The<i>Spitzer</i>View of Low‐Metallicity Star Formation. II. Mrk 996, a Blue Compact Dwarf Galaxy with an Extremely Dense Nucleus

Thuan, T. X.; Hunt, L. K.; Izotov, Y. I.

doi:10.1086/592720

Cited by 12 publications

(38 citation statements)

References 56 publications

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“…Using CLOUDY photo-ionized H ii region models for the range of the ionization parameter appropriate for Mrk 996 (logU = −3−2), we find that a broad Hα/Hβ flux ratio of ∼4.6 corresponds to an electron number density N e ∼ (1−5) × 10 6 cm −3 . This range of N e is consistent with that derived by Thuan et al (1996Thuan et al ( , 2008, but is somewhat lower than N e > ∼ 10 7 cm −3 obtained by James et al (2009) …”

Section: Collisional Excitation Of Hydrogen and Helium Linessupporting

confidence: 91%

“…That the density structure in Mrk 996 is not uniform has been discussed by Thuan et al (1996Thuan et al ( , 2008, and James et al (2009). Thuan et al (1996Thuan et al ( , 2008 had to invoke a CLOUDY model with two zones of different electron densities to account for the integrated optical, near-, and mid-infrared spectra of Mrk 996. Here, we use a method for mapping low-and high-density clouds in astrophysical nebulae recently devised by Steiner et al (2009a).…”

Section: Matching the Spatially Resolved Kinematics With The Two-densmentioning

confidence: 99%

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A Gemini/GMOS study of the physical conditions and kinematics of the blue compact dwarf galaxy Mrk 996

Telles

Thuan

Izotov

et al. 2014

A&A

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Aims. We present an integral field spectroscopic study with the Gemini Multi-Object Spectrograph (GMOS) of the unusual blue compact dwarf (BCD) galaxy Mrk 996. Methods. We show through velocity and dispersion maps, emission-line intensity and ratio maps, and by a new technique of electron density limit imaging that the ionization properties of different regions in Mrk 996 are correlated with their kinematic properties. Results. From the maps, we can spatially distinguish a very dense high-ionization zone with broad lines in the nuclear region, and a less dense low-ionization zone with narrow lines in the circumnuclear region. Four kinematically distinct systems of lines are identified in the integrated spectrum of Mrk 996, suggesting stellar wind outflows from a population of Wolf-Rayet (WR) stars in the nuclear region, superposed on an underlying rotation pattern. From the intensities of the blue and red bumps, we derive a population of ∼473 late nitrogen (WNL) stars and ∼98 early carbon (WCE) stars in the nucleus of Mrk 996, resulting in a high N(WR)/N(O+WR) of 0.19. We derive, for the outer narrow-line region, an oxygen abundance 12 + log (O/H) = 7.94 ± 0.30 (∼0.2 Z ) by using the direct T e method derived from the detected narrow [O iii]λ4363 line. The nucleus of Mrk 996 is, however, nitrogen-enhanced by a factor of ∼20, in agreement with previous CLOUDY modeling. This nitrogen enhancement is probably due to nitrogen-enriched WR ejecta, but also to enhanced nitrogen line emission in a high-density environment. Although we have made use here of two new methodsprincipal component analysis (PCA) tomography and a method for mapping low-and high-density clouds -to analyze our data, new methodology is needed to further exploit the wealth of information provided by integral field spectroscopy.

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Section: Collisional Excitation Of Hydrogen and Helium Linessupporting

confidence: 91%

Section: Matching the Spatially Resolved Kinematics With The Two-densmentioning

confidence: 99%

A Gemini/GMOS study of the physical conditions and kinematics of the blue compact dwarf galaxy Mrk 996

Telles

Thuan

Izotov

et al. 2014

A&A

Self Cite

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“…Finally, the equivalent widths of the Hβ emission line in our five CSFGs are very high, ∼ 350 -520Å (see section 3) implying a very high efficiency of the ionizing photon production ξ. These properties are similar to those of some local BCDs (Thuan 2008) and of the hypothetical dwarf galaxies thought to be the main contributors to the reionization of the Universe at redshifts z > 5.…”

Section: Selection Of Csfgs and Their Global Characteristicssupporting

confidence: 79%

“…Cardamone et al 2009;Izotov, Guseva & Thuan 2011;Izotov et al 2016c). In the literature, and depending on their redshifts, subsets of these galaxies have been variously called blue compact dwarf (BCD) galaxies at redshifts z < 0.1 (Loose & Thuan 1986;Thuan 2008) because of their blue colours, "Green peas" (GPs) which at redshifts of ∼ 0.1 -0.3 appear green on composite SDSS images (Cardamone et al 2009) and luminous compact galaxies (LCGs) with diverse colours in the wider redshift range from 0 to ∼0.6 (Izotov et al 2011).…”

Section: Introductionmentioning

confidence: 99%

LBT observations of compact star-forming galaxies with extremely high [O iii]/[O ii] flux ratios: He i emission-line ratios as diagnostics of Lyman continuum leakage

Izotov

Thuan

Гусева

2017

Monthly Notices of the Royal Astronomical Society

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We present Large Binocular Telescope spectrophotometric observations of five lowredshift (z < 0.070) compact star-forming galaxies (CSFGs) with extremely high emission-line ratios, ranging from 23 to 43. Galaxies with such high O 32 are thought to be promising candidates for leaking large amounts of Lyman continuum (LyC) radiation and, at high redshifts, for contributing to the reionization of the Universe. The equivalent widths EW(Hβ) of the Hβ emission line in the studied galaxies are very high, ∼ 350 -520Å, indicating very young ages for the star formation bursts, < 3 Myr. All galaxies are characterized by low oxygen abundances 12+logO/H = 7.46 -7.79 and low masses M ⋆ ∼ 10 6 -10 7 M ⊙ , much lower than the M ⋆ for known low-redshift LyC leaking galaxies, but probably more typical of the hypothetical population of low-luminosity dwarf LyC leakers at high redshifts. A broad Hα emission line is detected in the spectra of all CSFGs, possibly related to expansion motions of supernova remnants. Such rapid ionized gas motions would facilitate the escape of the resonant Lyα emission from the galaxy. We show that high O 32 may not be a sufficient condition for LyC leakage and propose new diagnostics based on the He i λ3889/λ6678 and λ7065/λ6678 emission-line flux ratios. Using these diagnostics we find that three CSFGs in our sample are likely to have density-bounded H ii regions and are thus leaking large amounts of LyC radiation. The amount of leaking LyC radiation is probably much lower in the other two CSFGs.

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“…The slight decrease in He/H in the core region might be attributable to the presence of He 2+ . We find this unlikely, however, since no nebular He ii λ4686 emission has been detected (none was reported in the recent study by Thuan, Hunt & Izotov 2008), and in addition, other similarly high excitation lines such as [Ar iv ] are not present in the spectrum of the galaxy. The derived He/H abundances at the centre of Mrk 996 agree well with those of TIL96 who calculated He + /H + ∼0.09 from the He i λ5876 emission within their FOS aperture.…”

Section: Chemical Abundancesmentioning

confidence: 52%

A VLT VIMOS study of the anomalous BCD Mrk 996: mapping the ionized gas kinematics and abundances

James

Tsamis

Barlow

et al. 2009

Monthly Notices of the Royal Astronomical Society

132

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A study of the blue compact dwarf (BCD) galaxy Mrk 996 based on high‐resolution optical Very Large Telescope Visible Multi‐Object Spectrograph integral field unit spectroscopy is presented. Mrk 996 displays multicomponent line emission, with most line profiles consisting of a narrow, central Gaussian [full width at half‐maximum (FWHM) ∼ 110 km s−1] with an underlying broad component (FWHM ∼ 400 km s−1). The broad H i Balmer component splits into two separate broad components inside a 1.5‐arcsec radius from the nucleus; these are attributed to a two‐armed minispiral. This spiral‐like nucleus rotates in the same sense as the extended narrow line ionized gas but is offset by ∼50 km s−1 from the systemic velocity of the galaxy. The rotation curve of Mrk 996 derived from the Hα narrow component yields a total mass of 5 × 108 M⊙ within a radius of 3 kpc. From the Hα luminosity we infer a global star formation rate of ∼2 M⊙ yr−1. The high excitation energy, high critical density [O iii]λ4363 and [N ii]λ5755 lines are only detected from the inner region and exist purely in broad component form, implying unusual excitation conditions. Surface brightness, radial velocity and FWHM maps for several emission components are presented. A separate physical analysis of the broad and narrow emission line regions is undertaken. We derive an upper limit of 10 000 K for the electron temperature of the narrow line gas, together with an electron density of 170 cm−3, typical of normal H ii regions. For the broad line component, measured [O iii] and [Fe iii] diagnostic line ratios are consistent with a temperature of 11 000 K and an electron density of 107 cm−3. The broad line emission regions show N/H and N/O enrichment factors of ∼20 relative to the narrow line regions, but no He/H, O/H, S/H or Ar/H enrichment is inferred. Previous studies indicated that Mrk 996 showed anomalously high N/O ratios compared with BCDs of a similar metallicity. Our multicomponent analysis yields a revised metallicity of ≥0.5 Z⊙ (12 + log O/H = 8.37) for both the narrow and broad gas components, significantly higher than previous studies. As a result the narrow line region's N/O ratio is now typical for the galaxy's metallicity. The narrow line component's N/O ratio peaks outside the core region, spatially correlating with ∼3‐Myr‐old stellar populations. The dominant line excitation mechanism is photoionization by the ∼3000 Wolf–Rayet stars and ∼150 000 O‐type stars estimated to be present in the core. This is indeed a peculiar BCD, with extremely dense zones of gas in the core, through which stellar outflows and possible shock fronts permeate contributing to the excitation of the broad line emission.

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TheSpitzerView of Low‐Metallicity Star Formation. II. Mrk 996, a Blue Compact Dwarf Galaxy with an Extremely Dense Nucleus

Cited by 12 publications

References 56 publications

A Gemini/GMOS study of the physical conditions and kinematics of the blue compact dwarf galaxy Mrk 996

A Gemini/GMOS study of the physical conditions and kinematics of the blue compact dwarf galaxy Mrk 996

LBT observations of compact star-forming galaxies with extremely high [O iii]/[O ii] flux ratios: He i emission-line ratios as diagnostics of Lyman continuum leakage

A VLT VIMOS study of the anomalous BCD Mrk 996: mapping the ionized gas kinematics and abundances

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