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Anahita Alavi

Publications and source records attributed to Anahita Alavi.

At least 37 records · Page 2Linked to original sources
arXiv2024

UVCANDELS: The role of dust on the stellar mass-size relation of disk galaxies at 0.5 $\leq z \leq$ 3.0

We use the Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields (UVCANDELS) to measure half-light radii in the rest-frame far-UV for $\sim$16,000 disk-like galaxies over $0.5\leq z \leq 3$. We compare these results to rest-frame optical sizes that we measure in a self-consistent way and find that the stellar mass-size relation of disk galaxies is steeper in the rest-frame UV than in the optical across our entire redshift range. We show that this is mainly driven by massive galaxies ($\gtrsim10^{10}$M$_\odot$), which we find to also be among the most dusty. Our results are consistent with the literature and have commonly been interpreted as evidence of inside-out growth wherein galaxies form their central structures first. However, they could also suggest that the centers of massive galaxies are more heavily attenuated than their outskirts. We distinguish between these scenarios by modeling and selecting galaxies at $z=2$ from the VELA simulation suite in a way that is consistent with UVCANDELS. We show that the effects of dust alone can account for the size differences we measure at $z=2$. This indicates that, at different wavelengths, size differences and the different slopes of the stellar mass-size relation do not constitute evidence for inside-out growth.

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arXiv2024

The UV luminosity function at 0.6 < z < 1 from UVCANDELS

UVCANDELS is a HST Cycle-26 Treasury Program awarded 164 orbits of primary ultraviolet (UV) F275W imaging and coordinated parallel optical F435W imaging in four CANDELS fields: GOODS-N, GOODS-S, EGS, and COSMOS, covering a total area of $\sim426$ arcmin$^2$. This is $\sim2.7$ times larger than the area covered by previous deep-field space UV data combined, reaching a depth of about 27 and 28 ABmag ($5σ$ in $0.2"$ apertures) for F275W and F435W, respectively. Along with the new photometric catalogs, we present an analysis of the rest-frame UV luminosity function (LF), relying on our UV-optimized aperture photometry method yielding a factor of $1.5\times$ increase than the H-isophot aperture photometry in the signal-to-noise ratios of galaxies in our F275W imaging. Using well tested photometric redshift measurements we identify 5810 galaxies at redshifts $0.6<z<1$, down to an absolute magnitude of $M_\text{UV} = -14.2$. In order to minimize the effect of uncertainties in estimating the completeness function, especially at the faint-end, we restrict our analysis to sources above $30\%$ completeness, which provides a final sample of 4726 galaxies at $-21.5<M_\text{UV}<-15.5$. We performed a maximum likelihood estimate to derive the best-fit parameters of the UV LF. We report a best-fit faint-end slope of $α= -1.359^{+0.041}_{-0.041}$ at $z \sim 0.8$. Creating sub-samples at $z\sim0.7$ and $z\sim0.9$, we observe a possible evolution of $α$ with redshift. The unobscured UV luminosity density at $M_\text{UV}<-10$ is derived as $ρ_\text{UV}=1.339^{+0.027}_{-0.030}\ (\times10^{26} \text{ergs/s/Hz/Mpc}^3)$ using our best-fit LF parameters. The new F275W and F435 photometric catalogs from UVCANDELS have been made publicly available on the Barbara A. Mikulski Archive for Space Telescopes (MAST).

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arXiv2024

The Rest-UV Spectral Properties of Dwarf Galaxies at $z\sim2$

Rest-UV spectroscopy can constrain properties of the stellar populations, outflows, covering fractions, and can indirectly constrain the Lyman continuum escape fraction of galaxies. Many works have studied the rest-UV spectra of more massive star forming galaxies and low-mass galaxies selected via strong nebular line emission or via Ly$α$ emission. However, studies of rest-UV spectroscopy have yet to be done on an unbiased sample at low mass during the epoch of peak star formation ($z\sim2$). We present a stacked rest-UV spectrum of a complete sample of 16 dwarf galaxies ($\rm \langle log(M^*/M_\odot)\rangle_{median} = 8.2$) at $z\sim2$. The rest-UV Keck/LRIS spectroscopy is complemented by rest-optical Keck/MOSFIRE spectroscopy and Hubble photometry. We find generally larger Ly$α$ equivalent widths ($\rm EW_{Lyα} = 11.2\;$Å) when compared with higher mass ($\rm \langle log(M^*/M_\odot)\rangle_{median} = 10.3$) composites from KBSS ($\rm EW_{Lyα} = -5\;$Å). The average low- and high-ionization absorption line EWs ($\rm EW_{LIS}$ and $\rm EW_{HIS}$, respectively) are weaker ($\rm EW_{LIS}$=-1.18 Å, $\rm EW_{HIS}=$-0.99 Å) in dwarf galaxies than in higher mass galaxies ($\rm EW_{LIS}$=-2.04 Å, $\rm EW_{HIS}=$-1.42 Å). The LIS absorption lines are optically thick and is thus a good tracer of the neutral hydrogen covering fraction. Both higher $\rm EW_{Lyα}$ and lower $\rm EW_{LIS}$ measurements imply that the escape fraction of ionizing radiation is larger in lower-mass galaxies at $z\sim2$.

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arXiv2024

Lyman Continuum Emission from AGN at 2.3$\lesssim$z$\lesssim$3.7 in the UVCANDELS Fields

We present the results of our search for Lyman continuum (LyC) emitting AGN at redshifts 2.3$\lesssim$z$\lesssim$4.9 from HST WFC3 F275W observations in the UVCANDELS fields. We also include LyC emission from AGN using HST WFC3 F225W, F275W, and F336W found in the ERS and HDUV data. We performed exhaustive queries of the Vizier database to locate AGN with high quality spectroscopic redshifts. In total, we found 51 AGN that met our criteria within the UVCANDELS and ERS footprints. Of these 51, we find 12 AGN had $\geq$4$σ$ detected LyC flux in the WFC3/UVIS images. Using space- and ground-based data from X-ray to radio, we fit the multi-wavelength photometric data of each AGN to a CIGALE SED and correlate various SED parameters to the LyC flux. KS-tests of the SED parameter distributions for the LyC-detected and non-detected AGN showed they are likely not distinct samples. However, we find that X-ray luminosity, star-formation onset age, and disk luminosity show strong correlations relative to their emitted LyC flux. We also find strong correlation of the LyC flux to several dust parameters, i.e., polar and toroidal dust emission, 6 $μm$ luminosity, and anti-correlation with metallicity and $A_{FUV}$. We simulate the LyC escape fraction ($f_{esc}$) using the CIGALE and IGM transmission models for the LyC-detected AGN and find an average $f_{esc}$$\simeq$18%, weighted by uncertainties. We stack the LyC flux of subsamples of AGN according to the wavelength continuum region in which they are detected and find no significant distinctions in their LyC emission, although our $sub-mm\ detected$ F336W sample shows the brightest stacked LyC flux. These findings indicate that LyC-production and -escape in AGN is more complicated than the simple assumption of thermal emission and a 100% escape fraction. Further testing of AGN models with larger samples than presented here is needed.

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arXiv2023

CEERS: 7.7 $μ$m PAH Star Formation Rate Calibration with JWST MIRI

We test the relationship between UV-derived star formation rates (SFRs) and the 7.7 $μ$m polycyclic aromatic hydrocarbon (PAH) luminosities from the integrated emission of galaxies at z ~ 0 - 2. We utilize multi-band photometry covering 0.2 - 160 $μ$m from HST, CFHT, JWST, Spitzer, and Herschel for galaxies in the Cosmic Evolution Early Release Science (CEERS) Survey. We perform spectral energy distribution (SED) modeling of these data to measure dust-corrected far-UV (FUV) luminosities, $L_{FUV}$, and UV-derived SFRs. We then fit SED models to the JWST/MIRI 7.7 - 21 $μ$m CEERS data to derive rest-frame 7.7 $μ$m luminosities, $L_{770}$, using the average flux density in the rest-frame MIRI F770W bandpass. We observe a correlation between $L_{770}$ and $L_{FUV}$, where log $L_{770}$ is proportional to (1.27+/-0.04) log $L_{FUV}$. $L_{770}$ diverges from this relation for galaxies at lower metallicities, lower dust obscuration, and for galaxies dominated by evolved stellar populations. We derive a "single-wavelength" SFR calibration for $L_{770}$ which has a scatter from model estimated SFRs (${σ_{ΔSFR}}$) of 0.24 dex. We derive a "multi-wavelength" calibration for the linear combination of the observed FUV luminosity (uncorrected for dust) and the rest-frame 7.7 $μ$m luminosity, which has a scatter of ${σ_{ΔSFR}}$ = 0.21 dex. The relatively small decrease in $σ$ suggests this is near the systematic accuracy of the total SFRs using either calibration. These results demonstrate that the rest-frame 7.7 $μ$m emission constrained by JWST/MIRI is a tracer of the SFR for distant galaxies to this accuracy, provided the galaxies are dominated by star-formation with moderate-to-high levels of attenuation and metallicity.

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arXiv2023

UV-Bright Star-Forming Clumps and Their Host Galaxies in UVCANDELS at 0.5 $\leq$ z $\leq$ 1

Giant star-forming clumps are a prominent feature of star-forming galaxies (SFGs) and contain important clues on galaxy formation and evolution. However, basic demographics of clumps and their host galaxies remain uncertain. Using the HST/WFC3 F275W images from the Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (UVCANDELS), we detect and analyze giant star-forming clumps in galaxies at 0.5 $\leq$ z $\leq$ 1, connecting two epochs when clumps are common (at cosmic high-noon, z $\sim$ 2) and rare (in the local universe). We construct a clump sample whose rest-frame 1600 Å luminosity is 3 times higher than the most luminous local HII regions (M$_{UV} \leq -$16 AB). In our sample, 35 $\pm$ 3$\%$ of low-mass galaxies (log[M$_{*}$/M$_{\odot}$] $<$ 10) are clumpy (i.e., containing at least one off-center clump). This fraction changes to 22 $\pm$ 3$\%$ and 22 $\pm$ 4$\%$ for intermediate (10 $\leq$ log[M$_{*}$/M$_{\odot}$] $\leq$ 10.5) and high-mass (log[M$_{*}$/M$_{\odot}$] $>$ 10.5) galaxies in agreement with previous studies. When compared to similar-mass non-clumpy SFGs, low- and intermediate-mass clumpy SFGs tend to have higher SFRs and bluer rest-frame U-V colors, while high-mass clumpy SFGs tend to be larger than non-clumpy SFGs. However, clumpy and non-clumpy SFGs have similar Sérsic index, indicating a similar underlying density profile. Furthermore, we investigate how UV luminosity of star-forming regions correlates with the physical properties of host galaxies. On average, more luminous star-forming regions reside in more luminous, smaller, and/or higher-specific SFR galaxies and are found closer to their hosts' galactic center.

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arXiv2023

An excursion into the core of the cluster lens Abell 1689

Abell 1689 is a well studied cluster of galaxies and one of the largest gravitational lens systems ever observed. We have obtained a reconstruction of the cluster Abell 1689 using Grale, a free-form lens inversion method that relies exclusively on the multiple image data. Non-inclusion of any data related to cluster member galaxies ensures an unbiased measure of the mass distribution, which is the most notable feature of free-form methods like Grale. We used two different sets of multiple image systems from the available strong lensing data - one containing only the secure systems (107 images), and the other containing all available systems, only excluding some very non-secure systems (151 images). For the very well-constrained central $\sim$100 kpc region of the cluster we made detailed comparison of the Grale reconstructed lensing mass and stellar mass retrieved by the Spectral Energy Distribution (SED) fitting software FAST++. We found a light-unaccompanied mass peak in this region, whose existence, while tentative, is favored by the distribution of nearby images that are local maxima in the Fermat potential. However, further tests, using different methodologies are needed to confirm the reality of this feature. If it shown to be real, this light-unaccompanied mass peak is consistent with dark matter self-interaction cross-section $σ\lesssim 1$cm$^2$/g, while being in tension with larger cross-sections.

astro-ph.CO↗
arXiv2023

Investigating the Dominant Environmental Quenching Process in UVCANDELS/COSMOS Groups

We explore how the fraction of quenched galaxies changes in groups of galaxies with respect to the distance to the center of the group, redshift, and stellar mass to determine the dominant process of environmental quenching in $0.2 < z < 0.8$ groups. We use new UV data from the UVCANDELS project in addition to existing multiband photometry to derive new galaxy physical properties of the group galaxies from the zCOSMOS 20k Group Catalog. Limiting our analysis to a complete sample of log$(M_*/M_{\odot})>10.56$ group galaxies we find that the probability of being quenched increases slowly with decreasing redshift, diverging from the stagnant field galaxy population. A corresponding analysis on how the probability of being quenched increases with time within groups suggests that the dominant environmental quenching process is characterized by slow ($\sim$Gyr) timescales. We find a quenching time of approximately $4.91^{+0.91}_{-1.47} $Gyrs, consistent with the slow processes of strangulation (Larson et al. 1980) and delayed-then-rapid quenching (Wetzel et al. 2013 arXiv:1206.3571v2 [astro-ph.CO]), although more data are needed to confirm this result.

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arXiv2023

A spatially resolved analysis of star-formation burstiness by comparing UV and H$α$ in galaxies at z$\sim$1 with UVCANDELS

The UltraViolet imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey Fields (UVCANDELS) program provides HST/UVIS F275W imaging for four CANDELS fields. We combine this UV imaging with existing HST/near-IR grism spectroscopy from 3D-HST$+$AGHAST to directly compare the resolved rest-frame UV and H$α$ emission for a sample of 979 galaxies at $0.7 1.5$), suggesting that bursty star-formation is likely prevalent in the outskirts of even the most massive galaxies but is likely over-shadowed by their brighter cores. Furthermore, we present the UV-to-H$α$ ratio as a function of galaxy surface brightness, a proxy for stellar mass surface density, and find that regions below $\sim$10$^{7.5}$ M$_\odot$ kpc$^{-2}$ are consistent with bursty star-formation, regardless of their galaxy stellar mass, potentially suggesting that local star-formation is independent of global galaxy properties at the smallest scales. Lastly, we find galaxies at $z>1.1$ to have bursty star-formation regardless of radius or surface brightness.

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arXiv2023

Fraction of Clumpy Star-Forming Galaxies at $0.5\leq z\leq 3$ in UVCANDELS: Dependence on Stellar Mass and Environment

High-resolution imaging of galaxies in rest-frame UV has revealed the existence of giant star-forming clumps prevalent in high redshift galaxies. Studying these sub-structures provides important information about their formation and evolution and informs theoretical galaxy evolution models. We present a new method to identify clumps in galaxies' high-resolution rest-frame UV images. Using imaging data from CANDELS and UVCANDELS, we identify star-forming clumps in an HST/F160W$\leq 25$ AB mag sample of 6767 galaxies at $0.5\leq z\leq 3$ in four fields, GOODS-N, GOODS-S, EGS, and COSMOS. We use a low-pass band filter in Fourier space to reconstruct the background image of a galaxy and detect small-scale features (clumps) on the background-subtracted image. Clumpy galaxies are defined as those having at least one off-center clump that contributes a minimum of 10$\%$ of the galaxy's total rest-frame UV flux. We measure the fraction of clumpy galaxies ($\rm f_{clumpy}$) as a function of stellar mass, redshift, and galaxy environment. Our results indicate that $\rm f_{clumpy}$ increases with redshift, reaching $\sim 65\%$ at $z\sim 1.5$. We also find that $\rm f_{clumpy}$ in low-mass galaxies ($\rm 9.5\leq log(M_*/M_\odot)\leq 10$) is 10$\%$ higher compared to that of their high-mass counterparts ($\rm log(M_*/M_\odot)>10.5$). Moreover, we find no evidence of significant environmental dependence of $\rm f_{clumpy}$ for galaxies at the redshift range of this study. Our results suggest that the fragmentation of gas clouds under violent disk instability remains the primary driving mechanism for clump formation, and incidents common in dense environments, such as mergers, are not the dominant processes.

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arXiv2023

CEERS: Spatially Resolved UV and mid-IR Star Formation in Galaxies at 0.2 < z < 2.5: The Picture from the Hubble and James Webb Space Telescopes

We present the mid-IR (MIR) morphologies for 64 star-forming galaxies at $0.2 10^{9}~M_\odot}$ using JWST MIRI observations from the Cosmic Evolution Early Release Science survey (CEERS). The MIRI bands span the MIR (7.7--21~$μ$m), enabling us to measure the effective radii ($R_{\rm{eff}}$) and Sérsic indexes of these SFGs at rest-frame 6.2 and 7.7 $μ$m, which contains strong emission from Polycyclic aromatic hydrocarbon (PAH) features, a well-established tracer of star formation in galaxies. We define a ``PAH-band'' as the MIRI bandpass that contains these features at the redshift of the galaxy. We then compare the galaxy morphologies in the PAH-bands to those in rest-frame Near-UV (NUV) using HST ACS/F435W or ACS/F606W and optical/near-IR using HST WFC3/F160W imaging from UVCANDELS and CANDELS, where the NUV-band and F160W trace the profile of (unobscured) massive stars and the stellar continuum, respectively. The $R_{\rm{eff}}$ of galaxies in the PAH-band are slightly smaller ($\sim$10\%) than those in F160W for galaxies with $\rm{M_*\gtrsim10^{9.5}~M_\odot}$ at $z\leq1.2$, but the PAH-band and F160W have a similar fractions of light within 1 kpc. In contrast, the $R_{\rm{eff}}$ of galaxies in the NUV-band are larger, with lower fractions of light within 1 kpc compared to F160W for galaxies at $z\leq1.2$. Using the MIRI data to estimate the $\rm{SFR_{\rm{IR}}}$ surface density, we find the correlation between the $\rm{SFR_{\rm{IR}}}$ surface density and stellar mass has a steeper slope than that of the $\rm{SFR_{\rm{UV}}}$ surface density and stellar mass, suggesting more massive galaxies having increasing amounts of obscured fraction of star formation in their inner regions. This paper demonstrates how the high-angular resolution data from JWST/MIRI can reveal new information about the morphology of obscured-star formation.

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arXiv2023

The Direct-Method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High-Noon

We present a Keck/MOSFIRE, rest-optical, composite spectrum of 16 typical, gravitationally-lensed, star-forming, dwarf galaxies at $1.7 \lesssim z \lesssim 2.6$ ($z_{\rm{mean}}=2.30$), all chosen independent of emission-line strength. These galaxies have a median stellar mass of $\log(M_\ast/\rm{M_\odot})_{\rm{med}} = 8.29^{+0.51}_{-0.43}$ and a median star formation rate of $\rm{SFR_{Hα}^{med} = 2.25^{+2.15}_{-1.26}\ M_\odot\ yr^{-1}}$. We measure the faint, electron-temperature-sensitive, [O III] $λ$4363 emission line at $2.5σ$ ($4.1σ$) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of $12+\log(\rm{O/H})_{\rm{direct}}=7.88^{+0.25}_{-0.22}$ ($0.15^{+0.12}_{-0.06}\ \rm{Z_\odot}$). We investigate the applicability at high-$z$ of locally-calibrated, oxygen-based, strong-line metallicity relations, finding that the local reference calibrations of arXiv:1805.08224 best reproduce ($\lesssim 0.12$ dex) our composite metallicity at fixed strong-line ratio. At fixed $M_\ast$, our composite is well-represented by the $z \sim 2.3$ direct-method stellar mass$\,-\,$gas-phase metallicity relation (MZR) of arXiv:1907.00013. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed $M_\ast$ and SFR, of the locally-defined fundamental metallicity relation. We measure the doublet ratio [O II] $λ$3729/[O II] $\lambda3726 = 1.56 \pm 0.32$ ($1.51 \pm 0.12$) and a corresponding electron density of $n_e = 1^{+215}_{-0}\ \rm{cm^{-3}}$ ($n_e = 1^{+74}_{-0}\ \rm{cm^{-3}}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies at $z \sim 2$.

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arXiv2022

Lyman Continuum Galaxy Candidates in COSMOS

Star-forming galaxies are the sources likely to have reionized the universe. As we cannot observe them directly due to the opacity of the intergalactic medium at $z\gtrsim5$, we study $z\sim3\text{--}5$ galaxies as proxies to place observational constraints on cosmic reionization. Using new deep \textit{Hubble Space Telescope} rest-frame UV F336W and F435W imaging (30-orbit, $\sim40$~arcmin$^2$, $\sim29\text{--}30$~mag depth at 5$σ$), we attempt to identify a sample of Lyman continuum galaxies (LCGs). These are individual sources that emit ionizing flux below the Lyman break ($<912~\textÅ$). This population would allow us to constrain cosmic reionization parameters such as the number density and escape fraction ($f_{\rm esc}$) of ionizing sources. We compile a comprehensive parent sample that does not rely on the Lyman-break technique for redshifts. We present three new spectroscopic candidates at $z\sim3.7\text{--}4.4$, and 32 new photometric candidates. The high-resolution multi-band HST imaging and new Keck/Low Resolution Imaging Spectrometer (LRIS) redshifts make these promising spectroscopic LCG candidates. Using both a traditional and probabilistic approach, we find the most likely $f_{\rm esc}$ values for the three spectroscopic LCG candidates are $>100\%$, and therefore not physical. We are unable to confirm the true nature of these sources with the best available imaging and direct blue Keck/LRIS spectroscopy. More spectra, especially from the new class of 30 m telescopes, will be required to build a statistical sample of LCGs to place firm observational constraints on cosmic reionization.

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arXiv2021

The mass-metallicity relation at cosmic noon in overdense environments: first results from the MAMMOTH-Grism HST slitless spectroscopic survey

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 $z=2.24\pm0.02$, We secure a sample of 36 protocluster member galaxies at $z\sim2.24$, showing strong nebular emission lines ([O III], H$β$ and [O II]) in their G141 spectra. Using the multi-wavelength broad-band deep imaging from HST and ground-based telescopes, we measure their stellar masses in the range of $[10^{9},10^{10.4}]M_\odot$, instantaneous star formation rates (SFR) from 10 to 240$M_\odot yr^{-1}$, and global gas-phase metallicities [$\frac{1}{3}$,1] of solar. Compared with similarly selected field galaxy sample at the same redshift, our galaxies show on average increased SFRs by $\sim$0.06dex and $\sim$0.18dex at $\sim$10$^{10.1}M_\odot$ and $\sim$10$^{9.8}M_\odot$, respectively. Using the stacked spectra of our sample galaxies, we derive the MZR in the BOSS1244 protocluster core as $12+\log({\rm O/H})=(0.136\pm0.018)\times\log(M_\ast/M_\odot)+(7.082\pm0.175)$, showing 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.

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arXiv2021

Spectroscopically Identified Emission Line Galaxy Pairs in the WISP survey

We identify a sample of spectroscopically measured emission line galaxy (ELG) pairs up to z=1.6 from the WFC3 Infrared Spectroscopic Parallels (WISP) survey. WISP obtained slitless, near-infrared grism spectroscopy along with direct imaging in the J and H bands by observing in the pure-parallel mode with the Wide Field Camera Three (WFC3) on the Hubble Space Telescope (HST). From our search of 419 WISP fields covering an area of ~0.5 deg$^{2}$, we find 413 ELG pair systems, mostly Halpha emitters. We then derive reliable star formation rates (SFRs) based on the attenuation-corrected Halpha fluxes. Compared to isolated galaxies, we find an average SFR enhancement of 40%-65%, which is stronger for major pairs and pairs with smaller velocity separations (Delta_v < 300 km/s). Based on the stacked spectra from various subsamples, we study the trends of emission line ratios in pairs, and find a general consistency with enhanced lower-ionization lines. We study the pair fraction among ELGs, and find a marginally significant increase with redshift $f \propto (1+z)^α$, where the power-law index α=0.58$\pm$0.17 from $z\sim$0.2 to $z\sim$1.6. The fraction of Active galactic Nuclei (AGNs), is found to be the same in the ELG pairs as compared to isolated ELGs.

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arXiv2021

The High Latitude Spectroscopic Survey on the Nancy Grace Roman Space Telescope

The Nancy Grace Roman Space Telescope will conduct a High Latitude Spectroscopic Survey (HLSS) over a large volume at high redshift, using the near-IR grism (1.0-1.93 $μ$m, $R=435-865$) and the 0.28 deg$^2$ wide field camera. We present a reference HLSS which maps 2000 deg$^2$ and achieves an emission line flux limit of 10$^{-16}$ erg/s/cm$^2$ at 6.5$σ$, requiring $\sim$0.6 yrs of observing time. We summarize the flowdown of the Roman science objectives to the science and technical requirements of the HLSS. We construct a mock redshift survey over the full HLSS volume by applying a semi-analytic galaxy formation model to a cosmological N-body simulation, and use this mock survey to create pixel-level simulations of 4 deg$^2$ of HLSS grism spectroscopy. We find that the reference HLSS would measure $\sim$ 10 million H$α$ galaxy redshifts that densely map large scale structure at $z=1-2$ and 2 million [OIII] galaxy redshifts that sparsely map structures at $z=2-3$. We forecast the performance of this survey for measurements of the cosmic expansion history with baryon acoustic oscillations and the growth of large scale structure with redshift space distortions. We also study possible deviations from the reference design, and find that a deep HLSS at $f_{\rm line}>7\times10^{-17}$erg/s/cm$^2$ over 4000 deg$^2$ (requiring $\sim$1.5 yrs of observing time) provides the most compelling stand-alone constraints on dark energy from Roman alone. This provides a useful reference for future optimizations. The reference survey, simulated data sets, and forecasts presented here will inform community decisions on the final scope and design of the Roman HLSS.

astro-ph.CO↗
arXiv2020

Lyman Continuum Escape Fraction from Low-mass Starbursts at z=1.3

We present a new constraint on the Lyman Continuum (LyC) escape fraction at z~1.3. We obtain deep, high sensitivity far-UV imaging with the Advanced Camera for Surveys (ACS) Solar Blind Channel (SBC) on the Hubble Space Telescope (HST), targeting 11 star-forming galaxies at 1.2 190 Å, low stellar mass (M* < 10^10 M_sun) and U-band magnitude of U<24.2. These criteria identify young, low metallicity star bursting populations similar to the primordial star-forming galaxies believed to have reionized the universe. We do not detect any LyC signal (with S/N >3) in the individual galaxies or in the stack in the far-UV images. We place $3σ$ limits on the relative escape fraction of individual galaxies to be f_{esc,rel}<[0.10-0.22] and a stacked $3σ$ limit of f_{esc,rel}<0.07. Comparing to the confirmed LyC emitters from the literature, the galaxies in our sample span similar ranges of various galaxy properties including stellar mass, dust attenuation, and star formation rate (SFR). In particular, we compare the distribution of H$α$ and [OIII] EWs of confirmed LyC emitters and non-detections including the galaxies in this study. Finally, we discuss if a dichotomy seen in the distribution of H$α$ EWs can perhaps distinguish the LyC emitters from the non-detections.

astro-ph.GA↗
arXiv2020

The Ionizing Photon Production Efficiency ($ξ_{ion}$) Of Lensed Dwarf Galaxies At $z \sim 2 $

We measure the ionizing photon production efficiency ($ξ_{ion}$) of low-mass galaxies ($10^{7.8}$-$10^{9.8}$ $M_{\odot}$) at $1.4<z<2.7$, allowing us to better understand the contribution of dwarf galaxies to the ionizing background and cosmic reionization. We target galaxies that are magnified by the strong lensing galaxy clusters Abell 1689, MACS J0717, and MACS J1149. We utilize Keck/MOSFIRE spectra to measure optical nebular emission line fluxes and HST imaging to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log(L$_{Hα}$ /L$_{UV}$) of galaxies in our sample to determine the typical log($ξ_{ion}$). Second, we take the logarithm of the total L$_{Hα}$ over the total L$_{UV}$. We prefer the latter as it provides the total ionizing UV luminosity density of galaxies when multiplied by the non-ionizing UV luminosity density from the UV luminosity function. log($ξ_{ion}$) calculated from the second method is $\sim$ 0.2 dex higher than the first method. We do not find any strong dependence between log($ξ_{ion}$) and stellar mass, M$_{UV}$ or UV spectral slope ($β$). We report a value of log($ξ_{ion}$) $\sim25.47\pm 0.09$ for our UV-complete sample ($-22<M_{UV}<-17.3$) and $\sim25.37\pm0.11$ for our mass-complete sample ($7.8<\log(M_*)<9.8)$. These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log($ξ_{ion}$) is $0.2-0.3$ dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity, age, or the prevalence of binary stars. We also find a correlation between log($ξ_{ion}$) and the equivalent widths of H$α$ and [OIII]$λ$5007 fluxes, confirming that these equivalent widths can be used to estimate $ξ_{ion}$.

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