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Register a new userYoung Stars in the Outer HI Disc of NGC 6822
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W.J.G. de Blok
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We present wide-field optical imaging covering the entire neutral hydrogen disc of the Local Group dwarf galaxy NGC 6822. These observations reveal the presence of numerous blue, young stars at large galactocentric radii well beyond R_25. Blue stars are also found that are associated with NGC 6822s companion HI cloud, indicating that star formation was triggered in the companion in the last 10^8 yr. In general, blue stars are present where the HI surface densities reach values > 5 x 10^20 cm^(-2). However, over one-third of the blue stars detected are found at lower surface densities. The young stars trace the distribution of the neutral hydrogen surprisingly well, but seem to be avoiding the supergiant HI shell in NGC 6822, setting a lower limit for its age of 10^8 yr. The extended distribution of young stars implies that stars can form at large galactocentric radii in dwarf galaxies; the HI is therefore not necessarily much more extended than the stellar population. This finding has important consequences for the chemical enrichment of the interstellar medium throughout (dwarf) galaxies
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We present a comprehensive study of massive young stellar objects (YSOs) in the metal-poor galaxy NGC 6822 using IRAC and MIPS data obtained from the {em Spitzer Space Telescope}. We find over 500 new YSO candidates in seven massive star-formation regions; these sources were selected using six colour-magnitude cuts. Via spectral energy distribution fitting to the data with YSO radiative transfer models we refine this list, identifying 105 high-confidence and 88 medium-confidence YSO candidates. For these sources we constrain their evolutionary state and estimate their physical properties. The majority of our YSO candidates are massive protostars with an accreting envelope in the initial stages of formation. We fit the mass distribution of the Stage I YSOs with a Kroupa initial mass function and determine a global star-formation rate of 0.039 $M_{odot} yr^{-1}$. This is higher than star-formation rate estimates based on integrated UV fluxes. The new YSO candidates are preferentially located in clusters which correspond to seven active high-mass star-formation regions which are strongly correlated with the 8 and 24 $mu$m emission from PAHs and warm dust. This analysis reveals an embedded high-mass star-formation region, Spitzer I, which hosts the highest number of massive YSO candidates in NGC 6822. The properties of Spitzer I suggest it is younger and more active than the other prominent H,{sc ii} and star-formation regions in the galaxy.
We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep color magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended HI structure in NGC 6822 is a warped disk inclined to the line of sight.
Dippers are a common class of young variable star exhibiting day-long dimmings with depths of up to several tens of percent. A standard explanation is that dippers host nearly edge-on (70 deg) protoplanetary discs that allow close-in (< 1 au) dust lifted slightly out of the midplane to partially occult the star. The identification of a face-on dipper disc and growing evidence of inner disc misalignments brings this scenario into question. Thus we uniformly (re)derive the inclinations of 24 dipper discs resolved with (sub-)mm interferometry from ALMA. We find that dipper disc inclinations are consistent with an isotropic distribution over 0-75 deg, above which the occurrence rate declines (likely an observational selection effect due to optically thick disc midplanes blocking their host stars). These findings indicate that the dipper phenomenon is unrelated to the outer (>10 au) disc resolved by ALMA and that inner disc misalignments may be common during the protoplanetary phase. More than one mechanism may contribute to the dipper phenomenon, including accretion-driven warps and broken discs caused by inclined (sub-)stellar or planetary companions.
In the pre-main-sequence stage, star-disc interactions have been shown to remove stellar angular momentum and regulate the rotation periods of stars with M2 and earlier spectral types. Whether disc regulation also extends to stars with later spectral types still remains a matter of debate. Here we present a star-disc interaction study in a sample of over 180 stars with spectral types M3 and later (corresponding to stellar masses $leq 0.3 M_odot$) in young stellar cluster NGC 2264. Combining rotation periods from the literature, new and literature spectral types, and newly presented deep Spitzer observations, we show that stars with masses below 0.3 $M_odot$ with discs also rotate slower than stars without a disc in the same mass regime. Our results demonstrate that disc-regulation still operates in these low-mass stars, although the efficiency of this process might be lower than in higher-mass objects. We confirm that stars with spectral types earlier and later than M2 have distinct period distributions and that stars with spectral types M5 and later rotate even faster M3 and M4-type stars.
We investigate the star formation threshold in NGC 6822, a nearby Local Group dwarf galaxy, on sub-kpc scales using high-resolution, wide-field, deep HI, Halpha and optical data. In a study of the HI velocity profiles we identify a cool and warm neutral component in the Interstellar Medium of NGC 6822. We show that the velocity dispersion of the cool component (~4 km/s) when used with a Toomre-Q criterion gives an optimal description of ongoing star formation in NGC 6822, superior to that using the more conventional dispersion value of 6 km/s. However, a simple constant surface density criterion for star formation gives an equally superior description. We also investigate the two-dimensional distribution of Q and the star formation threshold and find that these results also hold locally. The range in gas density in NGC 6822 is much larger than the range in critical density, and we argue that the conditions for star formation in NGC 6822 are fully driven by this density criterion. Star formation is local, and in NGC 6822 global rotational or shear parameters are apparently not important.
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