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Allison L. Strom

Publications and source records attributed to Allison L. Strom.

48 records · Page 3Linked to original sources
arXiv2018

Imaging Spectroscopy of Ionized Gaseous Nebulae around Optically Faint AGN at Redshift z ~ 2

We present Keck/OSIRIS laser guide-star assisted adaptive optics (LGSAO) integral field spectroscopy of [O III] 5007 Angstrom nebular emission from twelve galaxies hosting optically faint (R = 20 - 25; nuLnu ~ 10^44 -10^46 erg/s) active galactic nuclei (AGN) at redshift z ~ 2 - 3. In combination with deep Hubble Space Telescope Wide Field Camera 3 rest-frame optical imaging, Keck/MOSFIRE rest-optical spectroscopy, and Keck/KCWI rest-UV integral field spectroscopy we demonstrate that both the continuum and emission-line structure of these sources exhibit a wide range of morphologies from compact isolated point sources to double-AGN merging systems with extensive ~ 50 kpc tidal tails. One of the twelve galaxies previously known to exhibit a proximate damped LyA system coincident in redshift with the galaxy shows evidence for both an extended [O III] narrow-line emission region and spatially offset LyA emission (with morphologically distinct blueshifted and redshifted components) indicative of large scale gas flows photoionized by the central AGN. We do not find widespread evidence of star formation in the host galaxies surrounding these AGN; the [O III] velocity dispersions tend to be high (sigma = 100 - 500 km/s), the continuum morphologies are much more compact than a mass-matched star forming comparison sample, and the diagnostic nebular emission line ratios are dominated by an AGN-like ionizing spectrum. The sample is most consistent with a population of AGN that radiate at approximately their Eddington limit and photoionize extended [O III] nebulae whose characteristic sizes scale approximately as the square root of the AGN luminosity.

astro-ph.GA↗
arXiv2018

The redshift evolution of rest-UV spectroscopic properties in Lyman Break Galaxies at z ~ 2-4

We present the first comprehensive evolutionary analysis of the rest-frame UV spectroscopic properties of star-forming galaxies at z ~ 2-4. We match samples at different redshifts in UV luminosity and stellar mass, and perform systematic measurements of spectral features and stellar population modeling. By creating composite spectra grouped according to Ly$α$ equivalent width (EW), and various galaxy properties, we study the evolutionary trends among Ly$α$, low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Ly$α$ and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Ly$α$, LIS lines, and dust extinction are redshift-independent, as is the decoupling of Ly$α$ and HIS line strengths, and the bulk outflow kinematics as traced by LIS lines. Stronger Ly$α$ emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in average Ly$α$ strength with redshift, and the variation in Ly$α$ strength at fixed redshift, can be explained in terms of variations in neutral gas covering fraction and/or dust content in the ISM and CGM. However, based on the connection between Ly$α$ and CIII] emission strengths, we additionally find evidence for variations in the intrinsic production rate of Ly$α$ photons at the highest Ly$α$ EWs. The challenge now is to understand the observed evolution in neutral gas covering fraction and dust extinction within a coherent model for galaxy formation, and make robust predictions for the escape of ionizing radiation at z > 6.

astro-ph.GA↗
arXiv2017

Nebular Emission Line Ratios in z~2-3 Star-Forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio

We present a detailed study of the rest-optical (3600-7000 Angstrom) nebular spectra of ~380 star-forming galaxies at z~2-3 obtained with Keck/MOSFIRE as part of the Keck Baryonic Structure Survey (KBSS). The KBSS-MOSFIRE sample is representative of star-forming galaxies at these redshifts, with stellar masses M*=10^9-10^11.5 M_sun and star formation rates SFR=3-1000 M_sun/yr. We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: [O II]3727,3729, [Ne III]3869, H-beta, [O III]4960,5008, [N II]6549,6585, H-alpha, and [S II]6718,6732. Comparisons with observations of typical local galaxies from the Sloan Digital Sky Survey (SDSS) and between subsamples of KBSS-MOSFIRE show that high-redshift galaxies exhibit a number of significant differences in addition to the well-known offset in log([O III]/H-beta) and log([N II]/H-alpha). We argue that the primary difference between H II regions in z~2.3 galaxies and those at z~0 is an enhancement in the degree of nebular excitation, as measured by [O III]/H-beta and R23=log[([O III]+[O II])/H-beta]. At the same time, KBSS-MOSFIRE galaxies are ~10 times more massive than z~0 galaxies with similar ionizing spectra and have higher N/O (likely accompanied by higher O/H) at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed N/O and O/H relative to typical z~0 galaxies, consistent with Fe-poor stellar population models that include massive binaries, and highlight a population of massive, high-specific star formation rate galaxies at high-redshift with systematically different star formation histories than galaxies of similar stellar mass today.

astro-ph.GA↗
arXiv2016

The Rest-Frame Optical Spectroscopic Properties of Ly$α$-Emitters at $z\sim2.5$: The Physical Origins of Strong Ly$α$ Emission

We present rest-frame optical spectra of 60 faint ($R_{AB}\sim 27$; $L\sim0.1 L_*$) Ly$α$-selected galaxies (LAEs) at $z\approx2.56$. The average LAE is consistent with the extreme low-metallicity end of the continuum-selected galaxy distribution at $z\approx2-3$. In particular, the LAEs have extremely high [OIII] $λ$5008/H$β$ ratios (log([OIII]/H$β$) $\sim$ 0.8) and low [NII] $λ$6585/H$α$ ratios (log([NII]/H$α$) $<-1.15$). Using the [OIII] $λ$4364 auroral line, we find that the star-forming regions in faint LAEs are characterized by high electron temperatures ($T_e\approx1.8\times10^4$K), low oxygen abundances (12 + log(O/H) $\approx$ 8.04, $Z_{neb}\approx0.22Z_\odot$), and high excitations with respect to more luminous galaxies. Our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) $\approx$ 6.9$-$7.4 ($Z_{neb}\approx0.02-0.05Z_\odot$). We interpret these observations in light of new models of stellar evolution (including binary interactions). We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionization-era galaxies. Furthermore, we investigate physical trends accompanying Ly$α$ emission across the largest current sample of combined Ly$α$ and rest-optical galaxy spectroscopy, including 60 faint LAEs and 368 more luminous galaxies at similar redshifts. We find that Ly$α$ emission is strongly correlated with nebular excitation and ionization and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content.

astro-ph.GA↗
arXiv2016

Q1549-C25: A Clean Source of Lyman-Continuum Emission at $z=3.15$

We present observations of Q1549-C25, an ~L* star-forming galaxy at z=3.15 for which Lyman-continuum (LyC) radiation is significantly detected in deep Keck/LRIS spectroscopy. We find no evidence for contamination from a lower-redshift interloper close to the line of sight in the high signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25 in V_606, J_125, and H_160 Hubble Space Telescope (HST) imaging reveals that the object consists of a single, isolated component within 1". In combination, these data indicate Q1549-C25 as a clean spectroscopic detection of LyC radiation, only the second such object discovered to date at z~3. We model the spectral energy distribution (SED) of Q1549-C25, finding evidence for negligible dust extinction, an age (assuming continuous star formation) of ~1 Gyr, and a stellar mass of M_*=7.9x10^9 M_sun. Although it is not possible to derive strong constraints on the absolute escape fraction of LyC emission, f_esc(LyC), from a single object, we use simulations of intergalactic and circumgalactic absorption to infer f_esc(LyC)>=0.51 at 95% confidence. The combination of deep Keck/LRIS spectroscopy and HST imaging is required to assemble a larger sample of objects like Q1549-C25, and obtain robust constraints on the average f_esc(LyC) at z~3 and beyond.

astro-ph.GA↗
arXiv2016

A High Fraction of Ly-alpha-Emitters Among Galaxies with Extreme Emission Line Ratios at z ~ 2

Star-forming galaxies form a sequence in the [OIII]/H-beta vs. [NII]/H-alpha diagnostic diagram, with low metallicity, highly ionized galaxies falling in the upper left corner. Drawing from a large sample of UV-selected star-forming galaxies at z~2 with rest-frame optical nebular emission line measurements from Keck-MOSFIRE, we select the extreme ~5% of the galaxies lying in this upper left corner, requiring log([NII]/H-alpha) <= 1.1 and log([OIII]/H-beta) >= 0.75. These cuts identify galaxies with 12 + log(O/H) <~ 8.0, when oxygen abundances are measured via the O3N2 diagnostic. We study the Ly-alpha properties of the resulting sample of 14 galaxies. The mean (median) rest-frame Ly-alpha equivalent width is 39 (36) A, and 11 of the 14 objects (79%) are Ly-alpha-emitters (LAEs) with W_Lya > 20 A. We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at 2.0<z<2.6 identified without regard to their optical line ratios; this sample has mean (median) Ly-alpha equivalent width -1 (-4) A, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at Ly-alpha than those in the comparison sample, and have ~50% lower median oxygen abundances. Both factors are likely to facilitate the escape of Ly-alpha: in less dusty galaxies Ly-alpha photons are less likely to be absorbed during multiple scatterings, while the harder ionizing spectrum and higher ionization parameter associated with strong, low metallicity star formation may reduce the covering fraction or column density of neutral hydrogen, further easing Ly-alpha escape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to Ly-alpha photons across a range of redshifts.

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arXiv2015

Detection of hot, metal-enriched outflowing gas around $z\approx\,$2.3 star-forming galaxies in the Keck Baryonic Structure Survey

We use quasar absorption lines to study the physical conditions in the circumgalactic medium of redshift $z\approx 2.3$ star-forming galaxies taken from the Keck Baryonic Structure Survey (KBSS). In Turner et al. 2014 we used the pixel optical depth technique to show that absorption by HI and the metal ions OVI, NV, CIV, CIII and SiIV is strongly enhanced within $|Δv|\lesssim170$ km/s and projected distances $|d|\lesssim180$ proper kpc from sightlines to the background quasars. Here we demonstrate that the OVI absorption is also strongly enhanced at fixed HI, CIV, and SiIV optical depths, and that this enhancement extends out to $\sim350$ km/s. At fixed HI the increase in the median OVI optical depth near galaxies is 0.3-0.7 dex and is detected at 2--3-$σ$ confidence for all seven HI bins that have $\log_{10}τ_{\rm HI}\ge-1.5$. We use ionization models to show that the observed strength of OVI as a function of HI is consistent with enriched, photoionized gas for pixels with $τ_{\rm HI}\gtrsim10$. However, for pixels with $τ_{\rm HI} \lesssim 1$ this would lead to implausibly high metallicities at low densities if the gas were photoionized by the background radiation. This indicates that the galaxies are surrounded by gas that is sufficiently hot to be collisionally ionized ($T > 10^5\,$K) and that a substantial fraction of the hot gas has a metallicity $\gtrsim 10^{-1}$ of solar. Given the high metallicity and large velocity extent (out to $\sim1.5\times v_{\rm circ}$) of this gas, we conclude that we have detected hot, metal enriched outflows arising from star-forming galaxies.

astro-ph.GA↗
arXiv2015

Correlation between star formation activity and electron density of ionized gas at z=2.5

In the redshift interval of $2<z<3$, the physical conditions of the inter-stellar medium (ISM) in star-forming galaxies are likely to be different from those in the local Universe because of lower gaseous metallicities, higher gas fractions, and higher star formation activities. In fact, observations suggest that higher electron densities, higher ionization parameters, and harder UV radiation fields are common. In this paper, based on the spectra of H$α$-selected star-forming galaxies at $z=2.5$ taken with Multi-Object Spectrometer for InfraRed Exploration (MOSFIRE) on Keck-1 telescope, we measure electron densities ($n_e$) using the oxygen line ratio ( [OII]$λλ$3726,3729), and investigate the relationships between the electron density of ionized gas and other physical properties. As a result, we find that the specific star formation rate (sSFR) and the surface density of SFR ($Σ_\mathrm{SFR}$) are correlated with the electron density at $z=2.5$ for the first time. The $Σ_\mathrm{SFR}-n_e$ relation is likely to be linked to the star formation law in HII regions (where star formation activity is regulated by interstellar pressure). Moreover, we discuss the mode of star formation in those galaxies. The correlation between sSFR and $Σ_\mathrm{SFR}$ suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation ($Σ_\mathrm{SFR}$) and thus higher $n_e$ values as well.

astro-ph.CO↗
arXiv2015

Metal-line absorption around $z\approx$2.4 star-forming galaxies in the Keck Baryonic Structure Survey

(Abridged) We study metal absorption around 854 $z\approx$2.4 star-forming galaxies taken from the Keck Baryonic Structure Survey (KBSS). The galaxies examined in this work lie in the fields of 15 hyper-luminous background QSOs, with galaxy impact parameters ranging from 35 proper kpc (pkpc) to 2 proper Mpc (pMpc). Using the pixel optical depth technique, we present the first galaxy-centred 2-D maps of the median absorption by OVI, NV, CIV, CIII, and SIV, as well as updated results for HI. At small galactocentric radii we detect a strong enhancement of the absorption relative to randomly located regions that extend out to at least 180 pkpc in the transverse direction, and $\pm$240 km/s along the line-of-sight (LOS, ~1 pMpc in the case of pure Hubble flow) for all ions except NV. For CIV (and HI) we detect a significant enhancement of the absorption signal out to 2 pMpc in the transverse direction, corresponding to the maximum impact parameter in our sample. After normalising the median absorption profiles to account for variations in line strengths and detection limits, in the transverse direction we find no evidence for a sharp drop-off in metals distinct from that of HI. We argue instead that non-detection of some metal line species in the extended circumgalactic medium is consistent with differences in the detection sensitivity. We also present measurements of covering fractions and equivalent widths as a function of projected galaxy distance. Limiting the sample to the 340 galaxies with redshifts measured from nebular emission lines does not decrease the extent of the enhancement along the LOS compared to that in the transverse direction. This rules out redshift errors as the source of the observed redshift-space anisotropy and thus implies that we have detected the signature of gas peculiar velocities from infall, outflows, or virial motions for HI, OVI, CIV, CIII, and CIV.

astro-ph.CO↗
arXiv2014

The Ly-alpha Properties of Faint Galaxies at z2-3 with Systemic Redshifts and Velocity Dispersions from Keck-MOSFIRE

We study the Lya profiles of 36 spectroscopically-detected Lya-emitters (LAEs) at z2-3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical nebular emission lines. The sample has a median optical magnitude R=26.0, and ranges from R23 to R>27, corresponding to rest-frame UV absolute magnitudes M_UV-22 to M_UV>-18.2. Dynamical masses range from M_dyn<1.3e8 Msun to M_dyn=6.8e9 Msun, with a median value of M_dyn=6.3e8 Msun. Thirty of the 36 Lya emission lines are redshifted with respect to the systemic velocity with at least 1 sigma significance, and the velocity offset with respect to systemic dv_Lya is correlated with R-band magnitude, M_UV, and the velocity dispersion measured from nebular emission lines with >3 sigma significance: brighter galaxies with larger velocity dispersions tend to have larger values of dv_Lya. We also make use of a comparison sample of 122 UV-color-selected R<25.5 galaxies at z2, all with Lya emission and systemic redshifts measured from nebular emission lines. Using the combined LAE and comparison samples for a total of 158 individual galaxies, we find that dv_Lya is anti-correlated with the Lya equivalent width with 7 sigma significance. Our results are consistent with a scenario in which the Lya profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Lya photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars.

astro-ph.GA↗
arXiv2010

Submillimeter Variability and the Gamma-ray Connection in Fermi Blazars

We present multi-epoch observations from the Submillimeter Array (SMA) for a sample of 171 bright blazars, 43 of which were detected by Fermi during the first three months of observations. We explore the correlation between their gamma-ray properties and submillimeter observations of their parsec-scale jets, with a special emphasis on spectral index in both bands and the variability of the synchrotron component. Subclass is determined using a combination of Fermi designation and the Candidate Gamma-Ray Blazar Survey (CGRaBS), resulting in 35 BL Lac objects and 136 flat-spectrum radio quasars (FSRQs) in our total sample. We calculate submillimeter energy spectral indices using contemporaneous observations in the 1 mm and 850 micron bands during the months August-October 2008. The submillimeter light curves are modeled as first-order continuous autoregressive processes, from which we derive characteristic timescales. Our blazar sample exhibits no differences in submillimeter variability amplitude or characteristic timescale as a function of subclass or luminosity. All of the the light curves are consistent with being produced by a single process that accounts for both low and high states, and there is additional evidence that objects may be transitioning between blazar class during flaring epochs.

astro-ph.HE↗