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Imaging and Spectroastrometry with Photonic Lanterns


PL diagram
Fig.1 A schematic diagram of the standard 6 port photonic lantern. Kim et al. 2022

A photonic lantern (PL) is a tapered waveguide that gradually transitions from a multi-mode fiber (MMF) geometry to a bundle of single-mode fibers (SMFs). When a multi-moded light from the telescope is injected to the MMF end of a PL, it splits and gets confined in the SMF cores, which means the PL converts multi-moded beam into multiple single-moded beams. Due to the spatial filtering nature of the SMF, the output beams from the PL are highly stable, making it ideal for high-contrast imaging and spectroscopy. Also, SMF outputs of PLs have information on the input scene and are particularly sensitive to small angular scale variations. We are working on exploring the capability of PLs for two applications, interferometric imaging (Kim et al. submitted) and spectroastrometry (Kim et al. 2022). To learn more about potential science cases of PLs on extremely large telescopes, check out my AO4ELT7 poster.

Spectroastrometry is a method to study angular scales smaller than seeing- or diffraction-limits for objects whose morphology changes with wavelength (such as those exhibiting emission lines). Spectroastrometry has been used in many science cases, such as studying young binaries, stellar outflows, and broad-line region of quasars. Mostly, spectroastrometric studies have been carried out using either long-slit spectroscopy or image slicer-based integral field units. We studied the potential of another mode for spectroastrometry, which is using photonic lanterns. We performed numerical simulations and presented mock observation of binary system (accreting planet and a star) in Kim et al. 2022 and we are going to do on-sky observations!

By interferometrically combining the SMF outputs, we may be able to achieve a resolution limit smaller than λ/D. We are exploring the potential of achieving the interferometric resolution with a single telescope using a PL with a photonic chip beam combiner in the backend (Kim et al. submitted).

Measuring distances to the galaxies in the Local Volume


The tip of the red giant branch (TRGB) and the surface brightness fluctuations (SBF) are powerful techniques for measuring distances to nearby galaxies in that they use bright and old stellar populations, which are ubiquitous in every nearby galaxy. Knowledge of precise distances to galaxies is crucial, both for constraining cosmological parameters and for obtaining true galaxy properties. I have worked on measuring the value of Hubble constant and studying satellite galaxies of the NGC 4437 group during my master's.

Determination of the Local H0 using galaxies in front of the Virgo Cluster

Virgo infall
Fig.2 (Left) Map of the galaxies used for Virgo infall fitting. (Right) Hubble diagram of the sample galaxies in the Virgo cluster frame: Virgocentric velocity vs. Virgocentric distance. Radial velocities are transformed into Virgocentric velocities using (a) the minor attractor model and (b) the major attractor model.

There is a significant disagreement between the measurements of the Hubble constant from the early universe and from the late universe. Most measurements of the local H0 use distant galaxies hosting supernovae in order to minimize the effects of peculiar velocities. However, galaxies located between the Local Group and the Virgo Cluster (D = 16.5 Mpc) can also be used to obtain H0, if we simultaneously consider their peculiar motion and the Hubble flow. These galaxies show an infall pattern toward the center of the Virgo cluster. Using velocities and TRGB distances of 33 such galaxies and Virgo infall models, we determined an independent value of the H0 in Kim et al. 2020.

Dwarf Galaxies in NGC 4437 Group

NGC4437 group
Fig.3 (Left) Location of dwarf satellite candidate galaxies of the NGC 4437 group. (Right) Hyper Suprime-Cam g, r, i color images of the confirmed members.

In Kim & Lee 2021, we studied the use of SBF methods for dwarf galaxies and determined an empirical calibration for the Hyper Suprime-Cam i-band data, valid in a blue regime, 0.2 < (g-i) < 0.8. We then used our empirical SBF calibration to measure the distances to dwarf galaxies around NGC 4437 (D = 9.28 ±0.39 Mpc) and its pair NGC 4592 (D = 9.07 ± 0.27 Mpc) that we have found in Hyper Suprime-Cam Subaru Strategic Program Wide layer, which covers 5 deg x 4 deg around NGC 4437 (Kim et al. 2022). We confirmed five dwarf galaxies as members and compared the properties of the NGC 4437 group with other galaxy groups in the Local Volume.




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