arXiv subjects

Juergen Ott

Publications and source records attributed to Juergen Ott.

78 records · Page 5Linked to original sources

arXiv2005

Detection of neutral hydrogen in early-type dwarf galaxies of the Sculptor Group

We present our results of deep 21 cm line (HI) observations of five early and mixed-type dwarf galaxies in the nearby Sculptor group using the ATNF 64m Parkes Radio Telescope. Four of these objects, ESO294-G010, ESO410-G005, ESO540-G030, and ESO540-G032, were detected in HI with neutral hydrogen masses in the range of 2-9x10^5 M_{\odot} ($M_{HI}/L_{B}$ = 0.08, 0.13, 0.16, and 0.18, respectively). These HI masses are consistent with the gas mass expected from stellar outflows over a large period of time. Higher resolution radio data from the Australia Telescope Compact Array were further analysed to measure more accurate positions and the distribution of the HI gas. In the cases of dwarfs ESO294-G010 and ESO540-G030, we find significant offsets of 290 pc and 460 pc, respectively, between the position of the HI peak flux and the center of the stellar component. These offsets are likely to have internal cause such as the winds from star-forming regions. The fifth object, the spatially isolated dwarf elliptical Scl-dE1, remains undetected at our 3σlimit of 22.5 mJy km/s and thus must contain less than 10^5 M_{\odot} of neutral hydrogen. This leaves Scl-dE1 as the only Sculptor group galaxy known where no interstellar medium has been found to date. The object joins a list of similar systems including the Local Group dwarfs Tucana and Cetus that do not fit into the global picture of the morphology-density relation where gas-rich dwarf irregulars are in relative isolation and gas-deficient dwarf ellipticals are satellites of more luminous galaxies.

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arXiv2005

The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253

[abridged] We present interferometric maps of ammonia (NH3) of the nearby starburst galaxy NGC 253 [star formation rate: ~2.8 Mo yr^(-1)]. The observations have been taken with the Australia Telescope Compact Array and include the para-NH3 (1,1), (2,2), and the ortho-NH3 (3,3) and (6,6) inversion lines. Six major complexes of dense ammonia are identified, three of them on either side of the starburst center, out to projected galactocentric radii of \~250 pc. [...] The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 K toward the south-west and north-east [of the nucleus of NGC 253], respectively. The temperatures under which ammonia was formed in the past are with >~30 K also warmer toward the south-west than toward the north-east (~15-20 K). This is indicated by the ortho-to-para ammonia ratio which is ~1 and 1.5-2.5 toward the south-west and north-east, respectively. Ammonia column densities in the brightest complexes are in the range of 6-11x10^(14) cm^(-2), which adds up to a total ammonia mass of ~20 Mo, about evenly distributed toward both sides of the nucleus. [...] Toward the center of NGC 253, NH3 (1,1), (2,2), and (6,6) is detected in absorption against an unresolved continuum source. At the same location, however, ammonia (3,3) is found in emission which indicates maser activity. This would be the first detected extragalactic NH3 maser. Evidence for an expanding shell in the south-western complex is provided. [...] The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ~10^5 Mo which also heats the dense gas.

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arXiv2005

A Chandra X-ray survey of nearby dwarf starburst galaxies: I. Data reduction and results

(abridged) We present an analysis of Chandra X-ray observations of a sample of eight dwarf starburst galaxies (I Zw 18, VII Zw 403, NGC 1569, NGC 3077, NGC 4214, NGC 4449, NGC 5253, and He 2-10). Extended, diffuse X-ray emission is detected in all but two of the objects. Unresolved sources were found within all dwarf galaxies (total: 55 sources). These point sources are well fit by power law, thermal plasma, or black body models. Ten of the point sources exceed an X-ray luminosity of 10^39 erg s^-1 (ultraluminous X-ray sources). In those galaxies where diffuse X-ray emission is detected, this emission (with X-ray luminosities ranging from 4x10^38 erg s^-1 to 2x10^40 erg s^-1) contains most (60-80 per cent) of the X-ray photons. This diffuse emission can be well fit by MeKaL one-temperature thermal plasma models once the contribution from the unresolved point sources is subtracted properly. The diffuse X-ray component is significantly extended, reaching as far as 0.5-5 kpc into the outskirts of their hosts. Temperatures of various regions within the galaxies range from 1.6-7.6x10^6 K. With few exceptions, temperatures of the hot gas are remarkably uniform, hovering around 2-3x10^6 K. Temperatures of the coronal gas in the outer regions are in general ~2-3 times lower than those found in the central regions. Fits to the diffuse emission do not allow strong constraints to be put on the metallicities of the emitting plasmas. However, the derived metallicities are compatible with those determined from their HII regions. An alpha/Fe ratio of ~2 is indicated for the hot gas within at least three objects (NGC 1569, NGC 4449, and He 2-10).

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arXiv2005

A Chandra X-ray survey of nearby dwarf starburst galaxies: II. Starburst properties and outflows

(abridged) We present a comprehensive comparison of the X-ray properties of a sample of eight dwarf starburst galaxies observed with Chandra (I Zw 18, VII Zw 403, NGC 1569, NGC 3077, NGC 4214, NGC 4449, NGC 5253, He 2-10). In Paper I we presented in detail the data reduction and analysis of the individual galaxies. For the unresolved X-ray sources we find the following: point sources are in general located close to bright HII regions, rims of superbubbles, or young stellar clusters. The number of X-ray point sources appears to be a function of the current star formation rate and the blue luminosity of the hosts. Ultraluminous X-ray sources are only found in those dwarf galaxies which are currently interacting. The power law index of the combined cumulative X-ray point source luminosity function is alpha=0.24+/-0.06, shallower than that of more massive starburst galaxies (alpha=0.4-0.8) and of non-starburst galaxies (alpha~1.2). For those galaxies showing extended X-ray emission (6 out of the 8 galaxies), we derive the following: Superwinds develop along the steepest gradient of the HI distribution with volume densities of 0.02-0.06 cm^-3, pressures of 1-3x10^5 K cm^-3, thermal energies of 2-30x10^54 erg, and hot gas masses of 2-20x10^6 Mo (~1 per cent of the HI masses). The mechanical luminosities of the developing superwinds are energetic enough to overcome the gravitational potentials of their host galaxies. This scenario is supported by the overpressures of the hot gas compared to the ambient ISM. Extended HI envelopes such as tidal tails, however, may delay outflows on timescales exceeding those of the cooling time of the hot gas.

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arXiv2003

Chandra Observations of Expanding Shells in the Dwarf Starburst Galaxy NGC 3077

Deep Chandra observations (53 ks, ACIS-S3) of NGC 3077, a starburst dwarf galaxy in the M 81 triplet, resolve the X-ray emission from several supershells. The emission is brightest in the cavities defined by expanding shells detected previously in H alpha emission (Martin 1998). Thermal emission models fitted to the data imply temperatures ranging from ~1.3 to 4.9 x 10^(6) K and indicate that the strongest absorption is coincident with the densest clouds traced by CO emission. The fitted emission measures give pressures of P/k~10^(5-6) xi^(-0.5) f_(v)^(-0.5) K cm^(-3) (xi: metallicity of the hot gas in solar units, f_(v): volume filling factor). Despite these high pressures, the radial density profile of the hot gas is not as steep as that expected in a freely expanding wind (e.g., as seen in the neighboring starburst galaxy M 82) implying that the hot gas is still confined by the H alpha shells. The chaotic dynamical state of NGC 3077 undermines reliable estimates of the escape velocity. The more relevant quantity for the ultimate fate of the outflow is probably the gas density in the rich intragroup medium. Based on the HI distribution of NGC 3077 and a connected tidal tail we argue that the wind has the potential to leave the gravitational well of NGC 3077 to the north but not to the south. The total 0.3-6.0 keV X-ray luminosity is ~2-5 x 10^(39) erg s^(-1) (depending on the selected thermal plasma model). Most (~85%) of the X-ray luminosity in NGC 3077 comes from the hot interstellar gas; the remainder comes from six X-ray point sources. In spite of previous claims to the contrary, we do not find X-ray emission originating from the prominent tidal tail near NGC 3077.

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arXiv2001

Evidence for Blow-out in the Low-mass Dwarf Galaxy Holmberg I

We present radio and optical observations of Holmberg I (HoI), a member of the M81 group of galaxies (distance 3.6Mpc). HoI is a low-mass, low surface- brightness dwarf galaxy. High-resolution multi-array VLA HI observations reveal a supergiant shell (diameter: 1.7 kpc) which covers about half the optical extent of HoI and which comprises 75% of the total HI content (total HI mass: 1.1 10^8 M_o). We set a tentative upper limit to the dark matter content of < 3.1 10^8 M_o. The HI data are complemented by deep, optical UBV(RI)_c and Halpha observations obtained at the Calar Alto 2.2m telescope. We find M_HI/L_B = 1.1 M_o/L_B_o. The total visible (stars plus gas) mass of HoI adds up to 2.4 10^8 M_o. This leads to a total mass of < 5.5 10^8 M_o. The origin of HoI's peculiar HI morphology is discussed in terms of a supergiant shell created by strong stellar winds and supernova explosions (energy: equiv. 20-260 type II SN; age: 80+-20 Myr). The morphological center of HoI is offset by 0.75 kpc with respect to the dynamical center. Within the interior of the shell the light distribution is exponential with a rather shallow gradient and blue optical colors. Beyond a radius corresponding to an HI column density of 10^21 cm^-2, the putative star formation (SF) threshold, the disk becomes considerably redder and the slope for the exponential light distribution steepens. It is speculated that a generation of stars (age 15-30 Myr) is the result of secondary SF on the rim of the shell. Based on the global morphology and velocity dispersion as well as the location of the HII regions we find evidence for ram pressure within the M81 group. Finally, we discuss the likelihood of HoI having lost some of its interstellar material to the intergalactic medium (''blow-out'' scenario).

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