A Near-Infrared Extinction and Reddening Map Towards the Galactic Bulge Using UKIRT

TL;DR

This work directly measures per-pixel near-infrared extinction $A_{K_S}$ and reddening $E(H-K_S)$ toward the Galactic bulge by fitting the Red Clump luminosity function to UKIRT $H$ and $K$-band data, rather than assuming a universal extinction law. The authors construct $2′×2′$ maps over $-2.15^ om{ ext{a}} olinebreak[4]2.71^ om{ ext{a}} \\le l \\le 2.71^ om{ ext{a}}$ and $-2.69^ om{ ext{a}} olinebreak[4]2.03^ om{ ext{a}}$ with careful photometric calibration, quality cuts, and a sophisticated luminosity-function model that reduces to four free parameters per line of sight and uses an integral constraint to fix the overall normalization. By exploring parameter posteriors with MCMC, they obtain RC magnitudes and colors, enabling direct derivation of $A_{K_S}$ and $E(H-K_S)$, and they assess variations in the extinction law by comparing multiple prescriptions, finding a median $A_{K_S}/E(H-K_S) \\approx 1.4$ with significant latitude-dependent deviations. The results show notable field-to-field variation and incomplete sampling near the Galactic plane, yet the maps are broadly consistent with recent VVV-based extinctions (G18, S20) while exhibiting a translational offset relative to Z22 due to population and zero-point differences. These maps will aid bulge science and Roman GBTDS planning, and they underscore the necessity of field-specific extinction laws for precise dereddening in the inner Galaxy.

Abstract

The Galactic bulge is one of the most information-dense regions to study resolved stellar populations, variables, and transients, such as microlensing events. Studies toward the Galactic bulge are complicated by the large and variable extinction along the line of sight. We measure the near-infrared $A_{K_S}$ extinction and $E(H-K_S)$ reddening in this region using $H$- and $K$-band photometry obtained with the 2017 UKIRT microlensing survey. We fit the apparent magnitude and color distribution of bright giants in the bulge to recover the apparent magnitude and color of Red Clump stars, which are known to be standard candles and crayons. We present $2^\prime \times 2^\prime$ resolution maps in UKIRT fields between $-2.15^{\circ} \le l \le 2.71^{\circ}$ and $-2.69^{\circ} \le b \le 2.03^{\circ}$ of the $A_{K_S}$ extinction and the $E(H-K_S)$ reddening. We find large variations in the $K_S$-band extinction and $E(H-K_S)$ reddening on all the scales we probe. We find that a constant, standard extinction law is a poor representation of the relationship between the extinction and reddening we measure in fields of different latitudes. These maps will be useful for understanding the near-infrared extinction law for sight lines close to the Galactic plane, as well as for final field selection for the Nancy Grace Roman Space Telescope Galactic Bulge Time Domain Survey.

Figures (14)

• Figure 1: The fields observed by the UKIRT team during the 2017 and 2018 seasons. The 2019 season contains the same regions except for the top row of orange grids. Each square represents a grouping of the four CCDs on WFCAM. The black area shows the 438-day overguide scenario for the Roman GBTDS fields 2019ApJS..241....3Protac. The background is the Milky Way as seen by the Gaia Data Release 3 using mw-plotmw-plot.
• Figure 2: Illumination corrections for the 2017 UKIRT microlensing survey using the May and July 2017 products.
• Figure 3: $K$-band filter used by the UKIRT microlensing survey as well as $K_S$-band filters from VVV and 2MASS with transmission normalized to $1.0$ for all filters. Filter profiles were provided by the Spanish Visual Observatory's (SVO) Filter Profile Service 2020sea..confE.182R.
• Figure 4: Shown are two example fields of stars that passed the quality cuts. The brown is a field close to the GC with significant extinction and reddening, whereas the orange is a field away from the Galactic Plane where extinction and reddening have a lesser effect. The boxes enclose stars used in fitting the luminosity function to isolate the Red Clump outlined in Section \ref{['sec:fit']}.
• Figure 5: Fit results for two sample fields of standard, $2^\prime \times 2^\prime$ size. We stress that we do not bin the data in our analysis and that binning is solely for visualization. Panel (a) is a high-latitude field that effectively characterizes the Red Clump and background giant stars. Panel (b) is a low-latitude field that suffers from crowding and completeness issues. The exponential shape of the background giant branch stars is missing, resulting in $B$ pushing against the prior lower limit to account for the lack of stars. This field is an example where we only report a lower limit on the extinction. Corner plots of the MCMC sampling are provided in the right panels, with blue lines indicating values from the Maximum Likelihood Estimation.