arXiv subjects

Michele Fumagalli

Publications and source records attributed to Michele Fumagalli.

At least 109 records · Page 6Linked to original sources

arXiv2018

MCMC determination of the cosmic UV background at $z\simeq 0$ from H$α$ fluorescence

In a recent paper (Fumagalli et al. 2017) we reported on the detection of a diffuse H$α$ glow in the outskirts of the nearby, edge-on disc galaxy UGC 7321 observed with the Multi Unit Spectroscopic Explorer (MUSE) at the ESO Very Large Telescope. By interpreting the H$α$ emission as fluorescence arising from hydrogen ionised by an external (i.e., extragalactic) radiation field, we estimated the UV background (UVB) intensity in terms of HI ionisation rate (per ion) at $z\simeq 0$ to be in the range $Γ_{\rm HI}\sim 6-8\times 10^{-14}$ s$^{-1}$. In the present work, by performing radiative transfer calculations over a large set of models of the gaseous disc of UGC 7321, we refine our estimate and through an MCMC analysis derive a value for the photoionisaton rate of $Γ_{\rm HI}=7.27^{+2.93}_{-2.90}\times 10^{-14}$ s$^{-1}$. In particular, our analysis demonstrates that this value is robust against large variations in the galaxy model and that the uncertainties are mainly driven by the errors associated with the observed H$α$ surface brightness. Our measurement is consistent with several recent determinations of the same quantity by a completely independent technique (i.e., flux decrement analysis of the Ly$α$-forest), and support the notion that the low redshift UVB is largely dominated by active galactic nuclei (AGNs), possibly with no need of further contribution from star forming galaxies.

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arXiv2018

Theoretical predictions for IMF diagnostics in UV spectroscopy of star clusters

We explore the possibility of using UV spectroscopy in combination with broad-band photometry as diagnostic tools for understanding the shape of the Initial Mass Function (IMF) in unresolved stellar populations. Building on our previous work, we extend the Stochastically Lighting Up Galaxies code (slug) to include a high-resolution UV spectral synthesiser and equivalent width calculation capabilities. We first gain a qualitative understanding of how UV spectral features behave as the parameters that define a star cluster in slug (mass, age, extinction, and IMF slope alpha) are changed. We then exploit Bayesian inference techniques to recover the alpha values for clusters simulated with slug, using mock observations of these clusters comprised of broad-band photometry and equivalent width measurements of a selection of UV spectral features. We find some improvement when compared to attempts using broad-band photometry alone (with the interquartile range of the alpha posterior PDF shrinking by 32%), although we still do not yet fully break the known degeneracy between the cluster mass and alpha. Finally, we make predictions about how effective real observations will be by quantifying our ability to constrain alpha as a function of limiting equivalent width. We find that observations sensitive to a modest equivalent width of 9 A are sufficient to improve the recovery of the IMF slope parameter by 32% (interquartile range of posterior PDF median residuals), moving to 39% when we include all the significant spectral features in the wavelength range 900-3000 A.

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arXiv2018

A comparison of young star properties with local galactic environment for LEGUS/LITTLE THINGS dwarf irregular galaxies

We have explored the role environmental factors play in determining characteristics of young stellar objects in nearby dwarf irregular and Blue Compact Dwarf galaxies. Star clusters are characterized by concentrations, masses, and formation rates, OB associations by mass and mass surface density, O stars by their numbers and near-ultraviolet absolute magnitudes, and HII regions by Halpha surface brightnesses. These characteristics are compared to surrounding galactic pressure, stellar mass density, HI surface density, and star formation rate surface density. We find no trend of cluster characteristics with environmental properties, implying that larger scale effects are more important in determining cluster characteristics or that rapid dynamical evolution erases memory of the initial conditions. On the other hand, the most massive OB associations are found at higher pressure and HI surface density, and there is a trend of higher HII region Halpha surface brightness with higher pressure, suggesting that a higher concentration of massive stars and gas are found preferentially in regions of higher pressure. At low pressures we find massive stars but not bound clusters and OB associations. We do not find evidence for an increase of cluster formation efficiency as a function of star formation rate density. However, there is an increase in the ratio of the number of clusters to number of O stars with pressure, perhaps reflecting an increase in clustering properties with star formation rate.

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arXiv2018

The spectral and environment properties of $z\sim2.0-2.5$ quasar pairs

We present the first results from our survey of intervening and proximate Lyman limit systems (LLSs) at $z$$\sim$2.0-2.5 using the Wide Field Camera 3 on-board the Hubble Space Telescope. The quasars in our sample are projected pairs with proper transverse separations $R_\perp$$\leq$150 kpc and line of sight velocity separations $\lesssim$11,000 km/s. We construct a stacked ultraviolet (rest-frame wavelengths 700-2000Å) spectrum of pairs corrected for the intervening Lyman forest and Lyman continuum absorption. The observed spectral composite presents a moderate flux excess for the most prominent broad emission lines, a $\sim$30% decrease in flux at $λ$=800-900Å compared to a stack of brighter quasars not in pairs at similar redshifts, and lower values of the mean free path of the HI ionizing radiation for pairs ($λ_{\rm mfp}^{912}=140.7\pm20.2~h_{70}^{-1}$Mpc) compared to single quasars ($λ_{\rm mfp}^{912}=213.8\pm28~h_{70}^{-1}$Mpc) at the average redshift $z\simeq2.44$. From the modelling of LLS absorption in these pairs, we find a higher ($\sim$20%) incidence of proximate LLSs with $\log N_{\rm HI}\geq17.2$ at $δv$$<$5,000 km/s compared to single quasars ($\sim$6%). These two rates are different at the 5$σ$ level. Moreover, we find that optically-thick absorbers are equally shared between foreground and background quasars. Based on these pieces of evidence, we conclude that there is a moderate excess of gas absorbing Lyman continuum photons in our closely-projected quasar pairs compared to single quasars. We argue that this gas arises mostly within large-scale structures or partially neutral regions inside the dark matter haloes where these close pairs reside.

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arXiv2018

Quasars probing quasars X: The quasar pair spectral database

The rare close projection of two quasars on the sky provides the opportunity to study the host galaxy environment of a foreground quasar in absorption against the continuum emission of a background quasar. For over a decade the "Quasars probing quasars" series has utilized this technique to further the understanding of galaxy formation and evolution in the presence of a quasar at z>2, resolving scales as small as a galactic disc and from bound gas in the circumgalactic medium to the diffuse environs of intergalactic space. Presented here, is the public release of the quasar pair spectral database utilized in these studies. In addition to projected pairs at z>2, the database also includes quasar pair members at z<2, gravitational lens candidates and quasars closely separated in redshift that are useful for small-scale clustering studies. In total the database catalogs 5627 distinct objects, with 4083 lying within 5' of at least one other source. A spectral library contains 3582 optical and near-infrared spectra for 3028 of the cataloged sources. As well as reporting on 54 newly discovered quasar pairs, we outline the key contributions made by this series over the last ten years, summarize the imaging and spectroscopic data used for target selection, discuss the target selection methodologies, describe the database content and explore some avenues for future work. Full documentation for the spectral database, including download instructions, are supplied at the following URL http://specdb.readthedocs.io/en/latest/

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arXiv2018

A study of two dwarf irregular galaxies with asymmetrical star formation distributions

Two dwarf irregular galaxies DDO 187 and NGC 3738 exhibit a striking pattern of star formation: intense star formation is taking place in a large region occupying roughly half of the inner part of the optical galaxy. We use data on the HI distribution and kinematics and stellar images and colors to examine the properties of the environment in the high star formation rate (HSF) halves of the galaxies in comparison with the low star formation rate (LSF) halves. We find that the pressure and gas density are higher on the HSF sides by 30-70%. In addition we find in both galaxies that the HI velocity fields exhibit significant deviations from ordered rotation and there are large regions of high velocity dispersion and multiple velocity components in the gas beyond the inner regions of the galaxies. The conditions in the HSF regions are likely the result of large-scale external processes affecting the internal environment of the galaxies and enabling the current star formation there.

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arXiv2017

Witnessing galaxy assembly in an extended z~3 structure

We present new observations acquired with the Multi Unit Spectroscopic Explorer instrument on the Very Large Telescope in a quasar field that hosts a high column-density damped Lyα absorber (DLA) at z~3.25. We detect Lyα emission from a nebula at the redshift of the DLA with line luminosity (27+/-1)x1e41 erg/s, which extends over 37+/-1 kpc above a surface brightness limit of 6x1e-19 erg/s/cm2/arcsec2 at a projected distance of 30.5+/-0.5 kpc from the quasar sightline. Two clumps lie inside this nebula, both with Lyα rest-frame equivalent width > 50 A and with relative line-of-sight velocities aligned with two main absorption components seen in the DLA spectrum. In addition, we identify a compact galaxy at a projected distance of 19.1+/-0.5 kpc from the quasar sightline. The galaxy spectrum is noisy but consistent with that of a star-forming galaxy at the DLA redshift. We argue that the Lyα nebula is ionized by radiation from star formation inside the two clumps, or by radiation from the compact galaxy. In either case, these data imply the presence of a structure with size >>50 kpc inside which galaxies are assembling, a picture consistent with galaxy formation in groups and filaments as predicted by cosmological simulations such as the EAGLE simulations.

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arXiv2017

MUSE sneaks a peek at extreme ram-pressure events - III. Tomography of UGC 6697, a massive galaxy falling into Abell 1367

We present the MUSE observations of UGC 6697, a giant (Mstar 10^{10} Msol) spiral galaxy infalling in the nearby cluster Abell 1367. During its high velocity transit through the intracluster medium (ICM), the hydrodynamical interactions with the ICM produce a ~ 100 kpc tail of ionized gas that we map with a mosaic of five MUSE pointings up to 60 kpc from the galaxy. CGCG 97087N, a small companion that lies at few arcminutes in projection from UGC 6697, is also suffering from the hydrodynamic action of the ICM of the cluster. Along the whole extent of the tail we detect diffuse H$α$ emission and, to a lesser extent, H$β$, [OIII]$\lambda5007$, and [OI]$\lambda6300$. By comparing the kinematics and distribution of gas and stars (as traced by the CaII triplet) for both galaxies, we separate the ionized gas, as traced by the H$α$ line, in a component still bound to the galaxy and a component that is stripped. We find that the "onboard" component shows low velocity dispersion and line ratios consistent with photoionization by hot stars. The stripped gas is more turbulent, with velocity dispersions up to > 100 km/s}, and is excited by shocks as traced by high values of [OI]/H$α$ and [NII]/H$α$ ratio. In the tail of UGC 6697 we identify numerous bright compact knots with line ratios typical of HII regions. These are distributed along the only streams of stripped gas retaining low velocity dispersions (< 35 km/s). Despite being in the stripped gas, their physical properties do not differentiate from normal HII regions in galactic disks. We find evidence of a past fast encounter between the two galaxies in the form of a double tail emerging from CGCG 97087N that connects with UGC 6697. This encounter might have increased the efficiency of the stripping process, leaving the stellar distribution and kinematics unaltered.

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arXiv2017

A measurement of the z = 0 UV background from H$α$ fluorescence

We report the detection of extended Halpha emission from the tip of the HI disk of the nearby edge-on galaxy UGC 7321, observed with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope. The Halpha surface brightness fades rapidly where the HI column density drops below N(HI) = 10^19 cm^-2 , consistent with fluorescence arising at the ionisation front from gas that is photoionized by the extragalactic ultraviolet background (UVB). The surface brightness measured at this location is (1.2 +/- 0.5)x10^-19 erg/s/cm^2/arcsec^2, where the error is mostly systematic and results from the proximity of the signal to the edge of the MUSE field of view, and from the presence of a sky line next to the redshifted Halpha wavelength. By combining the Halpha and the HI 21 cm maps with a radiative transfer calculation of an exponential disk illuminated by the UVB, we derive a value for the HI photoionization rate of Gamma ~ (6-8)x10^-14 1/s . This value is consistent with transmission statistics of the Lyalpha forest and with recent models of a UVB which is dominated by quasars.

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arXiv2017

The COS-Halos Survey: Metallicities in the Low-Redshift Circumgalactic Medium

We analyze new far-ultraviolet spectra of 13 quasars from the z~0.2 COS-Halos survey that cover the HI Lyman limit of 14 circumgalactic medium (CGM) systems. These data yield precise estimates or more constraining limits than previous COS-Halos measurements on the HI column densities NHI. We then apply a Monte-Carlo Markov Chain approach on 32 systems from COS-Halos to estimate the metallicity of the cool (T~10^4K) CGM gas that gives rise to low-ionization state metal lines, under the assumption of photoionization equilibrium with the extragalactic UV background. The principle results are: (1) the CGM of field L* galaxies exhibits a declining HI surface density with impact parameter Rperp (at >99.5%$ confidence), (2) the transmission of ionizing radiation through CGM gas alone is 70+/-7%; (3) the metallicity distribution function of the cool CGM is unimodal with a median of 1/3 Z_Sun and a 95% interval from ~1/50 Z_Sun to over 3x solar. The incidence of metal poor (<1/100 Z_Sun) gas is low, implying any such gas discovered along quasar sightlines is typically unrelated to L* galaxies; (4) we find an unexpected increase in gas metallicity with declining NHI (at >99.9% confidence) and, therefore, also with increasing Rperp. The high metallicity at large radii implies early enrichment; (5) A non-parametric estimate of the cool CGM gas mass is M_CGM_cool = 9.2 +/- 4.3 10^10 Msun, which together with new mass estimates for the hot CGM may resolve the galactic missing baryons problem. Future analyses of halo gas should focus on the underlying astrophysics governing the CGM, rather than processes that simply expel the medium from the halo.

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arXiv2017

Angular momentum evolution of galaxies over the past 10-Gyr: A MUSE and KMOS dynamical survey of 400 star-forming galaxies from z=0.3-1.7

We present a MUSE and KMOS dynamical study 405 star-forming galaxies at redshift z=0.28-1.65 (median redshift z=0.84). Our sample are representative of star-forming, main-sequence galaxies, with star-formation rates of SFR=0.1-30Mo/yr and stellar masses M=10^8-10^11Mo. For 49+/-4% of our sample, the dynamics suggest rotational support, 24+/-3% are unresolved systems and 5+/-2% appear to be early-stage major mergers with components on 8-30kpc scales. The remaining 22+/-5% appear to be dynamically complex, irregular (or face-on systems). For galaxies whose dynamics suggest rotational support, we derive inclination corrected rotational velocities and show these systems lie on a similar scaling between stellar mass and specific angular momentum as local spirals with j*=J/M*\propto M^(2/3) but with a redshift evolution that scales as j*\propto M^{2/3}(1+z)^(-1). We identify a correlation between specific angular momentum and disk stability such that galaxies with the highest specific angular momentum, log(j*/M^(2/3))>2.5, are the most stable, with Toomre Q=1.10+/-0.18, compared to Q=0.53+/-0.22 for galaxies with log(j*/M^(2/3))<2.5. At a fixed mass, the HST morphologies of galaxies with the highest specific angular momentum resemble spiral galaxies, whilst those with low specific angular momentum are morphologically complex and dominated by several bright star-forming regions. This suggests that angular momentum plays a major role in defining the stability of gas disks: at z~1, massive galaxies that have disks with low specific angular momentum, appear to be globally unstable, clumpy and turbulent systems. In contrast, galaxies with high specific angular have evolved in to stable disks with spiral structures.

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arXiv2016

The DESI Experiment Part I: Science,Targeting, and Survey Design

DESI (Dark Energy Spectroscopic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations (BAO) and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. To trace the underlying dark matter distribution, spectroscopic targets will be selected in four classes from imaging data. We will measure luminous red galaxies up to $z=1.0$. To probe the Universe out to even higher redshift, DESI will target bright [O II] emission line galaxies up to $z=1.7$. Quasars will be targeted both as direct tracers of the underlying dark matter distribution and, at higher redshifts ($ 2.1 < z < 3.5$), for the Ly-$α$ forest absorption features in their spectra, which will be used to trace the distribution of neutral hydrogen. When moonlight prevents efficient observations of the faint targets of the baseline survey, DESI will conduct a magnitude-limited Bright Galaxy Survey comprising approximately 10 million galaxies with a median $z\approx 0.2$. In total, more than 30 million galaxy and quasar redshifts will be obtained to measure the BAO feature and determine the matter power spectrum, including redshift space distortions.

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arXiv2016

The DESI Experiment Part II: Instrument Design

DESI (Dark Energy Spectropic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. The DESI instrument is a robotically-actuated, fiber-fed spectrograph capable of taking up to 5,000 simultaneous spectra over a wavelength range from 360 nm to 980 nm. The fibers feed ten three-arm spectrographs with resolution $R= λ/Δλ$ between 2000 and 5500, depending on wavelength. The DESI instrument will be used to conduct a five-year survey designed to cover 14,000 deg$^2$. This powerful instrument will be installed at prime focus on the 4-m Mayall telescope in Kitt Peak, Arizona, along with a new optical corrector, which will provide a three-degree diameter field of view. The DESI collaboration will also deliver a spectroscopic pipeline and data management system to reduce and archive all data for eventual public use.

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arXiv2016

The Cosmic Evolution of the Metallicity Distribution of Ionized Gas Traced by Lyman Limit Systems

We present the first results from our KODIAQ Z survey aimed to determine the metallicity distribution and physical properties of the z>2 partial and full Lyman limit systems (pLLSs and LLSs; 16.2<log N(HI)<19), which are probed of the interface regions between the intergalactic medium (IGM) and galaxies. We study 31 HI-selected pLLSs and LLSs at 2.3<z<3.3 observed with Keck/HIRES in absorption against background QSOs. We compare the column densities of metal-ions to HI and use photoionization models to assess the metallicity. The metallicity distribution of the pLLSs/LLSs at 2.3<z<3.3 is consistent with a unimodal distribution peaking at [X/H]=-2. The metallicity distribution of these absorbers therefore evolves markedly with z since at z<1 it is bimodal with peaks at [X/H]=-1.8 and -0.3. There is a substantial fraction (25-41%) of pLLSs/LLSs with metallicities well below those of damped Lya absorbers (DLAs) at any studied z from z<1 to z~2-4, implying reservoirs of metal-poor cool, dense gas in the IGM/galaxy interface at all z. However, the gas probed by pLLSs and LLSs is rarely pristine, with a fraction 3-18% for pLLSs/LLSs with [X/H]<-3. We find C/alpha enhancement in several pLLSs and LLSs in the metallicity range -2<[X/H]<-0.5, where C/alpha is 2-5 times larger than observed in Galactic metal-poor stars or high redshift DLAs at similar metallicities. This is likely caused by preferential ejection of carbon from metal-poor galaxies into their surroundings.

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arXiv2016

Exploring Damped Lyman-$α$ System Host Galaxies using Gamma-ray Bursts

We present a sample of 45 Damped Lyman-$α$ system (DLA, \nhi\ $ \geq 2 \times 10^{20} {\rm cm}^{-2}$) counterparts (33 detections, 12 upper limits) which host gamma-ray bursts (GRB-DLAs) in order to investigate star-formation and metallicity within galaxies hosting DLAs. Our sample spans $z \sim 2-6$ and is nearly three times larger than any previously detected DLA counterparts survey based on quasar line-of-sight searches (QSO-DLAs). We report star formation rates (SFRs) from rest-frame UV photometry and SED modeling. We find that DLA counterpart SFRs are not correlated with either redshift or HI column density. Thanks to the combination of \hst\ and ground-based observations, we also investigate DLA host star-formation efficiency. Our GRB-DLA counterpart sample spans both higher efficiency and low efficiency star formation regions compared to the local Kennicutt-Schmidt relation, local star formation laws, and $z \sim 3$ cosmological simulations. We also compare the depletion times of our DLA hosts sample to other objects in the local Universe, our sample appears to deviate from the star formation efficiencies measured in local spiral and dwarf galaxies. Furthermore, we find similar efficiencies as local inner disks, SMC, and LBG outskirts. Finally, our enrichment time measurements show a spread of systems with under- and over-abundance of metals which may suggest that these systems had episodic star formation and a metal enrichment/depletion as a result of strong stellar feedback and/or metal inflow/outflow.

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arXiv2016

MUSE searches for galaxies near very metal-poor gas clouds at z~3: new constraints for cold accretion models

We report on the search for galaxies in the proximity of two very metal-poor gas clouds at z~3 towards the quasar Q0956+122. With a 5-hour MUSE integration in a ~500x500 kpc^2 region centred at the quasar position, we achieve a >80% complete spectroscopic survey of continuum-detected galaxies with m<25 mag and Lyα emitters with luminosity L>3e41 erg/s. We do not identify galaxies at the redshift of a z~3.2 Lyman limit system (LLS) with log Z/Zsun = -3.35 +/- 0.05, placing this gas cloud in the intergalactic medium or circumgalactic medium of a galaxy below our sensitivity limits. Conversely, we detect five Lyα emitters at the redshift of a pristine z~3.1 LLS with log Z/Zsun < -3.8, while ~0.4 sources were expected given the z~3 Lyα luminosity function. Both this high detection rate and the fact that at least three emitters appear aligned in projection with the LLS suggest that this pristine cloud is tracing a gas filament that is feeding one or multiple galaxies. Our observations uncover two different environments for metal-poor LLSs, implying a complex link between these absorbers and galaxy halos, which ongoing MUSE surveys will soon explore in detail. Moreover, in agreement with recent MUSE observations, we detected a ~90 kpc Lyα nebula at the quasar redshift and three Lyα emitters reminiscent of a "dark galaxy" population.

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arXiv2016

Robust automatic photometry of local galaxies from SDSS Dissecting the color magnitude relation with color profiles

We present an automatic procedure to perform reliable photometry of galaxies on SDSS images. We selected a sample of 5853 galaxies in the Coma and Virgo superclusters. For each galaxy, we derive Petrosian g and i magnitudes, surface brightness profiles and color profiles. Unlike the SDSS pipeline, our procedure is not affected by the well known shredding problem and efficiently extracts Petrosian magnitudes for all galaxies. Hence we derived magnitudes even from the population of galaxies missed by the SDSS that represents 25% of all Local supercluster galaxies and ~95% of galaxies with g < 11 mag. After correcting the g and i magnitudes for Galactic and internal extinction, the blue and red sequences in the color magnitude diagram are well separated, with similar slopes. In addition, we study (i) the color-magnitude diagrams in different galaxy regions, the inner (r <= 1 kpc), intermediate (0.2R{Pet} <=r<= 0.3R{Pet}) and outer, disk-dominated (r>=0.35R{Pet})) zone, and (ii), we compute template color profiles, discussing the dependences of the templates on the galaxy masses and on their morphological type. The two analyses consistently lead to a picture where elliptical galaxies show no color gradients, irrespective of their masses. Spirals, instead, display a steeper gradient in their color profiles with increasing mass, consistently with the growing relevance of a bulge and/or a bar component above 10^{10} Modot.

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arXiv2016

The Star Formation Rate Efficiency of Neutral Atomic-dominated Hydrogen Gas in the Outskirts of Star Forming Galaxies from z~1 to z~3

Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in Damped Ly-alpha Systems (DLAs) at z~3 is more than 10 times lower than predicted by the Kennicutt-Schmidt (KS) relation. To understand the origin of this deficit, and to investigate possible evolution with redshift and galaxy properties, we measure the SFR efficiency of atomic gas at z~1, z~2, and z~3 around star-forming galaxies. We use new robust photometric redshifts in the Hubble Ultra Deep Field to create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies' outskirts. We find that the SFR efficiency of HI gas at z>1 is ~1-3% of that predicted by the KS relation. Contrary to simulations and models that predict a reduced SFR efficiency with decreasing metallicity and thus with increasing redshift, we find no significant evolution in the SFR efficiency with redshift. Our analysis instead suggests that the reduced SFR efficiency is driven by the low molecular content of this atomic-dominated phase, with metallicity playing a secondary effect in regulating the conversion between atomic and molecular gas. This interpretation is supported by the similarity between the observed SFR efficiency and that observed in local atomic-dominated gas, such as in the outskirts of local spiral galaxies and local dwarf galaxies.

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