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Register a new userSHINING, A Survey of Far Infrared Lines in Nearby Galaxies. II: Line-Deficit Models, AGN impact, [CII]-SFR Scaling Relations, and Mass-Metallicity Relation in (U)LIRGS
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Rodrigo Herrera-Camus
Publication date
2018
fields
Physics
and research's language is
English
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The SHINING survey (Paper I; Herrera-Camus et al. 2018) offers a great opportunity to study the properties of the ionized and neutral media of galaxies from prototypical starbursts and active galactic nuclei (AGN) to heavily obscured objects. Based on Herschel/PACS observations of the main far-infrared (FIR) fine-structure lines, in this paper we analyze the physical mechanisms behind the observed line deficits in galaxies, the apparent offset of luminous infrared galaxies (LIRGs) from the mass-metallicity relation, and the scaling relations between [CII] 158 $mu$m line emission and star formation rate (SFR). Based on a toy model and the Cloudy code, we conclude that the increase in the ionization parameter with FIR surface brightness can explain the observed decrease in the line-to-FIR continuum ratio of galaxies. In the case of the [CII] line, the increase in the ionization parameter is accompanied by a reduction in the photoelectric heating efficiency and the inability of the line to track the increase in the FUV radiation field as galaxies become more compact and luminous. In the central $sim$kiloparsec regions of AGN galaxies we observe a significant increase in the [OI] 63 $mu$m/[CII] line ratio; the AGN impact on the line-to-FIR ratios fades on global scales. Based on extinction-insensitive metallicity measurements of LIRGs we confirm that they lie below the mass-metallicity relation, but the offset is smaller than those reported in studies that use optical-based metal abundances. Finally, we present scaling relations between [CII] emission and SFR in the context of the main-sequence of star-forming galaxies.
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We use the Herschel/PACS spectrometer to study the global and spatially resolved far-infrared (FIR) fine-structure line emission in a sample of 52 galaxies that constitute the SHINING survey. These galaxies include star-forming, active-galactic nuclei (AGN), and luminous infrared galaxies (LIRGs). We find an increasing number of galaxies (and kiloparsec size regions within galaxies) with low line-to-FIR continuum ratios as a function of increasing FIR luminosity ($L_{mathrm{FIR}}$), dust infrared color, $L_{mathrm{FIR}}$ to molecular gas mass ratio ($L_{mathrm{FIR}}/M_{mathrm{mol}}$), and FIR surface brightness ($Sigma_{mathrm{FIR}}$). The correlations between the [CII]/FIR or [OI]/FIR ratios with $Sigma_{mathrm{FIR}}$ are remarkably tight ($sim0.3$ dex scatter over almost four orders of magnitude in $Sigma_{mathrm{FIR}}$). We observe that galaxies with $L_{mathrm{FIR}}/M_{mathrm{mol}} gtrsim 80,L_{odot},M_{odot}^{-1}$ and $Sigma_{mathrm{FIR}}gtrsim10^{11}$ $L_{odot}$ kpc$^{-2}$ tend to have weak fine-structure line-to-FIR continuum ratios, and that LIRGs with infrared sizes $gtrsim1$ kpc have line-to-FIR ratios comparable to those observed in typical star-forming galaxies. We analyze the physical mechanisms driving these trends in Paper II (Herrera-Camus et al. 2018). The combined analysis of the [CII], [NII], and [OIII] lines reveals that the fraction of the [CII] line emission that arises from neutral gas increases from 60% to 90% in the most active star-forming regions and that the emission originating in the ionized gas is associated with low-ionization, diffuse gas rather than with dense gas in HII regions. Finally, we report the global and spatially resolved line fluxes of the SHINING galaxies to enable the comparison and planning of future local and high-$z$ studies.
We present [CII] 158um measurements from over 15,000 resolved regions within 54 nearby galaxies of the KINGFISH program to investigate the so-called [CII] line cooling deficit long known to occur in galaxies with different luminosities. The [CII]/TIR ratio ranges from above 1% to below 0.1% in the sample, with a mean value of 0.48+-0.21%. We find that the surface density of 24um emission dominates this trend, with [CII]/TIR dropping as nuInu{24um} increases. Deviations from this overall decline are correlated with changes in the gas phase metal abundance, with higher metallicity associated with deeper deficits at a fixed surface brightness. We supplement the local sample with resolved [CII] measurements from nearby luminous infrared galaxies and high redshift sources from z=1.8-6.4, and find that star formation rate density drives a continuous trend of deepening [CII] deficit across six orders of magnitude in SFRD. The tightness of this correlation suggests that an approximate star formation rate density can be estimated directly from global measurements of [CII]/TIR, and a relation is provided to do so. Several low-luminosity AGN hosts in the sample show additional and significant central suppression of [CII]/TIR, but these deficit enhancements occur not in those AGN with the highest X-ray luminosities, but instead those with the highest central starlight intensities. Taken together, these results demonstrate that the [CII] cooling line deficit in galaxies likely arises from local physical phenomena in interstellar gas.
We have observed the [CII] 158 micron line emission from the Galactic plane (-10 deg < l < 25 deg, |b| <= 3 deg) with the Balloon-borne Infrared Carbon Explorer (BICE). The observed longitudinal distribution of the [CII] line emission is clearly different from that of the far-infrared continuum emission; the Galactic center is not the dominant peak in the [CII] emission. Indeed, the ratio of the [CII] line emission to far-infrared continuum (I_[CII] / I_FIR) is systematically low within the central several hundred parsecs of the Galaxy. The observational results indicate that the abundance of the C+ ions themselves is low in the Galactic center. We attribute this low abundance mainly to soft UV radiation with fewer C-ionizing photons. This soft radiation field, together with the pervasively high molecular gas density, makes the molecular self-shielding more effective in the Galactic center. The self-shielding further reduces the abundance of C+ ions, and raises the temperature of molecular gas at the C+/C/CO transition zone.
We investigate the nature of the relation among stellar mass, star-formation rate, and gas-phase metallicity (the M$_*$-SFR-Z relation) at high redshifts using a sample of 260 star-forming galaxies at $zsim2.3$ from the MOSDEF survey. We present an analysis of the high-redshift M$_*$-SFR-Z relation based on several emission-line ratios for the first time. We show that a M$_*$-SFR-Z relation clearly exists at $zsim2.3$. The strength of this relation is similar to predictions from cosmological hydrodynamical simulations. By performing a direct comparison of stacks of $zsim0$ and $zsim2.3$ galaxies, we find that $zsim2.3$ galaxies have $sim0.1$ dex lower metallicity at fixed M$_*$ and SFR. In the context of chemical evolution models, this evolution of the M$_*$-SFR-Z relation suggests an increase with redshift of the mass-loading factor at fixed M$_*$, as well as a decrease in the metallicity of infalling gas that is likely due to a lower importance of gas recycling relative to accretion from the intergalactic medium at high redshifts. Performing this analysis simultaneously with multiple metallicity-sensitive line ratios allows us to rule out the evolution in physical conditions (e.g., N/O ratio, ionization parameter, and hardness of the ionizing spectrum) at fixed metallicity as the source of the observed trends with redshift and with SFR at fixed M$_*$ at $zsim2.3$. While this study highlights the promise of performing high-order tests of chemical evolution models at high redshifts, detailed quantitative comparisons ultimately await a full understanding of the evolution of metallicity calibrations with redshift.
We show that the mass-metallicity relation observed in the local universe is due to a more general relation between stellar mass M*, gas-phase metallicity and SFR. Local galaxies define a tight surface in this 3D space, the Fundamental Metallicity Relation (FMR), with a small residual dispersion of ~0.05 dex in metallicity, i.e, ~12%. At low stellar mass, metallicity decreases sharply with increasing SFR, while at high stellar mass, metallicity does not depend on SFR. High redshift galaxies, up to z~2.5 are found to follow the same FMR defined by local SDSS galaxies, with no indication of evolution. The evolution of the mass-metallicity relation observed up to z=2.5 is due to the fact that galaxies with progressively higher SFRs, and therefore lower metallicities, are selected at increasing redshifts, sampling different parts of the same FMR. By introducing the new quantity mu_alpha=log(M*)-alpha log(SFR), with alpha=0.32, we define a projection of the FMR that minimizes the metallicity scatter of local galaxies. The same quantity also cancels out any redshift evolution up to z~2.5, i.e, all galaxies have the same range of values of mu_0.32. At z>2.5, evolution of about 0.6 dex off the FMR is observed, with high-redshift galaxies showing lower metallicities. The existence of the FMR can be explained by the interplay of infall of pristine gas and outflow of enriched material. The former effect is responsible for the dependence of metallicity with SFR and is the dominant effect at high-redshift, while the latter introduces the dependence on stellar mass and dominates at low redshift. The combination of these two effects, together with the Schmidt-Kennicutt law, explains the shape of the FMR and the role of mu_0.32. The small metallicity scatter around the FMR supports the smooth infall scenario of gas accretion in the local universe.
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