A three-dimensional map of the Milky Way using classical Cepheid variable stars (1806.10653v3)

Abstract: The Milky Way is a barred spiral galaxy, with physical properties inferred from various tracers informed by the extrapolation of structures seen in other galaxies. However, the distances of these tracers are measured indirectly and are model-dependent. We constructed a map of the Milky Way in three-dimensions based on the positions and distances of thousands of classical Cepheid variable stars. This map shows the structure of our Galaxy's young stellar population, and allows us to constrain the warped shape of the Milky Way's disk. A simple model of star formation in the spiral arms reproduces the observed distribution of Cepheids.

• The paper refines the Milky Way’s 3D structure by leveraging the Cepheid period-luminosity relation, revealing significant disk warping starting at 8 kpc.
• It employs multi-source data from OGLE-IV, Gaia DR2, Spitzer, and WISE to map 2431 Cepheids, measure disk thickness of 73.5 pc, and analyze pulsation-period gradients.
• The study uncovers an inside-out formation pattern with age and metallicity variations, enhancing our understanding of spiral arm star formation and Galactic evolution.

A Three-Dimensional Map of the Milky Way Using Classical Cepheid Variable Stars

This paper presents a comprehensive paper focusing on the three-dimensional mapping of the Milky Way Galaxy utilizing classical Cepheid variable stars as distance indicators. Recognizing the limitations of previous Galaxy models, which often relied on indirect tracers with model dependencies, this research utilizes more definitive measurements from classical Cepheids to construct a more accurate depiction of the Galaxy’s young stellar structure.

The use of classical Cepheid variable stars as the main dataset stems from their well-established pulsation period-luminosity (P-L) relationship, which allows researchers to determine absolute magnitudes based on pulsation periods. These stars are not only luminous but are also young, typically less than 400 million years old, fitting their placement within regions of active star formation in the Galaxy's spiral arms.

The investigation analyzed data from multiple sources, notably the Optical Gravitational Lensing Experiment (OGLE-IV), further supplemented by catalogs such as Gaia DR2 and data from the Spitzer and WISE satellites for reduction of extinction effects. Over 2431 Galactic Cepheids were used, spanning distances as far as 20,000 parsecs from the Galactic center.

Key Findings:

1. Galactic Structure and Warping: The map reveals the Galaxy's non-uniform structural distribution and significant warping of the disk. This warp is observed to begin at approximately 8 kpc from the Galactic center, becoming more pronounced at 10 kpc and continuing outwards to 20 kpc. The warp tilts differentially across the Galaxy, being negative in some azimuthal directions and positive in others.
2. Disk Thickness and Flaring: The scale height of the young disk was measured at approximately 73.5 pc, with the Sun positioned about 14.5 pc above the Galactic plane. These figures are consistent with previous studies, suggesting a stable disk thickness within the sectors probed.
3. Pulsation Period and Metallicity: A gradient in the minimum pulsation periods of Cepheids correlates with Galactocentric distance, in line with a known radial metallicity gradient where inner regions are more metal-rich than the outer disk. The period gradient hints at the evolutionary stages and elemental composition variances across the Galaxy, with younger Cepheids being concentrated closer to the center.
4. Age Distribution and Star Formation: The majority of the Cepheids were formed between 50 and 250 million years ago, with a distribution suggesting an inside-out formation pattern: Cepheids near the center are generally younger. Distinct features within the detected Cepheid overdensities suggest a connection to past star formation episodes in the spiral arms, showing coherent regions likely aligned with spiral density waves in past epochs.

Implications and Future Research:

This mapping effort serves as a more precise depiction of the Milky Way's shape and stellar composition, particularly for young stars. By providing critical insights into the warp and extent of the stellar disk, the paper reinforces the need for utilizing direct measurements from stars like Cepheids to accurately model galactic structures.

Future work could potentially expand on the temporal understanding of Galactic evolution by cross-referencing with other stellar populations and constructing indirect models of historical Galactic rotations. Moreover, these findings set a precedent for using similar methods in other galaxies with Cepheid populations and could provide comparative insights into spiral galaxy morphology and kinematics across the universe.

The comprehensive nature of this paper exemplifies the significance of classical Cepheid stars in Galactic cartography and reaffirms their pivotal role in expanding our knowledge of the Milky Way's structure and history. As astronomical methods and equipment improve, including forthcoming observational missions, there is anticipation of even more refined models of stellar distribution and Galactic structure evolution.