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Register a new userThe X-ray Properties of the Cometary Blue Compact Dwarf galaxies Mrk 59 and Mrk 71
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We present XMM-Newton and Chandra observations of two low-metallicity cometary blue compact dwarf (BCD) galaxies, Mrk 59 and Mrk 71. The first BCD, Mrk 59, contains two ultraluminous X-ray (ULX) sources, IXO 72 and IXO 73, both associated with bright massive stars and H II complexes, as well as one fainter extended source associated with a massive H II complex at the head of the cometary structure. The low-metallicity of Mrk 59 appears to be responsible for the presence of the two ULXs. IXO 72 has varied little over the last 10 yr, while IXO 73 has demonstrated a variability factor of ~4 over the same period. The second BCD, Mrk 71, contains two faint X-ray point sources and two faint extended sources. One point source is likely a background AGN, while the other appears to be coincident with a very luminous star and a compact H II region at the head of the cometary structure. The two faint extended sources are also associated with massive H II complexes. Although both BCDs have the same metallicity, the three sources in Mrk 71 have X-ray luminosities ~1-2 orders of magnitude fainter than those in Mrk 59. The age of the starburst may play a role.
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New FUSE far-UV spectroscopy of the nearby metal-deficient (Zsun/8) cometary Blue Compact Dwarf (BCD) galaxy Markarian (Mrk) 59 is discussed. The data are used to investigate element abundances in its interstellar medium. The H I absorption lines are characterized by narrow cores which are interstellar in origin and by broad wings which are stellar in origin. The mean interstellar H I column density is ~ 7x10E20 cm-2 in Mrk 59. No H2 lines are seen and N(H2) is < 10E15 cm-2 at the 10 sigma level. The lack of diffuse H2 is due to the combined effect of a strong UV radiation field which destroys the H2 molecules and a low metallicity which leads to a scarcity of dust grains necessary for H2 formation. P-Cygni profiles of the S VI 933.4, 944.5 A and O VI 1031.9, 1037.6 A lines are seen, indicating the presence of very hot O stars and a stellar wind terminal velocity of ~ 1000 km/s. By fitting the line profiles with multiple components having each a velocity dispersion b = 7 km/s and spanning a radial velocity range of 100 km/s, some of which can be saturated, we derive heavy element abundances in the neutral gas. We find log N(O I)/N(H I) = -5.0+/-0.3 or [O I/H I] = -1.5 for the neutral gas, about a factor of 10 below the oxygen abundance of the supergiant H II region, implying self-enrichment of the latter.
Cometary Blue Compact Dwarf Galaxies (iI,C BCDs) are characterized by off-center starbursts close to the end of an elongated dwarf irregular (dI)-like host galaxy. This may either represent randomly enhanced star-forming activity of a dI, or may be caused by a set of special properties of such systems or their environment. For a first investigation of this issue, we analyse the nearby iI,C BCDs Markarian 59 and Markarian 71. Using deep ground-based spectrophotometric data and HST images, we derive physical properties, structure and ages of the starburst regions and the underlying stellar host galaxies. The metallicities show small scatter in the vicinity of the star-forming regions and along the major axis of Mrk 59 which suggests effective mixing of heavy elements on kpc scales. The surface brightness profiles of the underlying host galaxies in either iI,C BCD show an exponential decay with a central surface brightness and scale length that are intermediate between typical iE/nE BCDs and dIs. Spectral population synthesis models in combination with colour magnitude diagrams and colour profiles yield most likely formation ages of ~2 Gyr for the host galaxies in both iI,C BCDs, with upper limits of ~4 Gyr for Mrk 59 and ~3 Gyr for Mrk 71, i.e. significantly lower than the typical age of several Gyr derived for the host galaxies of iE/nE BCDs. These findings raise the question whether iI,C systems form a distinct class within BCDs with respect to the age and structure of their hosts, or whether they represent an evolutionary stage connecting young i0 BCDs and classical iE/nE BCDs. Properties of analogous objects studied in the local universe and at medium redshifts provide some support for this evolutionary hypothesis.
(Abridged) Aims: By means of optical Integral Field Spectroscopy (IFS) observations, we aim to disentangle and characterize the starburst component in the BCD Mrk 1418. In particular we propose to study the stellar and ionized gas morphology, to investigate the ionization mechanism(s) acting in the interstellar medium, to derive the physical parameters and abundances of the ionized gas. Methods: IFS observations of Mrk 1418 were carried out with PMAS at the 3.5 m telescope at CAHA. The central 16x16 were mapped. From these data we built maps of the most prominent emission lines, namely [OII], H-beta, [OIII], H-alpha, [NII] and [SII] as well as of several continuum bands, plus maps of the main line ratios: [OIII]/H-beta, [NII]/H-alpha, [SII]/H-alpha, and H-alpha/H-beta, and derived the physical parameters and gaseous metal abundances of the different star-forming regions detected in the field of view. Results: Mrk 1418 shows a distorted morphology both in the continuum and in the ionized gas maps; the current star-formation episode is taking place in five knots, distributed around the nucleus of the galaxy. The interstellar medium surrounding these knots is photo-ionized by stars, with no clear evidence for other excitation mechanisms. The galaxy displays an inhomogeneous dust distribution, with the high H-alpha/H-beta ratio in the central areas indicating a large amount of dust. The oxygen abundances derived for the individual star-forming knots are very similar, suggesting that the ionized interstellar medium is chemically homogeneous in O/H over spatial scales of hundreds of parsecs. This abundance (Z~0.4 Z_solar from the empirical calibrations) places Mrk 1418 among the high metallicity BCD group.
The optical spectroscopic and radio interferometric HI 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission line features corresponding to high ionization lines of HeII $lambda$4686 and CIV $lambda$5808 from young massive stars are detected. The ages of two prominent star forming regions in the galaxy are estimated as $sim$10 Myr and $sim$ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young star-burst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission lines features. A significant N/O enrichment is seen in the fainter star forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98$pm$0.07 and 7.46$pm$0.09 respectively. The galaxy has a large diffuse HI envelop. The HI images reveal disturbed gas kinematics and HI clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.
We present an integral field spectroscopic study of the central 2x2 kpc^2 of the blue compact dwarf galaxy Mrk 409, observed with the Potsdam MultiAperture Spectrophotometer. This study focuses on the morphology, two-dimensional chemical abundance pattern, excitation properties and kinematics of the ionized interstellar medium in the starburst component. We also investigate the nature of the extended ring of ionized gas emission surrounding the bright nuclear starburst region of Mrk 409. PMAS spectra of selected regions along the ring, interpreted with evolutionary and population synthesis models, indicate that their ionized emission is mainly due to a young stellar population with a total mass of ~1.5x10^6 M_sun, which started forming almost coevally ~10 Myr ago. This stellar component is likely confined to the collisional interface of a spherically expanding, starburst-driven super-bubble with denser, swept-up ambient gas, ~600 pc away from the central starburst nucleus. The spectroscopic properties of the latter imply a large extinction (C_H-beta>0.9), and the presence of an additional non-thermal ionization source, most likely a low-luminosity Active Galactic Nucleus. Mrk 409 shows a relatively large oxygen abundance (12+log(O/H)~8.4) and no chemical abundance gradients out to R~600 pc. The ionized gas kinematics displays an overall regular rotation on a northwest-southwest axis, with a maximum velocity of 60 km/s; the total mass inside the star-forming ring is about 1.4x10^9 M_sun.
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