arXiv subjects

S. Salim

Publications and source records attributed to S. Salim.

At least 19 recordsLinked to original sources

arXiv2026

The MAGPI Survey: Evidence for Non-Universal Resolved Dust Attenuation Relations Beyond the Local Universe

We study the spatially resolved relation between dust attenuation ($A_V$) and star formation rate surface density ($\Sigma_{\mathrm{SFR}}$) in galaxies from the MAGPI survey ($0.25 < z < 0.42$). Using Balmer-decrement-based attenuation maps for 178 galaxies, we investigate whether the locally calibrated resolved $A_V$--$\Sigma_{\mathrm{SFR}}$ relation remains valid at intermediate redshift by comparing MAGPI with the local relation measured from MaNGA. We find a clear positive correlation between $A_V$ and $\Sigma_{\mathrm{SFR}}$ in MAGPI, with systematically higher attenuation than in MaNGA at fixed $\Sigma_{\mathrm{SFR}}$. After matching galaxies in stellar mass ($M_{*}$) and offset from the star-forming main sequence ($\Delta$SFMS), MAGPI galaxies remain more attenuated than MaNGA galaxies at fixed $\Sigma_{\mathrm{SFR}}$. The attenuation excess is strongest for galaxies below the SFMS ($\Delta A_V \sim 0.40$ mag), weaker for galaxies on the SFMS ($\Delta A_V \sim 0.28$ mag), and minimal for galaxies above the SFMS ($\Delta A_V \sim 0.07$ mag). The dependence of the offset on $\Delta$SFMS suggests that nebular attenuation on kpc scales is regulated not only by local star formation activity, but also by the global evolutionary state of the host galaxy. Together, these results indicate that the resolved $A_V$--$\Sigma_{\mathrm{SFR}}$ relation is not universal, and that locally calibrated attenuation relations may not fully describe galaxies at intermediate redshift. This highlights the need for attenuation calibrations that account for galaxy population and redshift when interpreting spatially resolved galaxy properties.

astro-ph.GA↗

arXiv2024

Attenuation proxy hidden in surface brightness-colour diagrams. A new strategy for the LSST era

Large future sky surveys, such as the LSST, will provide optical photometry for billions of objects. This paper aims to construct a proxy for the far ultraviolet attenuation (AFUVp) from the optical data alone, enabling the rapid estimation of the star formation rate (SFR) for galaxies that lack UV or IR data. To mimic LSST observations, we use the deep panchromatic optical coverage of the SDSS Photometric Catalogue DR~12, complemented by the estimated physical properties for the SDSS galaxies from the GALEX-SDSS-WISE Legacy Catalog (GSWLC) and inclination information obtained from the SDSS DR7. We restricted our sample to the 0.025-0.1 z-spec range and investigated relations among surface brightness, colours, and dust attenuation in the far UV range for star-forming galaxies obtained from the spectral energy distribution (SED). {Dust attenuation is best correlated with (u-r) colour and the surface brightness in the u band ($\rm \mu_{u}$). We provide a dust attenuation proxy for galaxies on the star-forming main sequence, which can be used for the LSST or any other type of broadband optical survey. The mean ratio between the catalogue values of SFR and those estimated using optical-only SDSS data with the AFUVp prior calculated as $\Delta$SFR=log(SFR$_{\tiny{\mbox{this work}}}$/SFR$_{\tiny{}\texttt{GSWLC}}$) is found to be less than 0.1~dex, while runs without priors result in an SFR overestimation larger than 0.3~dex. The presence or absence of theAFUVp has a negligible influence on the stellar mass estimation (with $\Delta$M$_{star}$ in the range from 0 to $-0.15$ dex). Forthcoming deep optical observations of the LSST Deep Drilling Fields, which also have multi-wavelength data, will enable one to calibrate the obtained relation for higher redshift galaxies and, possibly, extend the study towards other types of galaxies, such as early-type galaxies off the main sequence.

astro-ph.GA↗

arXiv2023

A comparative study of the fundamental metallicity relation: the impact of methodology on its observed evolution

We used 5487 star-forming galaxies at redshift z~0.63 extracted from the VIPERS and 143774 comparison galaxies in the local Universe from the GSWLC catalog. We employed two families of methods: parametric and non-parametric. In the former approaches, we compared the FMR projections plagued by observational biases on differently constructed control samples at various redshifts. Then, the metallicity difference between different redshifts in M*-SFR bins. In the latter approach, we related the metallicity and the normalized sSFR. The methodologies implemented to construct fair, complete samples for studying the MZR and the FMR produced consistent results showing a small, but still statistically significant evolution of both relations up to z~0.63. In particular, we observed a systematic trend where the median metallicity of the sample at z=0.63 is lower than that of the local sample at the same M* and SFR. The average difference in the metallicity of the low and intermediate redshifts is approximately 1.8 times the metallicity standard deviation of the median, of the intermediate redshift sample, in M*-SFR bins. We confirmed this result using the Kolmogorov-Smirnov test. When we applied the M*-completeness criterion to catalogs, the metallicity difference in redshifts decreased to approximately 0.96 times the metallicity standard deviation of the median, thus not statistically significant. This result may be dominated by the limited parameter space, being the lower M* galaxies where the difference is larger out from the analysis. A careful reading of the results, and their underlying selection criteria, are crucial in studies of the mass-metallicity and FMRs.

astro-ph.GA↗

arXiv2023

The WHaD diagram: Classifying the ionizing source with one single emission line

The usual approach to classify the ionizing source using optical spectroscopy is based on the use of diagnostic diagrams that compares the relative strength of pairs of collisitional metallic lines (e.g., [O iii] and [N ii]) with respect to recombination hydrogen lines (e.g., Hβ and Hα). Despite of being accepted as the standard procedure, it present known problems, including confusion regimes and/or limitations related to the required signal-to-noise of the involved emission lines. These problems affect not only our intrinsic understanding of inter-stellar medium and its poroperties, but also fundamental galaxy properties, such as the star-formation rate and the oxygen abundance, and key questions just as the fraction of active galactic nuclei, among several others. We explore the existing alternatives in the literature to minimize the confusion among different ionizing sources and proposed a new simple diagram that uses the equivalent width and the velocity dispersion from one single emission line, Hα, to classify the ionizing sources. We use aperture limited and spatial resolved spectroscopic data in the nearby Universe (z{\sim}0.01) to demonstrate that the new diagram, that we called WHaD, segregates the different ionizing sources in a more efficient way that previously adopted procedures. A new set of regions are defined in this diagram to select betweeen different ionizing sources. The new proposed diagram is well placed to determine the ionizing source when only Hα is available, or when the signal-to-noise of the emission lines involved in the classical diagnostic diagrams (e.g., Hβ).

astro-ph.GA↗

arXiv2022

SIT 45: An interacting, compact, and star-forming isolated galaxy triplet

The merging system SIT 45 (UGC 12589) is an unusual isolated galaxy triplet, consisting of three merging late-type galaxies, out of 315 systems in the SIT (SDSS-based catalogue of Isolated Triplets). The main aims of this work are to study its dynamical evolution and star formation history (SFH), as well as its dependence on its local and large-scale environment. To study its dynamics, parameters such as the velocity dispersion ($σ_{v}$), the harmonic radius ($R_{H}$), the crossing time ($H_0t_c$), and the virial mass ($M_{vir}$), along with the compactness of the triplet ($S$) were considered. To constrain the SFH, we used CIGALE to fit its observed spectral energy distribution using multi-wavelength data from the ultraviolet to the infrared. According to its SFH, SIT 45 presents star-formation, where the galaxies also present recent ($\sim $200 Myr) star-formation increase, indicating that this activity may have been triggered by the interaction. Its dynamical configuration suggests that the system is highly evolved in comparison to the SIT. However this is not expected for systems composed of star-forming late-type galaxies, based on observations in compact groups. We conclude that SIT 45 is a system of three interacting galaxies that are evolving within the same dark matter halo, where its compact configuration is a consequence of the on-going interaction, rather than due to a long-term evolution (as suggested from its $H_0t_c$ value). We consider two scenarios for the present configuration of the triplet, one where one of the members is a tidal galaxy, and another where this galaxy arrives to the system after the interaction. Both scenarios need further exploration. The isolated triplet SIT 45 is therefore an ideal system to study short timescale mechanisms ($\sim 10^8$ years), such as starbursts triggered by interactions which are more frequent at higher redshift.

astro-ph.GA↗

arXiv2022

SFR estimations from z=0 to z=0.9 -- A comparison of SFR calibrators for star-forming galaxies

Using VIPERS, we estimated a set of SFR based on photometric and spectroscopic data. We used, as estimators, photometric bands from ultraviolet to mid-infrared, and spectral lines. Assuming a reference SFR obtained from the spectral energy distribution reconstructed with Code Investigating GALaxy Emission, we estimated the reliability of each band as an SFR tracer. We used GSWLC to trace the dependence of these SFR calibrators with redshift. The far and near UV, u-band and 24-$μ$m bands, as well as $L_{TIR}$, are found to be good SFR tracers up to $z\sim0.9$ with a strong dependence on the attenuation prescription used for the bluest bands (scatter of SFR of 0.26, 0.14, 0.15, 0.23, and 0.24dex for VIPERS, and 0.25, 0.24, 0.09, 0.12, and 0.12dex for GSWLC). The 8-$μ$m band provides only a rough estimate of the SFR as it depends on metallicity and polycyclic aromatic hydrocarbon properties (scatter of 0.23dex for VIPERS). We estimated the scatter of rest-frame luminosity estimations from CIGALE to be 0.26, 0.14, 0.12, 0.15, and 0.20dex for FUV, NUV, ugriz, K$_{\mathrm{s}}$, and 8-24$μ$m-$L_{\mathrm{TIR}}$). At intermediate redshift, the H$β$ line is a reliable SFR tracer (scatter of 0.19dex) and the [OII] line gives an equally good estimation when the metallicity from the $R_{23}$ parameter is taken into account (0.17 for VIPERS and 0.20dex for GSWLC). A calibration based on [OIII] retrieves the SFR only when additional information such as the metallicity or the ionization parameter of galaxies are used (0.26 for VIPERS and 0.20dex for GSWLC), diminishing its usability as a direct SFR tracer. Based on rest-frame luminosities estimated with CIGALE, we propose our own set of calibrations from FUV, NUV, u-band, 8, 24$μ$m, $L_{TIR}$, H$β$, [OII], and [OIII].

astro-ph.GA↗

arXiv2022

The fundamental metallicity relation from SDSS (z~0) to VIPERS (z~0.7) Data selection or evolution

Aims. We focus on how to homogenize the comparison between galaxy samples having different characteristics. We check the projections of the fundamental metallicity relation (FMR) and the evolution of these projections between a sample selected at $z\sim0$ (SDSS) and $z\sim0.7$ (VIPERS). We check, in particular, whether and to what extent selection criteria can affect the results. Methods. We checked the influence of different biases introduced either by physical constraints (evolution of the luminosity function and differences in the fraction of blue galaxies) or data selection (the signal-to-noise ratio and quality of the spectra) on the FMR and its projections. To separate the differences occurring due to the physical evolution of galaxies with redshift from the false evolution mimed by these biases, we first analyzed the effects of these biases individually on the SDSS sample, and next, starting from the SDSS data, we built a VIPERS-equivalent $z \sim 0$ sample, replicating the main characteristics of VIPERS sample at $z\sim0.7$ for a fair comparison. Results. We found that the FMR projections are all sensitive to biases introduced by the selection on S/N and the quality flags of the emission line measurements in the spectra, especially the $\left[\text{O{\,\sc{iii}}}\right]λ4959$ line. The exception is the metallicity versus the sSFR plane which is insensitive to these biases. The evolution of the luminosity function introduces a bias only in the plane metallicity versus the star formation rate (SFR) while the fraction of blue galaxies has no impact on results.

astro-ph.GA↗

arXiv2021

A Multi-Wavelength Study of Star Formation in 15 Local Star-Forming Galaxies

We have fit the far-ultraviolet (FUV) to mid-infrared (MIR) spectral energy distributions (SEDs) for several nearby galaxies ($<$ 20 Mpc). Global, radial, and local photometric measurements are explored to better understand how SED-derived star formation histories (SFHs) and classic star formation rate (SFR) tracers manifest at different scales. Surface brightness profiles and radial SED fitting provide insight into stellar population gradients in stellar discs and haloes. A double exponential SFH model is used in the SED fitting to better understand the distributions of young vs. old populations throughout these galaxies. Different regions of a galaxy often have undergone very different SFHs, either in strength, rate, timing, or some combination of all these factors. An analysis of individual stellar complexes within these galaxies shows a relationship between the ages of stellar clusters and how these clusters are distributed throughout the galaxy. These star formation properties are presented alongside previously published HI observations to provide a holistic picture of a small sample of nearby star-forming galaxies. The results presented here show that there is a wide variety of star formation gradients and average stellar age distributions that can manifest in a $Λ$CDM universe.

astro-ph.GA↗

arXiv2020

An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: Dust attenuation in high-redshift Lyman break Galaxies

We analyse 870um Atacama Large Millimetre Array (ALMA) dust continuum detections of 41 canonically-selected z~3 Lyman-break galaxies (LBGs), as well as 209 ALMA-undetected LBGs, in follow-up of SCUBA-2 mapping of the UKIDSS Ultra Deep Survey (UDS) field. We find that our ALMA-bright LBGs lie significantly off the locally calibrated IRX-beta relation and tend to have relatively bluer rest-frame UV slopes (as parametrised by beta), given their high values of the 'infrared excess' (IRX=L_IR/L_UV), relative to the average 'local' IRX-beta relation. We attribute this finding in part to the young ages of the underlying stellar populations but we find that the main reason behind the unusually blue UV slopes are the relatively shallow slopes of the corresponding dust attenuation curves. We show that, when stellar masses are being established via SED fitting, it is absolutely crucial to allow the attenuation curves to vary (rather than fixing it on Calzetti-like law), where we find that the inappropriate curves may underestimate the resulting stellar masses by a factor of ~2-3x on average. In addition, we find these LBGs to have relatively high specific star-formation rates (sSFRs), dominated by the dust component, as quantified via the fraction of obscured star formation ( f_obs = SFR_IR/SFR_(UV+IR)). We conclude that the ALMA-bright LBGs are, by selection, massive galaxies undergoing a burst of a star formation (large sSFRs, driven, for example, by secular or merger processes), with a likely geometrical disconnection of the dust and stars, responsible for producing shallow dust attenuation curves.

astro-ph.GA↗

arXiv2019

A Search for Optical Emission from Binary-Black-Hole Merger GW170814 with the Dark Energy Camera

Binary black hole (BBH) mergers found by the LIGO and Virgo detectors are of immense scientific interest to the astrophysics community, but are considered unlikely to be sources of electromagnetic emission. To test whether they have rapidly fading optical counterparts, we used the Dark Energy Camera to perform an $i$-band search for the BBH merger GW170814, the first gravitational wave detected by three interferometers. The 87-deg$^2$ localization region (at 90\% confidence) centered in the Dark Energy Survey (DES) footprint enabled us to image 86\% of the probable sky area to a depth of $i\sim 23$ mag and provide the most comprehensive dataset to search for EM emission from BBH mergers. To identify candidates, we perform difference imaging with our search images and with templates from pre-existing DES images. The analysis strategy and selection requirements were designed to remove supernovae and to identify transients that decline in the first two epochs. We find two candidates, each of which is spatially coincident with a star or a high-redshift galaxy in the DES catalogs, and they are thus unlikely to be associated with GW170814. Our search finds no candidates associated with GW170814, disfavoring rapidly declining optical emission from BBH mergers brighter than $i\sim 23$ mag ($L_{\rm optical} \sim 5\times10^{41}$ erg/s) 1-2 days after coalescence. In terms of GW sky map coverage, this is the most complete search for optical counterparts to BBH mergers to date

astro-ph.HE↗

arXiv2019

A New Generation of Cool White Dwarf Atmosphere Models. III. WD J2356$-$209: Accretion of a Planetesimal with an Unusual Composition

WD J2356$-$209 is a cool metal-polluted white dwarf whose visible spectrum is dominated by a strong and broad sodium feature. Although discovered nearly two decades ago, no detailed and realistic analysis of this star had yet been realized. In the absence of atmosphere models taking into account the nonideal high-density effects arising at the photosphere of WD J2356$-$209, the origin of its unique spectrum had remained nebulous. We use the cool white dwarf atmosphere code presented in the first paper of this series to finally reveal the secrets of this peculiar object and details about the planetesimal that polluted its atmosphere. Thanks to the improved input physics of our models, we find a solution that is in excellent agreement with the photometric observations and the visible spectrum. Our solution reveals that the photosphere of WD J2356$-$209 has a number density ratio of $\log\,{\rm Na/Ca}= 1.0 \pm 0.2$, which is the highest ever found in a white dwarf. Since we do not know how long ago the accretion episode stopped (if it has), we cannot precisely determine the composition nor the mass of the accreted planetesimal. Nevertheless, all scenarios considered indicate that its composition is incompatible with that of chondrite-like material and that its mass was at least $10^{21}\,{\rm g}$.

astro-ph.SR↗

arXiv2018

Comprehensive comparison of models for spectral energy distributions from 0.1 micron to 1 mm of nearby star-forming galaxies

We have fit the far-ultraviolet (FUV) to sub-millimeter (850 micron) spectral energy distributions (SEDs) of the 61 galaxies from the "Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel" (KINGFISH). The fitting has been performed using three models: the Code for Investigating GALaxy Evolution (CIGALE), the GRAphite-SILicate approach (GRASIL), and the Multi-wavelength Analysis of Galaxy PHYSical properties (MAGPHYS). We have analyzed the results of the three codes in terms of the SED shapes, and by comparing the derived quantities with simple "recipes" for stellar mass (Mstar), star-formation rate (SFR), dust mass (Mdust), and monochromatic luminosities. Although the algorithms rely on different assumptions for star-formation history, dust attenuation and dust reprocessing, they all well approximate the observed SEDs and are in generally good agreement for the associated quantities. However, the three codes show very different behavior in the mid-infrared regime, in particular between 25 and 70 micron where there are no observational constraints for the KINGFISH sample. We find that different algorithms give discordant SFR estimates for galaxies with low specific SFR, and that the standard "recipes" for calculating FUV absorption overestimate the extinction compared to the SED-fitting results. Results also suggest that assuming a "standard" constant stellar mass-to-light ratio overestimates Mstar relative to the SED fitting, and we provide new SED-based formulations for estimating Mstar from WISE W1 (3.4 micron) luminosities and colors. From a Principal Component Analysis of Mstar, SFR, Mdust, and O/H, we reproduce previous scaling relations among Mstar, SFR, and O/H, and find that Mdust can be predicted to within roughly 0.3 dex using only Mstar and SFR.

astro-ph.GA↗

arXiv2012

The Arizona CDFS Environment Survey (ACES): A Magellan/IMACS Spectroscopic Survey of the Chandra Deep Field South

We present the Arizona CDFS Environment Survey (ACES), a recently-completed spectroscopic redshift survey of the Chandra Deep Field South (CDFS) conducted using IMACS on the Magellan-Baade telescope. In total, the survey targeted 7277 unique sources down to a limiting magnitude of R = 24.1, yielding 5080 secure redshifts across the ~30' x 30' extended CDFS region. The ACES dataset delivers a significant increase to both the spatial coverage and the sampling density of the spectroscopic observations in the field. Combined with previously-published, spectroscopic redshifts, ACES now creates a highly-complete survey of the galaxy population at R < 23, enabling the local galaxy density (or environment) on relatively small scales (~1 Mpc) to be measured at z < 1 in one of the most heavily-studied and data-rich fields in the sky. Here, we describe the motivation, design, and implementation of the survey and present a preliminary redshift and environment catalog. In addition, we utilize the ACES spectroscopic redshift catalog to assess the quality of photometric redshifts from both the COMBO-17 and MUSYC imaging surveys of the CDFS.

astro-ph.CO↗

arXiv2012

A Dual Narrowband Survey for Hα Emitters at z=2.2: Demonstration of the Technique and Constraints on the Hα Luminosity Function

We present first results from a narrowband imaging program for intermediate redshift emission-line galaxies using the newly commissioned FourStar infrared camera at the 6.5m Magellan telescope. To enable prompt identification of Hα emitters, a pair of custom 1% filters, which sample low-airglow atmospheric windows at 1.19 μm and 2.10 μm, is used to detect both Hα and [OII]λ3727 emission from the same redshift volume at z=2.2. Initial observations are taken over a 130 arcmin^2 area in the CANDELS-COSMOS field. The exquisite image quality resulting from the combination of the instrument, telescope, and standard site conditions (~0.55" FWHM) allows the 1.19 μm and 2.10 μm data to probe 3σ emission-line depths down to 1.0e-17 erg/s/cm^2 and 1.2e-17 erg/s/cm^2 respectively, in less than 10 hours of integration time in each narrowband. For Hα at z=0.8 and z=2.2, these fluxes correspond to observed star formation rates of ~0.3 and ~4 Msun/yr respectively. We find 122 sources with a 1.19 μm excess, and 136 with a 2.10 μm excess, 41 of which show an excess in both bands. The dual narrowband technique, as implemented here, is estimated to identify about >80% of z=2.2 Hα emitters in the narrowband excess population. With the most secure such sample obtained to-date, we compute constraints on the faint-end slope of the z=2.2 Hα luminosity function. These "narrow-deep" FourStar observations have been obtained as part of the larger NewHα Survey, which will combine the data with "wide-shallow" imaging through a similar narrowband filter pair with NEWFIRM at the KPNO/CTIO 4m telescopes, to enable study of both luminous (but rare) and faint emission-line galaxies in the intermediate redshift universe. [Abridged]

astro-ph.CO↗

arXiv2011

Deep UV Luminosity Functions at the Infall Region of the Coma Cluster

We have used deep GALEX observations at the infall region of the Coma cluster to measure the faintest UV luminosity functions (LFs) presented for a rich galaxy cluster thus far. The Coma UV LFs are measured to M_UV = -10.5 in the GALEX FUV and NUV bands, or 3.5 mag fainter than previous studies, and reach the dwarf early-type galaxy population in Coma for the first time. The Schechter faint-end slopes (alpha = -1.39 in both GALEX bands) are shallower than reported in previous Coma UV LF studies owing to a flatter LF at faint magnitudes. A Gaussian-plus-Schechter model provides a slightly better parametrization of the UV LFs resulting in a faint-end slope of ~ -1.15 in both GALEX bands. The two-component model gives faint-end slopes shallower than -1 (a turnover) for the LFs constructed separately for passive and star forming galaxies. The UV LFs for star forming galaxies show a turnover at M_UV ~ -14 owing to a deficit of dwarf star forming galaxies in Coma with stellar masses below M*=10^8 Msun. A similar turnover is identified in recent UV LFs measured for the Virgo cluster suggesting this may be a common feature of local galaxy clusters, whereas the field UV LFs continue to rise at faint magnitudes. We did not identify an excess of passive galaxies as would be expected if the missing dwarf star forming galaxies were quenched inside the cluster. In fact, the LFs for both dwarf passive and star forming galaxies show the same turnover at faint magnitudes. We discuss the possible origin of the missing dwarf star forming galaxies in Coma and their expected properties based on comparisons to local field galaxies.

astro-ph.CO↗

arXiv2010

Host galaxy colour gradients and accretion disc obscuration in AEGIS z~1 X-ray-selected active galactic nuclei

We describe the effect of AGN light on host galaxy optical and UV-optical colours, as determined from X-ray-selected AGN host galaxies at z~1, and compare the AGN host galaxy colours to those of a control sample matched to the AGN sample in both redshift and stellar mass. We identify as X-ray-selected AGNs 8.7 +4/-3 per cent of the red-sequence control galaxies, 9.8 +/-3 per cent of the blue-cloud control galaxies, and 14.7 +4/-3 per cent of the green-valley control galaxies. The nuclear colours of AGN hosts are generally bluer than their outer colours, while the control galaxies exhibit redder nuclei. AGNs in blue-cloud host galaxies experience less X-ray obscuration, while AGNs in red-sequence hosts have more, which is the reverse of what is expected from general considerations of the interstellar medium. Outer and integrated colours of AGN hosts generally agree with the control galaxies, regardless of X-ray obscuration, but the nuclear colours of unobscured AGNs are typically much bluer, especially for X-ray luminous objects. Visible point sources are seen in many of these, indicating that the nuclear colours have been contaminated by AGN light and that obscuration of the X-ray radiation and visible light are therefore highly correlated. Red AGN hosts are typically slightly bluer than red-sequence control galaxies, which suggests that their stellar populations are slightly younger. We compare these colour data to current models of AGN formation. The unexpected trend of less X-ray obscuration in blue-cloud galaxies and more in red-sequence galaxies is problematic for all AGN feedback models, in which gas and dust is thought to be removed as star formation shuts down. [See paper for full abstract.]

astro-ph.CO↗

arXiv2010

The Effects of an AGN on Host Galaxy Colour and Morphology Measurements

We assess the effects of simulated active galactic nuclei (AGNs) on the colour and morphology measurements of their host galaxies. To test the morphology measurements, we select a sample of galaxies not known to host AGNs and add a series of point sources scaled to represent specified fractions of the observed V band light detected from the resulting systems; we then compare morphology measurements of the simulated systems to measurements of the original galaxies. AGN contributions >20 per cent bias most of the morphology measurements tested, though the extent of the apparent bias depends on the morphological characteristics of the original galaxies. We test colour measurements by adding to non-AGN galaxy spectra a quasar spectrum scaled to contribute specified fractions of the rest-frame B band light detected from the resulting systems. A quasar fraction of 5 per cent can move the NUV-r colour of an elliptical galaxy from the UV-optical red sequence to the green valley, and 20 per cent can move it into the blue cloud. Combining the colour and morphology results, we find that a galaxy/AGN system with an AGN contribution >20 per cent may appear bluer and more bulge-dominated than the underlying galaxy. We conclude that (1) bulge-dominated, E/S0/Sa, and early-type morphology classifications are accurate for red AGN host galaxies and may be accurate for blue host galaxies, unless the AGN manifests itself as a well-defined point source; and (2) although highly unobscured AGNs, such as the quasar used for our experiments, can significantly bias the measured colours of AGN host galaxies, it is possible to identify such systems by examining optical images of the hosts for the presence of a point source and/or measuring the level of nuclear obscuration.

astro-ph.CO↗

arXiv2009

The Galactic Center: A Laboratory for Fundamental Astrophysics and Galactic Nuclei

As the closest example of a galactic nucleus, the Galactic center presents an exquisite laboratory for learning about supermassive black holes (SMBH) and their environs. Detailed studies of stellar dynamics deep in the potential well of a galaxy, with exisiting and future large ground-based telescopes, offer several exciting directions in the coming decade. First, it will be possible to obtain precision measurements of the Galaxy's central potential, providing both a unique test of General Relativity (GR) and a detection of the extended dark matter distribution that is predicted to exist around the SMBH. Tests of gravity have not previously been possible on the mass scale of a SMBH. Similarly, only upper limits on the extended matter distribution on small scales currently exist; detection of dark matter on these scales is an important test of Lambda-CDM and the detection of stellar remnants would reveal a population that may dominate the stellar dynamics on the smallest scales. Second, our detailed view of the SMBH and its local gas and stellar environment provides insight into how SMBHs at the centers of galaxies form, grow and interact with their environs as well as on the exotic processes at work in the densest stellar clusters in the Universe. The key questions, still unanswered, of when and how SMBHs formed in the early universe, and the myriad ways in which feedback from SMBHs can affect structure formation, can be informed by directly observing the physical processes operating at the SMBH.

astro-ph.GA↗