Context.
In this work we study the stellar mass–metallicity relation (MZR) of an extended sample of star-forming galaxies in the local Universe and its possible dependence on the star formation rate ...(SFR).
Aims.
We selected a sample of approximately 195 000 Sloan Digital Sky Survey (SDSS) star-forming galaxies up to
z
= 0.22 with the aim of analysing the behaviour of the MZR with respect to SFR whilst taking into account the age of their stellar populations.
Methods.
For the first time, with this sample, we obtained aperture corrected oxygen and nitrogen-to-oxygen abundances (O/H and N/O, respectively) and SFR using the empirical prescriptions from the Calar Alto Legacy Integral Field Area (CALIFA) survey. To perform this study we also make use of the stellar mass of the galaxies and the parameter
D
n
(4000) as a proxy for the age of the stellar population.
Results.
We derive a robust MZR locus, which is found to be fully consistent with the ‘anchoring’ points of a selected set of well-studied nearby galaxies for which the chemical abundance has been derived using the direct method. We observe a complex relation between MZR and SFR across the whole range of galaxy mass and metallicity, where the slope changes seen in the O/H–SFR plane present a pattern that seems to be tuned to the stellar age of the galaxies, and therefore stellar age has to be taken into account in the stellar mass–metallicity–SFR relation.
Conclusions.
In order to provide an answer to the question of whether or not the MZR depends on the SFR, it is essential to take into account the age of the stellar populations of galaxies. A strong dependence of the MZR on SFR is observed mainly for star-forming galaxies with strong SFR values and low
D
n
(4000). The youngest galaxies of our SDSS sample show the highest SFR measured for their stellar mass.
Integral field spectroscopy obtained with the Potsdam Multi-Aperture Spectrophotometer Fiber Package (PPak) and the 3.5-m telescope at the Calar Alto Observatory has been used to study an outer H ii ...region complex in the well-studied galaxy NGC 6946. This technique provides detailed maps of the region in different emission lines, yielding spatially resolved information about the physical properties of the gas. The configuration was chosen to cover the whole spectrum from 3600 up to 10 000 Å, allowing the measurement of the near-infrared S iii lines. We selected four luminous knots to perform a detailed integrated spectroscopic analysis of these structures and of the whole PPak field of view (FOV). For all the knots the electron density has been found to be very similar and below 100 cm−3. The O iii electron temperature was measured in knots A, B, C and in the integrated PPak field, and was found to be around 8000 K. The temperatures of O ii and S iii were estimated in the four cases. The elemental abundances computed from the ‘direct method’ are typical of high-metallicity disc H ii regions, with a mean value of 12+log(O/H) = 8.65, comparable to what has been found in this galaxy by other authors for regions at similar galactocentric distance. Therefore a remarkable abundance uniformity is found despite the different excitations found throughout the nebula. However, due to the quality of the data, the electron temperatures and metallicities obtained have associated errors comparable to the typical dispersion found in empirical calibrations. Wolf–Rayet (WR) features have been detected in three of the knots, leading to a derived total number of WR stars of 125, 22 and 5 for knots A, C and B, respectively. The ratios of the numbers of WR to O stars are consistent with the prediction of Starburst99 for individual bursts with an age about 4 Myr. Knot D, with no WR features, shows weak Hα emission, low excitation and the lowest Hβ equivalent width, all of which points to a more evolved state. The integrated spectrum of the whole PPak FOV shows high excitation and a relatively evolved age that does not correspond to the individual knot evolutionary stages. Some effects associated with the loss of spatial resolution are also evidenced by the higher ionizing temperature that is deduced from the η′ parameter measured in the integrated PPak spectrum with respect to that of the individual knots.
Aims. The chemical evolution of galaxies on a cosmological timescale is still a matter of debate despite the increasing number of available data provided by spectroscopic surveys of star-forming ...galaxies at different redshifts. The fundamental relations involving metallicity, such as the mass − metallicity relation (MZR) or the fundamental metallicity relation, give controversial results about the reality of evolution of the chemical content of galaxies at a given stellar mass. In this work we shed some light on this issue using the completeness reached by the 20 k bright sample of the zCOSMOS survey and using for the first time the nitrogen-to-oxygen ratio (N/O) as a tracer of the gas phase chemical evolution of galaxies that is independent of the star formation rate. Methods. Emission-line galaxies both in the SDSS and 20 k zCOSMOS bright survey were used to study the evolution from the local Universe of the MZR up to a redshift of ~1.32, and the relation between stellar mass and N/O (MNOR) up to a redshift of ~0.42 using the N2S2 parameter. All the physical properties derived from stellar continuum and gas emission-lines, including stellar mass, star formation rates, metallicity and N/O, were calculated in a self-consistent way over the full redshift range. Results. We confirm the trend to find lower metallicities in galaxies of a given stellar mass in a younger Universe. This trend is even observed when taking possible effects into account that are due to the observed larger median star formation rates for galaxies at higher redshifts. We also find a significant evolution of the MNOR up to z ~ 0.4. Taking the slope of the O/H vs. N/O relation into account for the secondary-nitrogen production regime, the observed evolution of the MNOR is consistent with the trends found for both the MZR and its equivalent relation using new expressions to reduce its dependence on star formation rate.
Giant H ii regions (GHIIRs) in nearby galaxies are a local sample in which we can study in detail processes in the interaction of gas, dust and newly formed stars which are analogous to those which ...occurred in episodes of higher intensity in which much of the current stellar population was born. Here, we present an analysis of NGC 588, a GHIIR in M33, based on optical Integral Field Spectroscopy data obtained with the Potsdam Multi-Aperture Spectrophotometer at the 3.5-m telescope of the Calar Alto Observatory, CAHA, together with Spitzer infrared images at 8 and 24 μm. The extinction distribution measured in the optical shows complex structure, with three maxima which correlate in position with those of the emission at 24 and 8 μm. Furthermore, the Hα luminosity absorbed by the dust within the H ii region reproduces the structure observed in the 24-μm image, supporting the use of the 24-μm band as a valid tracer of recent star formation. A velocity difference of ∼50 km s−1 was measured between the areas of high and low surface brightness, which would be expected if NGC 588 were an evolved GHIIR. We have carefully identified the areas which contribute most to the line ratios measured in the integrated spectrum. Those line ratios which are used in diagnostic diagrams proposed by Baldwin, Phillips & Terlevich (i.e. the BPT diagrams) show a larger range of variation in the low surface brightness areas. The ranges are ∼0.5-1.2 dex for N iiλ6584/Hα, 0.7-1.7 dex for S iiλλ6717,6731/Hα and 0.3-0.5 dex for O iiiλ5007/Hβ, with higher values of N iiλ6584/Hα and S iiλλ6717,6731/Hα, and lower values of O iiiλ5007/Hβ in the areas of lower surface brightness. Ratios corresponding to large ionization parameter (U) are found between the peak of the emission in Hβ and the main ionizing source decreasing radially outwards within the region. Differences between the integrated and local values of the U tracers can be as high as ∼0.8 dex, notably when using O iiiλλ4959,5007/O iiλλ3726,3729 and in the high surface brightness spaxels. O iiλλ3726,3729/Hβ and O iiiλλ4959,5007/O iiλλ3726,3729 yield similar local values for the ionization parameter, which are consistent with those expected from the integrated spectrum of an H ii region ionized by a single star. The ratio S iiλλ6717,6731/Hα departs significantly from the range predicted by this scenario, indicating the complex ionization structure in GHIIRs. There is a significant scatter in derivations of the metallicity using strong line tracers as a function of position, caused by variations in the degree of ionization. The scatter is smaller for N2O3 which points to this tracer as a better metallicity tracer than N2. One interesting result emerges from our comparison between integrated and local line ratio values: measurements of the line ratios of GHIIRs in galaxies at distances ≳25 Mpc may be dominated by the ionization conditions in their low surface brightness areas.
ABSTRACT
Here we report the first spatially resolved spectroscopic study for the galaxy PHL 293B using the high-resolution GTC/MEGARA integral field unit (IFU). PHL 293B is a local, extremely ...metal-poor, high ionization galaxy. This makes PHL 293B an excellent analogue for galaxies in the early Universe. The MEGARA aperture (∼ 12.5 arcsec × 11.3 arcsec) covers the entire PHL 293B main body and its far-reaching ionized gas. We created and discussed maps of all relevant emission lines, line ratios, and physical–chemical properties of the ionized ISM. The narrow emission gas appears to be ionized mainly by massive stars according to the observed diagnostic line ratios, regardless of the position across the MEGARA aperture. We detected low intensity broad emission components and blueshifted absorptions in the Balmer lines (H α,H β) which are located in the brightest zone of the galaxy ISM. A chemically homogeneity, across hundreds of parsecs, is observed in O/H. We take the oxygen abundance 12+log (O/H) = 7.64 ± 0.06 derived from the PHL 293B integrated spectrum as the representative metallicity for the galaxy. Our IFU data reveal for the first time that the nebular He iiλ4686 emission from PHL 293B is spatially extended and coincident with the ionizing stellar cluster, and allow us to compute its absolute He ii ionizing photon flux. Wolf-Rayet bumps are not detected excluding therefore Wolf-Rayet stars as the main He ii excitation source. The origin of the nebular He iiλ4686 is discussed.
Deep, high-resolution spectroscopic observations have been obtained for six compact, strongly star-forming galaxies at redshift z ~ 0.1-0.3, most of them also known as green peas. Remarkably, these ...galaxies show complex emission-line profiles in the spectral region including H alpha , NII lambdalambda6548, 6584, and SII lambdalambda6717, 6731, consisting of the superposition of different kinematical components on a spatial extent of few kiloparsecs: a very broad line emission underlying more than one narrower component. For at least two of the observed galaxies some of these multiple components are resolved spatially in their two-dimensional spectra, whereas for another one a faint detached H alpha blob lacking stellar continuum is detected at the same recessional velocity ~7 kpc away from the galaxy. The individual narrower H alpha components show high intrinsic velocity dispersion (sigma ~ 30-80 km s super(-1)), suggesting together with unsharped masking Hubble Space Telescope images that star formation proceeds in an ensemble of several compact and turbulent clumps, with relative velocities of up to ~500 km s super(-1). The broad underlying H alpha components indicate in all cases large expansion velocities (full width zero intensity > or = 1000 km s copyright ) and very high luminosities (up to ~10 super(42) erg s super(-1)), probably showing the imprint of energetic outflows from supernovae. These intriguing results underline the importance of green peas for studying the assembly of low-mass galaxies near and far.
Based on SITELLE spectroscopy data, we studied the ionised gas emission for the 175 H
α
emission regions in the Stephan’s Quintet (SQ). In this paper we perform a detailed analysis of the star ...formation rate (SFR), oxygen abundance, and nitrogen-to-oxygen abundance ratio (N/O) of the SQ regions, with the intention of exploring the provenance and evolution of this complex structure. According to the BPT diagram, we found 91 HII, 17 composite, and 7 active galactic nucleus-like regions in SQ. Several regions are compatible with fast shocks models without a precursor for solar metallicity and low density (
n
= 0.1 cm
−3
), with velocities in the range of 175–300 km s
−1
. We derived the total SFR in SQ (log(SFR/
M
⊙
yr
−1
= 0.496)). Twenty-eight percent of the total SFR in SQ comes from starburst A, while 9% is in starburst B, and 45% comes from the regions with a radial velocity lower than 6160 km s
−1
. For this reason, we assume that the material prior to the collision with the new intruder does not show a high SFR, and therefore SQ was apparently quenched. When considering the integrated SFR for the whole SQ and the new intruder, we found that both zones have a SFR consistent with those obtained in the SDSS star-forming galaxies. At least two chemically different gas components cohabit in SQ where, on average, the regions with high radial velocities (
v
> 6160 km s
−1
) have lower values of oxygen abundance and N/O than those with low radial velocities (
v
≤ 6160 km s
−1
). The values found for the line ratios considered in this study, as well as in the oxygen abundance and N/O for the southern debris region and the northernmost tidal tail, are compatible with regions belonging to the outer part of the galaxies. We highlight the presence of inner-outer variation for metallicity and some emission line ratios along the new intruder strands and the young tidal tail south strand. Finally, the SQ H
α
regions are outside the galaxies because the interactions have dispersed the gas to the peripheral zones.
ABSTRACT New integral field spectroscopy has been obtained for IZw 18, the nearby lowest-metallicity galaxy considered to be our best local analog of systems forming at high redshift (z). Here we ...report the spatially resolved spectral map of the nebular He ii λ4686 emission in IZw 18, from which we derived for the first time its total He ii-ionizing flux. Nebular He ii emission implies the existence of a hard radiation field. He ii-emitters are observed to be more frequent among high-z galaxies than for local objects. Therefore, investigating the He ii-ionizing source(s) in IZw 18 may reveal the ionization processes at high z. He ii emission in star-forming galaxies has been suggested to be mainly associated with Wolf-Rayet stars (WRs), but WRs cannot satisfactorily explain the He ii-ionization at all times, particularly at the lowest metallicities. Shocks from supernova remnants, or X-ray binaries, have been proposed as additional potential sources of He ii-ionizing photons. Our data indicate that conventional He ii-ionizing sources (WRs, shocks, X-ray binaries) are not sufficient to explain the observed nebular He iiλ4686 emission in IZw 18. We find that the He ii-ionizing radiation expected from models for either low-metallicity super-massive O stars or rotating metal-free stars could account for the He ii-ionization budget measured, while only the latter models could explain the highest values of He iiλ4686/Hβ observed. The presence of such peculiar stars in IZw 18 is suggestive and further investigation in this regard is needed. This letter highlights that some of the clues of the early universe can be found here in our cosmic backyard.
The N2 index (NII lambda6584/H alpha ) is used to determine emission line galaxy metallicities at all redshifts, including high redshift, where galaxies tend to be metal-poor. The initial aim of this ...work was to improve the calibrations used to infer oxygen abundance from N2 by employing updated low-metallicity galaxy databases. We compare N2 and the metallicity determined using the direct method for the set of extremely metal-poor galaxies compiled by Morales-Luis et al. To our surprise, the oxygen abundance presents a tendency to be constant with N2, with a very large scatter. Consequently, we find that the existing N2 calibrators overestimate the oxygen abundance for most low-metallicity galaxies, and can therefore only be used to set upper limits to the true metallicity in low-metallicity galaxies. An explicit expression for this limit is given. In addition, we try to explain the observed scatter using photoionization models. It is mostly due to the different evolutionary state of the H II regions producing the emission lines, but it also arises due to differences in N/O among the galaxies.
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