Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster

Abstract

Context. The nuclear star cluster of the Galaxy is an important template for understanding its extragalactic counterparts, which can currently not be resolved into individual stars. Important drawbacks of observations of the Galactic center are, however, the presence of strong and spatially highly variable interstellar extinction and extreme crowding of the sources, which makes the use of adaptive optics techniques necessary. Both points pose serious obstacles to precise photometry that is needed for analyzing the stellar population. Aims. The aims of this work are to provide accurate photometry in multiple near-infrared broadband filters, to determine the powerlaw index of the extinction-law toward the central parsec of the Galaxy, to provide measurements of the absolute extinction toward the Galactic center, and finally to measure the spatial variability of extinction on arcsecond scales. Methods. We use observations of the central parsec of the Milky Way that were obtained with the near-infrared camera and adaptive optics system NAOS/CONICA at the ESO VLT unit telescope 4. The photometric method takes into account anisoplanatic effects and limits the corresponding systematic uncertainties to 2%. Absolute values for the extinction in the H, Ks, and L -bands as well as of the power-law indices of the H to Ks and Ks to L extinction-laws are measured based on the well-known properties of red clump stars. Extinction maps are derived based on H − Ks and Ks − L colors. Results. We present Ks-band photometry for ∼7700 stars, and additionally photometry for stars detected in the H and/or L -bands. From a number of recently published values we compute a mean distance of the Galactic center of R 0 = 8.03 ± 0.15 kpc, which has an uncertainty of just 2%. Based on this R 0 and on the RC method, we derive absolute mean extinction values toward the central parsec of the Galaxy of A H = 4.48 ± 0.13 mag, A Ks = 2.54 ± 0.12 mag, and A L = 1.27 ± 0.18 mag. We estimate values of the power-law indices of the extinction-law of α H−Ks = 2.21 ± 0.24 and α Ks−L = 1.34 ± 0.29. A Ks-band extinction map for the Galactic center is computed based on this extinction law and on stellar H − Ks colors. Both its statistical and systematic uncertainties are estimated to be <10%. Extinction in this map derived from stellar color excesses is found to vary on arcsecond scales, with a mean value of A Ks = 2.74 ± 0.30 mag. Mean extinction values in a circular region with 0.5 radius centered on Sagittarius A* are A H,SgrA * = 4.35 ± 0.12, A Ks,SgrA * = 2.46 ± 0.03, and A L ,SgrA * = 1.23 ± 0.08.

KMOS view of the Galactic centre

“…Five of them are probably foreground stars, as they have very blue colours (Id 436, 663, 1104, 3308, and 3339, (H − K S ) 0 = −2.04, −0.53, −0.52, −0.50, and −1.92 mag). For one of the O/B stars (Id 982) we had to assume a mean extinction value of A K S = 2.70 mag because this star is beyond the field of view of the extinction map of Schödel et al (2010). This means there is a large uncertainty in the star's colour of (H − K S ) 0 = 0.04 mag.…”

“…Therefore, any foreground star will be strongly over-corrected to very negative colours. The intrinsic colours of cluster members are in a very narrow range of about −0.13 mag < (H−K S ) < 0.38 mag (Schödel et al 2010(Schödel et al , 2014bDo et al 2013;Cox 2000, Table 7.6, and 7.8). We used this knowledge to identify foreground stars.…”

“…In the Galactic centre, extinction varies on arcsecond scales (e.g. Scoville et al 2003;Schödel et al 2010;Fritz et al 2011). The typical extinction values are about 2.5 mag in the K S -band, 4.5 mag in the H-band, and more than 7 mag in the J-band.…”

“…The typical extinction values are about 2.5 mag in the K S -band, 4.5 mag in the H-band, and more than 7 mag in the J-band. We used the extinction map and the extinction law derived from Schödel et al (2010) 1 for the extinction correction of the photometry. About 350 (∼30%) of the stars are outside the field of view of the extinction map.…”

SGR A* and Its Environment: Low-Mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow

“…Approximately 12,000 frames, each with a 0.056 s integration time, were combined, corresponding to an accumulated exposure time of about 800 s. The photometry was calibrated by assuming constant extinction between the L and M bands and the same magnitudes at both bands for the sources IRS 16C and IRS 16NW. Their L′-band fluxes were taken from Schödel et al (2010). The uncertainty of the zero point was estimated to be 0.15 mag.…”