154 research outputs found
Feeding IC 342: The nuclear spiral of a starburst galaxy
IC 342 is a large nearby (1.8 Mpc, Turner and Hurt, 1991, hereafter T&H) spiral galaxy undergoing a moderate nuclear starburst. T&H have previously mapped the inner arcminute in CO-13(1-0) using the Owens Valley Millimeter Interferometer and found evidence that the nuclear molecular gas takes the form of spiral arms in a density wave pattern. They suggest that radial streaming along the arms may channel gas from the exterior of the galaxy into the nucleus, feeding the starburst. We have mapped the CO-12(1-0) emission of the inner 2 kpc of IC 342 at 2.8 inch resolution using the Owens Valley Radio Observatory (OVRO) Millimeter Interferometer. The greater sensitivity of CO-12 observations has allowed us to trace the spiral pattern out to a total extent of greater than 1 kpc. The CO-12 observations extend considerably the structure observed at CO-13 and offer further evidence that a spiral density wave may extend from the disk into the nucleus of IC 342
Is a local bar a good place to find a companion? The near infrared morphology of Maffei 2
Maffei 2 is one of the closest large spiral galaxies lying just beyond the Local Group. It would probably be one of the most heavily studied galaxies in the sky were it not for the approximately 5 magnitudes of visual extinction resulting from its position behind the Galactic plane. It is the site of a burst of nuclear star formation indicated by strong infrared and radii continuum emission. Interferometric maps of CO-12 and CO-13 emission indicate that star formation is associated with a barlike structure consisting of arms of molecular gas that extend from within approximately 50 pc of the dynamical center out to a radius of at least 500 pc. HI maps have shown the galaxy to have an angular extent of approximately 15 feet and a neutral gas mass typical of a large spiral galaxy
UV Imaging Polarimetry of the peculiar Seyfert 2 galaxy Mrk 477
We present the results of UV imaging polarimetry of the Seyfert 2 galaxy Mrk
477 taken by the Faint Object Camera onboard the Hubble Space Telescope (HST).
From a previous HST UV image (lambda ~ 2180A), Mrk 477 has been known to have
a pointlike bright UV hotspot in the central region, peculiar among nearby
Seyfert 2 galaxies. There are also claims of UV/optical variability, unusual
for a Seyfert 2 galaxy. Our data show that there is an off-nuclear scattering
region ~ 0."6 (~ 500 pc) NE from the hotspot. The data, after the subtraction
of the instrumental effect due to this bright hotspot region, might indicate
that the scattered light is also detected in the central 0."2 radius region and
is extended to a very wide angle. The hotspot location is consistent with the
symmetry center of the PA pattern, which represents the location of the hidden
nucleus, but our data do not provide a strong upper limit to the distance
between the symmetry center and the hotspot. We have obtained high spatial
resolution color map of the continuum which shows that the nuclear spiral arm
of 0."4 scale (~ 300pc) is significantly bluer than the off-nuclear mirror and
the hotspot region. The nature of the hotspot is briefly discussed.Comment: To appear in Ap
UV Imaging Polarimetry of the Seyfert 2 Galaxy Mrk 3
We present UV imaging polarimetry data of the Seyfert 2 galaxy Mrk 3 taken by
the Hubble Space Telescope. The polarized flux is found to be extended to ~1
kpc from the nucleus, and the position angles of polarization are
centrosymmetric, confirming that the polarization is caused by scattering. We
determine the location of the hidden nucleus as the center of this
centrosymmetric pattern. From the polarization images taken in two broad bands,
we have obtained the color distribution of the polarized flux. Some regions
have blue polarized flux, consistent with optically-thin dust scattering, but
some bright knots have a color similar to that of Seyfert 1 nucleus. Also, the
recent Chandra X-ray observation suggests that the ratio of scattered UV flux
to scattered X-ray flux is rather similar to the intrinsic UV/X-ray ratio in a
Seyfert 1 nucleus, if the observed extended X-ray continuum is scattered light.
While the scattered X-ray would be essentially from electron scattering, the UV
slope and UV/X-ray ratio both being similar to Seyfert 1's would lead to two
possibilities as to the nature of the UV scatterers. One is that the UV may
also be scattered by electrons, in which case the scattering gas is somehow
dust-free. The other is that the UV is scattered by dust grains, but the
wavelength-independent UV scattering with low efficiency indicated by the UV
slope and UV/X-ray ratio would suggest that the grains reside in UV-opaque
clouds, or the dust might be mainly composed of large grains and lacks
small-grain population.Comment: 15 pages, 8 figures (plus 2 color versions of grayscale figures), To
appear in ApJ; minor corrections for the proofs of the manuscrip
Nuclear Bar Catalyzed Star Formation: 13^CO, C18^O and Molecular Gas Properties in the Nucleus of Maffei 2
(Abridged) We present resolution maps of CO, its isotopologues, and HCN from
in the center of Maffei 2. The J=1-0 rotational lines of 12^CO, 13^CO, C18^O
and HCN, and the J=2-1 lines of 13^CO and C18^O were observed with the OVRO and
BIMA arrays. The 2-1/1-0 line ratios of the isotopologues constrain the bulk of
the molecular gas to originate in low excitation, subthermal gas. From LVG
modeling, we infer that the central GMCs have n(H_2) ~10^2.75 cm^-3 and T_k ~
30 K. Continuum emission at 3.4 mm, 2.7 mm and 1.4 mm was mapped to determine
the distribution and amount of HII regions and dust. Column densities derived
from C18^O and 1.4 mm dust continuum fluxes indicate the CO conversion factor
in the center of Maffei 2 is lower than Galactic by factors of ~2-4. Gas
morphology and the clear ``parallelogram'' in the Position-Velocity diagram
shows that molecular gas orbits within the potential of a nuclear (~220 pc)
bar. The nuclear bar is distinct from the bar that governs the large scale
morphology of Maffei 2. Giant molecular clouds in the nucleus are nonspherical
and have large linewidths. Dense gas and star formation are concentrated at the
sites of the x_1-x_2 orbit intersections of the nuclear bar, suggesting that
the starburst is dynamically triggered.Comment: 50 pages, 14 figures, accepted for publication in Ap
UV Spectropolarimetry of Narrow-line Radio Galaxies
We present the results of UV spectropolarimetry (2000 - 3000A) and far-UV
spectroscopy (1500 - 2000A) of two low-redshift narrow-line radio galaxies
(NLRGs) taken with the Faint Object Spectrograph onboard the Hubble Space
Telescope (HST). Spectropolarimetry of several NLRGs has shown that, by the
presence of broad permitted lines in polarized flux spectrum, they have hidden
quasars seen through scattered light. Imaging polarimetry has shown that NLRGs
including our targets often have large scattering regions of a few kpc to >~10
kpc scale. This has posed a problem about the nature of the scatterers in these
radio galaxies. Their polarized continuum has the spectral index similar to or
no bluer than that of quasars, which favors electrons as the dominant
scattering particles. The large scattering region size, however, favors dust
scattering, because of its higher scattering efficiency compared to electrons.
In this paper, we investigate the polarized flux spectrum over a wide
wavelength range, combining our UV data with previous optical/infrared
polarimetry data. We infer that the scattering would be often caused by opaque
dust clouds in the NLRGs and this would be a part of the reason for the
apparently grey scattering. In the high-redshift radio galaxies, these opaque
clouds could be the proto-galactic subunits inferred to be seen in the HST
images. However, we still cannot rule out the possibility of electron
scattering, which could imply the existence of a large gas mass surrounding
these radio galaxies.Comment: 25 pages, 21 figures. To appear in Ap
Near and Mid-IR Photometry of the Pleiades, and a New List of Substellar Candidate Members
We make use of new near and mid-IR photometry of the Pleiades cluster in
order to help identify proposed cluster members. We also use the new photometry
with previously published photometry to define the single-star main sequence
locus at the age of the Pleiades in a variety of color-magnitude planes.
The new near and mid-IR photometry extend effectively two magnitudes deeper
than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new
set of candidate very low mass and sub-stellar mass members of the Pleiades in
the central square degree of the cluster. We identify 42 new candidate members
fainter than Ks =14 (corresponding to 0.1 Mo). These candidate members should
eventually allow a better estimate of the cluster mass function to be made down
to of order 0.04 solar masses.
We also use new IRAC data, in particular the images obtained at 8 um, in
order to comment briefly on interstellar dust in and near the Pleiades. We
confirm, as expected, that -- with one exception -- a sample of low mass stars
recently identified as having 24 um excesses due to debris disks do not have
significant excesses at IRAC wavelengths. However, evidence is also presented
that several of the Pleiades high mass stars are found to be impacting with
local condensations of the molecular cloud that is passing through the Pleiades
at the current epoch.Comment: Accepted to ApJS; data tables and embedded-figure version available
at http://spider.ipac.caltech.edu/staff/stauffer/pleiades07
Protostars in the Elephant Trunk Nebula
The optically-dark globule IC 1396A is revealed using Spitzer images at 3.6,
4.5, 5.8, 8, and 24 microns to be infrared-bright and to contain a set of
previously unknown protostars. The mid-infrared colors of the 24 microns
detected sources indicate several very young (Class I or 0) protostars and a
dozen Class II stars. Three of the new sources (IC 1396A: gamma, delta, and
epsilon) emit over 90% of their bolometric luminosities at wavelengths greater
than 3 microns, and they are located within ~0.02 pc of the ionization front at
the edge of the globule. Many of the sources have spectra that are still rising
at 24 microns. The two previously-known young stars LkHa 349 a and c are both
detected, with component c harboring a massive disk and component a being bare.
Of order 5% of the mass of material in the globule is presently in the form of
protostars in the 10^5 to 10^6 yr age range. This high star formation rate was
likely triggered by radiation from a nearby O star.Comment: Spitzer first ApJS special issue (in press
First Results from the ISOâIRAS Faint Galaxy Survey
We present the first results from the ISO-IRAS Faint Galaxy Survey (IIFGS), a program designed to obtain ISO observations of the most distant and luminous galaxies in the IRAS Faint Source Survey by filling short gaps in the ISO observing schedule with pairs of 12 ÎŒm ISOCAM and 90 ÎŒm ISOPHOT observations. As of 1997 October, over 500 sources have been observed, with an ISOCAM detection rate over 80%, covering over 1.25 deg^2 of sky to an 11.5 ÎŒm point-source completeness limit of approximately 1.0 mJy (corresponding to a ~10 Ï detection sensitivity). Observations are presented for nine sources detected by ISOPHOT and ISOCAM early in the survey for which we have ground-based G- and I-band images and optical spectroscopy. The ground-based data confirm that the IIFGS strategy efficiently detects moderate-redshift (z = 0.11-0.38 for this small sample) strong emission line galaxies with L_(60 ÎŒm) âł 10^(11) L_â; one of our sample has L_(60 ÎŒm) > 10^(12) L_â (H_0 = 75 km s^(-1) Mpc^(-1), Ω = 1). The infrared-optical spectral energy distributions are comparable to those of nearby luminous infrared galaxies, which span the range from pure starburst (e.g., Arp 220) to infrared QSO (Mrk 231). Two of the systems show signs of strong interaction, and four show active galactic nucleus (AGN)-like excitation; one of the AGNs, F15390+6038, which shows a high excitation Seyfert 2 spectrum, has an unusually warm far- to mid-infrared color and may be an obscured QSO. The IIFGS sample is one of the largest and deepest samples of infrared-luminous galaxies available, promising to be a rich sample for studying infrared-luminous galaxies up to z ~ 1 and for understanding the evolution of infrared galaxies and the star formation rate in the universe
The ISO-IRAS Faint Galaxy Survey
The ISO-IRAS Faint Galaxy Survey will obtain comprehensive
space- and ground-based observations of the most distant and luminous galaxies in the IRAS Faint Source Survey. ISO observations are obtained by filling short gaps in the ISO observing schedule with pairs of 11.5ÎŒm ISOCAM and 90ÎŒm ISOPHOT observations. As of the October 1997 date of this Conference, over 500 sources have been observed by ISO with an ISOCAM detection rate exceeding 803. Ground-based
spectrophotometry confirms that the IIFGS efficiently detects moderateredshift, strong emission line Luminous Infrared Galaxies. Spectrophotometry is currently available for 67 galaxies with 0.07 < z < 0. 7 and
L_(fir) > 10^(11) L_â. The galaxies are comparable to nearby LIGs, showing HII/Liner excitation; about 10% exhibit strong AGN characteristics. As a part of this survey we will cover over 1.25 square degrees of
sky to an 11.5ÎŒm limit of approximately l.0mJy, allowing a sensitive estimate of the 11.5ÎŒm logN-logS Relationship. Preliminary ll.5ÎŒm source counts suggest substantial evolution in the mid-infrared galaxy population
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