Abstract
The MAMMOTH–Grism slitless spectroscopic survey is a Hubble Space Telescope (HST) cycle 28 medium program, which is obtaining 45 orbits of WFC3/IR grism spectroscopy in the density peak ...regions of three massive galaxy protoclusters at
z
= 2–3 discovered using the MAMMOTH technique. We introduce this survey by presenting the first measurement of the mass–metallicity relation (MZR) at high redshift in overdense environments via grism spectroscopy. From the completed MAMMOTH–Grism observations in the field of the BOSS1244 protocluster at
z
= 2.24 ± 0.02, we secure a sample of 36 protocluster member galaxies at
z
≈ 2.24, showing strong nebular emission lines (O
III
, H
β
, and O
II
) in their G141 spectra. Using the multi-wavelength broadband deep imaging from HST and ground-based telescopes, we measure their stellar masses in the range of 10
9
, 10
10.4
M
⊙
, instantaneous star formation rates (SFR) from 10 to 240
M
⊙
yr
−1
, and global gas-phase metallicities
1
3
,
1
of solar. Compared with similarly selected field-galaxy samples at the same redshift, our galaxies show, on average, increased SFRs by ∼0.06 dex and ∼0.18 dex at ∼10
10.1
M
⊙
and ∼10
9.8
M
⊙
, respectively. Using the stacked spectra of our sample galaxies, we derive the MZR in the BOSS1244 protocluster core as
12
+
log
(
O
/
H
)
=
0.136
±
0.018
×
log
(
M
*
/
M
⊙
)
+
7.082
±
0.175
, showing a significantly shallower slope than that in the field. This shallow MZR slope is likely caused by the combined effects of efficient recycling of feedback-driven winds and cold-mode gas accretion in protocluster environments. The former effect helps low-mass galaxies residing in overdensities retain their metal production, whereas the latter effect dilutes the metal content of high-mass galaxies, making them more metal-poor than their coeval field counterparts.
We report the first measurements with sub-kiloparsec spatial resolution of strongly inverted gas-phase metallicity gradients in two dwarf galaxies at z ∼ 2. The galaxies have stellar masses ∼109 , ...specific star formation rate ∼20 Gyr−1, and global metallicity (1/4 solar), assuming the strong-line calibrations of O iii/Hβ and O ii/Hβ from Maiolino et al. Their radial metallicity gradients are measured to be highly inverted, i.e., 0.122 0.008 and 0.111 0.017 dex kpc−1, which is hitherto unseen at such small masses in similar redshift ranges. From the Hubble Space Telescope observations of the source nebular emission and stellar continuum, we present two-dimensional spatial maps of star formation rate surface density, stellar population age, and gas fraction, which show that our galaxies are currently undergoing rapid mass assembly via disk inside-out growth. More importantly, using a simple chemical evolution model, we find that the gas fractions for different metallicity regions cannot be explained by pure gas accretion. Our spatially resolved analysis based on a more advanced gas regulator model results in a spatial map of net gaseous outflows, triggered by active central starbursts, that potentially play a significant role in shaping the spatial distribution of metallicity by effectively transporting stellar nucleosynthesis yields outwards. The relation between wind mass loading factors and stellar surface densities measured in different regions of our galaxies shows that a single type of wind mechanism, driven by either energy or momentum conservation, cannot explain the entire galaxy. These sources present a unique constraint on the effects of gas flows on the early phase of disk growth from the perspective of spatially resolved chemical evolution within individual systems.
We study large-scale outflows in a sample of 96 star-forming galaxies at 1 lap z lap 2, using near-UV spectroscopy of Fe II and Mg II absorption and emission. The profiles of the M II ...lambdalambda2796, 2803 lines show much more variety than the Fe II profiles, which are always seen in absorption; Mg II ranges from strong emission to pure absorption, with emission more common in galaxies with blue UV slopes and at lower stellar masses. Outflow velocities, as traced by the centroids and maximum extent of the absorption lines, increase with increasing stellar mass with 2sigma-3sigma significance, in agreement with previous results. A comparison of the strengths of the Fe II absorption and Fe II* emission lines indicates that massive galaxies have more extended outflows and/or greater extinction, while two-dimensional composite spectra indicate that emission from the outflow is stronger at a radius of ~ 10 kpc in high-mass galaxies than in low-mass galaxies.
We present the hitherto largest sample of gas-phase metallicity radial gradients measured at sub-kpc resolution in star-forming galaxies in the redshift range of z 1.2, 2.3. These measurements are ...enabled by the synergy of slitless spectroscopy from the Hubble Space Telescope near-infrared channels and the lensing magnification from foreground galaxy clusters. Our sample consists of 76 galaxies with stellar mass ranging from 107 to 1010 , an instantaneous star formation rate in the range of 1, 100 yr−1, and global metallicity of solar. At a 2 confidence level, 15/76 galaxies in our sample show negative radial gradients, whereas 7/76 show inverted gradients. Combining ours and all other metallicity gradients obtained at a similar resolution currently available in the literature, we measure a negative mass dependence of Δlog(O/H)/ = (−0.020 0.007) + (−0.016 0.008) , with the intrinsic scatter being = 0.060 0.006 over 4 orders of magnitude in stellar mass. Our result is consistent with strong feedback, not secular processes, being the primary governor of the chemostructural evolution of star-forming galaxies during the disk mass assembly at cosmic noon. We also find that the intrinsic scatter of metallicity gradients increases with decreasing stellar mass and increasing specific star formation rate. This increase in the intrinsic scatter is likely caused by the combined effect of cold-mode gas accretion and merger-induced starbursts, with the latter more predominant in the dwarf mass regime of .
We combine deep Hubble Space Telescope grism spectroscopy with a new Bayesian method to derive maps of gas-phase metallicity for 10 star-forming galaxies at high redshift ( ). Exploiting lensing ...magnification by the foreground cluster MACS1149.6+2223, we reach sub-kiloparsec spatial resolution and push the limit of stellar mass associated with such high-z spatially resolved measurements below for the first time. Our maps exhibit diverse morphologies, indicative of various effects such as efficient radial mixing from tidal torques, rapid accretion of low-metallicity gas, and other physical processes that can affect the gas and metallicity distributions in individual galaxies. Based upon an exhaustive sample of all existing sub-kiloparesec resolution metallicity gradient measurements at high z, we find that predictions given by analytical chemical evolution models assuming a relatively extended star-formation profile in the early disk-formation phase can explain the majority of observed metallicity gradients, without involving galactic feedback or radial outflows. We observe a tentative correlation between stellar mass and metallicity gradients, consistent with the "downsizing" galaxy formation picture that more massive galaxies are more evolved into a later phase of disk growth, where they experience more coherent mass assembly at all radii and thus show shallower metallicity gradients. In addition to the spatially resolved analysis, we compile a sample of homogeneously cross-calibrated integrated metallicity measurements spanning three orders of magnitude in stellar mass at z ∼ 1.8. We use this sample to study the mass-metallicity relation (MZR) and find that the slope of the observed MZR can rule out the momentum-driven wind model at a 3 confidence level.
Abstract
We report the first spatially resolved measurements of gas-phase metallicity radial gradients in star-forming galaxies in overdense environments at
z
≳ 2. The spectroscopic data are acquired ...by the MAMMOTH-Grism survey, a Hubble Space Telescope (HST) cycle 28 medium program. This program is obtaining 45 orbits of WFC3/IR grism spectroscopy in the density peak regions of three massive galaxy protoclusters (BOSS 1244, BOSS 1542, and BOSS 1441) at
z
= 2–3. Our sample in the BOSS 1244 field consists of 20 galaxies with stellar mass ranging from 10
9.0
to 10
10.3
M
⊙
, star formation rate (SFR) from 10 to 240
M
⊙
yr
−1
, and global gas-phase metallicity (
12
+
log
(
O
/
H
)
) from 8.2 to 8.6. At 1
σ
confidence level, 2/20 galaxies in our sample show positive (inverted) gradients—the relative abundance of oxygen increasing with galactocentric radius, opposite the usual trend. Furthermore, 1/20 shows negative gradients, and 17/20 are consistent with flat gradients. This high fraction of flat/inverted gradients is uncommon in simulations and previous observations conducted in blank fields at similar redshifts. To understand this, we investigate the correlations among various observed properties of our sample galaxies. We find an anticorrelation between metallicity gradient and global metallicity of our galaxies residing in extreme overdensities, and a marked deficiency of metallicity in our massive galaxies as compared to their coeval field counterparts. We conclude that the cold-mode gas accretion plays an active role in shaping the chemical evolution of galaxies in the protocluster environments, diluting their central chemical abundance, and flattening/inverting their metallicity gradients.
Using new Keck DEIMOS spectroscopy, we examine the origin of the steep number counts of ultra-faint emission-line galaxies recently reported by Dressler et al. We confirm six Lyalpha emitters (LAEs), ...three of which have significant asymmetric line profiles with prominent wings extending 300-400 km s-sup 1 redward of the peak emission. With these six LAEs, we revise our previous estimate of the number of faint LAEs in the Dressler et al. survey. Combining these data with the density of bright LAEs in the Cosmic Evolution Survey and Subaru Deep Field provides the best constraints to date on the redshift 5.7 LAE luminosity function (LF). The nominal model, which uses z ~ 6 equivalent widths from the literature, falls short of the observed space density of LAEs at the bright end, possibly indicating a need for higher equivalent widths.
We study large-scale outflows in a sample of 96 star-forming galaxies at 1 lap z lap 2, using near-UV spectroscopy of Fe II and Mg II absorption and emission. The profiles of the M II ...lambdalambda2796, 2803 lines show much more variety than the Fe II profiles, which are always seen in absorption; Mg II ranges from strong emission to pure absorption, with emission more common in galaxies with blue UV slopes and at lower stellar masses. Outflow velocities, as traced by the centroids and maximum extent of the absorption lines, increase with increasing stellar mass with 2sigma-3sigma significance, in agreement with previous results. A comparison of the strengths of the Fe II absorption and Fe II* emission lines indicates that massive galaxies have more extended outflows and/or greater extinction, while two-dimensional composite spectra indicate that emission from the outflow is stronger at a radius of ~ 10 kpc in high-mass galaxies than in low-mass galaxies.
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