1,009 research outputs found
A Spitzer Unbiased Ultradeep Spectroscopic Survey
We carried out an unbiased, spectroscopic survey using the low-resolution
module of the infrared spectrograph (IRS) on board Spitzer targeting two 2.6
square arcminute regions in the GOODS-North field. IRS was used in spectral
mapping mode with 5 hours of effective integration time per pixel. One region
was covered between 14 and 21 microns and the other between 20 and 35 microns.
We extracted spectra for 45 sources. About 84% of the sources have reported
detections by GOODS at 24 microns, with a median F_nu(24um) ~ 100 uJy. All but
one source are detected in all four IRAC bands, 3.6 to 8 microns. We use a new
cross-correlation technique to measure redshifts and estimate IRS spectral
types; this was successful for ~60% of the spectra. Fourteen sources show
significant PAH emission, four mostly SiO absorption, eight present mixed
spectral signatures (low PAH and/or SiO) and two show a single line in
emission. For the remaining 17, no spectral features were detected. Redshifts
range from z ~ 0.2 to z ~ 2.2, with a median of 1. IR Luminosities are roughly
estimated from 24 microns flux densities, and have median values of 2.2 x
10^{11} L_{\odot} and 7.5 x 10^{11} L_{\odot} at z ~ 1 and z ~ 2 respectively.
This sample has fewer AGN than previous faint samples observed with IRS, which
we attribute to the fainter luminosities reached here.Comment: Published in Ap
High-ionization mid-infrared lines as black hole mass and bolometric luminosity indicators in active galactic nuclei
We present relations of the black hole mass and the optical luminosity with
the velocity dispersion and the luminosity of the [Ne V] and the [O IV]
high-ionization lines in the mid-infrared (MIR) for 28 reverberation-mapped
active galactic nuclei. We used high-resolution Spitzer Infrared Spectrograph
and Infrared Space Observatory Short Wavelength Spectrometer data to fit the
profiles of these MIR emission lines that originate from the narrow-line region
of the nucleus. We find that the lines are often resolved and that the velocity
dispersion of [Ne V] and [O IV] follows a relation similar to that between the
black hole mass and the bulge stellar velocity dispersion found for local
galaxies. The luminosity of the [Ne V] and the [O IV] lines in these sources is
correlated with that of the optical 5100A continuum and with the black hole
mass. Our results provide a means to derive black hole properties in various
types of active galactic nuclei, including highly obscured systems.Comment: accepted for publication in ApJ
Energetics of the molecular gas in the H_2 luminous radio galaxy 3C 326: Evidence for negative AGN feedback
We present a detailed analysis of the gas conditions in the H_2 luminous radio galaxy 3C 326 N at z ~ 0.1, which has a low star-formation
rate (SFR ~ 0.07 M_⊙ yr^(−1)) in spite of a gas surface density similar to those in starburst galaxies. Its star-formation efficiency
is likely a factor ~ 10−50 lower than those of ordinary star-forming galaxies. Combining new IRAM CO emission-line interferometry
with existing Spitzer mid-infrared spectroscopy, we find that the luminosity ratio of CO and pure rotational H_2 line emission is factors
10−100 lower than what is usually found. This suggests that most of the molecular gas is warm. The Na D absorption-line profile of
3C 326 N in the optical suggests an outflow with a terminal velocity of ~−1800 km s^(−1) and a mass outflow rate of 30−40 M_⊙ yr^(−1),
which cannot be explained by star formation. The mechanical power implied by the wind, of order 10^(43) erg s^(−1), is comparable to the
bolometric luminosity of the emission lines of ionized and molecular gas. To explain these observations, we propose a scenario where
a small fraction of the mechanical energy of the radio jet is deposited in the interstellar medium of 3C 326 N, which powers the outflow,
and the line emission through a mass, momentum and energy exchange between the different gas phases of the ISM. Dissipation times
are of order 10^(7−8) yrs, similar or greater than the typical jet lifetime. Small ratios of CO and PAH surface brightnesses in another 7 H_2
luminous radio galaxies suggest that a similar form of AGN feedback could be lowering star-formation efficiencies in these galaxies
in a similar way. The local demographics of radio-loud AGN suggests that secular gas cooling in massive early-type galaxies of
≥ 10^(11) M_⊙ could generally be regulated through a fundamentally similar form of “maintenance-phase” AGN feedback
Shocked Molecular Hydrogen in the 3C 326 Radio Galaxy System
The Spitzer spectrum of the giant FR II radio galaxy 3C 326 is dominated by
very strong molecular hydrogen emission lines on a faint IR continuum. The H2
emission originates in the northern component of a double-galaxy system
associated with 3C 326. The integrated luminosity in H2 pure-rotational lines
is 8.0E41 erg/s, which corresponds to 17% of the 8-70 micron luminosity of the
galaxy. A wide range of temperatures (125-1000 K) is measured from the H2 0-0
S(0)-S(7) transitions, leading to a warm H2 mass of 1.1E9 Msun. Low-excitation
ionic forbidden emission lines are consistent with an optical LINER
classification for the active nucleus, which is not luminous enough to power
the observed H2 emission. The H2 could be shock-heated by the radio jets, but
there is no direct indication of this. More likely, the H2 is shock-heated in a
tidal accretion flow induced by interaction with the southern companion galaxy.
The latter scenario is supported by an irregular morphology, tidal bridge, and
possible tidal tail imaged with IRAC at 3-9 micron. Unlike ULIRGs, which in
some cases exhibit H2 line luminosities of comparable strength, 3C 326 shows
little star-formation activity (~0.1 Msun/yr). This may represent an important
stage in galaxy evolution. Starburst activity and efficient accretion onto the
central supermassive black hole may be delayed until the shock-heated H2 can
kinematically settle and coolComment: 27 pages, 7 figures, accepted for publication in the Astrophysical
Journa
PAH Emission from Ultraluminous Infrared Galaxies
We explore the relationships between the Polycyclic Aromatic Hydrocarbon
(PAH) feature strengths, mid-infrared continuum luminosities, far-infrared
spectral slopes, optical spectroscopic classifications, and silicate optical
depths within a sample of 107 ULIRGs observed with the Infrared Spectrograph on
the Spitzer Space Telescope. The detected 6.2 micron PAH equivalent widths
(EQWs) in the sample span more than two orders of magnitude (0.006-0.8 micron),
and ULIRGs with HII-like optical spectra or steep far-infrared spectral slopes
(S_{25} / S_{60} < 0.2) typically have 6.2 micron PAH EQWs that are half that
of lower-luminosity starbursts. A significant fraction (~40-60%) of HII-like,
LINER-like, and cold ULIRGs have very weak PAH EQWs. Many of these ULIRGs also
have large (tau_{9.7} > 2.3) silicate optical depths. The far-infrared spectral
slope is strongly correlated with PAH EQW, but not with silicate optical depth.
In addition, the PAH EQW decreases with increasing rest-frame 24 micron
luminosity. We argue that this trend results primarily from dilution of the PAH
EQW by continuum emission from dust heated by a compact central source,
probably an AGN. High luminosity, high-redshift sources studied with Spitzer
appear to have a much larger range in PAH EQW than seen in local ULIRGs, which
is consistent with extremely luminous starburst systems being absent at low
redshift, but present at early epochs.Comment: 15 pages, 9 Figures; Accepted for publication in Ap
Mid-J CO Emission From NGC 891: Microturbulent Molecular Shocks in Normal Star Forming Galaxies
We have detected the CO(6-5), CO(7-6), and [CI] 370 micron lines from the
nuclear region of NGC 891 with our submillimeter grating spectrometer ZEUS on
the CSO. These lines provide constraints on photodissociation region (PDR) and
shock models that have been invoked to explain the H_2 S(0), S(1), and S(2)
lines observed with Spitzer. We analyze our data together with the H_2 lines,
CO(3-2), and IR continuum from the literature using a combined PDR/shock model.
We find that the mid-J CO originates almost entirely from shock-excited warm
molecular gas; contributions from PDRs are negligible. Also, almost all the H_2
S(2) and half of the S(1) line is predicted to emerge from shocks. Shocks with
a pre-shock density of 2x10^4 cm^-3 and velocities of 10 km/s and 20 km/s for
C-shocks and J-shocks, respectively, provide the best fit. In contrast, the
[CI] line emission arises exclusively from the PDR component, which is best
parameterized by a density of 3.2x10^3 cm^-3 and a FUV field of G_o = 100 for
both PDR/shock-type combinations. Our mid-J CO observations show that
turbulence is a very important heating source in molecular clouds, even in
normal quiescent galaxies. The most likely energy sources for the shocks are
supernovae or outflows from YSOs. The energetics of these shock sources favor
C-shock excitation of the lines.Comment: 18 pages, 2 figures, 6 tables, accepted by Ap
Intrinsic Absorption in the Spectrum of Mrk 279: Simultaneous Chandra, FUSE, and STIS Observations
We present a study of the intrinsic X-ray and far-ultraviolet absorption in
the Seyfert 1.5 galaxy Markarian 279 using simultaneous observations from the
Chandra X-ray Observatory, the Space Telescope Imaging Spectrograph aboard the
Hubble Space Telescope, and the Far Ultraviolet Spectroscopic Explorer (FUSE).
We also present FUSE observations made at three additional epochs. We detect
the Fe K-alpha emission line in the Chandra spectrum, and its flux is
consistent with the low X-ray continuum flux level of Mrk 279 at the time of
the observation. Due to low signal-to-noise ratios in the Chandra spectrum, no
O VII or O VIII absorption features are observable in the Chandra data, but the
UV spectra reveal strong and complex absorption from HI and high-ionization
species such as O VI, N V, and C IV, as well as from low-ionization species
such as C III, N III, C II, and N II in some velocity components. The far-UV
spectral coverage of the FUSE data provides information on high-order Lyman
series absorption, which we use to calculate the optical depths and line and
continuum covering fractions in the intrinsic HI absorbing gas in a
self-consistent fashion. The UV continuum flux of Mrk 279 decreases by a factor
of ~7.5 over the time spanning these observations and we discuss the
implications of the response of the absorption features to this change. From
arguments based on the velocities, profile shapes, covering fractions and
variability of the UV absorption, we conclude that some of the absorption
components, particularly those showing prominent low-ionization lines, are
likely associated with the host galaxy of Mrk 279, and possibly with its
interaction with a close companion galaxy, while the remainder arises in a
nuclear outflow.Comment: To appear in 2004 May ApJS; double-column format; 58 pages, incl. 29
figures, 9 tables; minor changes to tex
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