A Deep Search for a Strong Diffuse Interstellar Band in the Circumgalactic Medium

TL;DR

This work addresses whether the strong diffuse interstellar band DIB$\\lambda4430$ exists in the circumgalactic medium (CGM) traced by MgII absorbers. Using ~60,000 MgII spectra from SDSS DR16, the authors construct high-S/N rest-frame composites by normalizing quasar SEDs with NMF eigenspectra and aligning to MgII rest frames to measure $W_0^{\\lambda4430}$ via a fixed Gaussian profile, then compare to the Milky Way $W_0^{\\lambda4430}(E(B-V))$ relation. They find no detectable DIB$\\lambda4430$ in the CGM and report a best-fit scaling $W_0^{\\lambda4430} = A E(B-V)^{0.89}$ with $A = -0.16 \\pm 0.23$ Å, corresponding to a $5.3\\sigma$ deviation from MW expectations when extrapolated to CGM reddening. Synthetic injections confirm the method’s sensitivity and validate that non-detection is not due to pipeline limitations. The results imply environmental factors in the CGM (production vs destruction, radiation field, and ionization) suppress the DIB carrier relative to the ISM, highlighting the importance of studying DIBs across different astrophysical environments and foreshadowing the role of future surveys like DESI in expanding such analyses.

Abstract

We investigate the absorption signals of a strong diffuse interstellar band, DIB$\lambda4430$, in the circumgalactic medium (CGM) traced by MgII absorption lines. To this end, we make use of approximately 60,000 MgII absorption line spectra within $0.4<z<1.0$ compiled from the Sloan Digital Sky Surveys and obtain composite spectra with uncertainties for absorption line measurements being a few m$Å$. By using MgII absorption strength and dust reddening relation from the literature, we measure the DIB$\lambda4430$ absorption strength as a function of $\rm E(B-V)$ in the CGM, and compare the Milky Way DIB$\lambda4430$ - $\rm E(B-V)$ relation extrapolated down to the CGM $\rm E(B-V)$ region. Our results show no detectable signals of DIB$\lambda4430$ across the entire $\rm E(B-V)$ range in the CGM traced by MgII absorption lines. This lack of detection of DIB$\lambda4430$ in the CGM is inconsistent with the Milky Way signals by $\sim 5 \, σ$, indicating that the factors associated with different environments affect the abundance of the DIB$\lambda4430$ carrier.

Figures (2)

• Figure 1: Composite spectra as a function of $W_0^{\lambda2796}$. The blue solid lines show the composite spectra with bootstrap uncertainties indicated by the shaded bands. The red solid lines are the best-fit absorption profiles. Left: observed spectral regions around DIB$4430$. Right: spectral regions around 4220 $\rm \AA$ with synthetic absorption signals recovered.
• Figure 2: DIB$\lambda4430$ absorption strengths as a function of $W^{\lambda2796}_{0}$ and $E(B-V)$. The red data points show the measured absorption strengths at DIB4430 and the blue data points show the measured absorption strengths for the synthetic absorption profiles at 4220 Å. The corresponding color dashed lines show the best-fit power laws. The black solid line shows the Milky Way DIB$\lambda4430$ - E(B-V) relation which is recovered from the synthetic signals (blue line). The purple stars show 3$\sigma$ upper limits derived from damped Lyman-alpha systems Lawton2006.