The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+) (1402.7024v2)

Abstract: The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u, g, r, i and Halpha at 1 arcsec angular resolution. Its footprint spans the Galactic latitude range -5 < b < +5 at all longitudes south of the celestial equator. Extensions around the Galactic Centre to Galactic latitudes +/-10 bring in much of the Galactic Bulge. This ESO public survey, begun on 28th December 2011, reaches down to 20th magnitude (10-sigma) and will provide single-epoch digital optical photometry for around 300 million stars. The observing strategy and data pipelining is described, and an appraisal of the segmented narrowband Halpha filter in use is presented. Using model atmospheres and library spectra, we compute main-sequence (u - g), (g - r), (r - i) and (r - Halpha) stellar colours in the Vega system. We report on a preliminary validation of the photometry using test data obtained from two pointings overlapping the Sloan Digital Sky Survey. An example of the (u - g, g - r) and (r - Halpha, r - i) diagrams for a full VPHAS+ survey field is given. Attention is drawn to the opportunities for studies of compact nebulae and nebular morphologies that arise from the image quality being achieved. The value of the u band as the means to identify planetary-nebula central stars is demonstrated by the discovery of the central star of NGC 2899 in survey data. Thanks to its excellent imaging performance, the VST/OmegaCam combination used by this survey is a perfect vehicle for automated searches for reddened early-type stars, and will allow the discovery and analysis of compact binaries, white dwarfs and transient sources.

The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS+)

The VPHAS+ project is a photometric survey aimed at exploring the Southern Galactic Plane and Bulge. It utilizes the VST/OmegaCam to capture data in five optical bands: $u$, $g$, $r$, $i$, and a narrowband H$\alpha$ filter. This survey addresses a significant range within the Galactic Plane, $-5^{\rm o} < b < +5^{\rm o}$, and extends around the Galactic Bulge to latitudes of $\pm 10^{\circ}$. VPHAS+ intends to provide single-epoch digital optical photometry for approximately 300 million stars, reaching a depth of $\sim$20th magnitude at 10$\sigma$.

Observational Strategy

VPHAS+ commenced its observing campaign on 28th December 2011 and has since grown its dataset through a structured plan. The survey separates the acquisition of blue ($u, g$) from red ($i, H\alpha$) filters, using $r$ as a reference collected in both blocks for consistency. To counteract the VST's image quality fluctuations, particularly those due to atmospheric changes, blue data are gathered when the moon is less than half full, while red data acquisition can tolerate a more illuminated moon. Thus far, red data often precede blue data in completion.

Data-processing Pipelining

Data processing for VPHAS+ occurs through a robust pipeline. Key steps include bias subtraction, flatfielding, and catalog generation, which employs world coordinate systems (WCS) based on 2MASS catalog data. Photometric calibration follows standards to ensure all data conform to an internal magnitude scale rooted in the Vega system. Special attention is given to overcoming the significant scattered light challenges and systemic vignetting involved in observing with the VST.

Characterization of the Narrowband Hα Filter

The Hα filter employed in VPHAS+, labeled NB-659, was initially part of a segmented arrangement due to technical constraints on filter production size. Each segment varies in central wavelength slightly, presenting a unique dataset for each part of its coverage. This design choice means that attention to the radial velocity of sources is necessary to ensure accurate Hα photometry. Any variations detected are counteracted via illumination correction methods based on APASS catalog comparisons.

Photometric Verification

Photometric validation is necessary to check derived fluxes against known standards. Comparisons between VPHAS+ and SDSS photometry exhibit strong consensus for $g$, $r$, and $i$ bandpasses, while $u$ band data require further refinement. The challenges surrounding $u$ calibration are attributed to the complexities of nonlinear color transformations in the presence of reddened fields, suggesting constant monitoring and recalibration efforts are necessary to maintain survey reliability.

Color-Color Diagrams and Survey Data Utilization

Synthetic photometry depicts main-sequence and giant stellar populations in various color-color spaces. These serve as crucial frameworks for interpreting VPHAS+ data against predicted behaviors—powerful tools for stellar classification, dust mapping, and, importantly, isolating the rare stellar phases such as young stellar objects, compact binaries, and planetary nebula central stars.

Implications and Future Work

The survey's implications span multiple realms of galactic astrophysics. For instance, it can significantly enhance the census of southern emission line stars or offer new insights into the Galactic bulge's stellar populations. As observed in test scenarios, including studies of reflection nebulae and planetary nebulae, VPHAS+ data provide high-quality imaging differences when juxtaposed with older surveys such as the SHS. This potential extends into the domain of variability studies, offering previews of Gaia-like synergy.

In conclusion, VPHAS+ is a pivotal step forward for Southern Hemisphere Galactic Plane and Bulge exploration. It serves as an invaluable resource for wide-ranging astrophysical inquiries, particularly in assessing objects characterized by complex morphologies or photometric variabilities, and it sets a new standard in astronomical datasets by providing multi-band photometry of unprecedented breadth and consistency.