The 3D Structure of the Galactic Bulge
Publications of the Astronomical Society of Australia/Publications Astronomical Society of Australia,
2016
Journal Article
Interstellar extinction toward the Galactic Center (GC) is large and significantly differential. Its reddening and dimming effects in red clump (RC) stars in the Galactic Bulge can be exploited to ...better constrain the extinction law toward the innermost Galaxy. By virtue of a deep and complete catalog of more than 30 million objects at and obtained from VVV survey observations, we apply the RC method to infer the selective-to-total extinction ratios in the Z, Y, J, H, and Ks broadband near-infrared filters. The measured values are smaller than previously reported, and are not constant, with mean values of, e.g., and . We also obtain a ratio AZ:AY:AJ:AH: of 7.74:5.38:3.30:1.88:1.0, implying extinction toward the GC to follow a distribution as a function of wavelength steeper than previously reported, consistent with a power law in the near-infrared.
We investigate interstellar extinction curve variations towards ∼4 deg2 of the inner Milky Way in VIJKs
photometry from the OGLE-III (third phase of the Optical Gravitational Lensing Experiment) and ...VVV (VISTA Variables in the Via Lactea) surveys, with supporting evidence from diffuse interstellar bands and F435W, F625W photometry. We obtain independent measurements towards ∼2000 sightlines of AI, E(V − I), E(I − J) and E(J − Ks
), with median precision and accuracy of 2 per cent. We find that the variations in the extinction ratios AI
/E(V − I), E(I − J)/E(V − I) and E(J − Ks
)/E(V − I) are large (exceeding 20 per cent), significant and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how RV
is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve have at least two degrees of freedom, and not one (e.g. RV
), which we confirm with a principal component analysis. We derive a median value of 〈AV
/AKs
〉 = 13.44, which is ∼60 per cent higher than the ‘standard’ value. We show that the Wesenheit magnitude WI
= I − 1.61(I − J) is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that RV
variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of Type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by ∼100 per cent and lack sensitivity to extinction curve variations.
ABSTRACT
Red clump stars are commonly used to map the reddening and morphology of the inner regions of the Milky Way. We use the new photometric catalogues of the VISTA Variables in the Vía Láctea ...survey to achieve twice the spatial resolution of previous reddening maps for Galactic longitudes $\rm -10^{\circ }\lt l\lt 10^{\circ }$ and latitudes $\rm -1.5^{\circ }\lt b\lt 1.5^{\circ }$. We use these de-reddened catalogues to construct the $\rm K_{s}$ luminosity function around the red clump in the Galactic plane. We show that the secondary peak (fainter than the red clump) detected in these regions does not correspond to the bulge red-giant branch bump alone, as previously interpreted. Instead, this fainter clump corresponds largely to the over-density of red clump stars tracing the spiral arm structure behind the Galactic bar. This result suggests that studies aiming to characterize the bulge red-giant branch bump should avoid low galactic latitudes ($\rm |b|\lt 2^{\circ }$), where the background red clump population contributes significant contamination. It furthermore highlights the need to include this structural component in future modelling of the Galactic bar.
Abstract
In the context of a project aimed at characterizing the properties of the so-called Bulge Fossil Fragments (the fossil remnants of the bulge formation epoch), here we present the first ...determination of the metallicity distribution of Liller 1. For a sample of 64 individual member stars we used ESO-MUSE spectra to measure the equivalent width of the Ca II triplet and then derive the iron abundance. To test the validity of the adopted calibration in the metal-rich regime, the procedure was first applied to three reference bulge globular clusters (NGC 6569, NGC 6440, and NGC 6528). In all the three cases, we found single-component iron distributions, with abundance values fully in agreement with those reported in the literature. The application of the same methodology to Liller 1 yielded, instead, a clear bimodal iron distribution, with a subsolar component at Fe/H = −0.48 dex (
σ
= 0.22) and a supersolar component at Fe/H = +0.26 dex (
σ
= 0.17). The latter is found to be significantly more centrally concentrated than the metal-poor population, as expected in a self-enrichment scenario and in agreement with that found in another bulge system, Terzan 5. The obtained metallicity distribution is astonishingly similar to that predicted by the reconstructed star formation history of Liller 1, which is characterized by three main bursts and a low, but constant, activity of star formation over the entire lifetime. These findings provide further support to the possibility that, similar to Terzan 5, Liller 1 is also a Bulge Fossil Fragment.
Aims. The windows of low extinction in the Milky Way (MW) plane are rare but important because they enable us to place structural constraints on the opposite side of the Galaxy, which has hither to ...been done rarely. Methods. We use the near-infrared (near-IR) images of the VISTA Variables in the Vía Láctea (VVV) Survey to build extinction maps and to identify low extinction windows towards the Southern Galactic plane. Here we report the discovery of VVV WIN 1713−3939, a very interesting window with relatively uniform and low extinction conveniently placed very close to the Galactic plane. Results. The new window of roughly 30 arcmin diameter is located at Galactic coordinates (l, b) = (347.4, −0.4) deg. We analyse the VVV near-IR colour-magnitude diagrams in this window. The mean total near-IR extinction and reddening values measured for this window are AKs = 0.46 and E(J − Ks) = 0.95. The red clump giants within the window show a bimodal magnitude distribution in the Ks band, with peaks at Ks = 14.1 and 14.8 mag, corresponding to mean distances of D = 11.0 ± 2.4 and 14.8 ± 3.6 kpc, respectively. We discuss the origin of these red clump overdensities within the context of the MW disk structure.
Abstract
In the context of the ESO-VLT Multi-Instrument Kinematic Survey of Galactic globular clusters, here we present the line-of-sight velocity dispersion profile of NGC 6440, a massive globular ...cluster located in the Galactic bulge. By combining the data acquired with four different spectrographs, we obtained the radial velocity of a sample of ∼1800 individual stars distributed over the entire cluster extension, from ∼0.″1 to 778″ from the center. Using a properly selected sample of member stars with the most reliable radial velocity measures, we derived the velocity dispersion profile up to 250″ from the center. The profile is well described by the same King model that best fits the projected star density distribution, with a constant inner plateau (at
σ
0
∼ 12 km s
−1
) and no evidence of a central cusp or other significant deviations. Our data allowed us to study the presence of rotation only in the innermost regions of the cluster (
r
< 5″), revealing a well-defined pattern of ordered rotation with a position angle of the rotation axis of ∼132° ± 2° and an amplitude of ∼3 km s
−1
(corresponding to
V
rot
/
σ
0
∼ 0.3). In addition, a flattening of the system qualitatively consistent with the rotation signal has been detected in the central region.
Abstract
In the context of a project aimed at characterizing the properties of star clusters in the Galactic bulge, here we present the determination of the internal kinematics and structure of the ...massive globular cluster NGC 6569. The kinematics has been studied by means of an unprecedented spectroscopic data set acquired in the context of the ESO-VLT Multi-Instrument Kinematic Survey of Galactic globular clusters, combining the observations from four different spectrographs. We measured the line-of-sight velocity of a sample of almost 1300 stars distributed between ∼0.″8 and 770″ from the cluster center. From a subsample of high-quality measures, we determined the velocity dispersion profile of the system over its entire radial extension (from ∼5″ to ∼200″ from the center), finding the characteristic behavior usually observed in globular clusters, with a constant inner plateau and a declining trend at larger radii. The projected density profile of the cluster has been obtained from resolved star counts, by combining high-resolution photometric data in the center, and the Gaia EDR3 catalog radially extended out to
∼
20
′
for a proper sampling of the Galactic field background. The two profiles are properly reproduced by the same King model, from which we estimated updated values of the central velocity dispersion, main structural parameters (such as the King concentration, the core, half mass, and tidal radii), total mass, and relaxation times. Our analysis also reveals a hint of ordered rotation in an intermediate region of the cluster (40″ <
r
< 90″, corresponding to 2
r
c
<
r
< 4.5
r
c
), but additional data are required to properly assess this possibility.
Abstract
In Paper I, we showed that clumps in high-redshift galaxies, having a high star formation rate density (Σ
SFR
), produce disks with two tracks in the Fe/H–
α
/Fe chemical space, similar to ...that of the Milky Way’s (MW’s) thin+thick disks. Here we investigate the effect of clumps on the bulge’s chemistry. The chemistry of the MW’s bulge is comprised of a single track with two density peaks separated by a trough. We show that the bulge chemistry of an
N
-body + smoothed particle hydrodynamics clumpy simulation also has a single track. Star formation within the bulge is itself in the high-Σ
SFR
clumpy mode, which ensures that the bulge’s chemical track follows that of the thick disk at low Fe/H and then extends to high Fe/H, where it peaks. The peak at low metallicity instead is comprised of a mixture of in situ stars and stars accreted via clumps. As a result, the trough between the peaks occurs at the end of the thick disk track. We find that the high-metallicity peak dominates near the mid-plane and declines in relative importance with height, as in the MW. The bulge is already rapidly rotating by the end of the clump epoch, with higher rotation at low
α
/Fe. Thus clumpy star formation is able to simultaneously explain the chemodynamic trends of the MW’s bulge, thin+thick disks, and the splash.
Deep near-IR images from the VISTA Variables in the Vía Láctea (VVV) Survey were used to search for RR Lyrae stars in the Southern Galactic plane. A sizable sample of 404 RR Lyrae of type ab stars ...was identified across a thin slice of the fourth Galactic quadrant (295° < < 350°, −2 24 < b < −1 05). The sample's distance distribution exhibits a maximum density that occurs at the bulge tangent point, which implies that this primarily Oosterhoff type I population of RRab stars does not trace the bar delineated by their red clump counterparts. The bulge RR Lyrae population does not extend beyond ∼ 340°, and the sample's spatial distribution presents evidence of density enhancements and substructure that warrants further investigation. Indeed, the sample may be employed to evaluate Galactic evolution models, and is particularly lucrative since half of the discovered RR Lyrae are within reach of Gaia astrometric observations.
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