Propagation Of Star Formation Research Articles

Central regions of LIRGs: rings, hidden starbursts, Supernovae and star clusters

We study star formation (SF) in very active environments, in luminous IR galaxies, which are often interacting. A variety of phenomena are detected, such as central starbursts, circumnuclear SF, obscured SNe tracing the history of recent SF, massive super star clusters, and sites of strong off-nuclear SF. All of these can be ultimately used to define the sequence of triggering and propagation of star-formation and interplay with nuclear activity in the lives of gas rich galaxy interactions and mergers. In this paper we present analysis of high-spatial resolution integral field spectroscopy of central regions of two interacting LIRGs. We detect a nuclear 3.3 μm PAH ring around the core of NGC 1614 with thermal-IR IFU observations. The ring's characteristics and relation to the strong star-forming ring detected in recombination lines are presented, as well as a scenario of an outward expanding starburst likely initiated with a (minor) companion detected within a tidal feature. We then present NIR IFU observations of IRAS 19115-2124, aka the Bird, which is an intriguing triple encounter. The third component is a minor one, but, nevertheless, is the source of 3/4 of the SFR of the whole system. Gas inflows and outflows are detected in their nuclei locations. Finally, we briefly report on our on-going NIR adaptive optics imaging survey of several dozen LIRGs. We have detected highly obscured core-collapse SNe in the central kpc, and discuss the statistics of "missing SNe" due to dust extinction. We are also determining the characteristics of hundreds of super star clusters in and around the core regions of LIRGs, as a function of host-galaxy properties.

The Stellar Content of the Local Group Dwarf Galaxy Phoenix

We present new deep VI ground-based photometry of the Local Group dwarf galaxy Phoenix. Our results confirm that this galaxy is mainly dominated by red stars, with some blue plume stars indicating recent (100 Myr old) star formation in the central part of the galaxy. We have performed an analysis of the structural parameters of Phoenix based on an ESO/SRC scanned plate, in order to search for differentiated components. The elliptical isopleths show a sharp rotation of 90? of their major axis at radius r 115'' from the center, suggesting the existence of two components: an inner component facing in the east-west direction, which contains all the young stars, and an outer component oriented north-south, which seems to be predominantly populated by old stars. These results were then used to obtain the color-magnitude diagrams for three different regions of Phoenix in order to study the variation of the properties of its stellar population. The young population located in the central component of Phoenix shows a clear asymmetry in its distribution, with the younger blue plume stars predominantly located in the western half of the central component and the older core helium-burning stars predominantly situated in the east. This spatial variation could indicate a propagation of star formation across the central component. The H I cloud found at ~6' southwest by Young & Lo could have been involved in this process. We also find a decreasing gradient in the density of the intermediate-age population with the galactocentric radius, based on the number of stars populating the red clump in the color-magnitude diagram. Since no metallicity gradient is apparent, this indicates the presence of a substantial intermediate-age population in the central region of Phoenix that would be less abundant or absent in its outer regions. This result is also consistent with the gradient found in the number of horizontal-branch stars, whose frequency relative to red giant branch stars increases toward the outer part of the galaxy. These results, together with those of our morphological study, suggest the existence of an old, metal-poor population with a spheroidal distribution surrounding the younger inner component of Phoenix. This two-component structure may resemble the halo-disk structure observed in spirals, although more data, in particular on kinematics, are necessary to confirm this. We have estimated the average star formation rate for the last 1 Gyr and for the age interval 1?15 Gyr from the number of blue and red giant branch and asymptotic giant branch stars observed in the color-magnitude diagram. For the central region, the average past star formation rate is very similar to that for the last 1 Gyr. The recent star formation rate of Phoenix is also comparable to that displayed by typical dIrr galaxies, except perhaps for the fact that it lacks any strong very recent burst as exhibited by galaxies such as Sextans A or NGC 6822. The area-normalized star formation rate for the central region of Phoenix is in the range obtained by Hunter & Gallagher for their sample of dIrr galaxies. We have determined a distance modulus for Phoenix of (m - M)0 = 23.0 ? 0.1 using the tip of the red giant branch as a distance indicator. We find four short-period variable candidates from our photometry that might be anomalous Cepheids or W Vir stars. Finally, it is very unlikely that Phoenix has globular clusters, as is expected for a galaxy with such a faint absolute magnitude.

A Hybrid N-Body/Cellular Automaton Scheme for Modelling Propagating Star Formation in Galaxies

We present a numerical scheme for modelling star formation in galaxies based on the dual operation of gravitational instabilities and propagating star formation. Star formation is superposed on a mechanical N-body simulation of a collisional particle system by using a cellular automaton framework. The simulations involve 5000 particles representing gas clouds which can collide inelastically, and have been performed on a 2D square surface with quasiperiodic boundary conditions. The input of kinetic energy from star-forming regions leads to self-regulation of the global dynamics of star formation. The model naturally generates cloud complexes assembled from groups of cloud particles and simulates the ignition and propagation of star formation in such complexes leading to the formation of low density bubbles and expanding shells. The percentage of the model surface undergoing star formation and the ratio of the rates of stimulated to spontaneous star formation are shown to be consistent with models based on the theory of Stochastic Self-Propagating Star Formation (SSPSF). The code is shown to lead to star formation that propagates very nearly isotropically and can thus be adapted to the case of rotating disc galaxies without geometrical modification.

Young Star Clusters in Bright-rimmed Clouds: Small-Scale Sequential Star Formation?

A JHK imaging survey has been made for 44 bright-rimmed clouds associated with IRAS point sources detected by Sugitani et al. We have found small clusters of near-infrared sources having young stellar object (YSO) colors in some of these objects; most of the cluster members are considered to be older than the IRAS point sources and to be pre-main-sequence stars such as T Tauri stars. In at least six bright-rimmed clouds, the clusters are elongated toward the bright-rim tip or the exciting star(s) of the bright rim with the IRAS sources situated near the other end. There is a tendency for bluer (i.e., older) stars to be located closer to the exciting star(s) and for redder (i.e., younger) stars to be closer to the IRAS sources. This asymmetric distribution of the cluster members strongly suggests small-scale sequential star formation or propagation of star formation from the side of the exciting star(s) to the IRAS position in a few times 105 yr, as a result of the advance of the shock caused by the UV radiation from the exciting star(s).

The structure of the spiral arm S4 in andromeda galaxy

TheUBV photometry of 690 stars in the spiral arm S4 and the U magnitudes of 120 stars in the spiral arm S6 with the help of the 2 m RCC telescope of the Rozhen Observatory at the Bulgarian Academy of Sciences, has been used to obtain the colour-magnitude and colour-colour diagrams across the arms. Our age estimations are compared with van den Bergh's (1964). The age gradient across the S4 arm has been found. The colour excessEB-V is highest at the inner edge of the arm S4. From the age we have evaluated the velocity of star formation propagation across the arm S4 ω⊥ ≈60 km s−1 , pattern frequency Ωp ≈14 km s−1 kpc−1 and corotation radiusRc≈20 kpc. The structure of S4 along the arm is complicated. In the OB 82 region an age gradient is absent. The young associationOB 79b is located near the outer edge of S4 and it has a large absorptionAv≈2m.5 contrary to the density wave prediction. This association bears no relation to the spiral density wave and it is probably, supernova events that stimulated the star formation in it. The colour excessER-V is randomly distributed and the youngest stars are concentrated in the middle of the S6 arm. A value of pattern frequency Ωp = 12km s−1 kpc−1 andRc=12 kpc of our Galaxy has been obtained from the age distribution of the open clusters and cepheids across the Carina-Sagittarius arm. The spiral structure of M31 is compared with that of the galaxy. There is a similarity between S4 in M31 and Carina-Sagittarius in the Galaxy, and also between the S6 and Perseus arms. The Orion arm in the Galaxy bears no relation to the wave density.