arXiv subjects

Anna Feltre

Publications and source records attributed to Anna Feltre.

50 records · Page 3Linked to original sources

arXiv2018

The large and small scale properties of the intergalactic gas in the Slug Ly-alpha nebula revealed by MUSE HeII emission observations

With a projected size of about 450 kpc at z~2.3, the Slug Ly-alpha nebula is a rare laboratory to study, in emission, the properties of the intergalactic gas in the Cosmic Web. Since its discovery, the Slug has been the subject of several spectroscopic follow-ups to constrain the properties of the emitting gas. Here we report the results of a deep MUSE integral-field spectroscopic search for non-resonant, extended HeII1640 and metal emission. Extended HeII radiation is detected on scales of about 100 kpc, but only in some regions associated with the bright Ly-alpha emission and a continuum-detected source, implying large and abrupt variations in the line ratios across adjacent regions in projected space. The recent detection of associated H-alpha emission and similar abrupt variations in the Ly-alpha kinematics, strongly suggest that the HeII/Ly-alpha gradient is due to large variations in the physical distances between the associated quasar and these regions. This implies that the overall length of the emitting structure could extend to physical Mpc scales and be mostly oriented along our line of sight. At the same time, the relatively low HeII/Ly-alpha values suggest that the emitting gas has a broad density distribution that - if expressed in terms of a lognormal - implies dispersions as high as those expected in the interstellar medium of galaxies. These results strengthen the possibility that the density distribution of intergalactic gas at high-redshift is extremely clumpy and multiphase on scales below our current observational spatial resolution of a few physical kpc.

astro-ph.GA↗

arXiv2018

Synthetic nebular emission from massive galaxies II: ultraviolet-line diagnostics of dominant ionizing sources

We compute synthetic optical and ultraviolet (UV) emission-line properties of galaxies in a full cosmological framework by coupling, in post-processing, new-generation nebular-emission models with high-resolution, cosmological zoom-in simulations of massive galaxies. Our self-consistent modelling accounts for nebular emission from young stars and accreting black holes (BHs). We investigate which optical- and UV-line diagnostic diagrams can best help to discern between the main ionizing sources, as traced by the ratio of BH accretion to star formation rates in model galaxies, over a wide range of redshifts. At low redshift, simulated star-forming galaxies, galaxies dominated by active galactic nuclei and composite galaxies are appropriately differentiated by standard selection criteria in the classical [OIII]$λ$5007/H$β$ versus [NII]$λ$6584/H$α$ diagram. At redshifts $z \gt 1$, however, this optical diagram fails to discriminate between active and inactive galaxies at metallicities below $0.5\ Z_\odot$. To robustly classify the ionizing radiation of such metal-poor galaxies, which dominate in the early Universe, we confirm 3 previous, and propose 11 novel diagnostic diagrams based on equivalent widths and luminosity ratios of UV emission lines, such as EW(OIII]$λ$1663) versus OIII]$λ$1663/HeII$λ$1640, CIII]$λ$1908/HeII$λ$1640 versus OIII]$λ$1663/HeII$λ$1640, and CIV$λ$1550/CIII]$λ$1908 versus CIII]$λ$1908/CII$λ$2326. We formulate associated UV selection criteria and discuss some caveats of our results (e.g., uncertainties in the modelling of the HeII$λ$1640 line). These UV diagnostic diagrams are potentially important for the interpretation of high-quality spectra of very distant galaxies to be gathered by next-generation telescopes, such as the James Webb Space Telescope.

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arXiv2018

The MUSE Hubble Ultra Deep Field Survey: XII. MgII emission and absorption in star-forming galaxies

The physical origin of the near-ultraviolet MgII emission remains an under-explored domain, contrary to more typical emission lines detected in the spectra of star-forming galaxies. We explore the nebular and physical properties for a sample of 381 galaxies between 0.70 < z < 2.34 drawn from the MUSE Hubble Ultra Deep Survey. The spectra of these galaxies show a wide variety of profiles of the MgII 2796,2803 resonant doublet, from absorption to emission. We present a study on the main drivers for the detection of MgII emission in galaxy spectra. By exploiting photoionization models we verified that the emission-line ratios observed in galaxies with MgII in emission are consistent with nebular emission from HII regions. From a simultaneous analysis of MUSE spectra and ancillary HST information via spectral energy distribution (SED) fitting, we find that galaxies with MgII in emission have lower stellar masses, smaller sizes, bluer spectral slopes and lower optical depth than those with absorption. This leads us to suggest that MgII emission is a potential tracer of physical conditions not merely related to those of the ionized gas. We show that these differences in MgII emission/absorption can be explained in terms of a higher dust and neutral gas content in the interstellar medium (ISM) of galaxies showing MgII in absorption, confirming the extreme sensitivity of MgII to the presence of the neutral ISM. We conclude with an analogy between the MgII doublet and the Ly-alpha line, due to their resonant nature. Further investigations with current and future facilities, including JWST, are promising as the detection of MgII emission and its potential connection with Ly-alpha could provide new insights on the ISM content in the early Universe.

astro-ph.GA↗

arXiv2018

Physical properties and H-ionizing-photon production rates of extreme nearby star-forming regions

Measurements of the galaxy UV luminosity function at z>6 suggest that young stars hosted in low-mass star-forming galaxies produced the bulk of hydrogen-ionizing photons necessary to reionize the intergalactic medium (IGM) by redshift z~6. Whether star-forming galaxies dominated cosmic reionization, however, also depends on their stellar populations and interstellar medium properties, which set, among other things, the production rate of H-ionizing photons, $ξ_\text{ion}^\star$, and the fraction of these escaping into the IGM. Given the difficulty of constraining with existing observatories the physical properties of z>6 galaxies, in this work we focus on a sample of ten nearby objects showing UV spectral features comparable to those observed at z>6. We use the new-generation Beagle tool to model the UV-to-optical photometry and UV/optical emission lines of these Local 'analogues' of high-redshift galaxies, finding that our relatively simple, yet fully self-consistent, physical model can successfully reproduce the different observables considered. Our galaxies span a broad range of metallicities and are characterised by high ionization parameters, low dust attenuation, and very young stellar populations. Through our analysis, we derive a novel diagnostic of the production rate of H-ionizing photons per unit UV luminosity, $ξ_\text{ion}^\star$, based on the equivalent width of the bright $[\text{OIII}] λ4959,5007$ doublet, which does not require measurements of H-recombination lines. This new diagnostic can be used to estimate $ξ_\text{ion}^\star$ from future direct measurements of the $[\text{OIII}] λ4959,5007$ line using JWST/NIRSpec (out to z~9.5), and by exploiting the contamination by $\text{H}β+ [\text{OIII}] λ4959,5007$ of photometric observations of distant galaxies, for instance from existing Spitzer/IRAC data and from future ones with JWST/NIRCam.

astro-ph.GA↗

arXiv2017

The MUSE Hubble Ultra Deep Field Survey: VII. FeII* Emission in Star-Forming Galaxies

Non-resonant FeII* 2365, 2396, 2612, 2626 emission can potentially trace galactic winds in emission and provide useful constraints to wind models. From the 3'x3' mosaic of the Hubble Ultra Deep Field (UDF) obtained with the VLT/MUSE integral field spectrograph, we identify a statistical sample of 40 FeII* emitters and 50 MgII 2796, 2803 emitters from a sample of 271 [OII] 3726, 3729 emitters with reliable redshifts from z = 0.85 - 1.5 down to 2E-18 (3 sigma) ergs/s/cm^2 (for [OII]), covering the stellar mass range 10^8 - 10^11 Msun. The FeII* and MgII emitters follow the galaxy main sequence, but with a clear dichotomy. Galaxies with masses below 10^9 Msun and star formation rates (SFRs) of <1 Msun/year have MgII emission without accompanying FeII* emission, whereas galaxies with masses above 10^10 Msun and SFRs >10 Msun/year have FeII* emission without accompanying MgII emission. Between these two regimes, galaxies have both MgII and FeII* emission, typically with MgII P-Cygni profiles. Indeed, the MgII profile shows a progression along the main sequence from pure emission to P-Cygni profiles to strong absorption, due to resonant trapping. Combining the deep MUSE data with HST ancillary information, we find that galaxies with pure MgII emission profiles have lower star formation rate surface densities than those with either MgII P-Cygni profiles or FeII* emission. These spectral signatures produced through continuum scattering and fluorescence, MgII P-Cygni profiles and FeII* emission, are better candidates for tracing galactic outflows than pure MgII emission, which may originate from HII regions. We compare the absorption and emission rest-frame equivalent widths for pairs of FeII transitions to predictions from outflow models and find that the observations consistently have less total re-emission than absorption, suggesting either dust extinction or non-isotropic outflow geometries.

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arXiv2017

The MUSE Hubble Ultra Deep Field Survey: IV. Global properties of C III] emitters

The C III] 1907,1909 emission doublet has been proposed as an alternative to Lyman-alpha in redshift confirmations of galaxies at z > 6 since it is not attenuated by the largely neutral intergalactic medium at these redshifts and is believed to be strong in the young, vigorously star-forming galaxies present at these early cosmic times. We present a statistical sample of 17 C III]-emitting galaxies beyond z~1.5 using 30 hour deep VLT/MUSE integral field spectroscopy covering 2 square arcminutes in the Hubble Deep Field South (HDFS) and Ultra Deep Field (UDF), achieving C III] sensitivities of ~2e-17 erg/s/cm^2 in the HDFS and ~7e-18 erg/s/cm^2 in the UDF. The rest-frame equivalent widths range from 2 to 19 Angstroms. These 17 galaxies represent ~3% of the total sample of galaxies found between 1.5 < z < 4. They also show elevated star formation rates, lower dust attenuation, and younger mass-weighted ages than the general population of galaxies at the same redshifts. Combined with deep slitless grism spectroscopy from the HST/WFC3 in the UDF, we can tie the rest-frame ultraviolet C III] emission to rest-frame optical emission lines, namely [O III] 5007, finding a strong correlation between the two. Down to the flux limits that we observe (~1e-18 erg/s/cm^2 with the grism data in the UDF), all objects with a rest-frame [O III] 4959,5007 equivalent width in excess of 250 Angstroms, the so-called Extreme Emission Line Galaxies, have detections of C III] in our MUSE data. More detailed studies of the C III]-emitting population at these intermediate redshifts will be crucial to understand the physical conditions in galaxies at early cosmic times and to determine the utility of C III] as a redshift tracer.

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arXiv2017

Synthetic nebular emission from massive galaxies I: origin of the cosmic evolution of optical emission-line ratios

Galaxies occupy different regions of the [OIII]$\lambda5007$/H$β$-versus-[NII]$\lambda6584$/H$α$ emission-line ratio diagram in the distant and local Universe. We investigate the origin of this intriguing result by modelling self-consistently, for the first time, nebular emission from young stars, accreting black holes (BHs) and older, post-asymptotic-giant-branch (post-AGB) stellar populations in galaxy formation simulations in a full cosmological context. In post-processing, we couple new-generation nebular-emission models with high-resolution, cosmological zoom-in simulations of massive galaxies to explore which galaxy physical properties drive the cosmic evolution of the optical-line ratios [OIII]$\lambda5007$/H$β$, [NII]$\lambda6584$/H$α$, [SII]$λ\lambda6717,6731$/H$α$ and [OI]$\lambda6300$/H$α$. The line ratios of simulated galaxies agree well with observations of both star-forming and active local SDSS galaxies. Towards higher redshifts, at fixed galaxy stellar mass, the average [OIII]/H$β$ increases and [NII]/H$α$, [SII]/H$α$ and [OI]/H$α$ decrease -- widely consistent with observations. At fixed stellar mass, we identify star formation history, which controls nebular emission from young stars via the ionization parameter, as the primary driver of the cosmic evolution of [OIII]/H$β$ and [NII]/H$α$. For [SII]/H$α$ and [OI]/H$α$, this applies only to redshifts above $z=1.5$, the evolution at lower redshift being driven in roughly equal parts by nebular emission from AGN and post-AGB stars. Instead, changes in the hardness of ionizing radiation, ionized-gas density, the prevalence of BH accretion relative to star formation and the dust-to-metal mass ratio (whose impact on the gas-phase N/O ratio we model at fixed O/H) play at most a minor role in the cosmic evolution of simulated galaxy line ratios.

astro-ph.GA↗

arXiv2017

Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below $Z_\odot/2$. Such tests are particularly important for interpreting the surprising high-ionization UV line emission detected at $z>6$ in recent years. We present HST/COS ultraviolet spectra of ten nearby metal-poor star-forming galaxies selected to show He II emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance ($7.8<12+\log\mathrm{O/H}<8.5$) and present uniformly large specific star formation rates (sSFR $\sim 10^2$ $\mathrm{Gyr}^{-1}$). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C III] equivalent widths comparable to those seen in systems at $z\sim 6-7$. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below $12+\log\mathrm{O/H}\lesssim 8.0$ ($Z/Z_\odot \lesssim 1/5$) presenting minimal stellar wind features and prominent nebular emission in He II and C IV. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the $\mathrm{He^+}$-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C IV and He II powered by these stars may provide useful metallicity constraints in the reionization era.

astro-ph.GA↗

arXiv2016

Extreme star formation events in quasar hosts over ${\bf0.5<\textit{z}<4}$

We explore the relationship between active galactic nuclei and star formation in a sample of 513 optically luminous type 1 quasars up to redshifts of $\sim$4 hosting extremely high star formation rates (SFRs). The quasars are selected to be individually detected by the \textit{Herschel} SPIRE instrument at $> $3$σ$ at 250 $μ$m, leading to typical SFRs of order of 1000 M$_{\odot}$yr$^{-1}$. We find the average SFRs to increase by almost a factor 10 from $z\sim0.5$ to $z\sim3$, mirroring the rise in the comoving SFR density over the same epoch. However, we find that the SFRs remain approximately constant with increasing accretion luminosity for accretion luminosities above 10$^{12}$ L$_{\odot}$. We also find that the SFRs do not correlate with black hole mass. Both of these results are most plausibly explained by the existence of a self-regulation process by the starburst at high SFRs, which controls SFRs on time-scales comparable to or shorter than the AGN or starburst duty cycles. We additionally find that SFRs do not depend on Eddington ratio at any redshift, consistent with no relation between SFR and black hole growth rate per unit black hole mass. Finally, we find that high-ionisation broad absorption line (HiBAL) quasars have indistinguishable far-infrared properties to those of classical quasars, consistent with HiBAL quasars being normal quasars observed along a particular line of sight, with the outflows in HiBAL quasars not having any measurable effect on the star formation in their hosts.

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arXiv2015

Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths

In the context of observations of the rest-frame ultraviolet and optical emission from distant galaxies, we explore the emission-line properties of photoionization models of active and inactive galaxies. Our aim is to identify new line-ratio diagnostics to discriminate between gas photoionization by active galactic nuclei (AGN) and star formation. We use a standard photoionization code to compute the emission from AGN narrow-line regions and compare this with calculations of the nebular emission from star-forming galaxies achieved using the same code. We confirm the appropriateness of widely used optical spectral diagnostics of nuclear activity versus star formation and explore new diagnostics at ultraviolet wavelengths. We find that combinations of a collisionally excited metal line or line multiplet, such as CIV 1548,1551, OIII]1661,1666, NIII]1750, [SiIII]1883+[SiIII]1892 and [CIII]1907+CIII]1909, with the HeII 1640 recombination line are individually good discriminants of the nature of the ionizing source. Diagrams involving at least 3 of these lines allow an even more stringent distinction between active and inactive galaxies, as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation. Several line ratios involving Ne-based emission lines, such as [NeIV]2424, [NeIII]3343 and [NeV]3426, are also good diagnostics of nuclear activity. Our results provide a comprehensive framework to identify the sources of photoionization and physical conditions of the ionized gas from the ultraviolet and optical nebular emission from galaxies. This will be particularly useful to interpret observations of high-redshift galaxies with future facilities, such as the James Webb Space Telescope and extremely large ground-based telescopes.

astro-ph.GA↗

arXiv2015

Spectroscopic detection of CIV in a galaxy at z=7.045: Implications for the ionizing spectra of reionization-era galaxies

We present Keck/MOSFIRE observations of UV metal lines in four bright gravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The spectrum of A1703-zd6, a highly-magnified star forming galaxy with a Lyman-alpha redshift of z=7.045, reveals a confident detection of the nebular CIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are not detected in the three other galaxies. At z~2-3, nebular CIV emission is observed in just 1% of UV-selected galaxies. The presence of strong CIV emission in one of the small sample of galaxies targeted in this paper may indicate hard ionizing spectra are more common at z~7. The total estimated equivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3) are comparable to measurements of narrow-lined AGNs. Photoionization models show that the nebular CIV line can also be reproduced by a young stellar population, with very hot metal poor stars dominating the photon flux responsible for triply ionizing carbon. Regardless of the origin of the CIV, we show that the ionizing spectrum of A1703-zd6 is different from that of typical galaxies at z~2, producing more H ionizing photons per unit 1500A luminosity and a larger flux density at 30-50 eV. If such extreme radiation fields are typical in UV-selected systems at z>7, it would indicate that reionization-era galaxies are more efficient ionizing agents than previously thought. Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7 may trace a rare population with intense radiation fields capable of ionizing their surrounding hydrogen distribution. Additional constraints on high ionization emission lines in galaxies with and without Lyman-alpha detections will help clarify whether hard ionizing spectra are common in the reionization era.

astro-ph.GA↗

arXiv2015

A complete census of silicate features in the mid-infrared spectra of active galaxies

We present a comprehensive study of the silicate features at 9.7 and 18 micron of a sample of almost 800 active galactic nuclei (AGN) with available spectra from the Spitzer InfraRed Spectrograph (IRS). We measure the strength of the silicate feature at 9.7 micron, S9.7, before and after subtracting the host galaxy emission from the IRS spectra. The numbers of type 1 and 2 AGN with the feature in emission increase by 20 and 50%, respectively, once the host galaxy is removed, while 35% of objects with the feature originally in absorption exhibit it in even deeper absorption. The peak of S9.7, lambda_peak, has a bimodal distribution when the feature is in emission, with about 65% of the cases showing lambda_peak > 10.2 micron. Silicates can appear in emission in objects with mid-infrared (MIR) luminosity spanning over six orders of magnitude. The derived distributions of the strength of the silicate features at 9.7 and 18 micron provide a solid test bed for modeling the dust distribution in AGN. Clumpiness is needed in order to produce absorption features in unobscured AGN and can also cause the silicates to be in absorption at 9.7 micron and in emission at 18 micron in type 1 sources. We find the `cosmic' silicates of Ossenkopf et al. to be more consistent with the observations than Draine's `astronomical' silicates. Finally, we discuss the possibility of a foreground absorber to explain the deep silicate absorption features in the MIR spectra of some type 2 AGN.

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arXiv2013

The roles of star formation and AGN activity of IRS sources in the HerMES fields

In this work we explore the impact of the presence of an active galactic nucleus (AGN) on the mid- and far-infrared (IR) properties of galaxies as well as the effects of simultaneous AGN and starburst activity in these same galaxies. To do this we apply a multi-component, multi-band spectral synthesis technique to a sample of 250 micron selected galaxies of the Herschel Multi-tiered Extragalactic Survey, with IRS spectra available for all galaxies. Our results confirm that the inclusion of the IRS spectra plays a crucial role in the spectral analysis of galaxies with an AGN component improving the selection of the best-fit hot dust model (torus). We find a correlation between the obscured star formation rate (SFR) derived from the IR luminosity of the starburst component, SFR_IR and SFR_PAH, derived from the luminosity of the PAH features, L_PAH, with SFR_FIR taking higher values than SFR_PAH. The correlation is different for AGN- and starburst-dominated objects. The ratio of L_PAH to that of the starburst component, L_PAH/L_SB, is almost constant for AGN-dominated objects but decreases with increasing L_SB for starburst-dominated objects. SFR_FIR increases with the accretion luminosity, L_acc, with the increase less prominent for the very brightest, unobscured AGN-dominated sources. We find no correlation between the masses of the hot and cold dust components. We interpret this as a non-constant fraction of gas driven by the gravitational effects to the AGN while the starburst is ongoing. We also find no evidence of the AGN affecting the temperature of the cold dust component, though this conclusion is mostly based on objects with a non-dominant AGN component. We conclude that our findings do not provide evidence that the presence of AGN affects the star formation process in the host galaxy, but rather that the two phenomena occur simultaneously over a wide range of luminosities.

astro-ph.CO↗

arXiv2013

Modeling IR SED of AGN with Spitzer and Herschel data

One of the remaining open issues in the context of the analysis of Active Galactic Nuclei (AGN) is the evidence that nuclear gravitational accretion is often accompanied by a concurrent starburst (SB) activity. What is, in this picture, the role played by the obscuring dust around the nucleus and what do the state of the art AGN torus models have to say? Can the IR data provided by Spitzer and Herschel help us in extensively investigate both phenomena and, if so, how and with what limitations? In this paper we present our contribution to the efforts of answering these questions. We show some of the main results coming from a comparative study of various AGN SED modeling approaches, focusing mostly on the much-debated issue about the morphology of the dust distribution in the toroidal structure surrounding the AGN. We found that the properties of dust in AGN as measured by matching observations (be it broad band IR photometry or IR spectra) with models, strongly depend on the choiceof the dust distribution. Then, we present the spectral energy distribution (SED) fitting procedure we developed, making make the best use of Spitzer and Herschel SPIRE mid- and far-IR observations, to dig into the role played by the possible presence of an AGN on the host galaxy's properties.

astro-ph.CO↗