Several detailed observations, such as those carried out at the Canada-France-Hawaii Telescope (CFHT), have revealed prominent Low Surface Brightness (LSB) fine structures that lead to a change in ...the apparent morphology of galaxies. Previous photometry surveys have developed observational techniques which make use of the diffuse light detected in the external regions of galaxies. In these studies, the outer perturbations have been identified and classified. These include tidal tails, stellar streams, and shells. These structures serve as tracers for interacting events and merging events and retain some memory of the mass assembly of galaxies. Cosmological numerical simulations are required to estimate their visibility timescale, among other properties, in order to reconstruct the merger history of galaxies. In the present work, we analyze a hydrodynamical cosmological simulation to build up a comprehensive interpretation of the properties of fine structures. We present a census of several types of LSB fine structures compiled using a visual inspection of individual snapshots at various points in time. We reconstruct the evolution of the number of fine structures detected around an early-type galaxy and we compare it with the merger history of the galaxy. We find that most fine structures are associated with major and intermediate mass merger events. Their survival timescale ranges between 0.7 and 4 Gyr. Shells and streams remain visible for a longer time, while tidal tails have a shorter lifetime. These estimates for the survival time of collisional debris provide clues for the interpretation of the shape and frequency of fine structures observed in deep images with regard to their mass assembly. We find that the detectability of stellar streams is most sensitive at the surface brightness limit, demonstrating greater visibility at the deepest surface brightness level used in our simulation. We see between two and three times more streams based on a surface brightness cut of 33 mag arcsec−2 than with 29 mag arcsec−2. We find that the detection of shells is strongly dependent upon the projection angle.
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Aims. We present an analysis of the relation between the star formation rate (SFR) surface density (ΣSFR) and mass surface density of molecular gas (ΣH2), commonly referred to as the ...Kennicutt-Schmidt (K-S) relation, on its intrinsic spatial scale, i.e. the size of giant molecular clouds (~10−150 pc), in the central, high-density regions of four nearby low-luminosity active galactic nuclei (AGN). These are AGN extracted from the NUclei of GAlaxies (NUGA) survey. This study investigates the correlations and slopes of the K-S relation, as a function of spatial resolution and of the different 12CO emission lines used to trace ΣH2, and tests its validity in the high-density central regions of spiral galaxies. Methods. We used interferometric IRAM 12CO(1−0) and 12CO(2−1) and SMA 12CO(3−2) emission line maps to derive ΣH2 and HST–Hα images to estimate ΣSFR. Results. Each galaxy is characterized by a distinct molecular SF relation on spatial scales between 20 to 200 pc. The K-S relations can be sublinear, but also superlinear, with slopes ranging from ~0.5 to ~1.3; slopes are generally superlinear on spatial scales >100 pc and sublinear on smaller scales. Depletion times range from ~1 and 2 Gyr, which is compatible with results for nearby normal galaxies. These findings are valid independently of which transition – 12CO(1−0), 12CO(2−1), or 12CO(3−2) – is used to derive ΣH2. Because of either star-formation feedback, the lifetime of clouds, turbulent cascade, or magnetic fields, the K-S relation might be expected to degrade on small spatial scales (<100 pc). However, we find no clear evidence of this, even on scales as small as ~20 pc, and this might be because of the higher density of GMCs in galaxy centers that have to resist higher shear forces. The proportionality between ΣH2 and ΣSFR found between 10 and 100 M⊙ pc-2 is valid even at high densities, ~103 M⊙ pc-2. However, by adopting a common CO-to-H2 conversion factor (αCO), the central regions of the NUGA galaxies have higher ΣSFR for a given gas column than those expected from the models, with a behavior that lies between the mergers or high-redshift starburst systems and the more quiescent star-forming galaxies, assuming that the first ones require a lower value of αCO.
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ABSTRACT
We study the population of star-forming clumps in A521-sys1, a $z=1.04$ system gravitationally lensed by the foreground ($z=0.25$) cluster Abell 0521. The galaxy presents one complete ...counter-image with a mean magnification of $\rm \mu \sim 4$ and a wide arc containing two partial images of A521-sys1 with magnifications reaching $\mu > 20$, allowing the investigations of clumps down to scales of ${\rm R}_{\rm eff} < 50$ pc. We identify 18 unique clumps with a total of 45 multiple images. Intrinsic sizes and UV magnitudes reveal clumps with elevated surface brightnesses comparable to similar systems at redshifts $z\gtrsim 1.0$. Such clumps account for ${\sim}40\,\rm{per\ cent}$ of the galaxy UV luminosity implying a significant fraction of the recent star-formation activity is taking place there. Clump masses range from 106 to $10^9\,{\rm M}_\odot$ and sizes from tens to hundreds of parsec resulting in mass surface densities from 10 to $10^3\,{\rm M}_\odot\,{\rm pc}^{-2}$ with a median of ${\sim}10^2\,{\rm M}_\odot\,{\rm pc}^{-2}$. These properties suggest that we detect star formation taking place across a wide range of scale from cluster aggregates to giant star-forming complexes. We find ages of less than 100 Myr consistent with clumps being observed close to their natal region. The lack of galactocentric trends with mass, mass density, or age and the lack of old migrated clumps can be explained either by dissolution of clumps after few ∼100 Myr or by stellar evolution making them fall below the detectability limits of our data.
From cusps to cores: a stochastic model El-Zant, Amr A; Freundlich, Jonathan; Combes, Françoise
Monthly notices of the Royal Astronomical Society,
09/2016, Volume:
461, Issue:
2
Journal Article
Peer reviewed
Open access
The cold dark matter model of structure formation faces apparent problems on galactic scales. Several threads point to excessive halo concentration, including central densities that rise too steeply ...with decreasing radius. Yet, random fluctuations in the gaseous component can ‘heat’ the centres of haloes, decreasing their densities. We present a theoretical model deriving this effect from first principles: stochastic variations in the gas density are converted into potential fluctuations that act on the dark matter; the associated force correlation function is calculated and the corresponding stochastic equation solved. Assuming a power-law spectrum of fluctuations with maximal and minimal cutoff scales, we derive the velocity dispersion imparted to the halo particles and the relevant relaxation time. We further perform numerical simulations, with fluctuations realized as a Gaussian random field, which confirm the formation of a core within a time-scale comparable to that derived analytically. Non-radial collective modes enhance the energy transport process that erases the cusp, though the parametrizations of the analytical model persist. In our model, the dominant contribution to the dynamical coupling driving the cusp-core transformation comes from the largest scale fluctuations. Yet, the efficiency of the transformation is independent of the value of the largest scale and depends weakly (linearly) on the power-law exponent; it effectively depends on two parameters: the gas mass fraction and the normalization of the power spectrum. This suggests that cusp-core transformations observed in hydrodynamic simulations of galaxy formation may be understood and parametrized in simple terms, the physical and numerical complexities of the various implementations notwithstanding.
Abstract
We report a CO(
J
= 3−2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with the Atacama Large Millimeter/submillimeter Array. The observed ...1 kpc
2
region is at about 1.24 times the optical radius (
R
25
) of the disk, where CO(
J
= 2–1) was previously not detected. The detection and nondetection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A—with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from CO(3−2), range from 8.2 × 10
2
to 2.3 × 10
4
M
⊙
and from 2.7 × 10
2
to 7.5 × 10
3
M
⊙
, respectively. The most massive clouds appear similar to Orion A in star formation activity as well as in mass, as expected if the cloud mass structure is common. The overall low SF activity in the XUV disk could be due to the relative shortage of gas in the molecular phase. The clouds are distributed like chains up to 600 pc (or longer) in length, suggesting that the trigger of cloud formation is on large scales. The common cloud mass structure also justifies the use of high-
J
CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-
z
universe. This study is the first demonstration that CO(3−2) is an efficient tracer of molecular clouds even in low-metallicity environments.
We study the connection between the large-scale dynamics and the gas fuelling towards a central black hole via the analysis of a Milky Way-like simulation at subparsec resolution. This allows us to ...follow a set of processes at various scales (e.g. the triggering of inward gas motion towards inner resonances via the large-scale bar, the connection to the central black hole via minispirals) in a self-consistent manner. This simulation provides further insights on the role of shear for the inhibition of star formation within the bar in regions with significant amount of gas. We also witness the decoupling of the central gas and nuclear cluster from the large-scale disc, via interactions with the black hole. This break of symmetry in the mass distribution triggers the formation of gas clumps organized in a time-varying 250 pc ring-like structure, the black hole being offset by about 70 pc from its centre. Some clumps form stars, while most get disrupted or merge. Supernovae feedback further create bubbles and filaments, some of the gas being expelled to 100 pc or higher above the galaxy plane. This helps remove angular momentum from the gas, which gets closer to the central dark mass. Part of the gas raining down is being accreted, forming a 10 pc polar disc-like structure around the black hole, leading to an episode of star formation. This gives rise to multiple stellar populations with significantly different angular momentum vectors, and may lead to a natural intermittence in the fuelling of the black hole.
ABSTRACT
We present a novel algorithm to detect double nuclei galaxies (DNG) called Gothic (Graph-bOosTed iterated HIll Climbing) – that detects whether a given image of a galaxy has two or more ...closely separated nuclei. Our aim is to test for the presence of dual/multiple active galactic nuclei (AGN) in galaxies that visually represent a DNG. Although galaxy mergers are common, the detection of dual AGN is rare. Their detection is very important as they help us understand the formation of supermassive black hole (SMBH) binaries, SMBH growth and AGN feedback effects in multiple nuclei systems. There is thus a need for an algorithm to do a systematic survey of existing imaging data for the discovery of DNGs and dual AGNs. We have tested Gothic on an established sample of DNGs with a 100 per cent detection rate and subsequently conducted a blind search of 1 million SDSS DR16 galaxies (with spectroscopic data available) lying in the redshift range of z = 0 to 0.75. From the list of candidate DNGs found, we have detected 159 dual AGNs, of which 2 are triple AGN systems. Our results show that dual AGNs are not common, and triple AGN even rarer. The colour (u–r) magnitude plots of the DNGs indicate that star formation is quenched as the nuclei come closer and as the AGN fraction increases. The quenching is especially prominent for dual/triple AGN galaxies that lie at the extreme end of the red sequence.
We study the star formation rate (SFR) vs. molecular gas mass (
M
mol
) scaling relation from hundreds to thousands of parsec in two strongly lensed galaxies at redshift
z
∼ 1, the Cosmic Snake and ...A521. We trace the SFR using extinction-corrected rest-frame UV observations with the
Hubble
Space Telescope (HST), and
M
mol
using detections of the CO(4–3) line with the Atacama Large Millimeter/submillimeter Array (ALMA). The similar angular resolutions of our HST and ALMA observations of 0.15 − 0.2″ combined with magnifications reaching μ > 20 enable us to resolve structures in the galaxies of sizes lower than 100 pc. These resolutions are close to those of studies of nearby galaxies. This allows us to investigate for the first time the Kennicutt–Schmidt (KS) law (SFR–
M
mol
surface densities) at different spatial scales, from galactic scales to ∼100 pc scales, in galaxies at
z
∼ 1. At integrated scales we find that both galaxies satisfy the KS law defined by galaxies at redshifts between 1 and 2.5. We test the resolved KS (rKS) law in cells of sizes down to 200 pc in the two galaxies. We observe that this relationship generally holds in these
z
∼ 1 galaxies, although its scatter increases significantly with decreasing spatial scales. We check the scale dependence of the spatial correlation between the surface densities of SFR and
M
mol
by focusing on apertures centred on individual star-forming regions and molecular clouds. We conclude that star-forming regions and molecular clouds become spatially de-correlated at ≲1 kpc in the Cosmic Snake, whereas they appear de-correlated at all spatial scales (from 400 pc to 6 kpc) in A521.
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20.
Planck 2015 results Alves, Joao; Combes, Françoise; Ferrara, Andrea ...
Astronomy and astrophysics (Berlin),
10/2016, Volume:
594
Journal Article
Peer reviewed
Open access
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