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Register a new userEvidence for the Accretion of Gas in Star-Forming Galaxies: High N/O Abundances in Regions of Anomalously-Low Metallicity
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While all models for the evolution of galaxies require the accretion of gas to sustain their growth via on-going star formation, it has proven difficult to directly detect this inflowing material. In this paper we use data of nearby star-forming galaxies in the SDSS IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to search for evidence of accretion imprinted in the chemical composition of the interstellar medium. We measure both the O/H and N/O abundance ratios in regions previously identified as having anomalously low values of O/H. We show that the unusual locations of these regions in the N/O vs. O/H plane indicate that they have been created through the mixing of disk gas having higher metallicity with accreted gas having lower metallicity. Taken together with previous analysis on these anomalously low-metallicity regions, these results imply that accretion of metal-poor gas can probably sustain star formation in present-day late-type galaxies.
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We use data from 1222 late-type star-forming galaxies in the SDSS IV MaNGA survey to identify regions in which the gas-phase metallicity is anomalously-low compared to expectations from the tight empirical relation between metallicity and stellar surface mass-density at a given stellar mass. We find anomalously low metallicity (ALM) gas in 10% of the star-forming spaxels, and in 25% of the galaxies in the sample. The incidence rate of ALM gas increases strongly with both global and local measures of the specific star-formation rate, and is higher in lower mass galaxies and in the outer regions of galaxies. The incidence rate is also significantly higher in morphologically disturbed galaxies. We estimate that the lifetimes of the ALM regions are a few hundred Myr. We argue that the ALM gas has been delivered to its present location by a combination of interactions, mergers, and accretion from the halo, and that this infusion of gas stimulates star-formation. Given the estimated lifetime and duty cycle of such events, we estimate that the time-averaged accretion rate of ALM gas is similar to the star-formation rate in late type galaxies over the mass-range M$_* sim10^9$ to 10$^{10}$ M$_{odot}$.
We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show 0.5 dex metallicity decrements in inner regions with enhanced star-formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013) and interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications: (1) it proves that galaxies other than the local tadpoles present the same unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are rare, the physical mechanism responsible for them may sustain a significant part of the star-formation activity in the local Universe. We argue that the star-formation dependence of the mass-metallicity relationship, as well as other general properties followed by most local disk galaxies, are naturally interpreted as side effects of pristine gas infall. Alternatives to the metal poor gas accretion are examined too.
We present an analysis of the chemical abundance properties of $approx$650 star-forming galaxies at $z approx0.6-1.8$. Using integral-field observations from the $K$-band Multi-Object Spectrograph (KMOS), we quantify the [NII]/H$alpha$ emission-line ratio, a proxy for the gas-phase Oxygen abundance within the interstellar medium. We define the stellar mass-metallicity relation at $z approx0.6-1.0$ and $z approx1.2-1.8$ and analyse the correlation between the scatter in the relation and fundamental galaxy properties (e.g. H$alpha$ star-formation rate, H$alpha$ specific star-formation rate, rotation dominance, stellar continuum half-light radius and Hubble-type morphology). We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. We measure the radial dependence of gas-phase metallicity in the galaxies, establishing a median, beam smearing-corrected, metallicity gradient of $ Delta Z/ Delta R=0.002 pm0.004$ dex kpc$^{-1}$, indicating on average there is no significant dependence on radius. The metallicity gradient of a galaxy is independent of its rest-frame optical morphology, whilst correlating with its stellar mass and specific star-formation rate, in agreement with an inside-out model of galaxy evolution, as well as its rotation dominance. We quantify the evolution of metallicity gradients, comparing the distribution of $Delta Z/ Delta R$ in our sample with numerical simulations and observations at $z approx0-3$. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role redistributing metals.
The fundamental metallicity relation (FMR) of galaxies is a 3D relation between the gas-phase metallicity, stellar mass and star-formation rate (SFR). It has been studied so far only for galaxies identified as star-forming (SF) on the BPT diagrams (BPT-SF), but not for galaxies with LI(N)ER/AGN classification (BPT-non-SF), mainly due to the lack of diagnostics for estimating their gas-phase metallicities in the latter cases. We extend the FMR to BPT-non-SF galaxies. To this end, we exploit the recent nebular line empirical calibrations derived specifically for galaxies classified as non-SF in the BPT diagrams. Moreover, we study an alternative representation of the FMR where we consider the offsets in metallicity and SFR with respect to Main Sequence (MS) galaxies. We find that galaxies with SFR higher than the MS are more metal-poor than their counterparts on the MS, which is interpreted in terms of gas accretion, boosting star formation and diluting the metallicity. Low-mass galaxies below the MS (i.e. towards quiescence) have metallicities higher than their MS counterparts, which is interpreted in terms of starvation, (i.e. suppression of fresh gas supply) hampering star formation and reducing the dilution effect, hence resulting in a higher level of internal chemical enrichment. Massive galaxies below the MS have gas metallicity much closer to their MS counterparts and much lower than expected from their stellar metallicities; this result suggests a scenario where massive nearly-quiescent galaxies with LI(N)ER-like nebular emission have recently accreted gas from the circum/intergalactic medium.
We present a sample of low-redshift (z<0.133) candidates for extremely low-metallicity star-forming galaxies with oxygen abundances 12+logO/H<7.4 selected from the Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS). Three methods are used to derive their oxygen abundances. Among these methods two are based on strong [OII]3727, [OIII]4959, and [OIII]5007 emission lines, which we call strong-line and semi-empirical methods. These were applied for all galaxies. We have developed one of these methods, the strong-line method, in this paper. This method is specifically focused on the accurate determination of metallicity in extremely low-metallicity galaxies and may not be used at higher metallicities with12+logO/H>7.5. The third, the direct Te method, was applied for galaxies with detected [OIII]4363 emission lines. All three methods give consistent abundances and can be used in combination or separately for selection of lowest-metallicity candidates. However, the strong-line method is preferable for spectra with a poorly detected or undetected [OIII]4363 emission line. In total, our list of selected candidates for extremely low-metallicity galaxies includes 66 objects.
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