A view of the M81 galaxy group via the H-alpha window

We present images for 36 galaxies of the M81 group obtained in the H-alpha line. Estimates of the H-alpha flux and star formation rate, SFR, are avialable now for all the known members of the group with absolute magnitudes down to M_B = -10^m. The character of distribution of the galaxies over three paremeters: M_B, SFR, and total hydrogen mass permits us to draw the following conclusions as to evolution status of the group population. a) Spiral and irregular type galaxies would have time to generate their luminosity (baryon mass) during the cosmological time T_0=13.7 Gyr, but dwarf spheroidal objects are capable of reproducing only ~5% of their observed luminosity. b) S and Im, BCD galaxies possess the supply of gas sufficient to maintain their observed SFRs during only next (1/4 -1/3)T_0 years, while dIr and dSph populations have the mean gas depletion time about 3 T_0. c) There is indirect evidence that the star formation in Im, BCD and dIr galaxies proceeds in a mode of vigorous burst activity rather than in the form of a sluggish process. We note the dwarf tidal system near NGC 3077, the Garland, to have the highest SFR per unit luminosity among 150 galaxies of the Local volume with known SFRs. Being averaged over the local "cell of homogeneity" of 4 Mpc in diameter around M 81, the rate of star formation of the group, rho(SFR)= 0.165 M_sun/year*Mpc^3, proves to be 5-8 times higher than, that of the average global rate at Z =0.Comment: 10 pages, 4 figure (figure 1 is available at http://luna.sao.ru/halpha/articles/M81). Astronomical Journal, accepte

Probe of dark galaxies via disturbed/ lopsided isolated galaxies

Searching for lopsided/interacting objects among ~1500 isolated galaxies yields only eight strongly disturbed galaxies which may be explained as a result of their interaction with massive dark objects. We present results of spectral and photometric observations of these galaxies performed with the 6-m telescope that lead to significant restriction on cosmic abundance of dark galaxies.Comment: To appear in proceedings IAU Symp 244, 'Dark Galaxies and Lost Baryons', June 200

Small Bites: Star formation recipes in extreme dwarfs

We study the relationship between the gas column density (Sigma_HI) and the star formation rate surface density (Sigma_SFR) for a sample of extremely small (M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear stochasticity in the relation between the gas column density and star formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star formation, but the fraction of gas with ongoing star formation decreases as the gas column density decreases, and falls to about 50% at Sigma_HI ~ 3 M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency is at least a factor of ~ 2 smaller than typical of star forming regions in spirals. We also find that the ratio of H-alpha emission to FUV emission increases with increasing gas column density. This is unlikely to be due to increasing dust extinction because the required dust to gas ratios are too high. We suggest instead that this correlation arises because massive (i.e. H-alpha producing) stars are formed preferentially in regions with high gas density.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS Letters. The definitive version is available at www.blackwell- synergy.co

Atomic hydrogen, star formation and feedback in the lowest mass Blue Compact Dwarf galaxies

We present the results from a search for HI emission from a sample of newly discovered dwarf galaxies in the M81 group. HI is detected in three galaxies, all of which are classified as BCDs. The HI masses of these galaxies are ~ 10^6 M_sun, making these some of the lowest mass BCDs known. For these three galaxies FUV images (from GALEX) and H-alpha images (from the Russian 6m BTA telescope) are available.The H-alpha emission is very faint, and, in principle could be produced by a single O star. Further, in all cases we find offsets between the peak of the FUV emission and that of the H-alpha emission. Offsets between the most recent sites of star formation (i.e. those traced by H-alpha) and the older sites (i.e. those traced by FUV) would be natural if the star formation is stochastic. In spite of the expectation that the effects of mechanical feedback from star formation would be most directly seen in the smallest galaxies with low gravitational potentials, we only see tentative evidence of outflowing HI gas associated with the star forming region in one of the galaxies.Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRAS. The definitive version is available at www.blackwell-synergy.co

More galaxies in the Local Volume imaged in H-alpha

We have carried out an H-alpha flux measurement for 52 nearby galaxies as part of a general H-alpha imaging survey for the Local Volume sample of galaxies within 10 Mpc. Most of the objects are probable members of the groups around Maffei 2/IC 342, NGC 672/IC 1727, NGC 784, and the Orion galaxy. The measured H-alpha fluxes corrected for extinction are used to derive the galaxy star formation rate (SFR). We briefly discuss some basic scaling relations between SFR, hydrogen mass and absolute magnitude of the Local Volume galaxies. The total SFR density in the local (z = 0) universe is estimated to be (0.019+/-0.003) M_sun yr/Mpc^3.Comment: 29 pages, 6 figures, 7 tables (figures 2 and 3 is available at this this http://una.sao.ru/halpha/articles/Maffield),Astronomical Journal, accepte

The M 101 galaxy group as a node in a nearby cosmic filament

We performed a search for faint low-surface-brightness dwarf galaxies around the major spiral galaxy M 101 and in the large rectangular area within SGL = [30–80]° and SGB = [10–37]° spanning a chain of galaxies: M 63, M 51, M 101, and NGC 6503. We based our search on data from DESI Legacy Imaging Surveys. We discovered six new probable dwarf members of the complex. We present a list of 25 prospective members of the M 101 group and estimate the total mass and the total-mass-to-K-band-luminosity ratio of the group as (1.02 ± 0.42)×1012 M⊙ and (16.0 ± 6.5) M⊙/L⊙, respectively. We note that the average dark mass-to-luminosity ratio in the groups around M 63, M 51, and M 101 is (12 ± 4) M⊙/L⊙, which is almost an order of magnitude lower than the global cosmic ratio, (102 ± 5) M⊙/L⊙

Discovery of an ultra-diffuse galaxy in the pisces-perseus supercluster

We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located 10.°4 in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy ( μ V = 24.8 mag arcsec −2 ), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius ( R e ( V ) = 12″) and proximity (15′) to the well-known dwarf spheroidal galaxy And II. Its red color ( V − I = 1.0), shallow Sérsic index ( n V = 0.68), and the absence of detectable H α emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6 m telescope ( V h = 5450 ± 40 km s −1 ) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (∼78 Mpc), DGSAT I would have an R e ∼ 4.7 kpc and M V ∼ −16.3. Its properties resemble those of the ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy