Stellar content of the interacting galaxies IC 1727 and NGC 672

Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for an isolated pair of interacting galaxies, IC 1727 and NGC 672, and constructed their Hertzsprung-Russell diagrams. The galaxy IC 1727 exhibits a strong asymmetry in the apparent distribution of young stars. The distribution of old stars is smoother and less asymmetric. In the galaxy NGC672, there is no noticeable asymmetry in the distribution of stars of different ages. Based on the TRGB method, we have determined an accurate distance to each galaxy for the first time: D = 7.14 ± 0.10 Mpc for IC 1727 and D = 7.22 ± 0.10 Mpc for NGC 672, confirming that these galaxies are closely spaced. The luminosity functions of red supergiants and peripheral AGB stars in both galaxies have positionally coincident local maxima, suggesting the simultaneous enhancement of star formation in the two galaxies occurred in the intervals 20–30 and 450–700 Myr ago. The results obtained point to an enhancement of star formation processes in the interacting galaxies.     

On the Distance to Nearby Dwarf Galaxy NGC 1569

We present stellar photometry of two HST/WFPC2 fields situated at the far periphery of the poststarburst galaxy NGC 1569. Their F606W and F814W images have been taken from the HST data archive. Judging by the obtained color-magnitude diagrams, we suggest presence of the old disk population (old red giants) of NGC 1569, which extends as far as Field I at 6 kpc from the galaxy center. We use the tip of the red giant branch to estimate the galaxy distance. There are two possible solutions, one leading to a short distance, D = 1.95 ± 0.2 Mpc, and the other one to a longer distance, D = 2.8 ± 0.2 Mpc.     

Does the IC 342/Maffei Galaxy Group Really Exist?

Based on the archival images of the Hubble Space Telescope we performed stellar photometry in the infrared and visible ranges in the fields of the Maffei 1, Maffei 2 and IC 342 galaxies, the light from which undergoes strong absorption by the interstellar gas and dust clouds of our Galaxy. The resulting Hertzsprung-Russell diagrams reveal red giant branches and the positions of their upper boundaries and color indices. A comparison of color indices of the red giant branch with similar indices in the galaxies not experiencing extinction made it possible to determine the amount of extinction in Maffei 1, Maffei 2 and IC 342 as follows: A_V = 3.06, 3.65 and 0.95, respectively. We found that these galaxies are located at the distances of 6.6, 6.8 and 3.9 Mpc. The values obtained substantially differ from the distances to these same galaxies (3.4, 3.5 and 3.5 Mpc), measured by Wu et al. in 2014 using the same images. The measurements we carried out show that Maffei 1 and Maffei 2 do not form a single group with the IC 342 galaxy, as previously thought.     

Stellar population of the blue compact dwarf galaxy SBS 1415+437

The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated using the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 ^m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch αTRGB) was used to calculate the distance modulus, which turned out to be m ? M = 30.65 ± 0.08 ^m . The corresponding distance to the galaxy is D = 13.5 ± 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old population occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distribution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 × 10⁷ yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V ? I) _TRGB ≈ 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 10¹⁰ yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is [Fe/H] = ?2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to [Fe/H] = ?1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 × 10⁹ yr.     

Stellar content of the interacting galaxies of the M51 system

Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for more than 0.6 million stars in the interacting galaxies NGC 5194 and NGC 5195 of the M51 system. Stars of various ages have been identified on the constructed Hertzsprung-Russell diagram: blue and red supergiants, AGB stars, and red giants. The distance to M51 has been measured from the position of the tip of the red giant branch, D = 9.9 ± 0.7 Mpc. We have determined the change in the metallicity of red supergiants along the galactic radius in NGC 5194. Despite the gravitational interaction, the distribution of stars in NGC 5194 does not differ from that in isolated galaxies. The asymmetric stellar structures of NGC5195 (the so-called “feathers”) formed through the interaction of two galaxies have been found to consist mostly of AGB stars.     

A colour-agnitude diagram of the Pisces dwarf galaxy

We present B, V and R band CCD images of the dwarf galaxy LGS-3 (Pisces dwarf). About 180 stars and diffuse objects were found and processed in these frames. The colour-magnitude diagram shows a prominent red giant branch and a few blue stars only. Details of the morphological properties and colour-agnitude diagram indicate to an intermediate (between dIrr and dSph) type of LGS-3. We estimate the galaxy distance modulus to be about (m - M)₀ = 23.9 ± 0.3 mag on the basis of the tip of the red giant branch. We measured the galaxy colour to be about B–V = 0.8 mag and the linear diameter 700 pc, which is the smallest one in the Local group.     

Stellar content of the metal-poor galaxy DDO 68

Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for the metal-poor galaxy DDO 68. The apparent distributions of stars of different ages and stellarmetallicity determinations indicate that DDO68 is a systemof two galaxies that have different stellar metallicities (Z = 0.004 and 0.001) and are in the stage of interaction or merging. We have determined the distance to DDO 68, D = 12.0 ± 0.3 Mpc, which differs significantly from previous estimates of the distance to this system. A concentration of red giants is observed outside DDO 68. This can be interpreted as the periphery of a partially visible low-surface-brightness galaxy located at the same distance as DDO 68. Comparison of the constructed CM diagrams with theoretical isochrones from Bertelli et al. has allowed us to determine that the age of each galaxy is at least 10 Gyr.     

Investigation of the new Local Group galaxy VV 124

We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV124 (UGC4879) obtainedwith the 6-m BTAtelescope. The presence of a fewbright supergiants in the galaxy indicates that the current star formation process is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution to the background emission from the central galactic regions. The presence of young stars is also confirmed by the [OIII] emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is υ _h = −70 ± 15 km s⁻¹ and the velocity with which VV 124 falls into the Local Group is υ _LG = −12 ± 15 km s⁻¹. We confirm the distance to the galaxy (D = 1.1 ± 0.1 Mpc) and the metallicity of red giants ([Fe/H] = −1.37) found by Kopylov et al. (2008). VV 124 is located on the periphery of the Local Group approximately at the same distance from M31 and our Galaxy and is isolated from other galaxies. The galaxy LeoA nearest to it is 0.5 Mpc away.     

Stellar content and distances to the isolated spiral galaxies NGC 6503 and NGC 6946

Based on archival Hubble Space Telescope images, we have performed stellar photometry of several fields in the isolated spiral galaxies NGC 6503 and NGC 6946 with high peculiar velocities. Based on the TRGB method, we have determined the distances to the galaxies: D = 6.30 ± 0.10 Mpc for NGC 6503 and D = 6.72 ± 0.15 Mpc for NGC 6946. The current stellar content of the galaxies does not differ from that of other similar galaxies. The metallicity for young stars in NGC 6503 is Z = 0.02 (corresponding to the solar metallicity), while the metallicity for stars in NGC 6946 reaches Z = 0.05. Very few old globular clusters have been found in NGC 6946, while they have not been found at all in NGC 6503. The number density distribution of stars with different ages in NGC 6503 does not differ from the analogous distributions in other galaxies. The large sizes of the thick disk in NGC 6503, which is clearly seen up to 6 kpc from the galactic disk plane and whose possible extension is noticeable up to 8.6 kpc from the plane, are a difference. The sizes of the region occupied by red giants of the disk in NGC 6503 are 51 × 17 kpc, which are not much larger than the sizes of this galaxy from H I radio observations.     

Stellar subsystems of the galaxy NGC 1313

Based on archival Hubble Space Telescope (HST) ACS/WFC images, we have performed stellar photometry for eight fields of the spiral galaxy NGC 1313 and its satellite, the low-mass Sph/Irr galaxy AM0319-662. Stars of various ages have been identified on the constructed Hertzsprung–Russell diagrams: young supergiants, middle-aged stars, and old stars (red giants); their apparent distributions over the body of the galaxy are presented. The red supergiants and giants have been divided into groups with larger and smaller color indices, corresponding to a difference in stellar metallicity. These groups of stars are shown to have different spatial distributions and to belong to two galaxies, NGC1313 itself and the disrupted satellite. We have determined the distance to NGC 1313, D = 3.88 ± 0.07 Mpc, by the TRGB method from six fields. Our photometry of 2014 HST images has revealed an emerged charge transfer inefficiency on the ACS/WFC CCDs, which manifests itself as a dependence of the photometry of stars on their coordinates on the CCD.     

Vilnius Photometry of Red Giant Stars in the Open Cluster NGC 7789

Photoelectric Vilnius seven-color photometry is presented and analyzed for a sample of 24 red giant branch and clump stars in the open cluster NGC 7789. For each star we have determined photometric spectral type, absolute magnitude, interstellar reddening, effective temperature, metallicity, and surface gravity. From averages over the stars in the sample we find the mean reddening to the cluster E _Y−V = 0.21± 0.02 (s.d.), or E _B−V = 0.25, and the apparent distance modulus (m−M) _V = 12.21± 0.10 (s.d.), which yield a distance of 1840 pc. The mean overall metallicity is found to be [Fe/H] = −0.18± 0.09 (s.d.). The clump stars, on average, appear to be slightly more metal-rich than the other red giants, which is most probably caused by evolutionary changes of carbon and nitrogen molecular bands falling in the photometric passbands. A difference in mass between the two groups of stars has also been detected, which suggests that the clump stars might have undergone extra mass loss before reaching their core He-burning phase of evolution.     

Irregular dwarf galaxy KK 230: A relatively young galaxy at the periphery of the local group?

We present some results of the photometric analysis of the stellar population of the irregular dwarf galaxy KK 230 on the basis of the archive database of the Hubble space telescope. The color index-magnitude diagram for KK 230 gets to magnitude 27 ^m in the V and I bands, and it comprises stellar populations of various ages. The age of the youngest main-sequence stars is 3.2 × 10⁷ yr. These stars are distributed along the north-south direction in the picture plane, and this fact can be linked to the observed kinematics of the neutral gas in the galaxy. Older blue and red supergiants are no less than 1.6 × 10⁸ years old, and such an age implies that the star formation was episodic over the last several hundreds of millions of years. As judged from the position of the tip of the red giant branch, the distance modulus for KK 230 is m ? M = 26.5 ^m . The corresponding distance is D = 2 Mpc. Based on the average absolute magnitude M _I,RC and color index (V ? I)_I,RC of the red clump, we conclude that the majority of KK 230 stars have an age of no more than (2–3) × 10⁹ yr, their metallicity being Z ≈ 0.0004.     

Red giants in the blue compact galaxy IZw18

Archival Hubble Space Telescope (HST) data have been used for the photometry of stars in blue compact dwarf (BCD) galaxies of the IZw18 system. Applying the spatial selection of stars, we have detected red giants, stars older than 1–2 Gyr, in the galaxies. These red giants have allowed the distance to IZw18 to be reliably determined for the first time: D = 13.9 ± 1.2 Mpc. The presence of old stars in the galaxies of the IZw18 system refutes the hypothesis about the observed primary star formation in these galaxies.     

Magnetically collimated jets with high mass flux

We present HST /WFPC2 observations of UGC 4483, an irregular galaxy in the M81/NGC 2403 complex. Stellar photometry was carried out with HSTphot, and is complete to V ≃26.0 and I ≃24.7. We measure the red giant branch tip at I =23.56±0.10, and calculate a distance modulus of μ ₀=27.53±0.12 (corresponding to a distance of 3.2±0.2 Mpc), placing UGC 4483 within the NGC 2403 subgroup. We were able to measure properties of a previously known young star cluster in UGC 4483, finding integrated magnitudes of V =18.66±0.21 and I =18.54±0.10 for the stellar contribution (integrated light minus H α and [O  iii ] contribution), corresponding to an age of ∼10–15 Myr and an initial mass of ∼10⁴ M_⊙. This is consistent with the properties of the cluster's brightest stars, which were resolved in the data for the first time. Finally, a numerical analysis of the galaxy's stellar content yields a roughly constant star formation rate of 1.3×10⁻³ M_⊙ yr⁻¹ and mean metallicity of [Fe/H]=−1.3 dex from 15 Gyr ago to the present.     

Supernova remnants, planetary nebulae and the distance to NGC 4214

We present narrow band, continuum subtracted Hα, [S ii], Hβ, [O iii] and [O ii] data taken with the Wide Field Camera 3 on the Hubble Space Telescope in the nearby dwarf starburst galaxy NGC 4214. From these images, we identify seventeen new planetary nebula candidates, and seven supernova remnant candidates. We use the observed emission line luminosity function of the planetary nebulae to establish a new velocity-independent distance to NGC 4214. We conclude that the PNLF technique gives a reddening independent distance to NGC 4214 of 3.19±0.36 Mpc, and that our current best-estimate of the distance to this galaxy ids 2.98±0.13 Mpc.     

Stellar population of the irregular galaxy IC 10

Based on our observations with the 6-m BTA telescope at the Special Astrophysical Observatory, the Russian Academy of Sciences, and archival Hubble Space Telescope images, we have performed stellar photometry for several regions of the irregular galaxy IC 10, a member of the Local Group. Distance moduli with a median value of m − M = 24.47, D = 780 ± 40 kpc, have been obtained by the TRGB method for several regions of IC 10. We have revealed 57 star clusters with various masses and ages within the fields used. Comparison of the Hertzsprung-Russell diagrams for star clusters in IC 10 with theoretical isochrones has shown that this galaxy has an enhanced metallicity, which probably explains the high ratio of the numbers of carbon and nitrogen Wolf-Rayet stars (WC/WN). The size of the galaxy’s thick disk along its minor axis is 10′.5 and a more extended halo is observed outside this disk.     

Recent Changes in the Near-Infrared Structure of eta Carinae

I present the first results from a Hubble Space Telescope/NICMOS imaging study of the most metal-poor blue compact dwarf galaxy, I Zw 18. The near-infrared color-magnitude diagram (CMD) is dominated by two populations, one 10-20 Myr population of red supergiants and one 0.1-5 Gyr population of asymptotic giant branch stars. Stars older than 1 Gyr are required to explain the observed CMD at the adopted distance of 12.6 Mpc, showing that I Zw 18 is not a young galaxy. The results hold also if the distance to I Zw 18 is significantly larger. This rules out the possibility that I Zw 18 is a truly young galaxy formed recently in the local universe.     

New results on large-scale structures

A new version of the magnetic-tape catalog of ABELL clusters of galaxies is used to obtain redshift estimators and to generate two samples of clusters. A procedure for searching for superclusters of galaxies is applied and the results are given in tabular and graphic form. For a lmited homogeneous sample (distance 60–275 Mpc, galactic latitude B > 35°), 12 multiplets, having member clusters with known redshifts, are found. It is shown that the spatial covariance function for rich clusters has the form ξ = (r₀/r)γ with r₀ = 22.4 ± 1.8 Mpc and γ = 1.90 ± 0.25 for 3 Mpc ≲ r ≲ 80 Mpc.     

KDG218, a nearby ultra-diffuse galaxy

We present properties of the low-surface-brightness galaxy KDG218 observed with the HST/ACS. The galaxy has a half-light (effective) diameter of a _e = 47″ and a central surface brightness of SB _V (0) = 24.^m4/□″. The galaxy remains unresolved with the HST/ACS, which implies its distance of D > 13.1 Mpc and linear effective diameter of A _e > 3.0 kpc. We notice that KDG218 is most likely associated with a galaxy group around the massive lenticular NGC4958 galaxy at approximately 22 Mpc, or with the Virgo Southern Extension filament at approximately 16.5 Mpc. At these distances, the galaxy is classified as an ultra-diffuse galaxy (UDG) similar to those found in the Virgo, Fornax, and Coma clusters. We also present a sample of 15 UDG candidates in the Local Volume. These sample galaxies have the following mean parameters: 〈D〉 = 5.1 Mpc, 〈A _e 〉 = 4.8 kpc, and 〈SB _B (e)〉 = 27.^m4/□″. All the local UDG candidates reside near massive galaxies located in the regions with the mean stellar mass density (within 1 Mpc) about 50 times greater than the average cosmic density. The local fraction of UDGs does not exceed 1.5% of the Local Volume population. We notice that the presented sample of local UDGs is a heterogeneous one containing irregular, transition, and tidal types, as well as objects consisting of an old stellar population.     

M 101 group galaxies

Based on archival Hubble Space Telescope images, we have performed stellar photometry for the galaxy M 101 and other neighboring galaxies located at a small angular distance from M 101 and having radial velocities similar to that of M 101: M 51, M 63, NGC 5474, NGC 5477, UGC 9405, Ho IV, KUG1413+573, and others. Based on the TRGB method, we have determined the distances to these galaxies. We have found that the M 101 group lies at a distance of 6.8 Mpc and is a small compact galaxy group consisting of four galaxies: NGC 5474, NGC 5477, UGC 9405, and Ho IV. The bright massive galaxies M 51 and M 63 are considerably farther (D = 9.0 and 9.3 Mpc, respectively) than the M 101 group and do not belong to it. Applying the virial theorem to 27 objects (H II regions and galaxies),M 101 satellites located at different distances from the galaxy, has revealed an increase in the dynamical mass of M 101 with increasing sizes of the system of satellites used in calculating the mass. The maximum calculated mass of M 101 is 7.5 × 10¹¹ M _⊙. The dynamical mass of M 101 calculated on the basis of the four galaxies constituting the group is 6.2 × 10¹¹ M _⊙. The mass-to-light ratio for this mass is M/L = 18 (at the adopted luminosity of M 101, M _B = ?20.8).