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Magellanic System in OGLE-IV

OGLE-ing the Magellanic System: Stellar Populations in the Magellanic Bridge

We report the discovery of a stellar bridge consisting of the young population of stars, that forms a continuous connection between the Magellanic Clouds. This finding is based on number density maps for the young, intermediate-age, and old populations, in the region between the Magellanic Clouds, called the Magellanic Bridge. The maps are based on data gathered in the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV) and fully cover over 270 square degrees of the sky in the Magellanic Bridge area. This is the most extensive optical survey of this region up to date.

We find that the young population is present mainly in the western half of the Magellanic Bridge, which is consistent with previous findings. However, we discover a significant young population in the eastern part of the classical Bridge region, connecting both galaxies along δ≈-73.5 deg, thus showing that the Magellanic Bridge contains a continuous stream of stars. We also find that the young population distribution is clumped, with one of the major densities close to the SMC (α≈2 h, δ≈-74 deg), and the other, fairly isolated and located approximately mid-way between the Clouds (α≈2.9 h, δ≈-73.5 deg), which we call the OGLE island. These overdensities are well matched by HI surface density contours, although the newly found young population in the eastern Bridge is offset by ≈2 deg north from the highest HI density contour.

We observe a continuity of red clump stars between the Magellanic Clouds, which represent an intermediate-age population. Red clump stars are present mainly in the southern and central parts of the Magellanic Bridge, below its gaseous part, and their presence is reflected by a strong deviation from the radial density profiles of the two galaxies. This may indicate either a tidal stream of stars, or that the stellar halos of the two galaxies overlap. On the other hand, we do not observe such an overlap within an intermediate-age population represented by the top of the red giant branch and the asymptotic giant branch stars. We also see only minor mixing of the old populations of the Clouds in the southern part of the Bridge, represented by the lowest part of the red giant branch.

Spatial density map of the Young Population stars in the Magellanic Bridge region, in a Hammer equal-area projection centered at α=3.3 h and δ=-70 deg. The color-coded value of each "pixel" is a logarithm of the number of stars per square degree area, while each "pixel" area is approximately 0.335 deg². A median background level was estimated from 40 most southern fields to be 1 star/deg² with a standard deviation of 6 stars/deg². Detections weaker than 2σ above the median background level have been given the background color. All number densities are listed in Table (see below). An approximate location of the LMC disk and the main stellar body of the SMC are marked with white ellipses. The white cross marks the SMC center of the outer SMC population found by Nidever et al. (2011). Color contours mark neutral hydrogen (HI) emission integrated over the velocity range 80<v<400 km/s, where each contour represents the HI column density twice as large as the neighboring contour. HI column densities are in the range 10²⁰-4×10²¹ cm^-2. Data were taken from the LAB survey of Galactic HI (Kalberla et al. 2005).

Computer readable Table 2 from the Paper is available for download here: table2.dat

α	δ	l	b	YP	RC	RGB top	RGB mid	RGB low
[h]	[deg]	[deg]	[deg]	[stars/deg2]
1.3007	-77.937	301.145	-39.099	26	279	50	536	418
1.3171	-76.661	300.848	-40.355	45	773	65	851	698
1.3299	-75.385	300.545	-41.612	253	2448	185	2530	2111
1.3398	-74.109	300.234	-42.867	3902	4974	299	4820	3913
1.3615	-79.988	301.289	-37.044	7	171	48	301	211
1.3661	-66.440	298.154	-50.404	1	52	-5	85	150
see whole Table

• YP - young population (main sequence stars)
• RC - Red clump stars
• RGB top - stars in the top part of the Red Giant Branch (young population, mainly AGB stars)
• RGB mid - stars in the middle part of the Red Giant Branch (see Fig. 6 in the paper)
• RGB low - stars in the low part of the Red Giant Branch (old population)

PLEASE cite the following paper when using the data or referring to these OGLE results:
Skowron et al., 2014, The Astrophysical Journal, 795, 108, arXiv:1405.7364

Any comments about the data and the form of their presentation are welcome as they can improve the future releases of OGLE analysis. Send your messages to this address.

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