Outskirts Of Galaxies Research Articles

A unified model for the maximum mass scales of molecular clouds, stellar clusters and high-redshift clumps

We present a simple, self-consistent model to predict the maximum masses of giant molecular clouds (GMCs), stellar clusters and high-redshift clumps as a function of the galactic environment. Recent works have proposed that these maximum masses are set by shearing motions and centrifugal forces, but we show that this idea is inconsistent with the low masses observed across an important range of local-Universe environments, such as low-surface density galaxies and galaxy outskirts. Instead, we propose that feedback from young stars can disrupt clouds before the global collapse of the shear-limited area is completed. We develop a shear–feedback hybrid model that depends on three observable quantities: the gas surface density, the epicylic frequency and the Toomre parameter. The model is tested in four galactic environments: the Milky Way, the Local Group galaxy M31, the spiral galaxy M83 and the high-redshift galaxy zC406690. We demonstrate that our model simultaneously reproduces the observed maximum masses of GMCs, clumps and clusters in each of these environments. We find that clouds and clusters in M31 and in the Milky Way are feedback-limited beyond radii of 8.4 and 4 kpc, respectively, whereas the masses in M83 and zC406690 are shear-limited at all radii. In zC406690, the maximum cluster masses decrease further due to their inspiral by dynamical friction. These results illustrate that the maximum masses change from being shear-limited to being feedback-limited as galaxies become less gas rich and evolve towards low shear. This explains why high-redshift clumps are more massive than GMCs in the local Universe.

Physical Properties of Sub-galactic Clumps at 0.5 ≤ Z ≤ 1.5 in the UVUDF

Abstract We present an investigation of clumpy galaxies in the Hubble Ultra Deep Field at in the rest-frame far-ultraviolet (FUV) using Hubble Space Telescope Wide Field Camera 3 broadband imaging in F225W, F275W, and F336W. An analysis of 1404 galaxies yields 209 galaxies that host 403 kpc scale clumps. These host galaxies appear to be typical star-forming galaxies, with an average of 2 clumps per galaxy and reaching a maximum of 8 clumps. We measure the photometry of the clumps and determine the mass, age, and star formation rates (SFR) using the spectral energy distribution fitting code FAST. We find that clumps make an average contribution of 19% to the total rest-frame FUV flux of their host galaxy. Individually, clumps contribute a median of 5% to the host galaxy SFR and an average of ∼4% to the host galaxy mass, with total clump contributions to the host galaxy stellar mass ranging widely from lower than 1% up to 93%. Clumps in the outskirts of galaxies are typically younger, with higher SFRs, than clumps in the inner regions. The results are consistent with clump migration theories in which clumps form through violent gravitational instabilities in gas-rich turbulent disks, eventually migrate toward the center of the galaxies, and coalesce into the bulge.

The SLUGGS survey: using extended stellar kinematics to disentangle the formation histories of low-mass S0 galaxies

We utilise the DEIMOS instrument on the Keck telescope to measure the wide-field stellar kinematics of early-type galaxies as part of the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey. In this paper, we focus on some of the lowest stellar mass lenticular galaxies within this survey, namely NGC 2549, NGC 4474, NGC 4459 and NGC 7457, performing detailed kinematic analyses out to large radial distances of $\sim 2-3$ effective radii. For NGC 2549, we present the first analysis of data taken with the SuperSKiMS (Stellar Kinematics from Multiple Slits) technique. To better probe kinematic variations in the outskirts of the SLUGGS galaxies, we have defined a local measure of stellar spin. We use this parameter and identify a clear separation in the radial behaviour of stellar spin between lenticular and elliptical galaxies, thereby reinforcing the physically meaningful nature of their morphological classifications. We compare the kinematic properties of our galaxies with those from various simulated galaxies to extract plausible formation scenarios. By doing this for multiple simulations, we assess the consistency of the theoretical results. Comparisons to binary merger simulations show that low-mass lenticular galaxies generally resemble the spiral progenitors more than the merger remnants themselves, an indication that these galaxies are not formed through merger events. We find, however, that recent mergers cannot be ruled out for some lenticular galaxies.

THE INFORMATION CONTENT OF STELLAR HALOS: STELLAR POPULATION GRADIENTS AND ACCRETION HISTORIES IN EARLY-TYPE ILLUSTRIS GALAXIES

ABSTRACT Long dynamical timescales in the outskirts of galaxies preserve the information content of their accretion histories, for example in the form of stellar population gradients. We present a detailed analysis of the stellar halo properties of a statistically representative sample of early-type galaxies from the Illustris simulation, and show that stellar population gradients at large radii can indeed be used to infer basic properties of galactic accretion histories. We measure metallicity, age, and surface-brightness profiles in quiescent Illustris galaxies ranging from = 1010–2 × 1012 and show that they are in reasonable agreement with observations. At fixed mass, galaxies that accreted little of their stellar halo material tend to have steeper metallicity and surface-brightness profiles, between 2–4 effective radii ( ), than those with larger accreted fractions. Profiles of metallicity and surface-brightness in the stellar halo typically flatten from z = 1 to the present. This suggests that the accretion of stars into the stellar halo tends to flatten metallicity and surface-brightness profiles, a picture which is supported by the tight correlation between the two gradients in the stellar halo. We find no statistical evidence of additional information content related to accretion histories in stellar halo metallicity profiles, beyond what is contained in surface-brightness profiles. Age gradients in the stellar halo do not appear to be sensitive to galactic accretion histories, and none of the stellar population gradients studied are strongly correlated with the mean merger mass-ratio. Our findings relate specifically to regions of the stellar halo within 4 , but suggest that future observations that reach large radii outside galaxies (including to 10 and beyond) will have the best potential to constrain galactic accretion histories.

EXPLORING DAMPED Lyα SYSTEM HOST GALAXIES USING GAMMA-RAY BURSTS

ABSTRACT We present a sample of 45 Damped Lyα system (DLA; ) 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 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 spectral energy distribution modeling. We find that DLA counterpart SFRs are not correlated with either redshift or H i column density. Thanks to the combination of Hubble Space Telescope 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 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 Lyman-break galaxy 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.

The origin of the enhanced metallicity of satellite galaxies

Observations of galaxies in the local Universe have shown that both the ionized gas and the stars of satellites are more metal-rich than of equally massive centrals. To gain insight into the connection between this metallicity enhancement and other differences between centrals and satellites, such as their star formation rates, gas content, and growth history, we study the metallicities of >3600 galaxies with M_star > 10^10 M_sun in the cosmological hydrodynamical EAGLE 100 Mpc `Reference' simulation, including ~1500 in the vicinity of galaxy groups and clusters (M_200 >= 10^13 M_sun). The simulation predicts excess gas and stellar metallicities in satellites consistent with observations, except for stellar metallicities at M_star <~ 10^10.2 M_sun where the predicted excess is smaller than observed. The exact magnitude of the effect depends on galaxy selection, aperture, and on whether the metallicity is weighted by stellar mass or luminosity. The stellar metallicity excess in clusters is also sensitive to the efficiency scaling of star formation feedback. We identify stripping of low-metallicity gas from the galaxy outskirts, as well as suppression of metal-poor inflows towards the galaxy centre, as key drivers of the enhancement of gas metallicity. Stellar metallicities in satellites are higher than in the field as a direct consequence of the more metal-rich star forming gas, whereas stripping of stars and suppressed stellar mass growth, as well as differences in accreted vs. in-situ star formation between satellites and the field, are of secondary importance.

Star formation associated with neutral hydrogen in the outskirts of early-type galaxies

About 20 percent of all nearby early-type galaxies ($M_{\star} \gtrsim 6 \times 10^{9}$ M$_{\odot}$) outside the Virgo cluster are surrounded by a disc or ring of low-column-density neutral hydrogen (HI) gas with typical radii of tens of kpc, much larger than the stellar body. In order to understand the impact of these gas reservoirs on the host galaxies, we analyse the distribution of star formation out to large radii as a function of HI properties using GALEX UV and SDSS optical images. Our sample consists of 18 HI-rich galaxies as well as 55 control galaxies where no HI has been detected. In half of the HI-rich galaxies the radial UV profile changes slope at the position of the HI radial profile peak. To study the stellar populations, we calculate the FUV-NUV and UV-optical colours in two apertures, 1-3 and 3-10 R$_{eff}$ . We find that HI -rich galaxies are on average 0.5 and 0.8 mag bluer than the HI-poor ones, respectively. This indicates that a significant fraction of the UV emission traces recent star formation and is associated with the HI gas. Using FUV emission as a proxy for star formation, we estimate the integrated star formation rate in the outer regions (R > 1R$_{eff}$) to be on average $6 \times 10^{-3}$ M$_{\odot}$ yr$^{-1}$ for the HI-rich galaxies. This rate is too low to build a substantial stellar disc and, therefore, change the morphology of the host. We find that the star formation efficiency and the gas depletion time are similar to those at the outskirts of spirals.

Galaxy growth from redshift 5 to 0 at fixed comoving number density

Studying the average properties of galaxies at a fixed comoving number density over a wide redshift range has become a popular observational method, because it may trace the evolution of galaxies statistically. We test this method by comparing the evolution of galaxies at fixed number density and by following individual galaxies through cosmic time (z=0-5) in cosmological, hydrodynamical simulations from OWLS. Comparing progenitors, descendants, and galaxies selected at fixed number density at each redshift, we find differences of up to a factor of three for galaxy and interstellar medium (ISM) masses. The difference is somewhat larger for black hole masses. The scatter in ISM mass increases significantly towards low redshift with all selection techniques. We use the fixed number density technique to study the assembly of dark matter, gas, stars, and black holes and the evolution in accretion and star formation rates. We find three different regimes for massive galaxies, consistent with observations: at high redshift the gas accretion rate dominates, at intermediate redshifts the star formation rate is the highest, and at low redshift galaxies grow mostly through mergers. Quiescent galaxies have much lower ISM masses (by definition) and much higher black hole masses, but the stellar and halo masses are fairly similar. Without active galactic nucleus (AGN) feedback, massive galaxies are dominated by star formation down to z=0 and most of their stellar mass growth occurs in the centre. With AGN feedback, stellar mass is only added to the outskirts of galaxies by mergers and they grow inside-out.

THE STAR FORMATION RATE EFFICIENCY OF NEUTRAL ATOMIC-DOMINATED HYDROGEN GAS IN THE OUTSKIRTS OF STAR-FORMING GALAXIES FROM z ∼ 1 TO z ∼ 3

ABSTRACT Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in damped Lyα systems (DLAs) at 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 1, 2, and 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 H i gas at 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.

Outskirts of Local Group Dwarf Galaxies Revealed by Subaru Hyper Suprime-Cam

AbstractLocal Group galaxies are important targets since their stellar populations can be resolved, and their properties can be investigated in detail with the help of stellar evolutionary models. The newly-built instrument for the 8.2m Subaru Telescope, Hyper Suprime-Cam (HSC), which has a 1 Giga pixel CCD camera with 1.5 degrees field of view, is the best instrument for observing Local Group galaxies. We have carried out a survey for Local Group dwarf galaxies using HSC aiming to shed light on the outskirts of these galaxies. The survey covers target galaxies out beyond the tidal radii down to a depth unexplored by previous surveys. Thanks to the high spatial resolution and high sensitivity provided by the Subaru Telescope, we are able to investigate properties such as spatial distribution and stellar population from the very center of galaxies to the outskirts. In this article, I will show results for the dwarf irregular galaxy NGC 6822 and the dwarf spheroidal galaxy Ursa Minor.

Probing Galactic Outskirts with Dragonfly

AbstractWe describe the challenges inherent to low surface brightness imaging and present some early results from the Dragonfly Nearby Galaxies survey. Wide field, ultra-low surface brightness imaging (μg &gt; 31 mag arcsec−2) of the first eight galaxies in the survey reveals a rich variety in the distribution of stars in the outskirts of luminous nearby galaxies. The mean stellar halo mass fraction is 0.009 ± 0.005 with a peak-to-peak scatter of a factor of &gt; 100. Some galaxies in the sample feature strongly structured halos resembling that of M31, but three of the eight galaxies have halos that are completely undetected in our data. We conclude that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. While the outskirts of some galaxies are dominated by halos with the rich substructures predicted by numerical simulations, in other cases the outermost parts of galaxies are simply the extrapolated smooth starlight from enormous stellar disks that closely trace neutral gas morphology out to around 20 scale lengths.

The 2X-Hidisks of spiral galaxies

AbstractThe outskirts of galaxies — especially the very extended Hidisks of galaxies — are strongly affected by their local environment. I highlight the giant 2X-Hidisks of nearby galaxies (M 83, NGC 3621, and NGC 1512), studied as part of the Local Volume HiSurvey (LVHIS), their kinematics and relation to XUV disks, signatures of tidal interactions and accretion events, theMHI-DHIrelation as well as the formation of tidal dwarf galaxies. - Using multi-wavelength data, I create 3D visualisations of the gas and stars in galaxies, with the shape of their warped disks obtained through kinematic modelling of their Hivelocity fields.

Probing the Baryon Cycle in Galaxy Outskirts

AbstractGalaxies are born and grow within a cosmic ecosystem, in which they receive material from surrounding intergalactic gas via gravitationally-driven inflows and expel material via powerful galactic outflows. These processes, collectively referred to as the baryon cycle, are increasingly believed to govern galaxy growth over cosmic time. I discuss new insights on the baryon cycle using analytic models and hydrodynamical simulations of galaxy evolution, particularly emphasizing how galaxy outskirts are the prime locale within which to observe these processes in action by examining observational tracers such as rest-ultraviolet absorption lines and the neutral and molecular gas content of galaxies.

Jet-CO alignments in the environments high-z radio galaxies

AbstractIn the outskirts of massive high-redshift radio galaxies, powerful radio-jets often interact with ambient warm Lyα-emitting gas. We present the discovery of luminous reservoirs of cold molecular gas in these environments, based on CO(1-0) observations with the Australia Telescope Compact Array. The CO-emission is aligned with the radio jets, and found tens of kpc outside the host galaxy. These molecular gas reservoirs have CO luminosities in the range of those found in submm-galaxies (L'CO ~ 4-9 × 1010 K km/s pc2), but they lack any near-infrared counterpart in deep Spitzer imaging. These results suggest that jet-triggered feedback takes place in the circum-galactic environment of high-z radio galaxies. We prefer the interpretation that the CO-emitting gas is formed when the propagating jets enrich, shock and cool pre-existing dusty halo gas. We further argue that sensitive low-surface-brightness CO observations, using radio interferometers in very compact array-configurations, are essential to study the role of the cold molecular medium in the outskirts of massive high-z galaxies.

What can the outskirts of galaxies tell us about dark matter?

AbstractDeep observations of galaxy outskirts reveal faint extended stellar components (ESCs) of streams, shells, and halos, which are ghostly remnants of the tidal disruption of satellite galaxies. We use cosmological galaxy formation simulations in Cold Dark Matter (CDM) and Warm Dark Matter (WDM) models to explore how the dark matter model influences the spatial, kinematic, and orbital properties of ESCs. These reveal that the spherically averaged stellar mass density at large galacto-centric radius can be depressed by up to a factor of ~10 in WDM models relative to the CDM model, reflecting the anticipated suppressed abundance of satellite galaxies in WDM models. However, these differences are much smaller in WDM models that are compatible with observational limits, and are comparable in size to the system-to-system variation we find within the CDM model. This suggests that it will be challenging to place limits on dark matter using only the unresolved ESC.

Gas Kinematics in the Multiphase Circumgalactic Medium

AbstractWe use high-resolution Keck, VLT, or Hubble Space Telescope spectra of background quasars to examine the kinematic properties of the multiphase, metal-enriched circumgalactic medium in the outskirts of galaxies at 0.08 &lt; zgal &lt; 1.0, focusing on the low-ionization Mgii and high-ionization Ovi doublets. The absorption kinematics of low-ionization gas in the circumgalactic medium depend strongly on the star formation activity and the location about the host galaxy, where the largest velocity dispersions are associated with blue, face-on galaxies probed along the minor axis. Conversely, high-ionization gas kinematics are independent of galaxy star formation activity and orientation.

Supermassive black holes hidden in the outskirts of galaxies

AbstractUltra-Compact dwarf galaxies, which inhabit the outskirts of galaxies, may conceal a significant population of hitherto overlooked super-massive black holes.

GASP: Gas stripping and the outskirts of galaxies as a function of environment

AbstractWe present GASP, an ongoing ESO Large Program with MUSE aiming to study gas removal processes from galaxies at low redshift. GASP targets 100 galaxies with tails, tentacles and one-sided debris. MUSE data allows a detailed investigation of the ionized stripped gas, as well as of the gas and stars within the galaxy out to large distances from the galaxy center. We show the first results for two of the GASP galaxies that are striking cluster jellyfish galaxies of stellar masses ~ 1011M⊙.

Outer Disk Star Formation in HI selected Galaxies

AbstractThe HI in galaxies often extends past their conventionally defined optical extent. I report results from our team which has been probing low intensity star formation in outer disks using imaging in Hα and ultraviolet. Using a sample of hundreds of HI selected galaxies, we confirm that outer disk HII regions and extended UV disks are common. Hence outer disks are not dormant but are dimly forming stars. Although the ultraviolet light in galaxies is more centrally concentrated than the HI, the UV/HI ratio (the Star Formation Efficiency) is nearly constant, with a slight dependency on surface brightness. This result is well accounted for in a model where disks maintain a constant stability parameter Q. This model also accounts for how the ISM and star formation are distributed in the bright parts of galaxies, and how HI appears to trace the distribution of dark matter in galaxy outskirts.

The atomic gas in outer disks in semi-analytic models of galaxy formation

AbstractWe use semi-analytic models of galaxy formation L-Galaxies based on ΛCDM cosmology to study the HI gas component in galaxy outskirts. We adopt the radially-resolved version of the models by Fu et al. (2013), which includes both atomic and molecular gas component in interstellar medium. This model has been recently updated by Luo et al. (2016) to include cold gas stripping in the outer disk regions of the satellite galaxies by ram pressure. In our models, we can perfectly reproduce the HI size-mass relation, which is discovered by Broeils &amp; Rhee (1997) and confirmed by many subsequent observations. In our model, the reason for such tight correlation between HI size and mass is atomic-molecular phase conversion in high gas surface density regions while HI ionization in low gas surface density region, which leads to very narrow distribution of HI mean surface density. The models also reproduce the universal exponential HI radial profiles in galaxy outskirts found by Bluedisk (Wang et al. 2013), which arises from cold gas accretion onto the galaxy disks in exponentially profiles.