Searcharxiv⌕ Search
Home/Archive/arXiv

arXiv · astro-ph/9809091

Distance, structure and bright stellar content of the dwarf irregular galaxy UGC 685

Ulrich Hopp
View original source ↗Read source document ↗Export metadata ↓

Abstract

B,R, and J frames taken with the Calar Alto 3.5m telescope of the dwarf irregular galaxy UGC 685 are presented. The brightest part of the stellar population is resolved in B and R, very few also in J. The stellar color-magnitude diagram is discussed. An estimate of the distance to UGC 685 of 5.5 Mpc is derived based on the brightest blue supergiant stars. Most of the resolved bright stars show colors in the range $-0.1 \le B-R \le +0.7$. The continuum light of the unresolved stars is distributed rather regular in B, R, and J, showing only minor irregularities. This surface brightness distribution follows an exponential law in all three colors with a scale length $r_c = 0.33$ kpc. The central surface brightnesses are $21.57 \pm 0.09 mag/\Box''$, $20.65 \pm 0.06mag/\Box''$, and $20.11 \pm 0.11 mag/\Box''$, in B, R, and J, respectively. The surface brightness can be traced out to 5 $r_c$ in B and R. Thus, UGC 685 belongs to the class of dwarfs where the HI distribution is much more extended (here 2.6 times) than the optical (stellar) light distribution, but contrary to many objects of this type, it does not belong to the class of low-surface brightness objects. The overall colors are $B-R = + 0.97$, $B-J = +1.55$ and the magnitude is B = 14.55 ($M_B^o$ = -14.5). The classification as an irregular dwarf results from the few HII regions of UGC 685. On an H$α$ Calar Alto 2.2m telescope CCD image, I identified only 5 HII regions. The total H$α$ flux corresponds to a mildly on-going star formation with a rate of 0.003 \Msolar yr$^{-1}$, a low rate even in comparison to other dwarfs. The available data do not indicate any major deviation from this rather low star formation rate within the last $10^9$ yr.

Explore related subjects

astro-ph

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Ulrich Hopp. 1998-09-08. Distance, structure and bright stellar content of the dwarf irregular galaxy UGC 685. https://doi.org/10.1051/aas%3A1999134

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2023

Dark Energy is the Cosmological Quantum Vacuum Energy of Light Particles-The Axion and the Lightest Neutrino

We uncover the general mechanism producing the dark energy(DE). This is only based on well known quantum physics and cosmology. We show that the observed DE originates from the cosmological quantum vacuum of light particles which provides a continuous energy distribution able to reproduce the data. Bosons give positive contributions to the DE while fermions yield negative contributions. As usual in field theory, ultraviolet divergences are subtracted from the physical quantities. The subtractions respect the symmetries of the theory and we normalize the physical quantities to be zero for the Minkowski vacuum. The resulting finite contributions to the energy density and the pressure from the quantum vacuum grow as log a(t) where a(t) is the scale factor, while the particle contributions dilute as 1/a^3(t), as it must be for massive particles. The DE equation of state P = w(z)H turns to be w(z)<-1 with w(z) asymptotically reaching the value -1 from below.A scalar particle can produce the observed DE through its quantum cosmological vacuum provided:(i)its mass is of the order of 10^{-3} eV = 1 meV,(ii) it is very weakly coupled and (iii) it is stable on the time scale of the age of the universe. The axion vacuum thus appears as a natural candidate. The neutrino vacuum (especially the lightest mass eigenstate) can give negative contributions to the DE. We find that w(z=0) is slightly below -1 by an amount ranging from [-1.5 10^{-3}] to [-8 10^{-3}] and we predict the axion mass to be in the range between 4 and 5 meV. We find that the universe will expand in the future faster than the de Sitter universe, as an exponential in the square of the cosmic time. DE arises from the quantum vacua of light particles in FRW cosmological space time in an analogous way to the Casimir effect in Minkowski spacetime with non trivial boundaries.

astro-ph↗
arXiv2023

Scaling of Black Hole Accretion Discs from Gamma-Ray Bursts and Black Hole X-Ray Binaries to Active Galactic Nuclei

I consider how physical processes scale over eight orders of magnitude in black hole mass, from stellar masses in gamma-ray bursts (GRB) and black-hole X-ray binaries (BHXRB) to supermassive active galactic nuclei (AGN). Accretion rates onto stellar mass black holes range over more than sixteen orders of magnitude, from the lower luminosity BHXRB to GRB. These enormous parameter ranges correspond to qualitative as well as quantitative differences in behavior. The fundamental questions involve the balance between nonequilibrium and thermalized plasmas. When energy fluxes exceed a critical value $\sim 10^{29}$ erg/cm$^2$s, as in GRB, a black-body equilibrium pair plasma forms. At the lower fluxes found in AGN, BHXRB and microquasars, accretion power electrodynamically accelerates a small number of very energetic particles, explaining their non-thermal spectra and the high energy gamma-ray emission of blazars. Ultra-high energy cosmic rays may be accelerated by massive black holes, otherwise undetectable, with very low thermal luminosities. New-born fast high-field pulsars may be in the black-body equilibrium regime, resembling SGR in permanent outburst. I also consider the question, significant for the acceleration of nonthermal particles in GRB outflows, of whether collisionless plasmas interpenetrate rather than forming hydrodynamic shocks, and propose this as an alternative to internal shock models of GRB. A new appendix attempts to explain why AGN are, proportionally, more efficient accelerators of energetic particles than stellar mass black holes.

astro-ph↗