93 research outputs found
The Discovery of 1000 km/s Outflows in Massive Post-starburst Galaxies at z=0.6
Numerical simulations suggest that active galactic nuclei (AGNs) play an
important role in the formation of early-type galaxies by expelling gas and
dust in powerful galactic winds and quenching star formation. However, the
existence of AGN feedback capable of halting galaxy-wide star formation has yet
to be observationally confirmed. To investigate this question, we have obtained
spectra of 14 post-starburst galaxies at z~0.6 to search for evidence of
galactic winds. In 10/14 galaxies we detect Mg II 2796,2803 absorption lines
which are blueshifted by 490 - 2020 km/s with respect to the stars. The median
blueshift is 1140 km/s. We hypothesize that the outflowing gas represents a
fossil galactic wind launched near the peak of the galaxy's activity, a few 100
Myr ago. The velocities we measure are intermediate between those of luminous
starbursts and broad absorption line quasars, which suggests that feedback from
an AGN may have played a role in expelling cool gas and shutting down star
formation.Comment: 5 pages, 2 figures, accepted to ApJ Letter
Unobscured Type 2 Active Galactic Nuclei
Type 2 active galactic nuclei (AGNs) with intrinsically weak broad emission lines (BELs) would be exceptions to the unified model. After examining a number of proposed candidates critically, we find that the sample is contaminated significantly by objects with BELs of strengths indicating that they actually contain intermediate-type AGNs, plus a few Compton-thick sources as revealed by extremely low ratios of X-ray to nuclear IR luminosities. We develop quantitative metrics that show two (NGC 3147 and NGC 4594) of the remaining candidates to have BELs 2-3 orders of magnitude weaker than those of typical type 1 AGNs. Several more galaxies remain as candidates to have anomalously weak BELs, but this status cannot be confirmed with the existing information. Although the parent sample is poorly defined, the two confirmed objects are well under 1% of its total number of members, showing that the absence of a BEL is possible, but very uncommon in AGN. We evaluate these two objects in detail using multi-wavelength measurements including new IR data obtained with Spitzer and ground-based optical spectropolarimeteric observations. They have little X-ray extinction with N_H < ~10^(21) cm^(â2). Their IR spectra show strong silicate emission (NGC 4594) or weak aromatic features on a generally power-law continuum with a suggestion of silicates in emission (NGC 3147). No polarized BEL is detected in NGC 3147. These results indicate that the two unobscured type 2 objects have circumnuclear tori that are approximately face-on. Combined with their X-ray and optical/UV properties, this behavior implies that we have an unobscured view of the nuclei and thus that they have intrinsically weak BELs. We compare their properties with those of the other less-extreme candidates. We then compare the distributions of bolometric luminosities and accretion rates of these objects with theoretical models that predict weak BELs
Local Luminous Infrared Galaxies. III. Co-evolution of Black Hole Growth and Star Formation Activity?
Local luminous infrared (IR) galaxies (LIRGs) have both high star formation
rates (SFR) and a high AGN (Seyfert and AGN/starburst composite) incidence.
Therefore, they are ideal candidates to explore the co-evolution of black hole
(BH) growth and star formation (SF) activity, not necessarily associated with
major mergers. Here, we use Spitzer/IRS spectroscopy of a complete
volume-limited sample of local LIRGs (distances of <78Mpc). We estimate typical
BH masses of 3x10^7 M_sun using [NeIII]15.56micron and optical [OIII]5007A gas
velocity dispersions and literature stellar velocity dispersions. We find that
in a large fraction of local LIRGs the current SFR is taking place not only in
the inner nuclear ~1.5kpc region, as estimated from the nuclear 11.3micron PAH
luminosities, but also in the host galaxy. We next use the ratios between the
SFRs and BH accretion rates (BHAR) to study whether the SF activity and BH
growth are contemporaneous in local LIRGs. On average, local LIRGs have SFR to
BHAR ratios higher than those of optically selected Seyferts of similar AGN
luminosities. However, the majority of the IR-bright galaxies in the RSA
Seyfert sample behave like local LIRGs. Moreover, the AGN incidence tends to be
higher in local LIRGs with the lowest SFRs. All this suggests that in local
LIRGs there is a distinct IR-bright star forming phase taking place prior to
the bulk of the current BH growth (i.e., AGN phase). The latter is reflected
first as a composite and then as a Seyfert, and later as a non-LIRG optically
identified Seyfert nucleus with moderate SF in its host galaxy.Comment: Accepted for publication in Ap
High-Velocity Outflows Without Agn Feedback: Eddington-Limited Star Formation in Compact Massive Galaxies
We present the discovery of compact, obscured star formation in galaxies at z ~ 0.6 that exhibit 1000 km sâ1 outflows. Using optical morphologies from the Hubble Space Telescope and infrared photometry from the Wide-field Infrared Survey Explorer, we estimate star formation rate (SFR) surface densities that approach ÎŁSFR â 3000 M â yrâ1 kpcâ2, comparable to the Eddington limit from radiation pressure on dust grains. We argue that feedback associated with a compact starburst in the form of radiation pressure from massive stars and ram pressure from supernovae and stellar winds is sufficient to produce the high-velocity outflows we observe, without the need to invoke feedback from an active galactic nucleus
Galaxies Probing Galaxies at High Resolution: Co-Rotating Gas Associated with a Milky Way Analog at z=0.4
We present results on gas flows in the halo of a Milky Way-like galaxy at
z=0.413 based on high-resolution spectroscopy of a background galaxy. This is
the first study of circumgalactic gas at high spectral resolution towards an
extended background source (i.e., a galaxy rather than a quasar). Using
longslit spectroscopy of the foreground galaxy, we observe spatially extended H
alpha emission with circular rotation velocity v=270 km/s. Using echelle
spectroscopy of the background galaxy, we detect Mg II and Fe II absorption
lines at impact parameter rho=27 kpc that are blueshifted from systemic in the
sense of the foreground galaxy's rotation. The strongest absorber EW(2796) =
0.90 A has an estimated column density (N_H>10^19 cm-2) and line-of-sight
velocity dispersion (sigma=17 km/s) that are consistent with the observed
properties of extended H I disks in the local universe. Our analysis of the
rotation curve also suggests that this r=30 kpc gaseous disk is warped with
respect to the stellar disk. In addition, we detect two weak Mg II absorbers in
the halo with small velocity dispersions (sigma<10 km/s). While the exact
geometry is unclear, one component is consistent with an extraplanar gas cloud
near the disk-halo interface that is co-rotating with the disk, and the other
is consistent with a tidal feature similar to the Magellanic Stream. We can
place lower limits on the cloud sizes (l>0.4 kpc) for these absorbers given the
extended nature of the background source. We discuss the implications of these
results for models of the geometry and kinematics of gas in the circumgalactic
medium.Comment: 14 pages, 6 figures, submitted to ApJ, comments welcom
SDSS-IV MaNGA : star-formation-driven biconical outflows in the local universe
We present a sample of 48 nearby galaxies with central, biconical outflows identified by the Mapping Nearby Galaxies at APO survey. All considered galaxies have star-formation-driven biconical (SFB) central outflows, with no signs of an active galactic nucleus. We find that the SFB outflows require high central concentration of the star formation rate. This increases the gas velocity dispersion over the equilibrium limit and helps maintain the gas outflows. The central starbursts increase the metallicity, extinction, and the [α/Fe] ratio in the gas. A significant amount of young stellar population at the centers suggests that the SFBs are associated with the formation of young bulges in galaxies. More than 70% of SFB galaxies are group members or have companions with no prominent interaction, or show asymmetry of external isophotes. In 15% of SFB cases, stars and gas rotate in the opposite directions, which points at the gas infall from satellites as the primary reason for triggering the SFB phenomena
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