arXiv subjects

E. Sabbi

Publications and source records attributed to E. Sabbi.

At least 37 records · Page 2Linked to original sources

arXiv2022

Star Cluster Formation and Evolution in M101: An Investigation with the Legacy Extragalactic UV Survey

We present Hubble Space Telescope WFC3/UVIS (F275W, F336W) and ACS/WFC optical (F435W, F555W, and F814W) observations of the nearby grand-design spiral galaxy M101 as part of the Legacy Extragalactic UV Survey (LEGUS). Compact sources detected in at least four bands were classified by both human experts and the convolutional neural network StarcNet. Human experts classified the 2,351 brightest sources, retrieving $N_{c} = 965$ star clusters. StarcNet, trained on LEGUS data not including M101, classified all 4,725 sources detected in four bands, retrieving $N_{c} = 2,270$ star clusters. The combined catalog represents the most complete census to date of compact star clusters in M101. We find that for the 2,351 sources with both a visual- and ML-classification StarcNet is able to reproduce the human classifications at high levels of accuracy ($\sim 80-90\%$), which is equivalent to the level of agreement between human classifiers in LEGUS. The derived cluster age distribution implies a disruption rate of $dN/dτ\propto τ^{-0.45 \pm 0.14}$ over $10^{7} < τ< 10^{8.5}$yr for cluster masses $\geq 10^{3.55} M_{\odot}$ for the central region of M101 and $dN/dτ\propto τ^{-0.02 \pm 0.15}$ for cluster masses $\geq 10^{3.38} M_{\odot}$ in the northwest region of the galaxy. The trends we recover are weaker than those of other nearby spirals (e.g. M51) and starbursts, consistent with the M101 environment having a lower-density interstellar medium, and providing evidence in favor of environmentally-dependent cluster disruption in the central, southeast, and northwest regions of M101.

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arXiv2022

Ionization and Star Formation in the Giant HII Region SMC-N66

The NGC 346 young stellar system and associated N66 giant HII region in the Small Magellanic Cloud are the nearest example of a massive star forming event in a low metallicity ($Z\approx0.2Z_{\odot}$) galaxy. With an age of $\lesssim$3Myr this system provides a unique opportunity to study relationships between massive stars and their associated HII region. Using archival data, we derive a total H$α$ luminosity of L(H$α$)=4.1$\times$10$^{38}$ergs$^{-1}$ corresponding to an H-photoionization rate of 3$\times$10$^{50}$s$^{-1}$. A comparison with a predicted stellar ionization rate derived from the more than 50 known O-stars in NGC 346, including massive stars recently classified from HST FUV spectra, indicates an approximate ionization balance. Spectra obtained with SALT suggest the ionization structure of N66 could be consistent with some leakage of ionizing photons. Due to the low metallicity, the far ultraviolet luminosity from NGC 346 is not confined to the interstellar cloud associated with N66. Ionization extends through much of the spatial extent of the N66 cloud complex, and most of the cloud mass is not ionized. The stellar mass estimated from nebular L(H$α$) appears to be lower than masses derived from the census of resolved stars which may indicate a disconnect between the formation of high and low mass stars in this region. We briefly discuss implications of the properties of N66 for studies of star formation and stellar feedback in low metallicity environments.

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arXiv2022

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope III. Integral-field spectroscopy

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) offers the first opportunity to use integral-field spectroscopy from space at near-infrared wavelengths. More specifically, NIRSpec's integral-field unit can obtain spectra covering the wavelength range $0.6 - 5.3~μ$m for a contiguous 3.1 arcsec $\times$ 3.2 arcsec sky area at spectral resolutions of $R \approx 100$, 1000, and 2700. In this paper we describe the optical and mechanical design of the NIRSpec integral-field spectroscopy mode, together with its expected performance. We also discuss a few recommended observing strategies, some of which are driven by the fact that NIRSpec is a multipurpose instrument with a number of different observing modes, which are discussed in companion papers. We briefly discuss the data processing steps required to produce wavelength- and flux-calibrated data cubes that contain the spatial and spectral information. Lastly, we mention a few scientific topics that are bound to benefit from this highly innovative capability offered by JWST/NIRSpec.

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arXiv2022

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope IV. Capabilities and predicted performance for exoplanet characterization

The Near-Inrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) is a very versatile instrument, offering multiobject and integral field spectroscopy with varying spectral resolution ($\sim$30 to $\sim$3000) over a wide wavelength range from 0.6 to 5.3 micron, enabling scientists to study many science themes ranging from the first galaxies to bodies in our own Solar System. In addition to its integral field unit and support for multiobject spectroscopy, NIRSpec features several fixed slits and a wide aperture specifically designed to enable high precision time-series and transit as well as eclipse observations of exoplanets. In this paper we present its capabilities regarding time-series observations, in general, and transit and eclipse spectroscopy of exoplanets in particular. Due to JWST's large collecting area and NIRSpec's excellent throughput, spectral coverage, and detector performance, this mode will allow scientists to characterize the atmosphere of exoplanets with unprecedented sensitivity.

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arXiv2022

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities

We provide an overview of the design and capabilities of the near-infrared spectrograph (NIRSpec) onboard the James Webb Space Telescope. NIRSpec is designed to be capable of carrying out low-resolution ($R\!=30\!-330$) prism spectroscopy over the wavelength range $0.6-5.3\!~μ$m and higher resolution ($R\!=500\!-1340$ or $R\!=1320\!-3600$) grating spectroscopy over $0.7-5.2\!~μ$m, both in single-object mode employing any one of five fixed slits, or a 3.1$\times$3.2 arcsec$^2$ integral field unit, or in multiobject mode employing a novel programmable micro-shutter device covering a 3.6$\times$3.4~arcmin$^2$ field of view. The all-reflective optical chain of NIRSpec and the performance of its different components are described, and some of the trade-offs made in designing the instrument are touched upon. The faint-end spectrophotometric sensitivity expected of NIRSpec, as well as its dependency on the energetic particle environment that its two detector arrays are likely to be subjected to in orbit are also discussed.

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arXiv2022

The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)

We provide an overview of the capabilities and performance of the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) when used in its multi-object spectroscopy (MOS) mode employing a novel Micro Shutter Array (MSA) slit device. The MSA consists of four separate 98 arcsec $\times$ 91 arcsec quadrants each containing $365\times171$ individually addressable shutters whose open areas on the sky measure 0.20 arcsec $\times$ 0.46 arcsec on a 0.27 arcsec $\times$ 0.53 arcsec pitch. This is the first time that a configurable multi-object spectrograph has been available on a space mission. The levels of multiplexing achievable with NIRSpec MOS mode are quantified and we show that NIRSpec will be able to observe typically fifty to two hundred objects simultaneously with the pattern of close to a quarter of a million shutters provided by the MSA. This pattern is fixed and regular, and we identify the specific constraints that it yields for NIRSpec observation planning. We also present the data processing and calibration steps planned for the NIRSpec MOS data. The significant variation in size of the mostly diffraction-limited instrument point spread function over the large wavelength range of 0.6-5.3 $μ$m covered by the instrument, combined with the fact that most targets observed with the MSA cannot be expected to be perfectly centred within their respective slits, makes the spectrophotometric and wavelength calibration of the obtained spectra particularly complex. These challenges notwithstanding, the sensitivity and multiplexing capabilities anticipated of NIRSpec in MOS mode are unprecedented, and should enable significant progress to be made in addressing a wide range of outstanding astrophysical problems.

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arXiv2020

Looking for obscured young star clusters in NCG 1313

Using recently acquired HST NIR observations (J, Pa$β$ and H bands) of the nearby galaxy NGC1313, we investigate the timescales required by a young star cluster to emerge from its natal cloud. We search for extincted star clusters, potentially embedded in their natal cloud as either: 1. compact sources in regions with high H$α$/Pa$β$ extinctions; 2. compact HII regions that appear as point-like sources in the Pa$β$ emission map. The NUV--optical--NIR photometry of the candidate clusters is used to derive their ages, masses and extinctions via a least$-χ^2$ SED broad and narrow--band fitting process. The 100 clusters in the final samples have masses in the range $\rm \log_{10}(M/M_\odot)=2.5-3.5$ and moderate extinctions, $\rm E(B-V)\lesssim1.0$ mag. Focusing on the young clusters ($0-6$ Myr) we derive a weak correlation between extinction and age of the clusters. Almost half of the clusters have low extinctions, $\rm E(B-V)<0.25$ mag, already at very young ages ($\le3$ Myr), suggesting that dust is quickly removed from clusters. A stronger correlation is found between the morphology of the nebular emission (compact, partial or absent, both in H$α$ and Pa$β$) and cluster age. Relative fractions of clusters associated with a specific nebular morphology is used to estimate the typical timescales for clearing the natal gas cloud, resulting between 3 and 5 Myr, $\sim1$ Myr older than what estimated from NUV--optical--based cluster studies. This difference hints to a bias for optically--only based studies, which JWST will address in the coming years.

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arXiv2020

Star cluster formation in the most extreme environments: Insights from the HiPEEC survey

We present the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey. We fit HST NUV to NIR broadband and H$α$ fluxes, to derive star cluster ages, masses, extinctions and determine the star formation rate (SFR) of 6 merging galaxies. These systems are excellent laboratories to trace cluster formation under extreme gas physical conditions, rare in the local universe, but typical for star-forming galaxies at cosmic noon. We detect clusters with ages of 1-500 Myr and masses that exceed $10^7$ M$_\odot$. The recent cluster formation history and their distribution within the host galaxies suggest that systems like NGC34, NGC1614, NGC4194 are close to their final coalescing phase, while NGC3256, NGC3690, NGC6052 are at an earlier/intermediate stage. A Bayesian analysis of the cluster mass function in the age interval 1-100 Myr provides strong evidence in 4 of the 6 galaxies that an exponentially truncated power law better describes the observed mass distributions. For two galaxies, the fits are inconclusive due to low number statistics. We determine power-law slopes $β\sim-1.5$ to $-2.0$, and truncation masses, M$_c$, between $10^6$ and a few times $10^7$ M$_\odot$, among the highest values reported in the literature. Advanced mergers have higher M$_c$ than early/intermediate merger stage galaxies, suggesting rapid changes in the dense gas conditions during the merger. We compare the total stellar mass in clusters to the SFR of the galaxy, finding that these systems are among the most efficient environments to form star clusters in the local universe.

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arXiv2020

Time-domain study of the young massive cluster Westerlund 2 with the Hubble Space Telescope. I

Time-domain studies of pre-main sequence stars have long been used to investigate star properties during their early evolutionary phases and to trace the evolution of circumstellar environments. Historically these studies have been confined to the nearest, low-density, star forming regions. We used the Wide Field Camera 3 on board of the Hubble Space Telescope to extend, for the first time, the study of pre-main sequence variability to one of the few young massive clusters in the Milky Way, Westerlund 2. Our analysis reveals that at least 1/3 of the intermediate and low-mass pre-main sequence stars in Westerlund 2 are variable. Based on the characteristics of their light curves, we classified ~11% of the variable stars as weak-line T-Tauri candidates, ~ 52% as classical T-Tauri candidates, ~ 5% as dippers and ~26% as bursters. In addition, we found that 2% of the stars below 6Mo (~6% of the variables) are eclipsing binaries, with orbital periods shorter than 80 days. The spatial distribution of the different populations of variable pre-main sequence stars suggests that stellar feedback and UV-radiation from massive stars play an important role on the evolution of circumstellar and planetary disks.

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arXiv2020

VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud

The process of massive star ($M\geq8~M_\odot$) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud (LMC). We took advantage of the spectral resolution and wavelength coverage of X-shooter (300-2500 nm), mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect H$α,β$, [O I] 630.0 nm, Ca II infrared triplet, [Fe II] 1643.5 nm, fluorescent Fe II 1687.8 nm, H$_2$ 2121.8 nm, Br$γ$, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of 10 candidates as bona fide MYSOs. We compare our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We compute lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Br$γ$ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features such as fluorescent Fe II and CO bandheads which is consistent with the Galactic rate (40\%). Most of our MYSOs show outflow features.

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arXiv2019

Spatial segregation of massive clusters in dwarf galaxies

The relative average minimum projected separations of star clusters in the Legacy ExtraGalactic UV Survey (LEGUS) and in tidal dwarfs around the interacting galaxy NGC 5291 are determined as a function of cluster mass to look for cluster-cluster mass segregation. Class 2 and 3 LEGUS clusters, which have a more irregular internal structure than the compact and symmetric class 1 clusters, are found to be mass segregated in low mass galaxies, which means that the more massive clusters are systematically bunched together compared to the lower mass clusters. This mass segregation is not present in high-mass galaxies nor for class 1 clusters. We consider possible causes for this segregation including differences in cluster formation and scattering in the shallow gravitational potentials of low mass galaxies.

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arXiv2019

Star formation histories of the LEGUS dwarf galaxies (III): the non-bursty nature of 23 star forming dwarf galaxies

We derive the recent star formation histories of 23 active dwarf galaxies using HST observations from the Legacy Extragalactic UV Survey (LEGUS). We apply a color-magnitude diagram fitting technique using two independent sets of stellar models, PARSEC-COLIBRI and MIST. Despite the non-negligible recent activity, none of the 23 star forming dwarfs show enhancements in the last 100 Myr larger than three times the 100-Myr-average. The unweighted mean of the individual SFHs in the last 100 Myr is also consistent with a rather constant activity, irrespective of the atomic gas fraction. We confirm previous results that for dwarf galaxies the CMD-based average star formation rates (SFRs) are generally higher than the FUV-based SFR. For half of the sample, the 60-Myr-average CMD-based SFR is more than two times the FUV SFR. In contrast, we find remarkable agreement between the 10-Myr-average CMD-based SFR and the H$α$-based SFR. Finally, using core helium burning stars of intermediate mass we study the pattern of star formation spatial progression over the past 60 Myr, and speculate on the possible triggers and connections of the star formation activity with the environment in which these galaxies live. Approximately half of our galaxies show spatial progression of star formation in the last 60 Myr, and/or very recent diffuse and off-center activity compared to RGB stars.

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arXiv2019

Star Cluster Catalogs for the LEGUS Dwarf Galaxies

We present the star cluster catalogs for 17 dwarf and irregular galaxies in the $HST$ Treasury Program "Legacy ExtraGalactic UV Survey" (LEGUS). Cluster identification and photometry in this subsample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogs with accurate fluxes due to low cluster statistics. The colors and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogs are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of $-2$ independent of age and global SFR binning. The age distribution declines as a power law, with an index of $\approx-0.80\pm0.15$, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cutoff ($<2σ$) in the composite cluster mass function, and can rule out a truncation mass below $\approx10^{4.5}$M$_{\odot}$ but cannot rule out the existence of a truncation at higher masses.

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arXiv2019

Photometry and spectroscopy of faint candidate spectrophotometric standard DA white dwarfs

We present precise photometry and spectroscopy for 23 candidate spectrophotometric standard white dwarfs. The selected stars are distributed in the Northern hemisphere and around the celestial equators and are all fainter than r ~ 16.5 mag. This network of stars, when established as standards, together with the three Hubble Space Telescope primary CALSPEC white dwarfs, will provide a set of spectrophotometric standards to directly calibrate data products to better than 1%. These new faint standard white dwarfs will have enough signal-to-noise ratio in future deep photometric surveys and facilities to be measured accurately while still avoiding saturation in such surveys. They will also fall within the dynamic range of large telescopes and their instruments for the foreseeable future. This paper discusses the provenance of the observational data for our candidate standard stars. The comparison with models, reconciliation with reddening, and the consequent derivation of the full spectral energy density distributions for each of them is reserved for a subsequent paper.

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arXiv2018

Gaia DR2 reveals a very massive runaway star ejected from R136

A previous spectroscopic study identified the very massive O2 III star VFTS 16 in the Tarantula Nebula as a runaway star based on its peculiar line-of-sight velocity. We use the Gaia DR2 catalog to measure the relative proper motion of VFTS 16 and nearby bright stars to test if this star might have been ejected from the central cluster, R136, via dynamical ejection. We find that the position angle and magnitude of the relative proper motion (0.338 +/- 0.046 mas/yr, or approximately 80 +\- 11 km/s) of VFTS 16 are consistent with ejection from R136 approximately 1.5 +/- 0.2 Myr ago, very soon after the cluster was formed. There is some tension with the presumed age of VFTS 16 that, from published stellar parameters, cannot be greater than 0.9 +0.3/-0.2 Myr. Older ages for this star would appear to be prohibited due to the absence of He I lines in its optical spectrum, since this sets a firm lower limit on its effective temperature. The dynamical constraints may imply an unusual evolutionary history for this object, perhaps indicating it is a merger product. Gaia DR2 also confirms that another very massive star in the Tarantula Nebula, VFTS 72 (alias BI253; O2 III-V(n)((f*)), is also a runaway on the basis of its proper motion as measured by Gaia. While its tangential proper motion (0.392 +/-0.062 mas/yr or 93 +/-15 km/s) would be consistent with dynamical ejection from R136 approximately 1 Myr ago, its position angle is discrepant with this direction at the 2$σ$ level. From their Gaia DR2 proper motions we conclude that the two ~100 solar mass O2 stars, VFTS 16 and VFTS72, are fast runaway stars, with space velocities of around 100 km/s relative to R136 and the local massive star population. The dynamics of VFTS16 are consistent with it having been ejected from R136, and this star therefore sets a robust lower limit on the age of the central cluster of ~1.3 Myr.

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arXiv2018

Search For Star Cluster Age Gradients Across Spiral Arms of Three LEGUS Disk Galaxies

One of the main theories for explaining the formation of spiral arms in galaxies is the stationary density wave theory. This theory predicts the existence of an age gradient across the arms. We use the stellar cluster catalogues of the galaxies NGC 1566, M51a, and NGC 628 from the Legacy Extragalactic UV Survey (LEGUS) program. In order to test for the possible existence of an age sequence across the spiral arms, we quantified the azimuthal offset between star clusters of different ages in our target galaxies. We found that NGC 1566, a grand-design spiral galaxy with bisymmetric arms and a strong bar, shows a significant age gradient across the spiral arms that appears to be consistent with the prediction of the stationary density wave theory. In contrast, M51a with its two well-defined spiral arms and a weaker bar does not show an age gradient across the arms. In addition, a comparison with non LEGUS star cluster catalogues for M51a yields similar results. We believe that the spiral structure of M51a is not the result of a stationary density wave with a fixed pattern speed. Instead, tidal interactions could be the dominant mechanism for the formation of spiral arms. We also found no offset in the azimuthal distribution of star clusters with different ages across the weak spiral arms of NGC 628.

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arXiv2018

HST astrometry in the 30 Doradus region: II. Runaway stars from new proper motions in the Large Magellanic Cloud

We present a catalog of relative proper motions for 368,787 stars in the 30 Doradus region of the Large Magellanic Cloud (LMC), based on a dedicated two-epoch survey with the Hubble Space Telescope (HST) and supplemented with proper motions from our pilot archival study. We demonstrate that a relatively short epoch difference of 3 years is sufficient to reach a $\sim$0.1 mas yr$^{-1}$ level of precision or better. A number of stars have relative proper motions exceeding a 3-sigma error threshold, representing a mixture of Milky Way denizens and 17 potential LMC runaway stars. Based upon 183 VFTS OB-stars with the best proper motions, we conclude that none of them move faster than $\sim$0.3 mas yr$^{-1}$ in each coordinate -- equivalent to $\sim$70 km s$^{-1}$. Among the remaining 351 VFTS stars with less accurate proper motions, only one candidate OB runaway can be identified. We rule out any OB star in our sample moving at a tangential velocity exceeding $\sim$120 km s$^{-1}$. The most significant result of this study is finding 10 stars over wide range of masses, which appear to be ejected from the massive star cluster R136 in the tangential plane to angular distances from $35^{\prime\prime}$ out to $407^{\prime\prime}$, equivalent to 8-98 pc. The tangential velocities of these runaways appear to be correlated with apparent magnitude, indicating a possible dependence on the stellar mass.

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arXiv2018

Extinction Maps and Dust-to-Gas Ratios in Nearby Galaxies with LEGUS

We present a study of the dust-to-gas ratios in five nearby galaxies NGC 628 (M74), NGC 6503, NGC 7793, UGC 5139 (Holmberg I), and UGC 4305 (Holmberg II). Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from the Treasury program LEGUS (Legacy ExtraGalactic UV Survey) combined with archival HST/ACS data, we correct thousands of individual stars for extinction across these five galaxies using an isochrone-matching (reddening-free Q) method. We generate extinction maps for each galaxy from the individual stellar extinctions using both adaptive and fixed resolution techniques, and correlate these maps with neutral HI and CO gas maps from literature, including The HI Nearby Galaxy Survey (THINGS) and the HERA CO-Line Extragalactic Survey (HERACLES). We calculate dust-to-gas ratios and investigate variations in the dust-to-gas ratio with galaxy metallicity. We find a power law relationship between dust-to-gas ratio and metallicity, consistent with other studies of dust-to-gas ratio compared to metallicity. We find a change in the relation when H$_2$ is not included. This implies that underestimation of $N_{H_2}$ in low-metallicity dwarfs from a too-low CO-to-H$_2$ conversion factor $X_{CO}$ could have produced too low a slope in the derived relationship between dust-to-gas ratio and metallicity. We also compare our extinctions to those derived from fitting the spectral energy distribution (SED) using the Bayesian Extinction and Stellar Tool (BEAST) for NGC 7793 and find systematically lower extinctions from SED-fitting as compared to isochrone matching.

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