arXiv subjects

Nolan W. Koblischke

Publications and source records attributed to Nolan W. Koblischke.

2 recordsLinked to original sources

arXiv2024

Calibrating and standardizing the Tip of the Red Giant Branch in the Small Magellanic Cloud using small-amplitude red giants

We investigate the absolute calibration of the Tip of the Red Giant Branch (TRGB) in the Small Magellanic Cloud (SMC) using small amplitude red giant stars (SARGs) classified by the Optical Gravitational Lensing Experiment (OGLE). We show that all stars near the SMC's TRGB are SARGs. Distinguishing older and younger RGs near the Tip according to two period-luminosity sequences labeled A and B, we show many similarities among SARG populations of the LMC and the SMC, along with notable differences. Specifically, SMC SARGs have shorter periods due to lower metallicity and smaller amplitudes due to younger ages than LMC SARGs. We discover two period-color relations near the TRGB that span all A- and B-sequence stars in the OGLE-III footprints of the SMC and LMC, and we investigate using periods instead of color for TRGB standardization. Using variability derived information only, we trace the SMC's age and metallicity gradients and show the core to be populated by younger, more metal rich RGs. The B-sequence yields the brightest and most accurate calibration ($M_{\mathrm{F814W,syn}} = -4.057 \pm 0.019 (\mathrm{stat.}) \pm 0.029 (\mathrm{syst.})$ mag), which we use to measure the distance modulus difference between the Clouds and investigate metallicity effects. Distance measurements not informed by variability should employ the SARGs-based calibration based on all stars near the tip ($M_{\mathrm{F814W,syn}} = -4.024 \pm 0.041 (\mathrm{stat.}) \pm 0.029 (\mathrm{syst.})$ mag). Our work highlights the impact of RG population diversity on TRGB distance measurements. Further study is needed unravel these effects and improve TRGB standardization.

astro-ph.SR↗

arXiv2024

Small amplitude red giants elucidate the nature of the Tip of the Red Giant Branch as a standard candle

The tip of the red giant branch (TRGB) is an important standard candle for determining luminosity distances. Although several $10^5$ small amplitude red giant stars (SARGs) have been discovered, variability was previously considered irrelevant for the TRGB as a standard candle. Here, we show that all stars near the TRGB are SARGs that follow several period-luminosity sequences, of which sequence A is younger than sequence B as predicted by stellar evolution. We measure apparent TRGB magnitudes, m$_{\mathrm{TRGB}}$, in the Large Magellanic Cloud (LMC), using Sobel filters applied to photometry from the Optical Gravitational Lensing Experiment and the ESA Gaia mission, and we identify several weaknesses in a recent LMC-based TRGB calibration used to measure the Hubble constant. We consider four samples: all Red Giants (RGs), SARGs, and sequences A & B. The B-sequence is best suited for measuring distances to old RG populations, with M$_{\mathrm{F814W,0}}$ = -4.025 $\pm$ 0.014(stat.) $\pm$ 0.033(syst.) mag assuming the LMC's geometric distance. Control of systematics is demonstrated using detailed simulations. Population diversity affects m$_{\mathrm{TRGB}}$ at a level exceeding the stated precision: the SARG and A-sequence samples yield 0.039 mag and 0.085 mag fainter (at 5σ significance) m$_{\mathrm{TRGB}}$ values, respectively. Ensuring equivalent RG populations is crucial to measuring accurate TRGB distances. Additionally, luminosity function smoothing ($\sim$ 0.02 mag) and edge detection response weighting (as much as -0.06 mag) can further bias TRGB measurements, with the latter introducing a tip-contrast relation. We are optimistic that variable red giants will enable further improvements to the TRGB as a standard candle.

astro-ph.CO↗