Mapping the Northern Galactic Disk Warp with Classical Cepheids

D. M. Skowron, J. Skowron, P. Mróz, A. Udalski, P. Pietrukowicz, I. Soszyński, M. K. Szymański, R. Poleski, S. Kozłowski, K. Ulaczyk, K. Rybicki, P. Iwanek, M. Wrona and M. Gromadzki
Acta Astronomica, 69, 305 (arXiv:1912.11142)

We present an updated three dimensional map of the Milky Way based on a sample of 2431 classical Cepheid variable stars, supplemented with about 200 newly detected classical Cepheids from the OGLE survey. The new objects were discovered as a result of a dedicated observing campaign of the ~280 square degree extension of the OGLE footprint of the Galactic disk during 2018-2019 observing seasons (green contours in the image below). These regions cover the main part of the northern Galactic warp that has been deficient in Cepheids so far.

The on-sky view of the Milky Way in the Galactic coordinates, on top of the Milky Way image (Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO) with all known Classical Cepheids from the OGLE collection (yellow dots) and from other surveys (cyan dots). Newly detected objects are marked with magenta dots. The total number of objects is 2632. The white contour marks the past OGLE survey footprint of the Galactic plane while the green one – the extension presented in this work. This image can be distributed under the CC BY-SA 3.0 licence.

We use direct distances to the sample of over 2390 classical Cepheids to model the distribution of the young stellar population in the Milky Way and recalculate the parameters of the Galactic disk warp. Our data show that its northern part is very prominent and its amplitude is ~10% larger than that of the southern part.

Top view of the Milky Way with all 2390 Cepheids with mid-IR counterparts. The sample from Skowron et al. (2019) is marked with blue dots, while the newly detected objects are marked with red dots. The background image represents a four-arm spiral galaxy model consistent with neutral hydrogen measurements in our Galaxy. The Sun is marked with a yellow disk, while the dotted lines show the angular extent of the extended OGLE footprint ( l < 60 deg and l > 190 deg ).

Three dimensional view of the Milky Way in young stellar population. Dots mark positions of classical Cepheids from our sample. The grid represents our new model of the Galactic disk (Eq. 1). The viewing angles are 325 deg (top) and 170 deg (bottom), at an inclination angle 30 deg. The Sun is marked with a yellow dot, while the solid line represents the solar circle. The dotted lines divide parts of the Milky Way disk model warped South and North.

By combining Gaia astrometric data with the Galactic rotation curve and distances to Cepheids from our sample, we construct a map of the vertical component of the velocity vector for all Cepheids in the Milky Way disk. We find large-scale vertical motions with amplitudes of 10-20 km/s, such that Cepheids located in the northern warp exhibit large positive vertical velocity (toward the north Galactic pole), whereas those in the southern warp - negative vertical velocity (toward the south Galactic pole).

The vertical distance Z from the Galactic plane (left panel) showing the extent and amplitude of the Galactic warp. The vertical velocity distribution (right panel) is smoothed by calculating a median velocity within 3 kpc of 0.5x0.5 kpc wide bins. The dotted lines divide parts of the Milky Way disk model warped South and North, the dashed line box shows an area investigated by Poggio et al. (2018). The Sun is marked with a yellow dot while the Galactic center with a black dot.

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