The Carnegie Hubble Program: The Leavitt Law at 3.6 and 4.5 micron in the Milky Way

TL;DR

This paper calibrates the Galactic Cepheid Period-Luminosity relation at 3.6 and 4.5 microns using Spitzer data, aiming to improve the extragalactic distance scale and Hubble Constant measurement.

Contribution

It provides a new calibration of the Leavitt Law at mid-infrared wavelengths using Galactic Cepheids and geometric distances, enhancing the precision of cosmic distance measurements.

Findings

Calibrated the 3.6 micron PL zero-point as -5.80±0.03.

Derived a distance modulus of 18.48±0.04 mag for the LMC.

Identified CO absorption effects impacting 4.5 micron data for distance measurements.

Abstract

The Carnegie Hubble Program (CHP) is designed to calibrate the extragalactic distance scale using data from the post-cryogenic era of the Spitzer Space Telescope. The ultimate goal of the CHP is a systematic improvement in the distance scale leading to a determination of the Hubble Constant to within an accuracy of 2%. This paper focuses on the measurement and calibration of the Galactic Cepheid Period-Luminosity (Leavitt) Relation using the warm Spitzer IRAC 1 and 2 bands at 3.6 and 4.5 \mu m. We present photometric measurements covering the period range 4 - 70 days for 37 Galactic Cepheids. Data at 24 phase points were collected for each star. Three PL relations of the form M=a(Log(P)-1)+b are derived. The method adopted here takes the slope a to be -3.31, as determined from the Spitzer LMC data of Scowcroft et al. (2012). Using the geometric HST guide-star distances to ten Galactic Cepheids we find a calibrated 3.6 micron PL zero-point of -5.80\pm0.03. Together with our value for the LMC zero-point we determine a reddening-corrected distance modulus of 18.48\pm0.04 mag to the LMC. The mid-IR Period-Color diagram and the [3.6] - [4.5] color variation with phase are interpreted in terms of CO absorption at 4.5 \mu m. This situation compromises the use of the 4.5 \mu m data for distance determinations.