Laura Sturch

The Carnegie Hubble Program: The Leavitt Law at 3.6 μm and 4.5 μm in the Large Magellanic Cloud

The Carnegie Hubble Program is designed to improve the extragalactic distance scale using data from the post-cryogenic era of Spitzer. The ultimate goal is a determination of the Hubble constant to an accuracy of 2%. This paper is the first in a series on the Cepheid population of the Large Magellanic Cloud, and focusses on the period-luminosity (PL) relations (Leavitt laws) that will be used, in conjunction with observations of Milky Way Cepheids, to set the slope and zero point of the Cepheid distance scale in the mid-infrared. To this end, we have obtained uniformly sampled light curves for 85 LMC Cepheids, having periods between 6 and 140 days. PL and period-color relations are presented in the 3.6 μm and 4.5 μm bands. We demonstrate that the 3.6 μm band is a superb distance indicator. The cyclical variation of the [3.6]–[4.5] color has been measured for the first time. We attribute the amplitude and phase of the color curves to the dissociation and recombination of CO molecules in the Cepheids atmosphere. The CO affects only the 4.5 μm flux making it a potential metallicity indicator.

THE CARNEGIE HUBBLE PROGRAM

We present an overview of and preliminary results from an ongoing comprehensive program that has a goal of determining the Hubble constant to a systematic accuracy of ±2%. As part of this program, we are currently obtaining 3.6 μm data using the Infrared Array Camera on Spitzer, and the program is designed to include James Webb Space Telescope in the future. We demonstrate that the mid-infrared period-luminosity relation for Cepheids at 3.6 μm is the most accurate means of measuring Cepheid distances to date. At 3.6 μm, it is possible to minimize the known remaining systematic uncertainties in the Cepheid extragalactic distance scale. We discuss the advantages of 3.6 μm observations in minimizing systematic effects in the Cepheid calibration of H 0 including the absolute zero point, extinction corrections, and the effects of metallicity on the colors and magnitudes of Cepheids. We undertake three independent tests of the sensitivity of the mid-IR Cepheid Leavitt Law to metallicity, which when combined will allow a robust constraint on the effect. Finally, we provide a new mid-IR Tully-Fisher relation for spiral galaxies.

THE CEPHEID PERIOD-LUMINOSITY RELATION AT MID-INFRARED WAVELENGTHS. I. FIRST-EPOCH LMC DATA

We present the first mid-infrared period-luminosity (PL) relations for Large Magellanic Cloud (LMC) Cepheids. Single-epoch observations of 70 Cepheids were extracted from Spitzer IRAC observations at 3.6, 4.5, 5.8 and 8.0 μm, serendipitously obtained during the SAGE (Surveying the Agents of a Galaxys Evolution) imaging survey of the LMC. All four mid-infrared PL relations have nearly identical slopes over the period range 6-88 days, with a small scatter of only ±0.16 mag independent of period for all four of these wavelengths. We emphasize that differential reddening is not contributing significantly to the observed scatter, given the nearly 2 orders of magnitude reduced sensitivity of the mid-IR to extinction compared to the optical. Future observations, filling in the light curves for these Cepheids, should noticeably reduce the residual scatter. These attributes alone suggest that mid-infrared PL relations will provide a practical means of significantly improving the accuracy of Cepheid distances to nearby galaxies.

The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths. II. Second-Epoch LMC Data

We present revised and improved mid-infrared (mid-IR) period-luminosity (PL) relations for Large Magellanic Cloud (LMC) Cepheids based on double-epoch data of 70 Cepheids observed by Spitzer at 3.6, 4.5, 5.8, and 8.0 μm. The observed scatter at all wavelengths is found to decrease from ±0.17 mag to ±0.14 mag, which is fully consistent with the prediction that the total scatter is made up of roughly equal contributions from random sampling of the light curve and nearly uniform samplings of stars across the instability strip. It is calculated that the Cepheids in this sample have a full amplitude of about 0.4 mag and that their fully sampled, time-averaged magnitudes should eventually reveal mid-IR PL relations that each have intrinsic scatter at most at the ±0.12 mag level, and as low as ±0.08 mag after correcting for the tilt of the LMC.

THE CEPHEID PERIOD-LUMINOSITY RELATION (THE LEAVITT LAW) AT MID-INFRARED WAVELENGTHS. IV. CEPHEIDS IN IC 1613

We present mid-infrared Period-Luminosity relations for Cepheids in the Local Group galaxy IC1613. Using archival IRAC imaging data from Spitzer we were able to measure single-epoch magnitudes for five, 7 to 50-day, Cepheids at 3.6 and 4.5 microns. When fit to the calibrating relations, measured for the Large Magellanic Cloud Cepheids, the data give apparent distance moduli of 24.29 +/- 0.07 and 24.28 +/- 0.07 at 3.6 and 4.5 microns, respectively. A multi-wavelength fit to previously published BVRIJHK apparent moduli and the two IRAC moduli gives a true distance modulus of 24.27 +/- 0.02 mag with E(B-V) = 0.08 mag, and a corresponding metric distance of 715 kpc. Given that these results are based on single-phase observations derived from exposures having total integration times of only 1,000 sec/pixel we suggest that Cepheids out to about 2 Mpc are accessible to Spitzer with modest integration times during its warm mission. We identify the main limiting factor to this method to be crowding/contamination induced by the ubiquitous population of infrared-bright AGB stars.

ERRATUM: “THE CARNEGIE HUBBLE PROGRAM: THE LEAVITT LAW AT 3.6 μm and 4.5 μm IN THE LARGE MAGELLANIC CLOUD” (2011, ApJ, 743, 76)

Victoria Scowcroft1, Wendy L. Freedman1, Barry F. Madore1, Andrew J. Monson1, S. E. Persson1, Mark Seibert1, Jane R. Rigby2, and Laura Sturch3 1 Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101, USA; vs@obs.carnegiescience.edu, wendy@obs.carnegiescience.edu, barry@obs.carnegiescience.edu, amonson@obs.carnegiescience.edu, persson@obs.carnegiescience.edu, mseibert@obs.carnegiescience.edu 2 NASA Goddard Space Flight Center, Code 665, Greenbelt, MD 20771, USA; jane.r.rigby@nasa.gov 3 Astronomy Department, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA; lsturch@bu.edu Received 2012 January 3; published 2012 February 15

TYPHOON observations of the Lindsay-Shapley Ring

We present the first results of the TYPHOON program on the ring galaxy AM 0644-741. TYPHOON is a program for producing highly resolved spectrophotometric data cubes with wavelength coverage ranging from [OII] 3727A to 7000A. Using the first results of TYPHOON we will show its efficacy in producing images and from that we will create velocity maps, one of the many uses of TYPHOON results. From this program we will deduce the motion of gas in the ring structure of AM 0644-741 to better understand how the galaxy has evolved to its present state.