On the Use of the Index N2 to Derive the Metallicity in Metal-Poor Galaxies

Morales-Luis, A. B.; Montero, E. Pérez; Alméida, J. Sánchez; Muñoz–Tuñón, C.

doi:10.1088/0004-637x/797/2/81

Cited by 30 publications

(22 citation statements)

References 57 publications

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“…∼300 cm −3 we obtain are more typical of giant H II regions in the nearby Universe and are also consistent with the typical density (100 cm −3 ) found in low-metallicity local galaxies with compact H II regions (Morales-Luis et al 2014).…”

Section: Temperature and Density From Line Ratio Diagnosticssupporting

confidence: 90%

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

Patrício¹,

Richard²,

Verhamme

2015

Proc. IAU

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Spatially resolved studies of high-redshift galaxies, an essential insight into galaxy formation processes, have been mostly limited to stacking or unusually bright objects. We present here the study of a typical (L * , M = 6 × 10 9 M ) young lensed galaxy at z = 3.5, observed with Multi Unit Spectroscopic Explorer (MUSE), for which we obtain 2D resolved spatial information of Lyα and, for the first time, of C III] emission. The exceptional signal-to-noise ratio of the data reveals UV emission and absorption lines rarely seen at these redshifts, allowing us to derive important physical properties (T e ∼ 15600 K, n e ∼ 300 cm −3 , covering fraction f c ∼ 0.4) using multiple diagnostics. Inferred stellar and gas-phase metallicities point towards a low-metallicity object (Z stellar = ∼0.07 Z and Z ISM < 0.16 Z ). The Lyα emission extends over ∼10 kpc across the galaxy and presents a very uniform spectral profile, showing only a small velocity shift which is unrelated to the intrinsic kinematics of the nebular emission. The Lyα extension is approximately four times larger than the continuum emission, and makes this object comparable to low-mass LAEs at low redshift, and more compact than the Lyman-break galaxies and Lyα emitters usually studied at high redshift. We model the Lyα line and surface brightness profile using a radiative transfer code in an expanding gas shell, finding that this model provides a good description of both observables.Key words: techniques: imaging spectroscopy -galaxies: abundances -galaxies: highredshift -galaxies: individual: SMACSJ2031.8-4036. I N T RO D U C T I O NDuring the past few decades, our understanding of galaxy formation and evolution has made significant progress thanks to the hundreds of high-redshift (z > 3) galaxies which have been detected in dedicated observing campaigns (e.g. Shapley et al. 2003;Vanzella et al. 2009;Stark et al. 2013). The main spectral feature used to confirm the distances of these galaxies is the Lyα emission, since it E-mail: vera.patricio@univ-lyon1.fr (VP); johan.richard@univ-lyon1.fr (JR) is the brightest emission line we can observe in distant sources. Unfortunately, many of these objects are too faint to show any other emission line at rest-frame UV wavelengths or, even less likely, continuum and absorption lines, offering a limited picture of the characteristics of high-redshift galaxies. The complexity of the Lyα resonant process, which depends not only on the gas dynamics but also on gas density and dust content, also requires the observation of non-resonant lines in order to robustly probe the physical properties of such galaxies.Stacking techniques, which combine spectra or images of dozens or even hundreds of objects in order to increase signal-to-noise ratio (e.g.

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Section: Temperature and Density From Line Ratio Diagnosticssupporting

confidence: 90%

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

Patrício¹,

Richard²,

Verhamme

2015

Proc. IAU

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“…The range given for N2 in Denicoló et al (2002) is 7.2 < 12 + log(O/H) < 9.1 and the entire sample lies within this. Recently, however, Morales-Luis et al (2014) have shown that N2 does not correlate with metallicity for galaxies with 12 + log(O/H) < 7.6. This is not an issue for our sample as only three galaxies have such low metallicities.…”

Section: Metallicity Calibrationsmentioning

confidence: 97%

Merging galaxies produce outliers from the fundamental metallicity relation

Grønnow

Finlator

Christensen

2015

Mon. Not. R. Astron. Soc.

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From a large sample of ≈ 170, 000 local SDSS galaxies, we find that the Fundamental Metallicity Relation (FMR) has an overabundance of outliers, compared to what would be expected from a Gaussian distribution of residuals, with significantly lower metallicities than predicted from their stellar mass and star formation rate (SFR). This low-metallicity population has lower stellar masses, bimodial specific SFRs with enhanced star formation within the aperture and smaller half-light radii than the general sample, and is hence a physically distinct population. We show that they are consistent with being galaxies that are merging or have recently merged with a satellite galaxy. In this scenario, low-metallicity gas flows in from large radii, diluting the metallicity of star-forming regions and enhancing the specific SFR until the inflowing gas is processed and the metallicity has recovered. We introduce a simple model in which mergers with a mass ratio larger than a minimum dilute the central galaxy's metallicity by an amount that is proportional to the stellar mass ratio for a constant time, and show that it provides an excellent fit to the distribution of FMR residuals. We find the dilution time-scale to be τ = 1.568 +0.029 −0.027 Gyr, the average metallicity depression caused by a 1:1 merger to be α = 0.2480 +0.0017 −0.0020 dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be ξ min = 0.2030 +0.0127 −0.0095 (these are statistical errors only). From this we derive that the average metallicity depression caused by a merger with mass ratio between 1:5 and 1:1 is 0.114 dex.

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“…Sánchez Almeida et al (2013) found a drop in metallicity coinciding with its largest star-forming region, with the value of 12 + log(O/H) 7.5. This estimate was based on the strong-line ratio N2 (log([NII]λ6583/Hα)), which is known to have biases (e.g., Morales-Luis et al 2014). On the other hand, the SDSS spectrum of the region gives a metallicity of around 7.8 when using either the direct method (DM) or the HIICM procedure by Perez-Montero (2014) (Ruben García-Benito 2017, private communication).…”

Section: Observationsmentioning

confidence: 99%

NOEMA Observations of a Molecular Cloud in the Low-metallicity Galaxy Kiso 5639

Elmegreen

Herrera

Rubio

et al. 2018

ApJL

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A giant star-forming region in a metal-poor dwarf galaxy has been observed in optical lines with the 10-m Gran Telescopio Canarias and in the emission line of CO(1-0) with the NOEMA mm-wave interferometer. The metallicity was determined to be 12 + log(O/H) = 7.83 ± 0.09, from which we estimate a conversion factor of α CO ∼ 100 M pc −2 K km s −1 −1 and a molecular cloud mass of ∼ 2.9 × 10 7 M . This is an enormous concentration of molecular mass at one end of a small galaxy, suggesting a recent accretion. The molecular cloud properties seem normal: the surface density, 120 M pc −2 , is comparable to that of a standard giant molecular cloud, the cloud's virial ratio of ∼ 1.8 is in the star-formation range, and the gas consumption time, 0.5 Gyr, at the present star formation rate is typical for molecular regions. The low metallicity implies that the cloud has an average visual extinction of only 0.8 mag, which is close to the threshold for molecule formation. With such an extinction threshold, molecular clouds in metal-poor regions should have high surface densities and high internal pressures. If high pressure is associated with the formation of massive clusters, then metal-poor galaxies such as dwarfs in the early universe could have been the hosts of metal-poor globular clusters.

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On the Use of the Index N2 to Derive the Metallicity in Metal-Poor Galaxies

Cited by 30 publications

References 57 publications

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

Merging galaxies produce outliers from the fundamental metallicity relation

NOEMA Observations of a Molecular Cloud in the Low-metallicity Galaxy Kiso 5639

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