288 research outputs found
On the Unusual Depletions toward Sk 155, or What Are the Small Magellanic Cloud Dust Grains Made of?
The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial
galaxies at high redshifts, differs markedly from that in the Milky Way by
exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the
2175 Angstrom extinction feature, and a local minimum at ~12 micron in its
infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous
grains (i.e. polycyclic aromatic hydrocarbon molecules) which are ubiquitously
seen in the Milky Way. While current models for the SMC dust all rely heavily
on silicates, recent observations of the SMC sightline toward Sk 155 indicated
that Si and Mg are essentially undepleted and the depletions of Fe range from
mild to severe, suggesting that metallic grains and/or iron oxides, instead of
silicates, may dominate the SMC dust. However, in this Letter we apply the
Kramers-Kronig relation to demonstrate that neither metallic grains nor iron
oxides are capable of accounting for the observed extinction; silicates remain
as an important contributor to the extinction, consistent with current models
for the SMC dust.Comment: 12 pages, 3 figures; The Astrophysical Journal Letters, in pres
Physical conditions in the gas phases of the giant HII region LMC-N11 unveiled by Herschel - I. Diffuse [CII] and [OIII] emission in LMC-N11B
(Abridged) The Magellanic Clouds provide a nearby laboratory for metal-poor
dwarf galaxies. The low dust abundance enhances the penetration of UV photons
into the interstellar medium (ISM), resulting in a relatively larger filling
factor of the ionized gas. Furthermore, there is likely a hidden molecular gas
reservoir probed by the [CII]157um line. We present Herschel/PACS maps in
several tracers, [CII], [OI]63um,145um, [NII]122um, [NIII]57um, and [OIII]88um
in the HII region N11B in the Large Magellanic Cloud. Halpha and [OIII]5007A
images were used as complementary data to investigate the effect of dust
extinction. Observations were interpreted with photoionization models to infer
the gas conditions and estimate the ionized gas contribution to the [CII]
emission. Photodissociation regions (PDRs) are probed through polycyclic
aromatic hydrocarbons (PAHs). We first study the distribution and properties of
the ionized gas. We then constrain the origin of [CII]157um by comparing to
tracers of the low-excitation ionized gas and of PDRs. [OIII] is dominated by
extended emission from the high-excitation diffuse ionized gas; it is the
brightest far-infrared line, ~4 times brighter than [CII]. The extent of the
[OIII] emission suggests that the medium is rather fragmented, allowing far-UV
photons to permeate into the ISM to scales of >30pc. Furthermore, by comparing
[CII] with [NII], we find that 95% of [CII] arises in PDRs, except toward the
stellar cluster for which as much as 15% could arise in the ionized gas. We
find a remarkable correlation between [CII]+[OI] and PAH emission, with [CII]
dominating the cooling in diffuse PDRs and [OI] dominating in the densest PDRs.
The combination of [CII] and [OI] provides a proxy for the total gas cooling in
PDRs. Our results suggest that PAH emission describes better the PDR gas
heating as compared to the total infrared emission.Comment: Accepted for publication in Astronomy and Astrophysics. Fixed
inverted line ratio in Sect. 5.
The effects of star formation on the low-metallicity ISM: NGC4214 mapped with Herschel/PACS spectroscopy
We present Herschel/PACS spectroscopic maps of the dwarf galaxy NC4214
observed in 6 far infrared fine-structure lines: [C II] 158mu, [O III] 88mu, [O
I] 63mu, [O I] 146mu, [N II] 122mu, and [N II] 205mu. The maps are sampled to
the full telescope spatial resolution and reveal unprecedented detail on ~ 150
pc size scales. We detect [C II] emission over the whole mapped area, [O III]
being the most luminous FIR line. The ratio of [O III]/[C II] peaks at about 2
toward the sites of massive star formation, higher than ratios seen in dusty
starburst galaxies. The [C II]/CO ratios are 20 000 to 70 000 toward the 2
massive clusters, which are at least an order of magnitude larger than spiral
or dusty starbursts, and cannot be reconciled with single-slab PDR models.
Toward the 2 massive star-forming regions, we find that L[CII] is 0.5 to 0.8%
of the LTIR . All of the lines together contribute up to 2% of LTIR . These
extreme findings are a consequence of the lower metallicity and young,
massive-star formation commonly found in dwarf galaxies. These conditions
promote large-scale photodissociation into the molecular reservoir, which is
evident in the FIR line ratios. This illustrates the necessity to move to
multiphase models applicable to star-forming clusters or galaxies as a whole.Comment: Accepted for publication in the A&A Herschel Special Issu
The star formation properties of disk galaxies: Halpha imaging of galaxies in the Coma supercluster
We present integrated H alpha measurements obtained from imaging observations
of 98 late-type galaxies, primarily selected in the Coma supercluster. These
data, combined with H alpha photometry from the literature, include a magnitude
selected sample of spiral (Sa to Irr) galaxies belonging to the "Great Wall"
complete up to mp=15.4, thus composed of galaxies brighter than Mp=-18.8
(H0=100 km Mpc^-1 s^-1). The frequency distribution of the H alpha E.W.,
determined for the first time from an optically complete sample, is
approximately gaussian peaking at E.W. ~25 A. We find that, at the present
limiting luminosity, the star formation properties of spiral+Irr galaxies
members of the Coma and A1367 clusters do not differ significantly from those
of the isolated ones belonging to the Great Wall. The present analysis confirms
the well known increase of the current massive star formation rate (SFR) with
Hubble type. Moreover perhaps a more fundamental anticorrelation exists between
the SFR and the mass of disk galaxies: low-mass spirals and dwarf systems have
present SFRs ~50 times higher than giant spirals. This result is consistent
with the idea that disk galaxies are coeval, evolve as "closed systems" with
exponentially declining SFR and that the mass of their progenitor protogalaxies
is the principal parameter governing their evolution. Massive systems having
high initial efficiency of collapse, or a short collapse time-scale, have
retained little gas to feed the present epoch of star formation. These findings
support the conclusions of Gavazzi & Scodeggio (1996) who studyed the
color-mass relation of a local galaxy sample and agree with the analysis by
Cowie et al. (1996) who traced the star formation history of galaxies up to
z>1.Comment: 13 pages (LateX) + 24 figures + 4 tables. To appear in Astronomical
Journal, April 1998 issu
Towards an Understanding of the Mid-Infrared Surface Brightness of Normal Galaxies
We report a mid-infrared color and surface brightness analysis of IC 10, NGC
1313, and NGC 6946, three of the nearby galaxies studied under the Infrared
Space Observatory Key Project on Normal Galaxies. Images with < 9 arcsecond
(170 pc) resolution of these nearly face-on, late-type galaxies were obtained
using the LW2 (6.75 mu) and LW3 (15 mu) ISOCAM filters. Though their global
I_nu(6.75 mu)/I_nu(15 mu) flux ratios are similar and typical of normal
galaxies, they show distinct trends of this color ratio with mid-infrared
surface brightness. We find that I_nu(6.75 mu)/I_nu(15 mu) ~< 1 only occurs for
regions of intense heating activity where the continuum rises at 15 micron and
where PAH destruction can play an important role. The shape of the
color-surface brightness trend also appears to depend, to the second-order, on
the hardness of the ionizing radiation. We discuss these findings in the
context of a two-component model for the phases of the interstellar medium and
suggest that star formation intensity is largely responsible for the
mid-infrared surface brightness and colors within normal galaxies, whereas
differences in dust column density are the primary drivers of variations in the
mid-infrared surface brightness between the disks of normal galaxies.Comment: 19 pages, 6 figures, uses AAS LaTeX; to appear in the November
Astronomical Journa
Spitzer Quasar and ULIRG evolution study (QUEST): I. The origin of the far infrared continuum of QSOs
This paper addresses the origin of the far-infrared (FIR) continuum of QSOs,
based on the Quasar and ULIRG Evolution Study (QUEST) of nearby QSOs and ULIRGs
using observations with the Spitzer Space Telescope. For 27 Palomar-Green QSOs
at z <~ 0.3, we derive luminosities of diagnostic lines ([NeII]12.8um,
[NeV]14.3um, [OIV]25.9um) and emission features (PAH7.7um emission which is
related to star formation), as well as continuum luminosities over a range of
mid- to far-infrared wavelengths between 6 and 60um. We detect star-formation
related PAH emission in 11/26 QSOs and fine-structure line emission in all of
them, often in multiple lines. The detection of PAHs in the average spectrum of
sources which lack individual PAH detections provides further evidence for the
widespread presence of PAHs in QSOs. Similar PAH/FIR and [NeII]/FIR ratios are
found in QSOs and in starburst-dominated ULIRGs and lower luminosity
starbursts. We conclude that the typical QSO in our sample has at least 30% but
likely most of the far-infrared luminosity (~ 10^(10...12)Lsun) arising from
star formation, with a tendency for larger star formation contribution at the
largest FIR luminosities. In the QSO sample, we find correlations between most
of the quantities studied including combinations of AGN tracers and starburst
tracers. The common scaling of AGN and starburst luminosities (and fluxes) is
evidence for a starburst-AGN connection in luminous AGN. Strong correlations of
far-infrared continuum and starburst related quantities (PAH, low excitation
[NeII]) offer additional support for the starburst origin of far-infrared
emission.Comment: 39 pages, 8 figures, accepted for publication in Ap
Mid-Infrared Diagnostics of LINERs
We report results from the first mid-infrared spectroscopic study of a
comprehensive sample of 33 LINERs, observed with the Spitzer Space Telescope.
We compare the properties of two different LINER populations: infrared-faint
LINERs, with LINER emission arising mostly in compact nuclear regions, and
infrared-luminous LINERs, which often show spatially extended (non-AGN) LINER
emission. We show that these two populations can be easily distinguished by
their mid-infrared spectra in three different ways: (i) their mid-IR spectral
energy distributions (SEDs), (ii) the emission features of polycyclic aromatic
hydrocarbons (PAHs), and (iii) various combinations of IR fine-structure line
ratios. IR-luminous LINERs show mid-IR SEDs typical of starburst galaxies,
while the mid-IR SEDs of IR-faint LINERs are much bluer. PAH flux ratios are
significantly different in the two groups. Fine structure emission lines from
highly excited gas, such as [O IV], are detected in both populations,
suggesting the presence of an additional AGN also in a large fraction of
IR-bright LINERs, which contributes little to the combined mid-IR light. The
two LINER groups occupy different regions of mid-infrared emission-line
excitation diagrams. The positions of the various LINER types in our diagnostic
diagrams provide important clues regarding the power source of each LINER type.
Most of these mid-infrared diagnostics can be applied at low spectral
resolution, making AGN- and starburst-excited LINERs distinguishable also at
high redshifts.Comment: 11 pages, including 2 eps figures, accepted for publication in ApJ
PGC-1α induced browning promotes involution and inhibits lactation in mammary glands
The PPARγ coactivator 1α (PGC-1α) is a transcriptional regulator of mitochondrial biogenesis and oxidative metabolism. Recent studies have highlighted a fundamental role of PGC-1α in promoting breast cancer progression and metastasis, but the physiological role of this coactivator in the development of mammary glands is still unknown. First, we show that PGC-1α is highly expressed during puberty and involution, but nearly disappeared in pregnancy and lactation. Then, taking advantage of a newly generated transgenic mouse model with a stable and specific overexpression of PGC-1α in mammary glands, we demonstrate that the re-expression of this coactivator during the lactation stage leads to a precocious regression of the mammary glands. Thus, we propose that PGC-1α action is non-essential during pregnancy and lactation, whereas it is indispensable during involution. The rapid preadipocyte–adipocyte transition, together with an increased rate of apoptosis promotes a premature mammary glands involution that cause lactation defects and pup growth retardation. Overall, we provide new insights in the comprehension of female reproductive cycles and lactation deficiency, thus opening new roads for mothers that cannot breastfeed
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