1,124 research outputs found
Star Formation in Collision Debris: Insights from the modeling of their Spectral Energy Distribution
During galaxy-galaxy interactions, massive gas clouds can be injected into
the intergalactic medium which in turn become gravitationally bound, collapse
and form stars, star clusters or even dwarf galaxies. The objects resulting
from this process are both "pristine", as they are forming their first
generation of stars, and chemically evolved because the metallicity inherited
from their parent galaxies is high. Such characteristics make them particularly
interesting laboratories to study star formation. After having investigated
their star-forming properties, we use photospheric, nebular and dust modeling
to analyze here their spectral energy distribution (SED) from the
far-ultraviolet to the mid-infrared regime for a sample of 7 star-forming
regions. Our analysis confirms that the intergalactic star forming regions in
Stephan's Quintet, around Arp 105, and NGC 5291, appear devoid of stellar
populations older than 10^9 years. We also find an excess of light in the
near-infrared regime (from 2 to 4.5 microns) which cannot be attributed to
stellar photospheric or nebular contributions. This excess is correlated with
the star formation rate intensity suggesting that it is probably due to
emission by very small grains fluctuating in temperature as well as the
polycyclic aromatic hydrocarbons (PAH) line at 3.3 micron. Comparing the
attenuation via the Balmer decrement to the mid-infrared emission allows us to
check the reliability of the attenuation estimate. It suggests the presence of
embedded star forming regions in NGC 5291 and NGC 7252. Overall the SED of
star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble
more that of dusty star-forming regions in galactic disks than to that of
typical star-forming dwarf galaxies.Comment: 22 pages, 24 figures, accepted for publication in A
Plasma and Warm Dust in the Collisional Ring Galaxy VIIZw466 from VLA and ISO Observations
We present the first mid-infrared (Mid-IR) (m) and radio
continuum (20,~6 and 3.6 cm) observations of the star-forming
collisional ring galaxy VII Zw 466 and its host group made with the Infrared
Space Observatory and the NRAO Very Large Array. A search was also made for CO
line emission in two of the galaxies with the Onsala 20m radio telescope and
upper limits were placed on the mass of molecular gas in those galaxies. The
ring galaxy is believed to owe its morphology to a slightly off-center
collision between an `intruder' galaxy and a disk. An off-center collision is
predicted to generate a radially expanding density wave in the disk which
should show large azimuthal variations in overdensity, and have observational
consequences. The radio continuum emission shows the largest asymmetry,
exhibiting a crescent-shaped distribution consistent with either the trapping
of cosmic-ray particles in the target disk, or an enhanced supernova rate in
the compressed region. On the other hand, the ISO observations (especially
those made at m) show a more scattered distribution, with
emission centers associated with powerful star formation sites distributed more
uniformly around the ring. Low-signal to noise observations at
m show possible emission inside the ring, with little emission
directly associated with the \ion{H}{2} regions. The observations emphasize the
complex relationship between the generation of radio emission and the
development of star formation even in relatively simple and well understood
collisional scenarios.Comment: Accepted for publication in The Astrophysical Journal, 23 pages + 6
PS figure
Studying the evolution of galaxies in compact groups over the past 3 Gyr - II. The importance of environment in the suppression of star formation
We present an in depth study on the evolution of galaxy properties in compact
groups over the past 3 Gyr. We are using the largest multi-wavelength sample
to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift
range of 0.01<z<0.23. To derive the physical properties of the galaxies we rely
on ultraviolet (UV)-to-infrared spectral energy distribution modeling, using
CIGALE. Our results suggest that during the 3 Gyr period covered by our sample,
the star formation activity of galaxies in our groups has been substantially
reduced (3-10 times). Moreover, their star formation histories as well as their
UV-optical and mid-infrared colors are significantly different from those of
field and cluster galaxies, indicating that compact group galaxies spend more
time transitioning through the green valley. The morphological transformation
from late-type spirals into early-type galaxies occurs in the mid-infrared
transition zone rather than in the UV-optical green valley. We find evidence of
shocks in the emission line ratios and gas velocity dispersions of the
late-type galaxies located below the star forming main sequence. Our results
suggest that in addition to gas stripping, turbulence and shocks might play an
important role in suppressing the star formation in compact group galaxies.Comment: (Accepted for publication in MNRAS, date of submission November 18,
2015
Spatially Resolved Spitzer-IRS Spectroscopy of the Central Region of M82
We present high spatial resolution (~ 35 parsec) 5-38 um spectra of the
central region of M82, taken with the Spitzer Infrared Spectrograph. From these
spectra we determined the fluxes and equivalent widths of key diagnostic
features, such as the [NeII]12.8um, [NeIII]15.5um, and H_2 S(1)17.03um lines,
and the broad mid-IR polycyclic aromatic hydrocarbon (PAH) emission features in
six representative regions and analysed the spatial distribution of these lines
and their ratios across the central region. We find a good correlation of the
dust extinction with the CO 1-0 emission. The PAH emission follows closely the
ionization structure along the galactic disk. The observed variations of the
diagnostic PAH ratios across M82 can be explained by extinction effects, within
systematic uncertainties. The 16-18um PAH complex is very prominent, and its
equivalent width is enhanced outwards from the galactic plane. We interpret
this as a consequence of the variation of the UV radiation field. The EWs of
the 11.3um PAH feature and the H_2 S(1) line correlate closely, and we conclude
that shocks in the outflow regions have no measurable influence on the H_2
emission. The [NeIII]/[NeII] ratio is on average low at ~0.18, and shows little
variations across the plane, indicating that the dominant stellar population is
evolved (5 - 6 Myr) and well distributed. There is a slight increase of the
ratio with distance from the galactic plane of M82 which we attribute to a
decrease in gas density. Our observations indicate that the star formation rate
has decreased significantly in the last 5 Myr. The quantities of dust and
molecular gas in the central area of the galaxy argue against starvation and
for negative feedback processes, observable through the strong extra-planar
outflows.Comment: 15 pages, 12 figures, 3 tables, ApJ, emulateap
The Spitzer Spirals, Bridges, and Tails Interacting Galaxy Survey: Interaction-Induced Star Formation in the Mid-Infrared
We present Spitzer mid-infrared imaging of a sample of 35 tidally-distorted pre-merger interacting galaxy pairs selected from the Arp Atlas. We compare their global mid-infrared properties with those of normal galaxies from the SINGS Spitzer Legacy survey, and separate the disk emission from that of the tidal features. The [8.0 micron] - [24 micron], [3.6 micron] - [24 micron], and [5.8 micron] - [8.0 micron] colors of these optically-selected interacting galaxies are redder on average than those of spirals, implying enhancements to the mass-normalized star formation rates (SFRs) of a factor of ~2. Furthermore, the 24 micron emission in the Arp galaxies is more centrally concentrated than that in the spirals, suggesting that gas is being concentrated into the inner regions and fueling central star formation. No significant differences can be discerned in the shorter wavelength Spitzer colors of the Arp galaxies compared to the spirals, thus these quantities are less sensitive to star formation enhancements. No strong trend of Spitzer color with pair separation is visible in our sample; this may be because our sample was selected to be tidally disturbed. The tidal features contribute <10% of the total Spitzer fluxes on average. The SFRs implied for the Arp galaxies by the Spitzer 24 micron luminosities are relatively modest, ~1 M(sun)/yr on average
A blind HI survey of the M81 group
Results are presented of the first blind HI survey of the M81 group of
galaxies. The data were taken as part of the HI Jodrell All Sky Survey
(HIJASS). The survey reveals several new aspects to the complex morphology of
the HI distribution in the group. All four of the known dwarf irregular (dIrr)
galaxies close to M81 can be unambiguously seen in the HIJASS data. Each forms
part of the complex tidal structure in the area. We suggest that at least three
of these galaxies may have formed recently from the tidal debris in which they
are embedded. The structure connecting M81 to NGC2976 is revealed as a single
tidal bridge of mass approx. 2.1 x 10^8 Msol and projected spatial extent
approx. 80 kpc. Two `spurs' of HI projecting from the M81 complex to lower
declinations are traced over a considerably larger spatial and velocity extent
than by previous surveys. The dwarf elliptical (dE) galaxies BK5N and Kar 64
lie at the spatial extremity of one of these features and appear to be
associated with it. We suggest that these may be the remnants of dIrrs which
has been stripped of gas and transmuted into dEs by close gravitational
encounters with NGC3077. The nucleated dE galaxy Kar 61 is unambiguously
detected in HI for the first time and has an HI mass of approx.10^8 Msol,
further confirming it as a dE/dIrr transitional object. HIJASS has revealed one
new possible group member, HIJASS J1021+6842. This object contains approx. 2 x
10^7 Msol of HI and lies approx.105arcmin from IC2574. It has no optical
counterpart on the Digital Sky Survey.Comment: To be published in Astrophysical Journal Letters 9 pages, including 3
figure
The Role of a Hot Gas Environment on the Evolution of Galaxies
Most spiral galaxies are found in galaxy groups with low velocity
dispersions; most E/S0 galaxies are found in galaxy groups with relatively high
velocity dispersions. The mass of the hot gas we can observe in the E/S0 groups
via their thermal X-ray emission is, on average, as much as the baryonic mass
of the galaxies in these groups. By comparison, galaxy clusters have as much or
more hot gas than stellar mass. Hot gas in S-rich groups, however, is of low
enough temperature for its X-ray emission to suffer heavy absorption due to
Galactic HI and related observational effects, and hence is hard to detect. We
postulate that such lower temperature hot gas does exist in low velocity
dispersion, S-rich groups, and explore the consequences of this assumption. For
a wide range of metallicity and density, hot gas in S-rich groups can cool in
far less than a Hubble time. If such gas exists and can cool, especially when
interacting with HI in existing galaxies, then it can help link together a
number of disparate observations, both Galactic and extragalactic, that are
otherwise difficult to understand.Comment: 16 pages with one figure. ApJ Letters, in pres
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