اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدThe Metal-Enriched Outer Disk of NGC 2915
97
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present optical emission-line spectra for outlying HII regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be enriched to a metallicity of 0.4 Z_solar. An analysis of the metal yields shows that the outer disk of NGC 2915 is overabundant for its gas fraction, while the central star-foming core is similarly under-abundant for its gas fraction. Star formation rates derived from very deep ~14 ks GALEX FUV exposures indicate that the low-level of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider 3 plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC 2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.
قيم البحث
اقرأ أيضاً
We analyzed the radial surface brightness profile of the spiral galaxy NGC 7793 using HST/ACS images from the GHOSTS survey and a new HST/WFC3 image across the disk break. We used the photometry of resolved stars to select distinct populations coveri
ng a wide range of stellar ages. We found breaks in the radial profiles of all stellar populations at 280 (~5.1 kpc). Beyond this disk break, the profiles become steeper for younger populations. This same trend is seen in numerical simulations where the outer disk is formed almost entirely by radial migration. We also found that the older stars of NGC 7793 extend significantly farther than the underlying HI disk. They are thus unlikely to have formed entirely at their current radii, unless the gas disk was substantially larger in the past. These observations thus provide evidence for substantial stellar radial migration in late-type disks.
Using Hubble Space Telescope (HST) ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended HI disc. These observations reveal an elliptical di
stribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the HI gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published H{alpha} observations of the field, which show one faint HII region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope {alpha}=-2.85 and upper-mass limit M$_rm{u}$ = 60 M$_odot$. However, if we assume that all H{alpha} emission is confined to HII regions then the upper-mass limit is restricted to M$_rm{u}$ $le$20 M$_odot$. For the luminosity function fit to be correct we have to discount the H{alpha} observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with HI imaging we find the SFL has a power law index $N=1.53 pm 0.21$. Applying these results to the entire outer HI disc indicates that it contributes 11--28% of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.
We use deep (~27.5 mag V-band point-source limiting magnitude) V- and U-band LBT imaging to study the outer disk (beyond the optical radius R_25) of the non-interacting, face-on spiral galaxy NGC 3184 (D = 11.1 Mpc; R_25 = 11.1 kpc) and find that thi
s outer disk contains >1000 objects (or marginally-resolved knots) resembling star clusters with masses ~10^2 - 10^4 M_sun and ages up to ~1 Gyr. We find statistically significant numbers of these cluster-like knots extending to ~1.4 R_25, with the redder knots outnumbering bluer at the largest radii. We measure clustering among knots and find significant correlation to galactocentric radii of 1.5 R_25 for knot separations <1 kpc. The effective integrated surface brightness of this outer disk cluster population ranges from 30 - 32 mag arcsec^-2 in V. We compare the HI extent to that of the correlated knots and find that the clusters extend at least to the damped Lyman-alpha threshold of HI column density (2e20 cm^-2; 1.62 R_25). The blue knots are correlated with HI spiral structure to 1.5 R_25, while the red knots may be correlated with the outer fringes of the HI disk to 1.7 R_25. These results suggest that outer disks are well-populated, common, and long-lasting features of many nearby disk galaxies.
We present deep ACS images of 3 fields in the edge-on disk galaxy NGC 891, which extend from the plane of the disk to 12 kpc, and out to 25 kpc along the major axis. The photometry of individual stars reaches 2.5 magnitudes below the tip of the RGB.
We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disk and spheroid of NGC 891. The CMDs of thick disk and spheroid population are dominated by old RGB stars with a wide range of metallicities, from a metal-poor tail at [Fe/H] ~ -2.4 dex, up to about half-solar metallicity. The peak of the MDF of the thick disk is at -0.9 dex. The inner parts of the thick disk, within 14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of Delta(V-I)/Delta|Z| = 0.1 +/- 0.05 kpc^-1 is detected. This gradient is however accounted for by the mixing with the metal poor halo stars. No metallicity gradient along the major axis is present for thick disk stars, but strong variations of about 0.35 dex around the mean of [Fe/H] = -1.13 dex are found. The properties of the asymmetric MDFs of the thick disk stars show no significant changes in both the radial and the vertical directions. The stellar populations at solar cylinder-like distances show strikingly different properties from those of the Galaxy, suggesting that the accretion histories of both galaxies have been different. The spheroid population shows remarkably uniform stellar population properties. The median metallicity of the halo stellar population shows a shallow gradient from about -1.15 dex in the inner parts to -1.27 dex at 24 kpc distance from the centre. Similar to the thick disk stars, large variations around the mean relation are present.
We examine Herschel Space Observatory images of one nearby prototypical outer ring galaxy, NGC 1291, and show that the ring becomes more prominent at wavelengths longer than 160um. The mass of cool dust in the ring dominates the total dust mass of th
e galaxy, accounting for at least 70% of it. The temperature of the emitting dust in the ring (T=19.5+/-0.3K) is cooler than that of the inner galaxy (T=25.7+/-0.7K). We discuss several explanations for the difference in dust temperature, including age and density differences in the stellar populations of the ring versus the bulge.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
