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Register a new userExtragalactic archeology with the GHOSTS Survey I. - Age-resolved disk structure of nearby low-mass galaxies
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We study the individual evolution histories of three nearby low-mass edge-on galaxies (IC 5052, NGC4244, and NGC5023). Using resolved stellar populations, we constructed star count density maps for populations of different ages and analyzed the change of structural parameters with stellar age within each galaxy. We do not detect a separate thick disk in any of the three galaxies, even though our observations cover a wider range in equivalent surface brightness than any integrated light study. While scale heights increase with age, each population can be well described by a single disk. Two of the galaxies contain a very weak additional component, which we identify as the faint halo. The mass of these faint halos is lower than 1% of the mass of the disk. The three galaxies show low vertical heating rates, which are much lower than the heating rate of the Milky Way. This indicates that heating agents, such as giant molecular clouds and spiral structure, are weak in low-mass galaxies. All populations in the three galaxies exhibit no or only little flaring. While this finding is consistent with previous integrated light studies, it poses strong constraints on galaxy simulations, where strong flaring is often found as a result of interactions or radial migration.
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We study the stellar halo color properties of six nearby massive highly inclined disk galaxies using Hubble Space Telescope Advanced Camera for Surveys and Wide Field Camera 3 observations in both F606W and F814W filters from the GHOSTS survey. The observed fields, placed both along the minor and major axis of each galaxy, probe the stellar outskirts out to projected distances of ~ 50-70 kpc from their galactic centre along the minor axis. The 50% completeness levels of the color magnitude diagrams are typically at two mag below the tip of the red giant branch. We find that all galaxies have extended stellar halos out to ~ 50 kpc and two out to ~ 70 kpc. We determined the halo color distribution and color profile for each galaxy using the median colors of stars in the RGB. Within each galaxy we find variations in the median colors as a function of radius which likely indicates population variations, reflecting that their outskirts were built from several small accreted objects. We find that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear negative color gradient, reflecting a declining metallicity in their halos; the other have no significant color or population gradient. In addition, notwithstanding the modest sample size of galaxies, there is no strong correlation between their halo color/metallicity or gradient with galaxys properties such as rotational velocity or stellar mass. The diversity in halo color profiles observed in the GHOSTS galaxies qualitatively supports the predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of the stochasticity inherent in the assembling history of galaxies.
Context: We introduce the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) project and report the discovery of eleven low surface brightness (LSB) galaxies in the fields of the nearby galaxies NGC 2683, NGC 3628, NGC 4594 (M104), NGC 4631, NGC 5457 (M101), and NGC 7814. Aims: The DGSAT project aims to use the potential of small-sized telescopes to probe LSB features around large galaxies and to increase the sample size of the dwarf satellite galaxies in the Local Volume. Methods: Using long exposure images, fields of the target spiral galaxies are explored for extended low surface brightness objects. After identifying dwarf galaxy candidates, their observed properties are extracted by fitting models to their light profiles. Results: We find three, one, three, one, one, and two new LSB galaxies in the fields of NGC 2683, 3628, 4594, 4631, 5457, and 7814, respectively. In addition to the newly found galaxies, we analyse the structural properties of nine already known galaxies. All of these 20 dwarf galaxy candidates have effective surface brightnesses in the range $25.3lesssimmu_{e}lesssim28.8$ mag.arcsec$^{-2}$ and are fit with Sersic profiles with indices $nlesssim 1$. Assuming that they are in the vicinity of the above mentioned massive galaxies, their $r$-band absolute magnitudes, their effective radii, and their luminosities are in the ranges $-15.6 lesssim M_r lesssim -7.8$, 160 pc $lesssim R_e lesssim$ 4.1 kpc, and $0.1times 10^6 lesssimleft(frac{L}{L_{odot}}right)_rlesssim127 times 10^6$, respectively. To determine whether these LSB galaxies are indeed satellites of the above mentioned massive galaxies, their distances need to be determined via further observations. Conclusions: Using small telescopes, we are readily able to detect LSB galaxies with similar properties to the known dwarf galaxies of the Local Group.
We have performed two-dimensional multicomponent decomposition of 144 local barred spiral galaxies using 3.6 $mu {rm m}$ images from the Spitzer Survey of Stellar Structure in Galaxies. Our model fit includes up to four components (bulge, disk, bar, and a point source) and, most importantly, takes into account disk breaks. We find that ignoring the disk break and using a single disk scale length in the model fit for Type II (down-bending) disk galaxies can lead to differences of 40% in the disk scale length, 10% in bulge-to-total luminosity ratio (B/T), and 25% in bar-to-total luminosity ratios. We find that for galaxies with B/T $geq$ 0.1, the break radius to bar radius, $r_{rm br}/R_{rm bar}$, varies between 1 and 3, but as a function of B/T the ratio remains roughly constant. This suggests that in bulge-dominated galaxies the disk break is likely related to the outer Lindblad Resonance (OLR) of the bar, and thus moves outwards as the bar grows. For galaxies with small bulges, B/T $<$ 0.1, $r_{rm br}/R_{rm bar}$ spans a wide range from 1 to 6. This suggests that the mechanism that produces the break in these galaxies may be different from that in galaxies with more massive bulges. Consistent with previous studies, we conclude that disk breaks in galaxies with small bulges may originate from bar resonances that may be also coupled with the spiral arms, or be related to star formation thresholds.
The Legacy ExtraGalactic UV Survey (LEGUS) is a Cycle 21 Treasury program on the Hubble Space Telescope, aimed at the investigation of star formation and its relation with galactic environment in nearby galaxies, from the scales of individual stars to those of ~kpc-size clustered structures. Five-band imaging, from the near-ultraviolet to the I-band, with the Wide Field Camera 3, plus parallel optical imaging with the Advanced Camera for Surveys, is being collected for selected pointings of 50 galaxies within the local 12 Mpc. The filters used for the observations with the Wide Field Camera 3 are: F275W(2,704 A), F336W(3,355 A), F438W(4,325 A), F555W(5,308 A), and F814W(8,024 A); the parallel observations with the Advanced Camera for Surveys use the filters: F435W(4,328 A), F606W(5,921 A), and F814W(8,057 A). The multi-band images are yielding accurate recent (<~50 Myr) star formation histories from resolved massive stars and the extinction-corrected ages and masses of star clusters and associations. The extensive inventories of massive stars and clustered systems will be used to investigate the spatial and temporal evolution of star formation within galaxies. This will, in turn, inform theories of galaxy evolution and improve the understanding of the physical underpinning of the gas-star formation relation and the nature of star formation at high redshift. This paper describes the survey, its goals and observational strategy, and the initial science results. Because LEGUS will provide a reference survey and a foundation for future observations with JWST and with ALMA, a large number of data products are planned for delivery to the community.
We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 $times$ 10$^{5}$ Msun in the inner disks of five large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and $^{12}$CO (J=1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H$_{2}$ mass (or xcons) for each galaxy is 1-2 xcounits, all within a factor of two of the Milky Way disk value ($sim$2 xcounits). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-xco trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
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