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Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy (2016)

Y. I. Izotov ; I. Orlitova ; D. Schaerer ; [et al.]

Nature Publishing Group (NPG)

In: Nature. 529(7585): 178-80. doi: 10.1038/nature16456.

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Publication Date: 2016-01-15

Description: One of the key questions in observational cosmology is the identification of the sources responsible for ionization of the Universe after the cosmic 'Dark Ages', when the baryonic matter was neutral. The currently identified distant galaxies are insufficient to fully reionize the Universe by redshift z approximately 6 (refs 1-3), but low-mass, star-forming galaxies are thought to be responsible for the bulk of the ionizing radiation. As direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. Starburst galaxies at low redshifts, however, generally are opaque to Lyman continuum photons. Small escape fractions of about 1 to 3 per cent, insufficient to ionize much surrounding gas, have been detected only in three low-redshift galaxies. Here we report far-ultraviolet observations of the nearby low-mass star-forming galaxy J0925+1403. The galaxy is leaking ionizing radiation with an escape fraction of about 8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Izotov, Y I -- Orlitova, I -- Schaerer, D -- Thuan, T X -- Verhamme, A -- Guseva, N G -- Worseck, G -- England -- Nature. 2016 Jan 14;529(7585):178-80. doi: 10.1038/nature16456.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Zabolotnoho street, Kyiv 03680, Ukraine. ; Astronomical Institute, Czech Academy of Sciences, Boc ni II 1401, 141 00 Prague, Czech Republic. ; Observatoire de Geneve, Universite de Geneve, 51 Chemin des Maillettes, 1290 Versoix, Switzerland. ; CNRS, IRAP, 14 Avenue East Belin, 31400 Toulouse, France. ; Astronomy Department, University of Virginia, PO Box 400325, Charlottesville, Virginia 22904, USA. ; Max-Planck-Institut fur Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26762455" target="_blank"〉PubMed〈/a〉

Print ISSN: 0028-0836

Electronic ISSN: 1476-4687

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Published by Nature Publishing Group (NPG)

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10 μm Imaging of the Blue Compact Galaxy HE 2–10 (1994)

Sauvage, M. ; Lagage, P. O. ; Thuan, T. X.

Springer

Experimental astronomy 3 (1994), S. 165-166

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ISSN: 1572-9508

Keywords: Infrared: 10 μm ; Interstellar medium: dust ; Star formation ; Galaxies: dwarf, individual: He 2–10

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract We present the first results of a survey of Blue Compact Galaxies with a 10 μm array. Blue Compact Galaxies (BCGs) are dwarf galaxies experiencing an intense burst of star formation. As dwarf systems, their main characteristics is a low heavy-elements abundance. Although dust is thought to be less abundant in these galaxies than in normal spirals, the presence of a compact starburst region favors a detection in the infrared, and 10 μm imaging is perfectly suited to star formation studies in BCGs since it focuses on the hottest dust inside the star-forming regions.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00430150

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Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies (2016)

Izotov, Y. I., Schaerer, D., Thuan, T. X., Worseck, G., Guseva, N. G., Orlitova, I., Verhamme, A.

Oxford University Press

In: Monthly Notices of the Royal Astronomical Society

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Publication Date: 2016-07-24

Description: Following our first detection reported in Izotov et al., we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope . These galaxies, at redshifts of z ~ 0.3, are characterized by high emission-line flux ratios [O iii ] 5007/[O ii ] 3727 5. The escape fractions of the LyC radiation f esc (LyC) in these galaxies are in the range of ~6–13 per cent, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Ly α emission lines are detected in the spectra of all four galaxies, compatible with predictions for LyC leakers. We find escape fractions of Ly α, f esc (Ly α) ~ 20–40 per cent, among the highest known for Ly α emitting galaxies. Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the centre and exponential discs in the outskirts with disc scalelengths α in the range ~0.6–1.4 kpc. Our galaxies are characterized by low metallicity, ~1/8–1/5 solar, low stellar mass ~(0.2–4) x 10 9 M , high star formation rates, SFR ~ 14–36 M yr –1 , and high SFR densities, ~ 2–35 M yr –1 kpc –2 . These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al., reveal that a selection for compact star-forming galaxies showing high [O iii ] 5007/[O ii ] 3727 ratios appears to pick up very efficiently sources with escaping LyC radiation: all five of our selected galaxies are LyC leakers.

Print ISSN: 0035-8711

Electronic ISSN: 1365-2966

Topics: Physics

Published by Oxford University Press

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Near-infrared spectroscopy of a large sample of low-metallicity blue compact dwarf galaxies (2016)

Izotov, Y. I., Thuan, T. X.

Oxford University Press

In: Monthly Notices of the Royal Astronomical Society

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Publication Date: 2016-01-21

Description: We present near-infrared (NIR) spectroscopic observations in the wavelength range 0.90–2.40 μm of 18 low-metallicity blue compact dwarf (BCD) galaxies and six H ii regions in spiral and interacting galaxies. Hydrogen and helium emission lines are detected in all spectra, while H 2 and iron emission lines are detected in most spectra. The NIR data for all objects have been supplemented by optical spectra. In all objects, except perhaps for the highest metallicity ones, we find that the extinctions A ( V ) in the optical and NIR ranges are similar, implying that the NIR hydrogen emission lines in low-metallicity BCDs do not reveal more star formation than seen in the optical. We conclude that emission-line spectra of low-metallicity BCDs in the ~0.36–2.40 μm wavelength range are emitted by a relatively transparent ionized gas. The H 2 emission-line fluxes can be accounted for by fluorescence in most of the observed galaxies. We find a decrease of the H 2 2.122 μm emission line relative to the Br line with increasing ionization parameter. This indicates an efficient destruction of H 2 by the stellar ultraviolet radiation. The intensities of the [Fe ii ] 1.257 and 1.644 μm emission lines in the spectra of all galaxies, but one, are consistent with the predictions of cloudy stellar photoionization models. There is thus no need to invoke shock excitation for these lines, and they are not necessarily shock indicators in low-metallicity high-excitation BCDs. The intensity of the He i 2.058 μm emission line is lower in high-excitation BCDs with lower neutral gas column densities and higher turbulent motions.

Print ISSN: 0035-8711

Electronic ISSN: 1365-2966

Topics: Physics

Published by Oxford University Press

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A new determination of the primordial He abundance using the He I {lambda}10830 A emission line: cosmological implications (2014)

Izotov, Y. I., Thuan, T. X., Guseva, N. G.

Oxford University Press

In: Monthly Notices of the Royal Astronomical Society

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Publication Date: 2014-10-02

Description: We present near-infrared (NIR) spectroscopic observations of the high-intensity He i 10830 Å emission line in 45 low-metallicity H ii regions. We combined these NIR data with spectroscopic data in the optical range to derive the primordial He abundance. The use of the He i 10830 Å line, the intensity of which is very sensitive to the density of the H ii region, greatly improves the determination of the physical conditions in the He + zone. This results in a considerably tighter Y –O/H linear regression compared to all previous studies. We extracted a final sample of 28 H ii regions with Hβ equivalent width EW(Hβ) ≥ 150 Å, excitation parameter O 2 + /O ≥ 0.8, and with helium mass fraction Y derived with an accuracy better than 3 per cent. With this final sample we derived a primordial 4 He mass fraction Y p = 0.2551 ± 0.0022. The derived value of Y p is higher than the one predicted by the standard big bang nucleosynthesis model. Using our derived Y p together with D/H = (2.53 ± 0.04) 10 –5 , and the 2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density b h 2 = 0.0240 ± 0.0017 (68 per cent confidence level, CL), ± 0.0028 (95.4 per cent CL), ± 0.0034 (99 per cent CL) and an effective number of neutrino species N eff = 3.58 ± 0.25 (68 per cent CL), ± 0.40 (95.4 per cent CL), ± 0.50 (99 per cent CL). A non-standard value of N eff is preferred at the 99 per cent CL, implying the possible existence of additional types of neutrino species.

Print ISSN: 0035-8711

Electronic ISSN: 1365-2966

Topics: Physics

Published by Oxford University Press

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The H I content of extremely metal-deficient blue compact dwarf galaxies (2016)

Thuan, T. X., Goehring, K. M., Hibbard, J. E., Izotov, Y. I., Hunt, L. K.

Oxford University Press

In: Monthly Notices of the Royal Astronomical Society

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Publication Date: 2016-10-16

Description: We have obtained new H i observations with the 100 m Green Bank Telescope (GBT) for a sample of 29 extremely metal-deficient star-forming blue compact dwarf (BCD) galaxies, selected from the Sloan Digital Sky Survey (SDSS) spectral data base to be extremely metal-deficient (12 + log O/H ≤ 7.6). Neutral hydrogen was detected in 28 galaxies, a 97 per cent detection rate. Combining the H i data with SDSS optical spectra for the BCD sample and adding complementary galaxy samples from the literature to extend the metallicity and mass ranges, we have studied how the H i content of a galaxy varies with various global galaxian properties. There is a clear trend of increasing gas mass fraction with decreasing metallicity, mass and luminosity. We obtain the relation M (H i )/ L g $L_{\rm g}^{-0.3}$ , in agreement with previous studies based on samples with a smaller luminosity range. The median gas mass fraction f gas for the GBT sample is equal to 0.94 while the mean gas mass fraction is 0.90±0.15, with a lower limit of ~0.65. The H i depletion time is independent of metallicity, with a large scatter around the median value of 3.4 Gyr. The ratio of the baryonic mass to the dynamical mass of the metal-deficient BCDs varies from 0.05 to 0.80, with a median value of ~0.2. About 65 per cent of the BCDs in our sample have an effective yield larger than the true yield, implying that the neutral gas envelope in BCDs is more metal-deficient by a factor of 1.5–20, as compared to the ionized gas.

Print ISSN: 0035-8711

Electronic ISSN: 1365-2966

Topics: Physics

Published by Oxford University Press

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The X-ray properties of the cometary blue compact dwarf galaxies Mrk 59 and Mrk 71 (2014)

Thuan, T. X., Bauer, F. E., Izotov, Y. I.

Oxford University Press

In: Monthly Notices of the Royal Astronomical Society

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Publication Date: 2014-05-14

Description: 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 sources (ULXs), 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.

Print ISSN: 0035-8711

Electronic ISSN: 1365-2966

Topics: Physics

Published by Oxford University Press

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