before JWST flies. This applies scientifically, to be prepared for the questions we want to answer with JWST, and observationally, because JWSTs lifetime is limited and a classic problem in targeted spectroscopy has been the turn-around time for designing surveys and for deciding which classes of objects to target. This white paper is released publicly to stimulate open discussion of future large HST programs.
The Advanced Camera for Surveys on-board HST is equipped with a set of one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150 Ang. to 10500 Ang. The G800L grism provides optical spectroscopy between 5500 Ang. and 1 micron with a mean dispersion of 39 Ang./pix and 24 Ang./pix (in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on-board calibration lamps for wavelength and narrow band flat-fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention to the treatment of the field dependence of the grism flat-field, wavelength solution and sensitivity in both Channels.
Lucky imaging is a high-resolution astronomical image recovery technique with two classic implementation algorithms, i.e. image selecting, shifting and adding in image space and data selecting and image synthesizing in Fourier space. This paper proposes a novel lucky imaging algorithm where with space-domain and frequency-domain selection rates as a link, the two classic algorithms are combined successfully, making each algorithm a proper subset of the novel hybrid algorithm. Experimental results show that with the same experiment dataset and platform, the high-resolution image obtained by the proposed algorithm is superior to that obtained by the two classic algorithms. This paper also proposes a new lucky image selection and storage scheme, which can greatly save computer memory and enable lucky imaging algorithm to be implemented in a common desktop or laptop with small memory and to process astronomical images with more frames and larger size. Besides, through simulation analysis, this paper discusses the binary star detection limits of the novel lucky imaging algorithm and traditional ones under different atmospheric conditions.
We present hitherto the largest sample of gas-phase metallicity radial gradients measured at sub-kiloparsec resolution in star-forming galaxies in the redshift range of $zin[1.2, 2.3]$. These measurements are enabled by the synergy of slitless spectroscopy from the Hubble Space Telescope near-infrared channels and the lensing magnification from foreground galaxy clusters. Our sample consists of 76 galaxies with stellar mass ranging from 10$^7$ to 10$^{10}$ $M_odot$, instantaneous star-formation rate in the range of [1, 100] $M_odot$/yr, and global metallicity [$frac{1}{12}$, 2] solar. At 2-$sigma$ confidence level, 15/76 galaxies in our sample show negative radial gradients, whereas 7/76 show inverted gradients. Combining ours and all other metallicity gradients obtained at similar resolution currently available in the literature, we measure a negative mass dependence of $Deltalog({rm O/H})/Delta r~ [mathrm{dex~kpc^{-1}}] = left(-0.020pm0.007right) + left(-0.016pm0.008right) log(M_ast/10^{9.4} M_odot)$ with the intrinsic scatter being $sigma=0.060pm0.006$ over four orders of magnitude in stellar mass. Our result is consistent with strong feedback, not secular processes, being the primary governor of the chemo-structural evolution of star-forming galaxies during the disk mass assembly at cosmic noon. We also find that the intrinsic scatter of metallicity gradients increases with decreasing stellar mass and increasing specific star-formation rate. This increase in the intrinsic scatter is likely caused by the combined effect of cold-mode gas accretion and merger-induced starbursts, with the latter more predominant in the dwarf mass regime of $M_astlesssim10^9 M_odot$.
We quantified and calibrated the metallicity and temperature sensitivities of colors derived from nine Wide Field Camera 3 (WFC3) filters aboard the Hubble Space Telescope using Dartmouth isochrones and Kurucz atmospheres models. The theoretical isochrone colors were tested and calibrated against observations of five well studied galactic clusters: M92, NGC 6752, NGC 104, NGC 5927, and NGC 6791, all of which have spectroscopically determined metallicities spanning -2.30 < [Fe/H] < +0.4. We found empirical corrections to the Dartmouth isochrone grid for each of the following color magnitude diagrams (CMD) (F555W--F814W, F814W), (F336W-F555W, F814W), (F390M-F555W, F814W) and (F390W-F555W, F814W). Using the empirical corrections we tested the accuracy and spread of the photometric metallicities assigned from CMDs and color-color diagrams (which are necessary to break the age-metallicity degeneracy). Testing three color-color diagrams [(F336W-F555W),(F390M-F555W),(F390W-F555W), vs (F555W-F814W)], we found the colors (F390M-F555W) and (F390W-F555W), to be the best suited to measure photometric metallicities. The color (F390W-F555W) requires much less integration time, but generally produces wider metallicity distributions, and, at very-low metallicity, the MDF from (F390W-F555W) is ~60% wider than that from (F390M-F555W). Using the calibrated isochrones we recovered the overall cluster metallicity to within ~0.1 dex in [Fe/H] when using CMDs (i.e. when the distance, reddening and ages are approximately known). The measured metallicity distribution function (MDF) from color-color diagrams show this method measures metallicities of stellar clusters of unknown age and metallicity with an accuracy of ~0.2 - 0.5 dex using F336W--F555W, ~0.15 - 0.25 dex using F390M-F555W, and ~0.2 - 0.4 dex with F390W-F555W, with the larger uncertainty pertaining to the lowest metallicity range.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
