Using classical Cepheids to study the far side of the Milky Way disk. I. Spectroscopic classification and the metallicity gradient

Context. Much of what we know about the Milky Way disk is based on studies of the solar vicinity. The structure, kinematics, and chemical composition of the far side of the Galactic disk, beyond the bulge, are still to be revealed.
Aims: Classical Cepheids (CCs) are young and luminous standard candles. We aim to use a well-characterized sample of these variable stars to study the present-time properties of the far side of the Galactic disk.
Methods: A sample of 45 Cepheid variable star candidates were selected from near-infrared time series photometry obtained by the VVV survey. We characterized this sample using high quality near-infrared spectra obtained with VLT/X-shooter. The spectroscopic data was used to derive radial velocities and iron abundances for all the sample Cepheids. This allowed us to separate the CCs, which are metal rich and with kinematics consistent with the disk rotation, from type II Cepheids (T2Cs), which are more metal poor and with different kinematics.
Results: We estimated individual distances and extinctions using VVV photometry and period-luminosity relations, reporting the characterization of 30 CCs located on the far side of the Galactic disk, plus 8 T2Cs mainly located in the bulge region, of which 10 CCs and 4 T2Cs are new discoveries. The remaining seven stars are probably misclassified foreground ellipsoidal binaries. This is the first sizeable sample of CCs in this distant region of our Galaxy that has been spectroscopically confirmed. We use their positions, kinematics, and metallicities to confirm that the general properties of the far disk are similar to those of the well-studied disk on the solar side of the Galaxy. In addition, we derive for the first time the radial metallicity gradient on the disk's far side. Considering all the CCs with R_GC < 17 kpc, we measure a gradient with a slope of -0.062 dex kpc^-1 and an intercept of +0.59 dex, which is in agreement with previous determinations based on CCs on the near side of the disk.

Based on observations collected at the European Southern Observatory under ESO programs 095.B-0444(A), 179.B-2002.