Merging galaxies produce outliers from the Fundamental Metallicity Relation

From a large sample of $\approx 170,000$ 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 $\tau=1.568_{-0.027}^{+0.029}$ Gyr, the average metallicity depression caused by a 1:1 merger to be $\alpha=0.2480_{-0.0020}^{+0.0017}$ dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be $\xi_\text{min}=0.2030_{-0.0095}^{+0.0127}$ (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.Comment: 14 pages, 10 figures, published in MNRAS, updated to be essentially identical to the published versio

When Does the Intergalactic Medium Become Enriched?

We use cosmological hydrodynamic simulations including galactic feedback based on observations of local starbursts to find a self-consistent evolutionary model capable of fitting the observations of the intergalactic metallicity history as traced by C IV between z=6.0->1.5. Our main finding is that despite the relative invariance in the measurement of Omega(C IV) as well as the column density and linewidth distributions over this range, continual feedback from star formation-driven winds are able to reproduce the observations, while an early enrichment scenario where a majority of the metals are injected into the IGM at z>6 is disfavored. The constancy of the C IV observations results from a rising IGM metallicity content balanced by a declining C IV ionization fraction due to a 1) decreasing physical densities, 2) increasing ionization background strength, and 3) metals becoming more shock-heated at lower redshift. Our models predict that ~20x more metals are injected into the IGM between z=6->2 than at z>6. We show that the median C IV absorber at z=2 traces metals injected 1 Gyr earlier indicating that the typical metals traced by C IV are neither from very early times nor from very recent feedback.Comment: 6 pages, 3 figures, to appear in the proceedings of "Chemodynamics: from the First Stars to Local Galaxies", Lyon, France, July 10-14, 200

Probing the Metal Enrichment of the Intergalactic Medium at z=5−6z=5-6 Using the Hubble Space Telescope

We test the galactic outflow model by probing associated galaxies of four strong intergalactic CIV absorbers at $z=5$--6 using the Hubble Space Telescope (HST) ACS ramp narrowband filters. The four strong CIV absorbers reside at $z=5.74$, $5.52$, $4.95$, and $4.87$, with column densities ranging from $N_{\rm{CIV}}=10^{13.8}$ cm$^{-2}$ to $10^{14.8}$ cm$^{-2}$. At $z=5.74$, we detect an i-dropout Ly$\alpha$ emitter (LAE) candidate with a projected impact parameter of 42 physical kpc from the CIV absorber. This LAE candidate has a Ly$\alpha$-based star formation rate (SFR$_{\rm{Ly\alpha}}$) of 2 $M_\odot$ yr$^{-1}$ and a UV-based SFR of 4 $M_\odot$ yr$^{-1}$. Although we cannot completely rule out that this $i$-dropout emitter may be an [OII] interloper, its measured properties are consistent with the CIV powering galaxy at $z=5.74$. For CIV absorbers at $z=4.95$ and $z=4.87$, although we detect two LAE candidates with impact parameters of 160 kpc and 200 kpc, such distances are larger than that predicted from the simulations. Therefore we treat them as non-detections. For the system at $z=5.52$, we do not detect LAE candidates, placing a 3-$\sigma$ upper limit of SFR$_{\rm{Ly\alpha}}\approx 1.5\ M_\odot$ yr$^{-1}$. In summary, in these four cases, we only detect one plausible CIV source at $z=5.74$. Combining the modest SFR of the one detection and the three non-detections, our HST observations strongly support that smaller galaxies (SFR$_{\rm{Ly\alpha}} \lesssim 2\ M_\odot$ yr$^{-1}$) are main sources of intergalactic CIV absorbers, and such small galaxies play a major role in the metal enrichment of the intergalactic medium at $z\gtrsim5$.Comment: Accepted for Publications in ApJ

Reionization in Technicolor

We present the Technicolor Dawn simulations, a suite of cosmological radiation-hydrodynamic simulations of the first 1.2 billion years. By modeling a spatially-inhomogeneous UVB on-the-fly with 24 frequencies and resolving dark matter halos down to $10^8 M_\odot$ within 12 $h^{-1}$ Mpc volumes, our simulations unify observations of the intergalactic and circumgalactic media, galaxies, and reionization into a common framework. The only empirically-tuned parameter, the fraction $f_{\mathrm{esc,gal}}(z)$ of ionizing photons that escape the interstellar medium, is adjusted to match observations of the Lyman-$\alpha$ forest and the cosmic microwave background. With this single calibration, our simulations reproduce the history of reionization; the stellar mass-star formation rate relation of galaxies; the number density and metallicity of damped Lyman-$\alpha$ absorbers (DLAs) at $z\sim5$; the abundance of weak metal absorbers; the ultraviolet background (UVB) amplitude; and the Lyman-$\alpha$ flux power spectrum at $z=5.4$. The galaxy stellar mass and UV luminosity functions are underproduced by $\leq2\times$, suggesting an overly vigorous feedback model. The mean transmission in the Lyman-$\alpha$ forest is underproduced at $z<6$, indicating tension between measurements of the UVB amplitude and Lyman-$\alpha$ transmission. The observed SiIV column density distribution is reasonably well-reproduced ($\sim 1\sigma$ low). By contrast, CIV remains significantly underproduced despite being boosted by an intense $>4$ Ryd UVB. Solving this problem by increasing metal yields would overproduce both weak absorbers and DLA metallicities. Instead, the observed strength of high-ionization emission from high-redshift galaxies and absorption from their environments suggest that the ionizing flux from conventional stellar population models is too soft.Comment: 24 pages, 17 figures, accepted to MNRA

The environments and hosts of metal absorption at z>5

A growing population of metal absorbers are observed at z>5, many showing strong evolution in incidence approaching the epoch of hydrogen reionization. Follow-up surveys examining fields around these metals have resulted in galaxy detections but the direct physical relationship between the detected galaxies and absorbers is unclear. Upcoming observations will illuminate this galaxy-absorber relationship, but the theoretical framework for interpreting these observations is lacking. To inform future z>5 studies, we define the expected relationship between metals and galaxies using the Technicolor Dawn simulation to model metal absorption from z=5-7, encompassing the end of reionization. We find that metal absorber types and strengths are slightly better associated with their environment than with the traits of their host galaxies, as absorption system strengths are more strongly correlated with the local galaxy overdensity than the stellar mass of their host galaxy. For redshifts prior to the end of the epoch of reionization, strong high ionization transitions like C IV are more spatially correlated with brighter galaxies on scales of a few hundred proper kpc than are low ionization systems, due to the former's preference for environments with higher UVB amplitudes and those ions' relative rarity at z>6. Post-reionization, the galaxy counts near these high-ionization ions are reduced, and increase surrounding certain low-ionization ions due to a combination of their relative abundances and preferred denser gas phase. We conclude that galaxy-absorber relationships are expected to evolve rapidly such that high-ionization absorbers are better tracers of galaxies pre-reionization while low-ionization absorbers are better post-reionization.Comment: Accepted to MNRA

The Physical Properties and Detectability of Reionization-Epoch Galaxies

We present predictions drawn from cosmological hydrodynamic simulations for the physical, photometric and emission line properties of galaxies present during the latter stages of reionization from z=9-6. We find significant numbers of galaxies that have stellar masses exceeding 10^8 Mo during this epoch, with metallicities exceeding one-thirtieth solar. Far from primeval "first-star" objects,these objects are likely to have reionized their infall regions prior to z=9, are dominated by atomic rather than molecular cooling, and are not expected to be forming Population III stars. Galaxies exhibit a slowly evolving comoving autocorrelation length from z=9-6, continuing a trend seen at lower redshifts in which the rapidly dropping bias counteracts the rapidly increasing matter clustering. These sources can be marginally detected using current instruments, but modest increases in sensitivity or survey area would yield significantly increased samples. We compare to current observations of the z~6 rest-UV and Ly-alpha line luminosity functions, and find good agreement. We also compare with the z~7 object observed by Egami et al., and find that such systems are ubiquitous in our simulations. The intrinsic Ly-alpha luminosity function evolves slowly from z=9-6, meaning that it should also be possible to detect these objects with upcoming narrow band surveys such as DAzLE. We make predictions for near-IR surveys with JWST, and show that while a high density of sources will be found, Population III objects may remain elusive. We present and compare simulations with several recipes for superwind feedback, and show that while our broad conclusions are insensitive to this choice, a feedback model based on momentum-driven winds is favored in comparisons with available data. (abridged)Comment: 20 pages, MNRAS, accepted. Expanded discussions, conclusions unchange