Chapter IV: Circumgalactic Ly 𝛼 Emission and Its Host Galaxies
4.1 Continuum Subtraction
We conduct continuum-subtraction on the spaxels near the galaxies to isolate Ly𝛼 emission from the stellar continuum. One issue we encountered in Chen et al. (2021) (hereafter C21) is the over-subtraction of the Ly𝛼profile blueward of the systemic redshift. This issue is caused by the fact that a simple interpolation of a constant continuum level between wavelengths above and below Ly𝛼 does not capture the complexity of the real spectra. Therefore, a more complex method is needed.
In this chapter, stellar-continuum subtraction is performed using stellar population synthesis (SPS) models, namely the “Binary Population and Spectral Synthesis”
(BPASS) v2.2 models (Stanway and Eldridge, 2018) with an upper mass cutoff of 100𝑀, assuming a stellar metallicity 𝑍∗ = 0.002𝑍, and adopting a constant star-formation history (SFH) with age𝑡 = 108years. The above assumptions were previously found to be suitable to describe the KBSS galaxy sample in the FUV continuum by Steidel et al. (2016) and Theios et al. (2019). As shown below, the models work acceptably well for our purpose – to remove the stellar continuum near Ly𝛼in composite spectra.
We apply the mean of Monte-Carlo models of “IGM+CGM” H i transmission curve as described by Steidel et al. (2018) (hereafter S18) at the redshift of the object to the SPS model. The curve was generated as the mean from Monte-Carlo simulations based on the best-fit H i incidence rate from Rudie et al. (2013). The attenuated spectrum is then reddened using an SMC extinction curve using the best-fit𝐸(𝐵−𝑉) required to match the observed the spectral shape of the 1D-extracted spectrum from the KCWI data1. The fits were conducted with iterative sigma-clipping with 𝑁𝜎 = 2.5. Emission and absorption lines arising from the ISM and CGM gas are masked using the templates suggested by S18.
As shown by S18, H i absorption in the ISM has a crucial impact on the continuum shape near Ly𝛼 as well. Therefore, as a final step in the continuum modeling, we applied the “holes” model of S18 to the modeled spectra. This model assumes that the observed spectrum consists of two distinct components. One is covered by low-ionization gas including dust, which reddens the continuum shape; the second component is assumed to be uncomtaminated by the ISM, emerging through holes.
S18 found that the fraction of the galaxy continuum covered by optically thick H i (𝑓𝑐) is linearly correlated with the observed Ly𝛼equivalent width [𝑊𝜆(Ly𝛼)]. Pahl et al. (2021) subsequently updated the correlation parameters after removing a small number of blended objects, which gives,
1− 𝑓𝑐 =0.58[𝑊𝜆(Ly𝛼)/110Å]. (4.1) We adopt this correlation for individual galaxies in our sample, where 𝑊𝜆(Ly𝛼) were measured from 1D-extracted KCWI spectra. We further assumed that the H i column density, log(𝑁HI/cm−2) = 20.61, the best-fit value found by S18 for the
“holes” model to the composite KLCS spectrum. The Doppler width and systemic velocity offset of this ISM component are assumed to be 𝑏 = 125 km s−1 and 𝑣sys =−100 km s−1, as in S18.
1The extraction method for 1D continuum spectra were described in C21. It generates spectra that are similar to that observed by slit spectrographs and optimizes the S/N for the continuum.
1160 1180 1200 1220 1240 1260 1280 (Å)
0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Normalised F CIII 1175 SiII 1190,92 SiII* 1197 SiIII 1206 Ly NV 1240 SiII 1260 SiII* 1265
All Galaxies (N = 110) Error Spectrum Continuum Model
1160 1180 1200 1220 1240 1260 1280 (Å)
1.00 0.75 0.50 0.25 0.00 0.25 0.50 0.75 1.00
CIII 1175 SiII 1190,92 SiII* 1197 SiIII 1206 Ly NV 1240 SiII 1260 SiII* 1265
Continuum Subtracted Error
15000 10000 5000v (km s0 1)5000 10000 15000
15000 10000 5000v (km s0 1)5000 10000 15000
Figure 4.1: Left: The composite 1D spectrum of the full sample (black) and the composite stellar continuum model (orange). Before stacking, each individual galaxy spectrum was normalized with𝐹𝜆(1450 Å) =1, and the stacking method was a sigma-clipped mean with𝑁𝜎 =2.5. Right: The composite continuum-subtracted spectrum (brown). In both panels, the grey spectrum is the 1𝜎error spectrum derived from bootstrap resampling. The vertical lines with labels mark the selected spectral features in the range, where different colors indicate their primary physical origin:
ISM (green), nebular (blue), stellar (red), and non-resonant emission (purple). Our continuum model works well in continuum subtraction.
Finally, we emphasize that the model assumptions above were previously most successful for fitting with composite spectra (e.g., S18). Most notably, the IGM + CGM opacity curve is the average absorption strength of Ly𝛼 absorbers in the foreground Ly𝛼 forest at a given redshift that lowers the continuum level in the composite spectra. However, we needed to develop a method for fitting spectra individually, for which discrete Ly𝛼forest absorption lines are present, rather than a smoothly leveled continuum, in order to measure Ly𝛼emission robustly as shown in §4.2. We experimented with multiple methods, including fitting only the portion of spectra redward of Ly𝛼, and/or fitting without accounting for IGM + CGM attenuation. In the end, we found that using iterative sigma clipping results in fits that are most consistent with fits to composite spectra over our redshift range. The difference between the the two versions of continuum spectra is < 5% (RMS) near Ly𝛼.
Following the procedures outlined above, a continuum model is generated for every object in the sample; this continuum model is scaled to match the flux within the continuum footprint in the KCWI cube before being subtracted. Figure 4.1 shows the composite 1D spectrum, the best-fit model continuum spectrum, and the
1180 1200 1220 1240 1260 (Å)
0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Normalised F CIII 1175 SiII 1190,92 SiII* 1197 SiIII 1206 Ly NV 1240 SiII 1260
Fiducial (Age = 108.0) Age = 107.7 Age = 109.0
1180 1200 1220 1240 1260 (Å)
Fiducial (Z*= 0.002Z ) Z*= 0.001Z
Z*= 0.004Z
CIII 1175 SiII 1190,92 SiII* 1197 SiIII 1206 Ly NV 1240 SiII 1260
1180 1200 1220 1240 1260 (Å)
Fiducial No IGM+CGM No ISM
CIII 1175 SiII 1190,92 SiII* 1197 SiIII 1206 Ly NV 1240 SiII 1260
10000 5000v (km s0 1)5000 10000
10000 5000v (km s0 1)5000 10000
10000 5000v (km s0 1)5000 10000
Figure 4.2: This figure shows how different parameters or assumptions affect the modeled continuum spectrum. The black and orange spectra are the same as in Figure 4.1, which show the composite 1D spectra of the galaxies and their fiducial continuum models. From left to right: The change of the continuum spectrum by varying age (left) and stellar metallicity (middle), and by ignoring the IGM + CGM component or the ISM component (right). Our fiducial model closely resembles the continuum around Ly𝛼in the composite 1D spectrum.
continuum-subtracted spectrum, for all galaxies in the KCWI sample. Our model spectrum follows well with the continuum near Ly𝛼. Figure 4.2 shows how age, 𝑍∗, and the application of IGM + CGM and ISM attenuation affect the modeled continuum. The assumed age of the constant-star-formation model has a significant impact near Ly𝛼, especially in the wings; it is somewhat degenerate to the assumed ISM column density. The impact of𝑍∗is minimal near Ly𝛼except in the wings for models with extremely low metallicity. Note that without the H i covering of the ISM component, the model would significantly underpredict the absorption strength within 5000 km s−1 of Ly𝛼. By ignoring the IGM + CGM opacity, the predicted continuum would be considerably higher than that observed blueward of Ly𝛼. In summary, continuum models constructed using the assumptions above are suitable and adequate for isolating the Ly𝛼emission component from the stellar continuum.